Water-equivalent plastic scintillation detectors for high-energy beam dosimetry: I. Physical characteristics and theoretical consideration

Development of a three-dimensional scintillation detector for pencil beam verification in proton therapy patient-specific quality assurance

BACKGROUND

Pencil Beam Scanning proton therapy has many advantages from a therapeutic point of view, but raises technical constraints in terms of treatment verification. The treatment relies on a large number of planned pencil beams (PB) (up to thousands), whose delivery is divided in several low-intensity pulses delivered a high frequency (1 kHz in this study).

PURPOSE

The purpose of this study was to develop a three-dimensional quality assurance system allowing to verify all the PBs' characteristics (position, energy, intensity in terms of delivered monitor unit-MU) of patient treatment plans on a pulse-by-pulse or a PB-by-PB basis.

METHODS

A system named SCICOPRO has been developed. It is based on a 10 × 10 × 10 cm3 scintillator cube and a fast camera, synchronized with beam delivery, recording two views (direct and using a mirror) of the scintillation distribution generated by the pulses. A specific calibration and analysis process allowed to extract the characteristics of all the pulses delivered during the treatment, and consequently of all the PBs. The system uncertainties, defined here as average value + standard deviation, were characterized with a customized irradiation plan at different PB intensities (0.02, 0.1, and 1 MU) and with two patient's treatment plans of three beams each. The system's ability to detect potential treatment delivery problems, such as positioning errors of the treatment table in this work (1° rotations and a 2 mm translation), was assessed by calculating the confidence intervals (CI) for the different characteristics and evaluating the proportion of PBs within these intervals.

RESULTS

The performances of SCICOPRO were evaluated on a pulse-by-pulse basis. They showed a very good signal-to-noise ratio for all the pulse intensities (between 2 × 10-3 MU and 150 × 10-3 MU) allowing uncertainties smaller than 580 µm for the position, 180 keV for the energy and 3% for the intensity on patients treatment plans. The position and energy uncertainties were found to be little dependent from the pulse intensities whereas the intensity uncertainty depends on the pulses number and intensity distribution. Finally, treatment plans evaluations showed that 98% of the PBs were within the CIs with a nominal positioning against 83% or less with the table positioning errors, thus proving the ability of SCICOPRO to detect this kind of errors.

CONCLUSION

The high acquisition rate and the very high sensitivity of the system developed in this work allowed to record pulses of intensities as low as 2 × 10-3 MU. SCICOPRO was thus able to measure all the characteristics of the spots of a treatment (position, energy, intensity) in a single measurement, making it possible to verify their compliance with the treatment plan. SCICOPRO thus proved to be a fast and accurate tool that would be useful for patient-specific quality assurance (PSQA) on a pulse-by-pulse or PB-by-PB verification basis.

Feasibility study for the development of multilayered solar cells for proton linear energy transfer depth profile measurement

BACKGROUND

Previous study proposed a method to measure linear energy transfer (LET) at specific points using the quenching magnitude of thin film solar cells. This study was conducted to propose a more advanced method for measuring the LET distribution.

PURPOSE

This study focuses on evaluating the feasibility of estimating the proton LET distribution in proton therapy. The feasibility of measuring the proton LET and dose distribution simultaneously using a single-channel configuration comprising two solar cells with distinct quenching constants is investigated with the objective of paving the way for enhanced proton therapy dosimetry.

METHODS

Two solar cells with different quenching constants were used to estimate the proton LET distribution. Detector characteristics (e.g., dose linearity and dose-rate dependency) of the solar cells were evaluated to assess their suitability for dosimetry applications. First, using a reference beam condition, the quenching constants of the two solar cells were determined according to the modified Birks equation. The signal ratios of the two solar cells were then evaluated according to proton LET in relation to the estimated quenching constants. The proton LET distributions of six test beams were obtained by measuring the signal ratios of the two solar cells at each depth, and the ratios were evaluated by comparing them with those calculated by Monte Carlo simulation.

RESULTS

The detector characterization of the two solar cells including dose linearity and dose-rate dependence affirmed their suitability for use in dosimetry applications. The maximum difference between the LET measured using the two solar cells and that calculated by Monte Carlo simulation was 2.34 keV/µm. In the case of the dose distribution measured using the method proposed in this study, the maximum difference between range measured using the proposed method and that measured using a multilayered ionization chamber was 0.7 mm. The expected accuracy of simultaneous LET and dose distribution measurement using the method proposed in this study were estimated to be 3.82%. The signal ratios of the two solar cells, which are related to quenching constants, demonstrated the feasibility of measuring LET and dose distribution simultaneously.

CONCLUSION

The feasibility of measuring proton LET and dose distribution simultaneously using two solar cells with different quenching constants was demonstrated. Although the method proposed in this study was evaluated using a single channel by varying the measuring depth, the results suggest that the proton LET and dose distribution can be simultaneously measured if the detector is configured in a multichannel form. We believe that the results presented in this study provide the envisioned transition to a multichannel configuration, with the promise of substantially advancing proton therapy's accuracy and efficacy in cancer treatment.

Characterization of a commercial plastic scintillator for electron FLASH dosimetry

PURPOSE

This study investigated the potential of a commercially available plastic scintillator, the Exradin W2, as a real-time dosimeter for ultra-high-dose-rate (UHDR) electron beams. This work aimed to characterize this system's performance under UHDR conditions and addressed limitations inherent to other conventional dosimetry systems.

METHODS AND MATERIALS

We assessed the W2's performance as a UHDR electron dosimeter using a 16 MeV UHDR electron beam from the FLASH research extension (FLEX) system. Additionally, the vendor provided a beta firmware upgrade to better handle the processing of the high signal generated in the UHDR environment. We evaluated the W2 regarding dose-per-pulse, pulse repetition rate, charge versus distance, and pulse linearity. Absorbed dose measurements were compared against those from a plane-parallel ionization chamber, optically stimulated luminescent dosimeters and radiochromic film.

RESULTS

We observed that the 1 × 1 mm W2 scintillator with the MAX SD was more suitable for UHDR dosimetry compared to the 1 × 3 mm W2 scintillator, capable of matching film measurements within 2% accuracy for dose-per-pulse up to 3.6 Gy/pulse. The W2 accurately ascertained the inverse square relationship regarding charge versus virtual source distance with R2 of ∼1.00 for all channels. Pulse linearity was accurately measured with the W2, demonstrating a proportional response to the delivered pulse number. There was no discernible impact on the measured charge of the W2 when switching between the available repetition rates of the FLEX system (18-180 pulses/s), solidifying consistent beam output across pulse frequencies.

CONCLUSIONS

This study tested a commercial plastic scintillator detector in a UHDR electron beam, paving the way for its potential use as a real-time, patient-specific dosimetry tool for future FLASH radiotherapy treatments. Further research is warranted to test and improve the signal processing of the W2 dosimetry system to accurately measure in UHDR environments using exceedingly high dose-per-pulse and pulse numbers.

Cerenkov free micro-dosimetry in small-field radiation therapy technique

Objective. Optical fiber-based scintillating dosimetry is a recent promising technique owing to the miniature size dosimeter and quality measurement in modern radiation therapy treatment. Despite several advantages, the major issue of using scintillating dosimeters is the Cerenkov effect and predominantly requires extra measurement corrections. Therefore, this work highlighted a novel micro-dosimetry technique to ensure Cerenkov-free measurement in radiation therapy treatment protocol by investigating several dosimetric characteristics.Approach.A micro-dosimetry technique was proposed with the performance evaluation of a novel infrared inorganic scintillator detector (IR-ISD). The detector essentially consists of a micro-scintillating head based on IR-emitting micro-clusters with a sensitive volume of 1.5 × 10-6mm3. The proposed system was evaluated under the 6 MV LINAC beam used in patient treatment. Overall measurements were performed using IBATMwater tank phantoms by following TRS-398 protocol for radiotherapy. Cerenkov measurements were performed for different small fields from 0.5 × 0.5 cm2to 10 × 10 cm2under LINAC. In addition, several dosimetric parameters such as percentage depth dose (PDD), high lateral resolution beam profiling, dose linearity, dose rate linearity, repeatability, reproducibility, and field output factor were investigated to realize the performance of the novel detector.Main results. This study highlighted a complete removal of the Cerenkov effect using a point-like miniature detector, especially for small field radiation therapy treatment. Measurements demonstrated that IR-ISD has acceptable behavior with dose rate variability (maximum standard deviation ∼0.18%) for the dose rate of 20-1000 cGy s-1. An entire linear response (R2= 1) was obtained for the dose delivered within the range of 4-1000 cGy, using a selected field size of 1 × 1 cm2. Perfect repeatability (max 0.06% variation from average) with day-to-day reproducibility (0.10% average variation) was observed. PDD profiles obtained in the water tank present almost identical behavior to the reference dosimeter with a build-up maximum depth dose at 1.5 cm. The small field of 0.5 × 0.5 cm2profiles have been characterized with a high lateral resolution of 100µm.Significance. Unlike recent plastic scintillation detector systems, the proposed micro-dosimetry system in this study requires no Cerenkov corrections and showed efficient performance for several dosimetric parameters. Therefore, it is expected that considering the detector correction factors, the IR-ISD system can be a suitable dose measurement tool, such as in small-field dose measurements, high and low gradient dose verification, and, by extension, in microbeam radiation and FLASH radiation therapy.

A comprehensive investigation of the performance of a commercial scintillator system for applications in electron FLASH radiotherapy

BACKGROUND

Dosimetry in ultra-high dose rate (UHDR) beamlines is significantly challenged by limitations in real-time monitoring and accurate measurement of beam output, beam parameters, and delivered doses using conventional radiation detectors, which exhibit dependencies in ultra-high dose-rate (UHDR) and high dose-per-pulse (DPP) beamline conditions.

PURPOSE

In this study, we characterized the response of the Exradin W2 plastic scintillator (Standard Imaging, Inc.), a water-equivalent detector that provides measurements with a time resolution of 100 Hz, to determine its feasibility for use in UHDR electron beamlines.

METHODS

The W2 scintillator was exposed to an UHDR electron beam with different beam parameters by varying the pulse repetition frequency (PRF), pulse width (PW), and pulse amplitude settings of an electron UHDR linear accelerator system. The response of the W2 scintillator was evaluated as a function of the total integrated dose delivered, DPP, and mean and instantaneous dose rate. To account for detector radiation damage, the signal sensitivity (pC/Gy) of the W2 scintillator was measured and tracked as a function of dose history.

RESULTS

The W2 scintillator demonstrated mean dose rate independence and linearity as a function of integrated dose and DPP for DPP ≤ 1.5 Gy (R2 > 0.99) and PRF ≤ 90 Hz. At DPP > 1.5 Gy, nonlinear behavior and signal saturation in the blue and green signals as a function of DPP, PRF, and integrated dose became apparent. In the absence of Cerenkov correction, the W2 scintillator exhibited PW dependence, even at DPP values <1.5 Gy, with a difference of up to 31% and 54% in the measured blue and green signal for PWs ranging from 0.5 to 3.6 µs. The change in signal sensitivity of the W2 scintillator as a function of accumulated dose was approximately 4%/kGy and 0.3%/kGy for the measured blue and green signal responses, respectively, as a function of integrated dose history.

CONCLUSION

The Exradin W2 scintillator can provide output measurements that are both dose rate independent and linear in response if the DPP is kept ≤1.5 Gy (corresponding to a mean dose rate up to 290 Gy/s in the used system), as long as proper calibration is performed to account for PW and changes in signal sensitivity as a function of accumulated dose. For DPP > 1.5 Gy, the W2 scintillator's response becomes nonlinear, likely due to limitations in the electrometer related to the high signal intensity.

Performance characterization of a novel hybrid dosimetry insert for simultaneous spatial, temporal, and motion-included dosimetry for MR-linac

BACKGROUND

Several (online) adaptive radiotherapy procedures are available to maximize healthy tissue sparing in the presence of inter/intrafractional motion during stereotactic body radiotherapy (SBRT) on an MR-linac. The increased treatment complexity and the motion-delivery interplay during these treatments require MR-compatible motion phantoms with time-resolved dosimeters to validate end-to-end workflows. This is not possible with currently available phantoms.

PURPOSE

Here, we demonstrate a new commercial hybrid film-scintillator cassette, combining high spatial resolution radiochromic film with four time-resolved plastic scintillator dosimeters (PSDs) in an MRI-compatible motion phantom.

METHODS

First, the PSD's performance for consistency, dose linearity, and pulse repetition frequency (PRF) dependence was evaluated using an RW3 solid water slab phantom. We then demonstrated the MRI4D scintillator cassette's suitability for time-resolved and motion-included quality assurance for adapt-to-shape (ATS), trailing, gating, and multileaf collimator (MLC) tracking adaptations on a 1.5 T MR-linac. To do this, the cassette was inserted into the Quasar MRI4D phantom, which we used statically or programmed with artificial and patient-derived motion. Simultaneously with dose measurements, the beam-gating latency was estimated from the time difference between the target entering/leaving the gating window and the beam-on/off times derived from the time-resolved dose measurements.

RESULTS

Experiments revealed excellent detector consistency (standard deviation ≤ $\le$ 0.6%), dose linearity (R2 = 1), and only very low PRF dependence ( ≤ $\le$ 0.4%). The dosimetry cassette demonstrated a near-perfect agreement during an ATS workflow between the time-resolved PSD and treatment planning system (TPS) dose (0%-2%). The high spatial resolution film measurements confirmed this with a 1%/1-mm local gamma pass-rate of 90%. When trailing patient-derived prostate motion for a prostate SBRT delivery, the time-resolved cassette measurements demonstrated how trailing mitigated the motion-induced dose reductions from 1%-17% to 1%-2% compared to TPS dose. The cassette's simultaneously measured spatial dose distribution highlighted the dosimetric gain of trailing by improving the 3%/3-mm local gamma pass-rates from 80% to 97% compared to the static dose. Similarly, the cassette demonstrated the benefit of real-time adaptations when compensating patient-derived respiratory motion by showing how the TPS dose was restored from 2%-56% to 0%-12% (gating) and 1%-26% to 1%-7% (MLC tracking) differences. Larger differences are explainable by TPS-PSD coregistration uncertainty combined with a steep dose gradient outside the PTV. The cassette also demonstrated how the spatial dose distributions were drastically improved by the real-time adaptations with 1%/1-mm local gamma pass-rates that were increased from 8 to 79% (gating) and from 35 to 89% (MLC tracking). The cassette-determined beam-gating latency agreed within ≤ $\le$ 12 ms with the ground truth latency measurement. Film and PSD dose agreed well for most cases (differences relative to TPS dose < $<$ 4%), while film-PSD coregistration uncertainty caused relative differences of 5%-8%.

CONCLUSIONS

This study demonstrates the excellent suitability of a new commercial hybrid film-scintillator cassette for simultaneous spatial, temporal, and motion-included dosimetry.

Development and characterisation of a plastic scintillator dosemeter in high-energy photon beams

The radioluminescent (RL) dosemeter is excellent for real-time radiation measurement and can be used in various applications. A plastic scintillator is often the choice sensor because of its size and tissue equivalency. This study aims to characterise a novel plastic scintillator irradiated with high-energy photon beams. An RL dosimetry system was developed using the plastic scintillator. The RL dosimetry system was irradiated using a linear accelerator to characterise the dose linearity, dose rate, energy dependency and depth dose. The developed system showed a linear response toward the dose and dose rate. An energy dependency factor of 1.06 was observed. Depth dose measurement showed a mean deviation of 1.21% from the treatment planning system. The response and characteristics of the plastic scintillator show that it may be used as an alternative in an RL dosimetry system.

Influence of magnetic field on a novel scintillation dosimeter in a 1.5 T MR-linac

For commissioning and quality assurance for adaptive workflows on the MR-linac, a dosimeter which can measure time-resolved dose during MR image acquisition is desired. The Blue Physics model 10 scintillation dosimeter is potentially an ideal detector for such measurements. However, some detectors can be influenced by the magnetic field of the MR-linac. To assess the calibration methods and magnetic field dependency of the Blue Physics scintillator in the 1.5 T MR-linac. Several calibration methods were assessed for robustness. Detector characteristics and the influence of the calibration methods were assessed based on dose reproducibility, dose linearity, dose rate dependency, relative output factor (ROF), percentage depth dose profile, axial rotation and the radial detector orientation with respect to the magnetic field. The potential application of time-resolved dynamic dose measurements during MRI acquisition was assessed. A variation of calibration factors was observed for different calibration methods. Dose reproducibility, dose linearity and dose rate stability were all found to be within tolerance and were not significantly affected by different calibration methods. Measurements with the detector showed good correspondence with reference chambers. The ROF and radial orientation dependence measurements were influenced by the calibration method used. Axial detector dependence was assessed and relative readout differences of up to 2.5% were observed. A maximum readout difference of 10.8% was obtained when rotating the detector with respect to the magnetic field. Importantly, measurements with and without MR image acquisition were consistent for both static and dynamic situations. The Blue Physics scintillation detector is suitable for relative dosimetry in the 1.5 T MR-linac when measurements are within or close to calibration conditions.

Characteristics of a plastic scintillation detector in photon beam dosimetry

BACKGROUND

Plastic scintillating detectors (PSD) have gained popularity due to small size and are ideally suited in small-field dosimetry due to no correction needed and hence detector reading can be compared to dose. Likewise, these detectors are active and water equivalent. A new PSD from Blue Physics is characterized in photon beam.

PURPOSE

Innovation in small-field dosimetry detector has led us to examine Blue Physics PSD (BP-PSD) for use in photon beams from linear accelerator.

METHODS

BP-PSD was acquired and its characteristics were evaluated in photon beams from a Varian TrueBeam. Data were collected in a 3D water tank. Standard parameters; dose, dose rate, energy, angular dependence and temperature dependence were studied. Depth dose, profiles and output in a reference condition as well as small fields were measured.

RESULTS

BP-PSD is versatile and provides data very similar to an ion chamber when Cerenkov radiation is properly accounted. This device measures data pulse by pulse which very few detectors can perform. The differences between ion chamber data and PSD are < 2% in most cases. The angular dependence is a bit pronounces to 1.5% which is due to PSD housing. Depth dose and profiles are comparable within < 1% to an ion chamber. For small fields this detector provides suitable field output factor compared to other detectors and Monte Carlo (MC) simulated data without any added correction factor.

CONCLUSIONS

The characteristics of Blue Physics PSD is uniquely suitable in photon beam and more so in small fields. The data are reproducible compared to ion chamber for most parameters and ideally suitable for small-field dosimetry without any correction factor.

Small field output correction factors at 18 MV

BACKGROUND

The response of various detectors in the radiotherapy energy range has been investigated, especially for 6 and 10 MV energies for small fields, and is summarized in TRS-483. However, data for accelerator energies above 10 MV are sparse or unavailable for many detectors, especially for the energy of 18 MV. Small variations in field output factors for the commissioning of a treatment planning system can have a high impact on calculation of dose distributions.

PURPOSE

Many studies describe an energy dependence of the response for a large number of detectors. We wanted to close the gap for the 18 MV energy regime and determined field output correction factors for different detectors at 18 MV.

METHODS

An ELEKTA Versa HD accelerator at 18 MV was used together with a PTW MP3 water phantom at an SSD of 90 cm. The following detectors were examined: PTW Semiflex 31021, PinPoint 3D 31022, diode 60012, diode 60008 and microDiamond 60019, Sun Nuclear EDGE detector, IBA PFD, SFD, Razor Chamber, Razor Nano Chamber and Razor Diode, Standard Imaging Scintillator Exradin W2 1x3, W2 1x1 and Gafchromic EBT3 film. The dose response was determined at a depth of 10 cm for square fields between 0.5 and 10 cm side length. As reference data a composure of radiochromic film data for small fields (s ≤ 3 $s\le 3$  cm) and data of all compatible chambers for larger fields (s ≥ 3 $s\ge 3$  cm) was used. The effective field sizes of small fields were determined from profiles obtained on radiochromic film. The obtained field output correction factors obey the rules of the TRS-483 protocol.

RESULTS

The W2 1x1 scintillator and the Razor Chamber showed the smallest deviations from the reference curve. The shielded diodes (diode 60008, EDGE detector) showed the highest over-response at small fields, followed by PFD, microDiamond and the unshielded diodes (diode 60012, SFD). The ionization chambers exhibited the well-known volume effect, that is, strong under-response at small fields of up to 9% for the PinPoint 3D, 7% for the Razor Chamber and up to 30% for the Semiflex detector for the smallest studied field size. The small chambers showed a polarity effect in axial orientation, especially the Razor Nano Chamber. Corrections at 18 MV are generally larger than those provided by TRS-483, continuing the trend of increasing corrections between 6 and 10 MV also at a higher accelerator energy. Only the PinPoint 3D Chamber showed a slightly smaller correction.

CONCLUSIONS

Field output correction factors were determined for square field sizes between 0.5 and 10 cm at 18 MV. Most detectors needed a larger correction than at 6 and 10 MV. Thus, the use of correction factors will improve beam data for 18 MV.

Real-time in vivo dose measurement using ruby-based fibre optic dosimetry during internal radiation therapy

In vivo dosimetry (IVD) in a commonly used liver cancer treatment of selective internal radiation therapy (SIRT) has been done based on the post-treatment image-based dosimetry approach. Real-time IVD is necessary to verify the dose delivery and detect errors during the treatment for better patient outcomes. This study aims to develop a fibre optic dosimeter (FOD) for in vivo real-time dose rate measurement during internal beta radiation therapy, e.g., SIRT. A ruby fibre optic probe was prepared and studied the radioluminescence (RL) characteristics, including its major challenge of stem effect arising from Cherenkov radiation and luminescence from the irradiated fibre. The stem signal was suppressed adequately using the stem removal technique of optical filtering, and only 2.3 ± 1.1% stem signal was contributed to the measured RL signal. A linear dose rate response was observed during the exposure of the ruby probe to varying dose rates using a 6 MeV electron beam and a positron-emitting radionuclide fluorine-18. The ruby exhibited a temporally non-constant RL signal, which increased the RL signal by 0.84 ± 0.29 counts/sec2 during the irradiation of the maximum dose rate used in this study of 9 Gy/min for 2 min. The ability of ruby FOD to measure the absolute dose rate with sufficient stem effect suppression and the linear RL dose rate response indicates its suitability for real-time IVD during internal beta radiation therapy. Future work will investigate the time-dependent RL characteristic of ruby and validate post-treatment image-based dosimetry using ruby-based FOD.

Characterization of a 0.8 mm3Medscint plastic scintillator detector system for small field dosimetry

Objective. Plastic scintillator detectors (PSDs) have demonstrated ability to meet requirements of small field dosimetry. Medscint developed a 1 mm long, 1 mm diameter cylindrical PSD with effective volume of 0.8 mm3. Clinically relevant, small field dosimetric properties of this detector, combined with a novel scintillation dosimetry system-HYPERSCINT RP-200, and HYPERDOSE analysis software were evaluated in this study.Approach. This novel scintillator-based dosimetry system was characterized with 6 MV-WFF and 10 MV-FFF x-ray beams delivered by Varian TrueBeamTMlinear accelerator. The detector was characterized for leakage, short-term repeatability, dose response linearity, angular response, dose rate response, and field size dependence for radiation field sizes of 0.25 × 0.25 to 10 × 10 cm2. Measured detector specific output ratios were compared with microDiamond output factors to determine small field output correction factors,kQclin,Qmsrfclin,fmsr.Main results. The dosimetry system showed excellent short-term repeatability with standard deviation of only 0.04 ± 0.01%. It demonstrated good dose linearity with variations less than 1.0% for 14.4 cGy and above. The dosimetry system was found to be independent of dose rate and angle of irradiation, with deviations for both below 0.5%. Leakage was found to be comparable to background readings. For 6 MV-WFF energy beams, detector specific output ratios for field sizes down to 1 × 1 cm2agreed with output factors measured with PTW TN60019 microDiamond, thus,kQclin,Qmsrfclin,fmsrequates to unity for these field sizes. For 10 MV-FFF energy beams, detector specific output ratios for field sizes down to 2 × 2 cm2agreed with PTW TN60019 microDiamond output factors, thus,kQclin,Qmsrfclin,fmsrequates to unity for these field sizes.kQclin,Qmsrfclin,fmsrfor field sizes down to 0.5 × 0.5 cm2were determined to be within 6% of unity for both 6 MV-WFF and 10 MV-FFF energy beams.Significance. The HYPERSCINT RP-200 dosimetry system coupled with a 0.8 mm3PSD showed excellent dosimetric properties and was found to be clinically relevant for relative dosimetry down to field sizes of 0.5 × 0.5 cm2and potentially smaller.

Ultra-fast, high spatial resolution single-pulse scintillation imaging of synchrocyclotron pencil beam scanning proton delivery

Objective.To demonstrates the ability of an ultra-fast imaging system to measure high resolution spatial and temporal beam characteristics of a synchrocyclotron proton pencil beam scanning (PBS) system.Approach.An ultra-fast (1 kHz frame rate), intensified CMOS camera was triggered by a scintillation sheet coupled to a remote trigger unit for beam on detection. The camera was calibrated using the linear (R2> 0.9922) dose response of a single spot beam to varying currents. Film taken for the single spot beam was used to produce a scintillation intensity to absolute dose calibration.Main results. Spatial alignment was confirmed with the film, where thexandy-profiles of the single spot cumulative image agreed within 1 mm. A sample brain patient plan was analyzed to demonstrate dose and temporal accuracy for a clinically-relevant plan, through agreement within 1 mm to the planned and delivered spot locations. The cumulative dose agreed with the planned dose with a gamma passing rate of 97.5% (2 mm/3%, 10% dose threshold).Significance. This is the first system able to capture single-pulse spatial and temporal information for the unique pulse structure of a synchrocyclotron PBS systems at conventional dose rates, enabled by the ultra-fast sampling frame rate of this camera. This study indicates that, with continued camera development and testing, target applications in clinical and FLASH proton beam characterization and validation are possible.

Performance of the HYPERSCINT scintillation dosimetry research platform for the 1.5 T MR-linac

Objective.Adaptive radiotherapy techniques available on the MR-linac, such as daily plan adaptation, gating, and dynamic tracking, require versatile dosimetric detectors to validate end-to-end workflows. Plastic scintillator detectors (PSDs) offer great potential with features including: water equivalency, MRI-compatibility, and time-resolved dose measurements. Here, we characterize the performance of the HYPERSCINT RP-200 PSD (MedScint, Quebec, CA) in a 1.5 T MR-linac, and we demonstrate its suitability for dosimetry, including in a moving target.Approach.Standard techniques of detector testing were performed using a Beamscan water tank (PTW, Freiburg, DE) and compared to microDiamond (PTW, Freiburg, DE) readings. Orientation dependency was tested using the same phantom. An RW3 solid water phantom was used to evaluate detector consistency, dose linearity, and dose rate dependence. To determine the sensitivity to motion and to MRI scanning, the Quasar MRI^4Dphantom (Modus, London, ON) was used statically or with sinusoidal motion (A= 10 mm,T= 4 s) to compare PSD and Semiflex ionization chamber (PTW, Freiburg, DE) readings. Conformal beams from gantry 0° and 90° were used as well as a 15-beam 8 × 7.5 Gy lung IMRT plan.Main results.Measured profiles, PDD curves and field-size dependence were consistent with the microDiamond readings with differences well within our clinical tolerances. The angular dependence gave variations up to 0.8% when not irradiating directly from behind the scintillation point. Experiments revealed excellent detector consistency between repeated measurements (SD = 0.06%), near-perfect dose linearity (R²= 1) and a dose rate dependence <0.3%. Dosimetric effects of MRI scanning (≤0.3%) and motion (≤1.3%) were minimal. Measurements were consistent with the Semiflex (differences ≤1%), and with the treatment planning system with differences of 0.8% and 0.4%, with and without motion.Significance.This study demonstrates the suitability of the HYPERSCINT PSD for accurate time-resolved dosimetry measurements in the 1.5 T MR-linac, including during MR scanning and target motion.

Determination of the proton LET using thin film solar cells coated with scintillating powder

PURPOSE

The amount of luminescent light detected in a scintillator is reduced with increased proton linear energy transfer (LET) despite receiving the same proton dose, through a phenomenon called quenching. This study evaluated the ability of a solar cell coated with scintillating powder (SC-SP) to measure therapeutic proton LET by measuring the quenching effect of the scintillating powder using a solar cell while simultaneously measuring the dose of the proton beam.

METHODS

SC-SP was composed of a flexible thin film solar cell and scintillating powder. The LET and dose of the pristine Bragg peak in the 14 cm range were calculated using a validated Monte Carlo model of a double scattering proton beam nozzle. The SC-SP was evaluated by measuring the proton beam under the same conditions at specific depths using SC-SP and Markus chamber. Finally, the 10 and 20 cm range pristine Bragg peaks and 5 cm spread-out Bragg peak (SOBP) in the 14 cm range were measured using the SC-SP and the Markus chamber. LETs measured using the SC-SP were compared with those calculated using Monte Carlo simulations.

RESULTS

The quenching factors of the SC-SP and solar cell alone, which were slopes of linear fit obtained from quenching correction factors according to LET, were 0.027 and 0.070 µm/keV (R² : 0.974 and 0.975). For pristine Bragg peaks in the 10 and 20 cm ranges, the maximum differences between LETs measured using the SC-SP and calculated using Monte Carlo simulations were 0.5 keV/µm (15.7%) and 1.2 keV/µm (12.0%), respectively. For a 5 cm SOBP proton beam, the LET measured using the SC-SP and calculated using Monte Carlo simulations differed by up to 1.9 keV/µm (18.7%).

CONCLUSIONS

Comparisons of LETs for pristine Bragg peaks and SOBP between measured using the SC-SP and calculated using Monte Carlo simulations indicated that the solar cell-based system could simultaneously measure both LET and dose in real-time and is cost-effective.

Small field output factor measurement and verification for CyberKnife robotic radiotherapy and radiosurgery system using 3D polymer gel, ionization chamber, diode, diamond and scintillator detectors, Gafchromic film and Monte Carlo simulation

The dosimetry of small fields has become tremendously important with the advent of intensity-modulated radiation therapy (IMRT) and stereotactic radiosurgery, where small field segments or very small fields are used to treat tumors. With high dose gradients in the stereotactic radiosurgery or radiotherapy treatment, small field dosimetry becomes challenging due to the lack of lateral electronic equilibrium in the field, x-ray source occlusion, and detector volume averaging. Small volume and tissue-equivalent detectors are recommended to overcome the challenges. With the lack of a perfect radiation detector, studies on available detectors are ongoing with reasonable disagreement and uncertainties. The joint IAEA and AAPM international code of practice (CoP) for small field dosimetry, TRS 483 (Alfonso et al., 2017) provides guidelines and recommendations for the dosimetry of small static fields in external beam radiotherapy. The CoP provides a methodology for field output factor (FOF) measurements and use of field output correction factors for a series of small field detectors and strongly recommends additional measurements, data collection and verification for CyberKnife (CK) robotic stereotactic radiotherapy/radiosurgery system using the listed detectors and more new detectors so that the FOFs can be implemented clinically. The present investigation is focused on using 3D gel along with some other commercially available detectors for the measurement and verification of field output factors (FOFs) for the small fields available in the CK system. The FOF verification was performed through a comparison with published data and Monte Carlo simulation. The results of this study have proved the suitability of an in-house developed 3D polymer gel dosimeter, several commercially available detectors, and Gafchromic films as a part of small field dosimetric measurements for the CK system.

Characterization of the Plastic Scintillator Detector System Exradin W2 in a High Dose Rate Flattening-Filter-Free Photon Beam

(1) Background: The Exradin W2 is a commercially available scintillator detector designed for reference and relative dosimetry in small fields. In this work, we investigated the performance of the W2 scintillator in a 10 MV flattening-filter-free photon beam and compared it to the performance of ion chambers designed for small field measurements. (2) Methods: We measured beam profiles and percent depth dose curves with each detector and investigated the linearity of each system based on dose per pulse (DPP) and pulse repetition frequency. (3) Results: We found excellent agreement between the W2 scintillator and the ion chambers for beam profiles and percent depth dose curves. Our results also showed that the two-voltage method of calculating the ion recombination correction factor was sufficient to correct for the ion recombination effect of ion chambers, even at the highest DPP. (4) Conclusions: These findings show that the W2 scintillator shows excellent agreement with ion chambers in high DPP conditions.

Mitigating the uncertainty in small field dosimetry by leveraging machine learning strategies

Small field dosimetry is significantly different from the dosimetry of broad beams due to loss of electron side scatter equilibrium, source occlusion, and effects related to the choice of detector. However, use of small fields is increasing with the increase in indications for intensity-modulated radiation therapy and stereotactic body radiation therapy, and thus the need for accurate dosimetry is ever more important. Here we propose to leverage machine learning (ML) strategies to reduce the uncertainties and increase the accuracy in determining small field output factors (OFs). Linac OFs from a Varian TrueBeam STx were calculated either by the treatment planning system (TPS) or measured with a W1 scintillator detector at various multi-leaf collimator (MLC) positions, jaw positions, and with and without contribution from leaf-end transmission. The fields were defined by the MLCs with the jaws at various positions. Field sizes between 5 and 100 mm were evaluated. Separate ML regression models were generated based on the TPS calculated or the measured datasets. Accurate predictions of small field OFs at different field sizes (FSs) were achieved independent of jaw and MLC position. A mean and maximum % relative error of 0.38 ± 0.39% and 3.62%, respectively, for the best-performing models based on the measured datasets were found. The prediction accuracy was independent of contribution from leaf-end transmission. Several ML models for predicting small field OFs were generated, validated, and tested. Incorporating these models into the dose calculation workflow could greatly increase the accuracy and robustness of dose calculations for any radiotherapy delivery technique that relies heavily on small fields.

Direct in-water radiation dose measurements using Cherenkov emission corrected signals from polarization imaging for a clinical radiotherapy application

Cherenkov emission (CE) is a visible blueish light emitted in water mediums irradiated by most radiotherapy treatment beams. However, CE is produced anisotropically which currently imposes a geometrical constraint uncertainty for dose measurements. In this work, polarization imaging is proposed and described as a method enabling precise 2D dose measurements using CE. CE produced in a water tank is imaged from four polarization angles using a camera coupled to a rotating polarizer. Using Malus' law, the polarized component of CE is isolated and corrected with Monte Carlo calculated CE polar and azimuthal angular distributions. Projected dose measurements resulting from polarization-corrected CE are compared to equivalent radiochromic film measurements. Overall, agreement between polarized corrected CE signal and films measurements is found to be within 3%, for projected percent depth dose (PPDD) and profiles at the different tested energies ([Formula: see text]: 6 and [Formula: see text], e[Formula: see text]: 6 and 18[Formula: see text]). In comparison, raw Cherenkov emission presented deviations up 60% for electron beam PPDDs and 20% for photon beams PPDDs. Finally, a degree of linear polarization between 29% and 47% was measured for CE in comparison to [Formula: see text]% for scintillation. Hence, polarization imaging is found to be a promising and powerful method for improved radio-luminescent dose measurements with possible extensions to signal separation.

On the use of polychromatic cameras for high spatial resolution spectral dose measurements

Objective. Despite the demonstrated benefits of hyperspectral formalism for stem effect corrections in the context of fiber dose measurements, this approach has not been yet translated into volumetric measurements where cameras are typically used for their distinguishing spatial resolution. This work investigates demosaicing algorithms for polychromatic cameras based spectral imaging. Approach. The scintillation and Cherenkov signals produced in a radioluminescent phantom are imaged by a polychromatic camera and isolated using the spectral formalism. To do so, five demosaicing algorithms are investigated from calibration to measurements: a clustering method and four interpolation algorithms. The resulting accuracy of scintillation and Cherenkov images is evaluated with measurements of the differences (mean ± standard deviation) between the obtained and expected signals from profiles drawn across a scintillation spot. Signal-to-noise ratio and signal-to-background ratio are further measured and compared in the resulting scintillation images. Finally, the resulting differences on the scintillation signal from a 0.2 × 0.2 cm2 region-of-interest (ROI) were reported. Main results. Clustering, OpenCV, bilinear, Malvar and Menon demosaicing algorithms respectively yielded differences of 3 ± 5%, 1 ± 3%, 1 ± 3%, 1 ± 2% and 2 ± 4% in the resulting scintillation images. For the Cherenkov images, all algorithms provided differences below 1%. All methods enabled measurements over the detectability (SBR > 2) and sensitivity (SNR > 5) thresholds with the bilinear algorithm providing the best SNR value. Clustering, OpenCV, bilinear, Malvar and Menon demosaicing algorithms respectively provided differences on the ROI analysis of 7 ± 5%, 3 ± 2%, 3 ± 2%, 4 ± 2%, 7 ± 3%. Significance. Radioluminescent signals can accurately be isolated using a single polychromatic camera. Moreover, demosaicing using a bilinear kernel provided the best results and enabled Cherenkov signal subtraction while preserving the full spatial resolution of the camera.

Lead-doped scintillator dosimeters for detection of ultrahigh dose-rate x-rays

Objective. Lead-doped scintillator dosimeters may be well suited for the dosimetry of FLASH-capable x-ray radiotherapy beams. Our study explores the dose rate dependence and temporal resolution of scintillators that makes them promising in the accurate detection of ultrahigh dose-rate (UHDR) x-rays. Approach. We investigated the response of scintillators with four material compositions to UHDR x-rays produced by a conventional x-ray tube. Scintillator output was measured using the HYPERSCINT-RP100 dosimetry research platform. Measurements were acquired at high frame rates (400 fps) which allowed for accurate dose measurements of sub-second radiation exposures from 1 to 100 ms. Dose-rate dependence was assessed by scaling tube current of the x-ray tube. Scintillator measurements were validated against Monte Carlo simulations of the probe geometries and UHDR x-ray system. Calibration factors converting dose-to-medium to dose-to-water were obtained from simulation data of plastic and lead-doped scintillator materials. Main Results. The results of this work suggest that lead-doped scintillators were dose-rate independent for UHDR x-rays from 1.1 to 40.1 Gy s−1 and capable of measuring conventional radiotherapy dose-rates (0.1 Gy s−1) at extended distance from the x-ray focal spot. Dose-to-water measured with a 5% lead-doped scintillator detector agreed with simulations within 0.6%. Significance. Lead-doped scintillators may be a valuable tool for the accurate real-time dosimetry of FLASH-capable UHDR x-ray beams.

Ultra-high dose rate electron beams and the FLASH effect: From preclinical evidence to a new radiotherapy paradigm

In their seminal paper from 2014, Fauvadon et al. coined the term FLASH irradiation to describe ultra-high-dose rate irradiation with dose rates greater than 40 Gy/s, which results in delivery times of fractions of a second. The experiments presented in that paper were performed with a high-dose-per-pulse 4.5 MeV electron beam, and the results served as the basis for the modern-day field of FLASH radiation therapy (RT). In this article, we review the studies that have been published after those early experiments, demonstrating the robust effects of FLASH RT on normal tissue sparing in preclinical models. We also outline the various irradiation parameters that have been used. Although the robustness of the biological response has been established, the mechanisms behind the FLASH effect are currently under investigation in a number of laboratories. However, differences in the magnitude of the FLASH effect between experiments in different labs have been reported. Reasons for these differences even within the same animal model are currently unknown, but likely has to do with the marked differences in irradiation parameter settings used. Here, we show that these parameters are often not reported, which complicates large multistudy comparisons. For this reason, we propose a new standard for beam parameter reporting and discuss a systematic path to the clinical translation of FLASH RT.

Pre‐clinical and clinical evaluation of the HYPERSCINT plastic scintillation dosimetry research platform for in vivo dosimetry during radiotherapy

Purpose

The purpose of this work is to evaluate the Hyperscint‐RP100 scintillation dosimetry research platform (Hyperscint‐RP100, Medscint Inc., Quebec, QC, Canada) designed for clinical quality assurance (QA) for use in in vivo dosimetry measurements.

Methods

The pre‐clinical evaluation of the scintillator was performed using a Varian TrueBeam linear accelerator. Dependency on field size, depth, dose, dose rate, and temperature were evaluated in a water tank and compared to calibration data from commissioning and annual QA. Angularity was evaluated with a 3D printed phantom. The clinical evaluation was first performed in two cadaver dogs, and then in three companion animal dogs receiving radiation therapy for nasal tumors. A treatment planning CT scan was performed for cadavers and clinical patients. Prior to treatment, the probe was inserted into the radiation field. Radiation was then delivered and measured with the scintillator. For cadavers, the treatment was repeated after making an intentional shift in patient position to simulate a treatment error.

Results

In the preclinical measurements the dose differed from annual measurements as follows: field size −0.77 to 0.43%, depth dose −0.36 to 1.14%, dose −0.54 to 2.93%, dose rate 0.3 to 3.6%, and angularity −1.18 to 0.01%. Temperature dependency required a correction factor of 0.11%/°C. In the two cadavers, the dose differed by −1.17 to 0.91%. The device correctly detected the treatment error when the heads were intentionally laterally shifted. In three canine clinical patients treated in multiple fractions, the detected dose ranged from 98.33 to 103.15%.

Conclusion

Results of this new device are promising although more work is necessary to fully validate it for clinical dosimetry.

Measurement of scattered rays from different materials using an inorganic scintillator based optical fiber sensor and its application in radiotherapy

Measurements using an Optical Fiber OFS including an inorganic scintillator placed on the surface of a phantom show that the particle energy distribution inside the phantom remains unchanged. The backscattered intensity measured using an Optical Fiber Sensor (OFS) exhibits a linear relationship with the total radiation dose delivered to the phantom, and this relationship shows that the OFS can be used for indirect dose measurement when located on the surface of the phantom i.e. that arising from the energetic backscattered electrons and photons. Such a device can therefore be used as a clinicalin-vivodosimeter, being located on the patient's body surface. In addition, the measurement results for the same OFS located inside and outside the radiation field of a compound water based phantom are analyzed. The differences in measurement of the fluorescence signal in response to various tissue materials representing bone or tumor tissue in the irradiation field are strongly related to the material's ability to block the scattered rays from the water phantom, as well as the scattered x-rays generated by the material located within the phantom.

Measuring small field profiles and output factors with a stemless plastic scintillator array

PURPOSE

To fabricate a 1D stemless plastic scintillation detector (SPSD) array using organic photodiodes and to use the detector to measure small field profiles and output factors.

METHODS

An organic photodiode array was fabricated by spin coating a mixture of P3HT and PCBM organic semiconductors onto an ITO-coated glass substrate and depositing aluminum top contacts. Four bulk scintillators of various dimensions were placed on top of the photodiode array. A fifth scintillator was used that had been segmented by laser etching and the septa filled with black paint. Each detector array was first calibrated using a reference field of 95 cm SSD, 5 cm depth, and 10 × 10 cm² field size for a 6 MV photon beam. After calibration, profiles were measured for three small field sizes: 0.5 × 0.5 cm² , 1 × 1 cm² , and 2 × 2 cm² . Using the central pixel of the array, output factors were measured for field sizes of 0.5 × 0.5 cm² to 25 × 25 cm² . Small field profiles were compared to film measurements and output factors compared to ion chamber measurements.

RESULTS

The segmented scintillator measured profiles that were in good agreement with film for all three field sizes. Output factors agreed to within 1.2% of ion chamber over the field size range of 1 × 1 cm² to 25 × 25 cm² . At 0.5 × 0.5 cm² the segmented scintillator underestimated the output factor compared to film and a microDiamond detector. Bulk scintillators failed to produce a good agreement with film for measured profiles and deviations from ion chamber for output factors were apparent at field sizes below 5 × 5 cm² . In comparison to a bulk scintillator of dimensions 5 × 5 × 0.5 cm³ the etched scintillator saw a reduction of 5.1, 7.1, and 10.5 times the signal for field sizes of 0.5 × 0.5 cm² , 1 × 1 cm² , and 2 × 2 cm² , respectively. The reduction of signal comes from reduced cross-talk that was present in all of the bulk scintillator geometries to various degrees.

CONCLUSION

A 1D SPSD array was demonstrated with various scintillator designs. The etched scintillator array demonstrated excellent small field profile measurements when compared to film and output factors (down to 1 × 1 cm² field size) when compared to micro ion chamber and diamond detector measurements.

On the use of machine learning methods for mPSD calibration in HDR brachytherapy

We sought to evaluate the feasibility of using machine learning (ML) algorithms for multipoint plastic scintillator detector (mPSD) calibration in high-dose-rate (HDR) brachytherapy. Dose measurements were conducted under HDR brachytherapy conditions. The dosimetry system consisted of an optimized 1-mm-core mPSD and a compact assembly of photomultiplier tubes coupled with dichroic mirrors and filters. An ¹⁹²Ir source was remotely controlled and sent to various positions in a homemade PMMA holder, ensuring 0.1-mm positional accuracy. Dose measurements covering a range of 0.5 to 12 cm of source displacement were carried out according to TG-43 U1 recommendations. Individual scintillator doses were decoupled using a linear regression model, a random forest estimator, and artificial neural network algorithms. The dose predicted by the TG-43U1 formalism was used as the reference for system calibration and ML algorithm training. The performance of the different algorithms was evaluated using different sample sizes and distances to the source for the mPSD system calibration. We found that the calibration conditions influenced the accuracy in predicting the measured dose. The decoupling methods' deviations from the expected TG-43 U1 dose generally remained below 20%. However, the dose prediction with the three algorithms was accurate to within 7% relative to the dose predicted by the TG-43 U1 formalism when measurements were performed in the same range of distances used for calibration. In such cases, the predictions with random forest exhibited minimal deviations (<2%). However, the performance random forest was compromised when the predictions were done beyond the range of distances used for calibration. Because the linear regression algorithm can extrapolate the data, the dose prediction by the linear regression was less influenced by the calibration conditions than random forest. The linear regression algorithm's behavior along the distances to the source was smoother than those for the random forest and neural network algorithms, but the observed deviations were more significant than those for the neural network and random forest algorithms. The number of available measurements for training purposes influenced the random forest and neural network models the most. Their accuracy tended to converge toward deviation values close to 1% from a number of dwell positions greater than 100. In performing HDR brachytherapy dose measurements with an optimized mPSD system, ML algorithms are good alternatives for precise dose reporting and treatment assessment during this kind of cancer treatment.

Characterization of an x-ray tube-based ultrahigh dose-rate system for in vitro irradiations

PURPOSE

To present an x-ray tube system capable of in vitro ultrahigh dose-rate (UHDR) irradiation of small < 0.3 mm samples and to characterize it by means of a plastic scintillation detector (PSD).

METHODS AND MATERIALS

A conventional x-ray tube was modified for the delivery of short UHDR irradiations. A beam shutter system with a sample holder was designed and installed in a close proximity of an x-ray tube window to enable <1 s irradiations at UHDR. The dosimetry was performed with a small 0.5-mm long 0.5-mm in diameter PSD irradiated with 80, 100, and 120 kVp beams and beam currents of 1-37.5 mA. The PSD signal was recorded at frame rates of 20 and 50 fps for shutter exposure between 100 and 1125 ms. Irradiation reproducibility was studied with the PSD. The x-ray tube irradiation setup was modeled with Monte Carlo (MC) and dose on a surface of a phantom was also measured with films. The effect of dose delivery uncertainty to 300-μm spheroids due to positioning and spheroid size was evaluated.

RESULTS

MC simulations showed good agreement with PSD measurements acquired at both frame rates of 20 and 50 fps in terms of beam temporal profile. PSD-measured dose exhibited excellent linearity as a function of instantaneous dose rate from 3.1 to 118.0 Gy/s as well as shutter exposure time from 100 and 1125 ms for all investigated beam energies. PSD absorbed dose for the 80, 100, and 120 kVp beams agreed with MC simulations to within 5%. The total delivered doses ranged from 0.4 Gy for a 1-mA, 80 kVp beam, and 100 ms shutter exposure to 166.9 Gy for a 37.5-mA, 80 kVp beam, and a 1125 ms exposure. PSD irradiation reproducibility was < 0.5%. Simulated and measured dose fall off agreed and it was steep along the axis of the shutter slit (1%/0.1 mm) and with depth (2%/0.1 mm at 1-mm depth). Spheroid positioning uncertainty of 300 μm resulted in dose difference of < 3% for x and y shifts but up to 7% uncertainty for a z-shift parallel to the beam axis. A 16% difference in spheroid size resulted in <5% dose difference in spheroid absorbed dose.

CONCLUSIONS

We have presented a cost-effective x-ray tube-based system with a beam shutter designed for in vitro UHDR delivery and reaching dose rates of up to 118.0 Gy/s. The described shutter system can be easily implemented at other institutions, which might enable new researchers to investigate the radiobiology of UHDR irradiations in vitro.

Optical Filter-Embedded Fiber-Optic Radiation Sensor for Ultra-High Dose Rate Electron Beam Dosimetry

FLASH radiotherapy is an emerging radiotherapy technique used to spare normal tissues. It employs ultra-high dose rate radiation beams over 40 Gy/s, which is significantly higher than those of conventional radiotherapy. In this study, a fiber-optic radiation sensor (FORS) was fabricated using a plastic scintillator, an optical filter, and a plastic optical fiber to measure the ultra-high dose rate electron beams over 40 Gy/s used in FLASH radiotherapy. The radiation-induced emissions, such as Cherenkov radiation and fluorescence generated in a transmitting optical fiber, were spectrally discriminated from the light outputs of the FORS. To evaluate the linearity and dose rate dependence of the FORS, the outputs of the fiber-optic radiation sensor were measured according to distances from an electron scattering device, and the results were compared with those of an ionization chamber and radiochromic films. Finally, the percentage depth doses were obtained using the FORS as a function of depth in a water phantom. This study found that ultra-high dose rate electron beams over 40 Gy/s could be measured in real time using a FORS.

Plastic scintillation dosimeter with a conical mirror for measuring 3D dose distribution

PURPOSE

To test the measurement technique of the three-dimensional (3D) dose distribution measured image by capturing the scintillation light generated using a plastic scintillator and a scintillating screen.

METHODS

Our imaging system constituted a column shaped plastic scintillator covered by a Gd₂ O₂ S:Tb scintillating screen, a conical mirror and a cooled CCD camera. The scintillator was irradiated with 6 MV photon beams. Meanwhile, the irradiated plan was prepared for the static field plans, two-field plan (2F plan) and the conformal arc plan (CA plan). The 2F plan contained 16 mm² and 10 mm² fields irradiated from gantry angles of 0° and 25°, respectively. The gantry was rotated counterclockwise from 45° to 315° for the CA plan. The field size was then obtained as 10 mm² . A Monte Carlo simulation was performed in the experimental geometry to obtain the calculated 3D dose distribution as the reference data. Dose response was acquired by comparing between the reference and the measurement. The dose rate dependence was verified by irradiating the same MU value at different dose rates ranging from 100 to 600 MU/min. Deconvolution processing was applied to the measured images for the correction of light blurring. The measured 3D dose distribution was reconstructed from each measured image. Gamma analysis was performed to these 3D dose distributions. The gamma criteria were 3% for the dose difference, 2 mm for the distance-to-agreement and 10% for the threshold.

RESULTS

Dose response for the scintillation light was linear. The variation in the light intensity for the dose rate ranging from 100 to 600 MU/min was less than 0.5%, while our system presents dose rate independence. For the 3D dose measurement, blurring of light through deconvolution processing worked well. The 3D gamma passing rate (3D GPR) for the 10 × 10 mm² , 16 × 16 mm² , and 20 × 20 mm² fields were observed to be 99.3%, 98.8%, and 97.8%, respectively. Reproducibility of measurement was verified. The 3D GPR results for the 2F plan and the CA plan were 99.7% and 100%, respectively.

CONCLUSIONS

We developed a plastic scintillation dosimeter and demonstrated that our system concept can act as a suitable technique for measuring the 3D dose distribution from the gamma results. In the future, we will attempt to measure the 4D dose distribution for clinical volumetric modulated arc radiation therapy (VMAT)-SBRTplans.

Increase in the intensity of an optical signal with fluorescein during proton and carbon-ion irradiation

Purpose

Although the imaging of luminescence emitted in water during irradiation of protons and carbon ions is a useful method for range and dose estimations, the intensity of the images is relatively low due to the low photon production of the luminescence phenomenon. Therefore, a relatively long time is required for the imaging. Since a fluorescent dye, fluorescein, may increase the intensity of the optical signal, we measured the luminescence images of water with different concentrations of fluorescein during irradiation of protons and carbon ions and compared the results with those by measurements with water.

Methods

A cooled charge‐coupled device (CCD) camera was used for imaging a water phantom with different concentrations of fluorescein from 0.0063 to 0.025 mg/cm³, in addition to a water phantom without fluorescein during irradiation of 150‐MeV protons and 241.5‐MeV/n carbon ions.

Results

For both protons and carbon ions, the intensity of the luminescence images increased as the concentration of fluorescein increased. With a fluorescein concentration of 0.025 mg/cm³, the intensities increased to more than 10 times those of water for both protons and carbon ions. Although the shape of the depth profiles of luminescence images of water with fluorescein appeared similar to that of water for protons, those for carbon ions were different from those of water due to the increase in the Cherenkov light component at shallow depths by the decrease in the angular dependencies of the Cherenkov light.

Conclusion

We confirmed the increase in intensity of the luminescence of water by adding fluorescein for particle ions. With a small amount of Cherenkov light contamination in the images, such as protons, the relative distributions of the luminescence images with fluorescein were similar to that of water and will be used for range or dose determination in a short time.

Miniaturized scintillator dosimeter for small field radiation therapy

The concept of a miniaturized inorganic scintillator detector is demonstrated in the analysis of the small static photon fields used in external radiation therapy. Such a detector is constituted by a 0.25 mm diameter and 0.48 mm long inorganic scintillating cell (1.6 × 10^-5cm³detection volume) efficiently coupled to a narrow 125μm diameter silica optical fiber using a tiny photonic interface (an optical antenna). The response of our miniaturized scintillator detector (MSD) under 6 MV bremsstrahlung beam of various sizes (from 1 × 1 cm²to 4 × 4 cm²) is compared to that of two high resolution reference probes, namely, a micro-diamond detector and a dedicated silicon diode. The spurious Cerenkov signal transmitted through our bare detector is rejected with a basic spectral filtering. The MSD shows a linear response regarding the dose, a repeatability within 0.1% and a radial directional dependence of 0.36% (standard deviations). Beam profiling at 5 cm depth with the MSD and the micro-diamond detector shows a mismatch in the measurement of the full widths at 80% and 50% of the maximum which does not exceed 0.25 mm. The same difference range is found between the micro-diamond detector and a silicon diode. The deviation of the percentage depth dose between the MSD and micro-diamond detector remains below 2.3% within the first fifteen centimeters of the decay region for field sizes of 1 × 1 cm², 2 × 2 cm²and 3 × 3 cm²(0.76% between the silicon diode and the micro-diamond in the same field range). The 2D dose mapping of a 0.6 × 0.6 cm²photon field evidences the strong 3D character of the radiation-matter interaction in small photon field regime. From a beam-probe convolution theory, we predict that our probe overestimates the beam width by 0.06%, making our detector a right compromise between high resolution, compactness, flexibility and ease of use. The MSD overcomes problem of volume averaging, stem effects, and despite its water non-equivalence it is expected to minimize electron fluence perturbation due to its extreme compactness. Such a detector thus has the potential to become a valuable dose verification tool in small field radiation therapy, and by extension in Brachytherapy, FLASH-radiotherapy and microbeam radiation therapy.

Comparative optic and dosimetric characterization of the HYPERSCINT scintillation dosimetry research platform for multipoint applications

This study introduces the HYPERSCINT research platform (HYPERSCINT-RP100, Medscint Inc., Quebec, Canada), the first commercially available scintillation dosimetry platform capable of multi-point dosimetry through the hyperspectral approach. Optic and dosimetric performances of the system were investigated through comparison with another commercially available solution, the Ocean Optics QE65Pro spectrometer. The optical characterization was accomplished by measuring the linearity of the signal as a function of integration time, photon detection efficiency and spectral resolution for both systems under the same conditions. Dosimetric performances were then evaluated with a 3-point plastic scintillator detector (mPSD) in terms of signal to noise ratio (SNR) and signal to background ratio (SBR) associated with each scintillator. The latter were subsequently compared with those found in the literature for the Exradin W1, a single-point plastic scintillator detector. Finally, various beam measurements were realized with the HYPERSCINT platform to evaluate its ability to perform clinical photon beam dosimetry. Both systems were found to be comparable in terms of linearity of the signal as a function of the intensity. Although the QE65Pro possesses a higher spectral resolution, the detection efficiency of the HYPERSCINT is up to 1000 time greater. Dosimetric measurements shows that the latter also offers a better SNR and SBR, surpassing even the SNR of the Exradin W1 single-point PSD. While doses ranging from 1 to 600 cGy were accurately measured within 2.1% of the predicted dose using the HYPERSCINT platform coupled to the mPSD, the Ocean optics spectrometer shows discrepancies up to 86% under 50cGy. Similarly, depth dose, full width at half maximum region of the beam profile and output factors were all accurately measured within 2.3% of the predicted dose using the HYPERSCINT platform and exhibit an average difference of 0.5%, 1.6% and 0.6%, respectively.

Characterization of YAG:Ce phosphor dosimeter by the co-precipitation method for radiotherapy

Yttrium aluminum garnet (YAG) doped with Ce was synthesized via the co-precipitation method with NH₄HCO₃ as the precipitant. The spectroscopic properties and the effects of the Ce doping concentration and sintering atmosphere on the crystal phase were investigated. The dosimeter of YAG:Ce phosphor material was prepared to study the radioluminescence (RL) characteristics of a clinical linear accelerator. A satisfying linear relationship between the radiation dose and RL signal was obtained, which provided a reference for the YAG:Ce phosphor material used in radiotherapy and real-time remote radiation detection.

Dosimetric characterization of a small-scale (Zn,Cd)S:Ag inorganic scintillating detector to be used in radiotherapy

PURPOSE

In modern radiotherapy techniques, to ensure an accurate beam modeling process, dosimeters with high accuracy and spatial resolution are required. Therefore, this work aims to propose a simple, robust, and a small-scale fiber-integrated X-ray inorganic detector and investigate the dosimetric characteristics used in radiotherapy.

METHODS

The detector is based on red-emitting silver-activated zinc-cadmium sulfide (Zn,Cd)S:Ag nanoclusters and the proposed system has been tested under 6 MV photons with standard dose rate used in the patient treatment protocol. The article presents the performances of the detector in terms of dose linearity, repeatability, reproducibility, percentage depth dose distribution, and field output factor. A comparative study is shown using a microdiamond dosimeter and considering data from recent literature.

RESULTS

We accurately measured a small field beam profile of 0.5 × 0.5 cm² at a spatial resolution of 100 µm using a LINAC system. The dose linearity at 400 MU/min has shown less than 0.53% and 1.10% deviations from perfect linearity for the regular and smallest field. Percentage depth dose measurement agrees with microdiamond measurements within 1.30% and 2.94%, respectively for regular to small field beams. Besides, the stem effect analysis shows a negligible contribution in the measurements for fields smaller than 3x3 cm². This study highlights the drastic decrease of the convolution effect using a point-like detector, especially in small dimension beam characterization. Field output factor has shown a good agreement while comparing it with the microdiamond dosimeter.

CONCLUSION

All the results presented here anticipated that the developed detector can accurately measure delivered dose to the region of interest, claim accurate depth dose distribution hence it can be a suitable candidate for beam characterization and quality assurance of LINAC system.

High resolution small-scale inorganic scintillator detector: HDR brachytherapy application

PURPOSE

Brachytherapy (BT) deals with high gradient internal dose irradiation made up of a complex system where the source is placed nearby the tumor to destroy cancerous cells. A primary concern of clinical safety in BT is quality assurance to ensure the best matches between the delivered and prescribed doses targeting small volume tumors and sparing surrounding healthy tissues. Hence, the purpose of this study is to evaluate the performance of a point size inorganic scintillator detector (ISD) in terms of high dose rate brachytherapy (HDR-BT) treatment.

METHODS

A prototype of the dose verification system has been developed based on scintillating dosimetry to measure a high dose rate while using an ¹⁹² Ir BT source. The associated dose rate is measured in photons/s employing a highly sensitive photon counter (design data: 20 photons/s). Dose measurement was performed as a function of source-to-detector distance according to TG43U1 recommendations. Overall measurements were carried out inside water phantoms keeping the ISD along the BT needle; a minimum of 0.1 cm distance was maintained between each measurement point. The planned dwell times were measured accurately from the difference of two adjacent times of transit. The ISD system performances were also evaluated in terms of dose linearity, energy dependency, scintillation stability, signal-to-noise ratio (SNR), and signal-to-background ratio (SBR). Finally, a comparison was presented between the ISD measurements and results obtained from TG43 reference dataset.

RESULTS

The detection efficiency of the ISD was verified by measuring the planned dwell times at different dwell positions. Measurements demonstrated that the ISD has a perfectly linear behavior with dose rate (R²  = 1) and shows high SNR (>35) and SBR (>36) values even at the lowest dose rate investigated at around 10 cm from the source. Standard deviation (1σ) remains within 0.03% of signal magnitude, and less than 0.01% STEM signal was monitored at 0.1 cm source-to-detector distance. Stability of 0.54% is achieved, and afterglow stays less than 1% of the total signal in all the irradiations. Excellent symmetrical behavior of the dose rate regarding source position was observed at different radiation planes. Finally, a comparison with TG-43 reference dataset shows that corrected measurements agreed with simulation data within 1.2% and 1.3%, and valid for the source-to-detector distance greater than 0.25 cm.

CONCLUSION

The proposed ISD in this study anticipated that the system could be promoted to validate with further clinical investigations. It allows an appropriate dose verification with dwell time estimation during source tracking and suitable dose measurement with a high spatial resolution both nearby (high dose gradient) and far (low dose gradient) from the source position.

Monte Carlo modeling of the influence of strong magnetic fields on the stem-effect in plastic scintillation detectors used in radiotherapy dosimetry

PURPOSE

To investigate the impact of strong magnetic fields on the stem-effect in plastic scintillation detectors (PSDs) using Monte Carlo methods.

METHODS

Prior to building the light guide model, the properties of the Čerenkov process in GEANT4 were investigated by simulating depth-dose and depth-Čerenkov emission profiles in water as functions of Čerenkov process input parameters. In addition, profile simulations were performed for magnetic field strengths ranging from 0 T to 1.5 T. A PMMA light guide was constructed in GEANT4 using data from the manufacturer and literature. Simulations were performed with the model as functions of depth and fiber-beam angle where the simulated stem-effect spectrum and the Čerenkov light ratio (CLR) were scored and compared to measured data in the literature. The light guide optical properties were iteratively adjusted until agreement between the simulated and measured data was achieved. Simulations were performed with the validated model as functions of depth and magnetic field strength and the simulated data were compared to measured data in the literature. The model was also used to evaluate the sensitivity of the CLR to the various optical properties of the light guide in different irradiation conditions.

RESULTS

No significant changes in the depth-dose or depth-Čerenkov emission profiles were observed with step-size restrictions imposed by the Čerenkov process input parameters, which was attributed to the condensed history algorithm and transport parameters used in this work. Similar changes in the depth-dose and depth-Čerenkov emission profiles were observed with increasing magnetic field strength, which indicates the Čerenkov process is not adversely impacted by the presence of the magnetic field. Following optimization of the light guide optical properties, agreement within two standard deviations was observed between the simulated and measured optical data for all validation geometries considered. Agreement within one standard deviation was observed between the simulated and measured data for all depths and field strengths ≥0 T whereas discrepancies were observed for magnetic field strengths <-0.35 T. These significant differences were attributed to insufficient measurement data for this irradiation configuration during model validation. Of the light guide optical properties investigated, the fluorescence signal had the greatest impact on the CLR sensitivity to the magnetic field.

CONCLUSIONS

No significant change in the Čerenkov emission per dose in water was observed for magnetic field strengths up to 1.5 T. The nominal fiber fluorescence signal was found to have a significant impact on the CLR sensitivity to varying irradiation conditions where changes up to 11.7% were observed whereas the mirror reflectivity and fiber attenuation had a modest impact with maximum CLR changes of 2.6% and 1.2% relative to 0 T, respectively. The results of this work suggest light guides with low fiber fluorescence should be used with PSDs for dosimetry measurements in magnetic fields to minimize the impact of the magnetic field on the CLR correction.

Physics and biology of ultrahigh dose-rate (FLASH) radiotherapy: a topical review

Ultrahigh dose-rate radiotherapy (RT), or 'FLASH' therapy, has gained significant momentum following various in vivo studies published since 2014 which have demonstrated a reduction in normal tissue toxicity and similar tumor control for FLASH-RT when compared with conventional dose-rate RT. Subsequent studies have sought to investigate the potential for FLASH normal tissue protection and the literature has been since been inundated with publications on FLASH therapies. Today, FLASH-RT is considered by some as having the potential to 'revolutionize radiotherapy'. FLASH-RT is considered by some as having the potential to 'revolutionize radiotherapy'. The goal of this review article is to present the current state of this intriguing RT technique and to review existing publications on FLASH-RT in terms of its physical and biological aspects. In the physics section, the current landscape of ultrahigh dose-rate radiation delivery and dosimetry is presented. Specifically, electron, photon and proton radiation sources capable of delivering ultrahigh dose-rates along with their beam delivery parameters are thoroughly discussed. Additionally, the benefits and drawbacks of radiation detectors suitable for dosimetry in FLASH-RT are presented. The biology section comprises a summary of pioneering in vitro ultrahigh dose-rate studies performed in the 1960s and early 1970s and continues with a summary of the recent literature investigating normal and tumor tissue responses in electron, photon and proton beams. The section is concluded with possible mechanistic explanations of the FLASH normal-tissue protection effect (FLASH effect). Finally, challenges associated with clinical translation of FLASH-RT and its future prospects are critically discussed; specifically, proposed treatment machines and publications on treatment planning for FLASH-RT are reviewed.

Verification system for intensity-modulated radiation therapy with scintillator

In the preparation of intensity-modulated radiation therapy (IMRT), patient-specific verification is widely employed to optimize the treatment. To accurately estimate the accumulated dose and obtain the field-by-field or segment-by-segment verification, an original IMRT verification tool using scintillator light and an analysis workflow was developed in this study. The raw light distribution was calibrated with respect to the irradiated field size dependency and light diffusion in the water. The calibrated distribution was converted to dose quantity and subsequently compared with the results of the clinically employed plan. A criterion of 2-mm dose-to-agreement and 3% dose difference was specified in the gamma analysis with a 10% dose threshold. By applying the light diffusion calibration, the maximum dose difference was corrected from 7.7 cGy to 3.9 cGy around the field edge for a 60 cGy dose, 7 × 7 cm² irradiation field, and 10 MV beam energy. Equivalent performance was confirmed in the chromodynamic film. The average dose difference and gamma pass rate of the accumulated dose distributions in six patients were 0.8 ± 4.5 cGy and 97.4%, respectively. In the field-by-field analysis, the average dose difference and gamma pass rate in seven fields of Patient 1 were 0.2 ± 1.2 cGy and 93.9%, respectively. In the segment-by-segment analysis, the average dose difference and gamma pass rate in nine segments of Patient 1 and a 305° gantry angle were - 0.03 ± 0.2 cGy and 93.9%, respectively. This system allowed the simultaneous and independent analysis of each field or segment in the accumulated dose analysis.

Feasibility of improving patient's safety with in vivo dose tracking in intracavitary and interstitial HDR brachytherapy

PURPOSE

The in vivo dosimetric monitoring in HDR brachytherapy is important for improving patient safety. However, there are very limited options available for clinical application. In this study, we present a new in vivo dose measurement system with a plastic scintillating detector (PSD) for GYN HDR brachytherapy.

METHODS

An FDA approved PSD system, called OARtrac (AngioDynamics, Latham, NY), was used with various applicators for in vivo dose measurements for GYN patients. An institutional workflow was established for the clinical implementation of the dosimetric system. Action levels were proposed based on the measurement and system uncertainty for measurement deviations. From October 2018 to September 2019, a total of 75 measurements (48 fractions) were acquired from 14 patients who underwent HDR brachytherapy using either a multichannel cylinder, Venezia applicator, or Syed-Neblett template. The PSDs were placed in predetermined catheters/channels. A planning CT was acquired for treatment planning in Oncentra (Elekta, Version-4.5.2) TPS. The PSDs were contoured on the CT images, and the PSD D_90% values were used as the expected doses for comparison with the measured doses.

RESULTS

The mean difference from patient measurements was -0.22% ± 5.98%, with 26% being the largest deviation from the expected value (Syed case). Large deviations were observed when detectors were placed in the area where dose rates were less than 1 cGy/s.

CONCLUSIONS

The establishment of clinical workflow for the in vivo dosimetry for both the intracavitary and interstitial GYN HDR brachytherapy will potentially improve the safety of the patient treatment.

Characterization of a Low-Cost Plastic Fiber Array Detector for Proton Beam Dosimetry

The Pencil Beam Scanning (PBS) technique in proton therapy uses fast magnets to scan the tumor volume rapidly. Changing the proton energy allows changing to layers in the third dimension, hence scanning the same volume several times. The PBS approach permits adapting the speed and/or current to modulate the delivered dose. We built a simple prototype that measures the dose distribution in a single step. The active detection material consists of a single layer of scintillating fibers (i.e., 1D) with an active length of 100 mm, a width of 18.25 mm, and an insignificant space (20 μm) between them. A commercial CMOS-based camera detects the scintillation light. Short exposure times allow running the camera at high frame rates, thus, monitoring the beam motion. A simple image processing method extracts the dose information from each fiber of the array. The prototype would allow scaling the concept to multiple layers read out by the same camera, such that the costs do not scale with the dimensions of the fiber array. Presented here are the characteristics of the prototype, studied under two modalities: spatial resolution, linearity, and energy dependence, characterized at the Center for Proton Therapy (Paul Scherrer Institute); the dose rate response, measured at an electron accelerator (Swiss Federal Institute of Metrology).

Advances on inorganic scintillator-based optic fiber dosimeters

This article presents a new perspective on the development of inorganic scintillator-based fiber dosimeters (IOSFDs) for medical radiotherapy dosimetry (RTD) focusing on real-time in vivo dosimetry. The scintillator-based optical fiber dosimeters (SFD) are compact, free of electromagnetic interference, radiation-resistant, and robust. They have shown great potential for real-time in vivo RTD. Compared with organic scintillators (OSs), inorganic scintillators (IOSs) have larger X-ray absorption and higher light output. Variable IOSs with maximum emission peaks in the red part of the spectrum offer convenient stem effect removal. This article outlines the main advantages and disadvantages of utilizing IOSs for SFD fabrication. IOSFDs with different configurations are presented, and their use for dosimetry in X-ray RT, brachytherapy (BT), proton therapy (PT), and boron neutron capture therapy (BNCT) is reviewed. Challenges including the percentage depth dose (PDD) deviation from the standard ion chamber (IC) measurement, the angular dependence, and the Cherenkov effect are discussed in detail; methods to overcome these problems are also presented.

Fabrication and characterization of a stemless plastic scintillation detector

PURPOSE

To fabricate a stemless plastic scintillation detector (SPSD) and characterize its linearity and reproducibility, and its dependence on energy and dose per pulse; and to apply it to clinical PDD and output factor measurements.

METHODS

An organic bulk heterojunction photodiode was fabricated by spin coating a blend of P3HT and PCBM onto an ITO-coated glass substrate and depositing aluminum top contacts. Eljen scintillators (~5 × 5 × 5 mm³ ; EJ-204, EJ-208, and EJ-260) or Saint-Gobain scintillators (~3 × 3 × 2 mm³ ; BC-400 and BC-412) were placed on the opposite side of the glass using a silicone grease (optical coupling agent) creating the SPSD. Energy dependence was measured by using 100, 180, and 300 kVp photon beams from an orthovoltage treatment unit (Xstrahl 300) and 6 and 10 MV photons from a Varian TrueBeam linear accelerator. Linearity, dose per pulse dependence, output factors, and PDDs were measured using a 6 MV photon beam. PDDs and output factors were compared to ion chamber measurements. A control device was fabricated by substituting polystyrene (PS) for the P3HT/PCBM layer. No photocurrent should be generated in the control device and so any current measured is due to Compton current in the electrodes, wires, and surroundings from the irradiation. Output factors were corrected by subtracting the signal measured using the control device from the photodiode measured signal to yield the photocurrent.

RESULTS

Each SPSD had excellent linearity with dose having an r² of 1 and sensitivities of 1.07 nC/cGy, 1.04 nC/cGy, 1.00 nC/cGy and 0.10 nC/cGy, and 0.10 nC/cGy for EJ-204, EJ-208, EJ-260 (5 × 5 × 5 mm³ volumes), BC-400, and BC-412 (3 × 3 × 2 mm³ volumes), respectively. No significant dose per pulse dependence was measured. Output factors matched within 1% for the large scintillators for field sizes of 5 × 5 cm² to 25 × 25 cm² , but there was a large under-response at field sizes below 3 × 3 cm² . After correcting the signal of the small scintillators by subtracting the current measured using the PS control, the output factors agreed with the ion chamber measurements within 1% from field sizes 1 × 1 cm² to 20 × 20 cm² . The impact of Cerenkov emissions in the scintillator was effectively corrected with a simple reflective coating on the scintillator. In comparison to a 6 MV photon beam, the large scintillator SPSDs exhibited 37%, 52%, and 73% of the response at energies 100 kVp, 180 kVp and 300 kVp, respectively.

CONCLUSION

The principle of the SPSD was demonstrated. Devices had excellent linearity, reproducibility, and no significant dose per pulse dependence, and a simple reflective coating was sufficient to correct for Cerenkov emissions from within the scintillator. The devices demonstrated similar energy dependence to other scintillator detectors used in a radiotherapy setting.

Characterization of novel 3D printed plastic scintillation dosimeters

We propose a new methodology for the fabrication and evaluation of scintillating detector elements using a consumer grade fusion deposition modeling (FDM) 3D printer. In this study we performed a comprehensive investigation into both the effects of the 3D printing process on the scintillation light output of 3D printed plastic scintillation dosimeters (PSDs) and their associated dosimetric properties. Fabrication properties including print variability, layer thickness, anisotropy and extrusion temperature were assessed for 1 cm³ printed samples. We then examined the stability, dose linearity, dose rate proportionality, energy dependence and reproducibility of the 3D printed PSDs compared to benchmarks set by commercially available products. Experimental results indicate that the shape of the emission spectrum of the 3D printed PSDs do not show significant spectral differences when compared to the emission spectrum of the commercial sample. However, the magnitude of scintillation light output was found to be strongly dependent on the parameters of the fabrication process. Dosimetric testing indicates that the 3D printed PSDs share many desirable properties with current commercially available PSDs such as dose linearity, dose rate independence, energy independence in the MV range, repeatability, and stability. These results demonstrate that not only does 3D printing offer a new avenue for the production and manufacturing of PSDs but also allows for further investigation into the application of 3D printing in dosimetry. Such investigations could include options for 3D printed, patient-specific scintillating dosimeters that may be used as standalone dosimeters or incorporated into existing 3D printed patient devices (e.g. bolus or immobilization) used during the delivery of radiation therapy.

Characterization of an inorganic scintillator for small-field dosimetry in MR-guided radiotherapy

INTRODUCTION

Aim of this study is to dosimetrically characterize a new inorganic scintillator designed for magnetic resonance-guided radiotherapy (MRgRT) in the presence of 0.35 tesla magnetic field (B).

METHODS

The detector was characterized in terms of signal to noise ratio (SNR), reproducibility, dose linearity, angular response, and dependence by energy, field size, and B orientation using a 6 MV magnetic resonance (MR)-Linac and a water tank. Field size dependence was investigated by measuring the output factor (OF) at 1.5 cm. The results were compared with those measured using other detectors (ion chamber and synthetic diamond) and those calculated using a Monte Carlo (MC) algorithm. Energy dependence was investigated by acquiring a percentage depth dose (PDD) curve at two field sizes (3.32 × 3.32 and 9.96 × 9.96 cm2 ) and repeating the OF measurements at 5 and 10 cm depths.

RESULTS

The mean SNR was 116.3 ± 0.6. Detector repeatability was within 1%, angular dependence was <2% and its response variation based on the orientation with respect to the B lines was <1%. The detector has a temporal resolution of 10 Hz and it showed a linear response (R2  = 1) in the dose range investigated. All the OF values measured at 1.5 cm depth using the scintillator are in accordance within 1% with those measured with other detectors and are calculated using the MC algorithm. PDD values are in accordance with MC algorithm only for 3.32 × 3.32 cm2 field. Numerical models can be applied to compensate for energy dependence in case of larger fields.

CONCLUSION

The inorganic scintillator in the present form can represent a valuable detector for small-field dosimetry and periodic quality controls at MR-Linacs such as dose stability, OFs, and dose linearity. In particular, the detector can be effectively used for small-field dosimetry at 1.5 cm depth and for PDD measurements if the field dimension of 3.32 × 3.32 cm2 is not exceeded.