Determination of intrinsic energy dependence of point-like inorganic scintillation detector in brachytherapy

BACKGROUND

Inorganic scintillation detectors (ISDs) are promising for in vivo dosimetry in brachytherapy (BT). ISDs have fast response, providing time resolved dose rate information, and high sensitivity, attributed to high atomic numbers. However, the conversion of the detector signal to absorbed dose-to-water is highly dependent on the energy spectrum of the incident radiation. This dependence is comprised of absorbed dose energy dependence, obtainable with Monte Carlo (MC) simulation, and the absorbed dose-to-signal conversion efficiency or intrinsic energy dependence requiring measurements. Studies have indicated negligible intrinsic energy dependence of ZnSe:O-based ISDs in Ir-192 BT. A full characterization has not been performed earlier.

PURPOSE

This study characterizes the intrinsic energy dependence of ZnSe:O-based ISDs for kV X-ray radiation qualities, with energies relevant for BT.

METHODS

Three point-like ISDs made from fiber-coupled cuboid ZnSe:O-based scintillators were calibrated at the Swedish National Metrology Laboratory for ionizing radiation. The calibration was done in terms of air kerma free-in-air,K air ${K}_{{\mathrm{air}}}$ , in 13 X-ray radiation qualities, Q $Q$ , from 25 to 300 kVp (CCRI 25-250 kV and ISO 4037 N-series), and in terms of absorbed dose to water,D w ${D}_{\mathrm{w}}$ , in a Co-60 beam,Q 0 ${Q}_0$ . The mean absorbed dose to the ISDs, relative toK air ${K}_{{\mathrm{air}}}$ andD w ${D}_{\mathrm{w}}$ , were obtained with the MC code TOPAS (Geant4) using X-ray spectra obtained with SpekPy software and laboratory filtration data and a generic Co-60 source. The intrinsic energy dependence was determined as a function of effective photon energy,E e f f ${E}_{eff}$ , (relative to Co-60). The angular dependence of the ISD signal was measured in a 25 kVp (0.20 mm Al HVL) and 135 kVp beam (0.48 mm Cu HVL), by rotating the ISDs 180° around the fiber's longitudinal axis (perpendicular to the beam). A full 360° was not performed due to setup limitations. The impact of detector design was quantified with MC simulation.

RESULTS

Above 30 keVE e f f ${E}_{eff}$ the intrinsic energy dependence varied with less than 5 ± 4% from unity for all detectors (with the uncertainty expressed as the mean of all expanded measurement uncertainties for individualE e f f ${E}_{eff}$ above 30 keV, k = 2). Below 30 keV, it decreased with up to 17% and inter-detector variations of 13% were observed, likely due to differences in detector geometry not captured by the simulations using nominal geometry. In the 25 kVp radiation quality, the ISD signal varied with 24% over a ∼45° rotation. For 135 kVp, the corresponding variation was below 3%. Assuming a 0.05 mm thicker layer of reflective paint around the sensitive volume changed the absorbed dose with 6.3% at the lowestE e f f ${E}_{eff}$ , and with less than 2% at higher energies.

CONCLUSION

The study suggests that the ISDs have an intrinsic energy dependence relative to Co-60 lower than 5 ± 4% in radiation qualities withE e f f ${E}_{eff}\ $ > 30 keV. Therefore, they could in principle be calibrated in a Co-60 beam quality and transferred to such radiation qualities with correction factors determined only by the absorbed dose energy dependence obtained from MC simulations. This encourages exploration of the ISDs' applications in intensity modulated BT with Yb-169 or other novel intermediate energy isotopes.

A simple calibration routine for small inorganic scintillation detectors for in vivo dosimetry during brachytherapy

BACKGROUND

In vivo dosimetry (IVD) is rarely performed in brachytherapy (BT), allowing potential dose misadministration to go unnoticed. This study presents a clinical routine-calibration method of detectors for IVD in high (HDR) and pulsed dose rate (PDR) Ir-192 BT.

PURPOSE

To evaluate the dosimetric precision and feasibility of an in-clinic calibration routine of detectors for IVD in afterloading BT.

METHODS

Calibrations were performed in a PMMA phantom with two needles inserted 20 mm apart. The source was loaded in one of the needles at 15 dwells for 10 s. The detector was placed in the other needle, and its signal was recorded. The mean signal at each dwell position was fitted to the expected dose rate with the calibration factor and the detector's longitudinal position being free parameters. The method was tested with an inorganic scintillation detector using one Ir-192 FlexiSource HDR and two Ir-192 GammaMedPlus PDR sources and followed by validation measurements in water.

RESULTS

The standard measurement uncertainty (k = 1) of the calibration factor in absolute terms (Gy/s) was 3.2/3.4% for the HDR/PDR source. The uncertainty was dominated by source strength uncertainty, and the precision of the method was <1%. The mean ± 1SD of the difference in measured and expected dose rate during validation was 1.5 ± 4.7% (HDR) and 0.0 ± 4.1% (PDR) with a positional uncertainty in the setup of 0.33/0.23 mm (HDR/PDR) (k = 1).

CONCLUSION

A precise and feasible in-clinic calibration method for IVD and source strength consistency tests in BT was presented.

Establishing a fingerprinting method for fast catheter identification in HDR brachytherapy in vivo dosimetry

PURPOSE

To use quantities measurable during in vivo dosimetry to build unique channel identifiers, that enable detection of brachytherapy errors.

MATERIALS AND METHODS

Treatment plan of 360 patients with prostate cancer who underwent high-dose-rate brachytherapy (range, 16-25 catheters; mean, 17) were used. A single point virtual dosimeter was placed at multiple positions within the treatment geometry, and the source-dosimeter distance and dwell time were determined for each dwell position in each catheter. These values were compared across all catheters, dwell position by dwell position, simulating a treatment delivery. A catheter was considered uniquely identified if, for a given dwell position, no other catheters had the same measured values. The minimum number of dwell positions needed to identify a specific catheter and the optimal dosimeter location uniquely were determined. The radial (r) and vertical (z) dimensions of the source-dosimeter distance were also examined for their utility in discriminating catheters.

RESULTS

Using a virtual dosimeter with no uncertainties, all catheters were identified in 359 of the 360 cases with 9 dwell position measurements. When only the dwell time were measured, all catheters were uniquely identified after 1 dwell position. With a 2-mm spatial accuracy (r,z), all catheters were identified in 94% of the plans. Simultaneous measurement of source-dosimeter distance and dwell time ensured full catheter identification in all plans ranging from 2 to 6 dwell positions. The number of dwell positions needed to uniquely identify all catheters was lower when the distance from the implant center was higher.

CONCLUSIONS

The most efficient fingerprinting approach involved combining source-dosimeter distance (i.e., source tracking) and dwell time. The further the dosimeter is placed from the center of the implant the better it can uniquely identify catheters.

Monte Carlo characterization of high atomic number inorganic scintillators for in vivo dosimetry in 192 Ir brachytherapy

BACKGROUND

There is increased interest in vivo dosimetry for ¹⁹² Ir brachytherapy (BT) treatments using high atomic number (Z) inorganic scintillators. Their high light output enables construction of small detectors with negligible stem effect and simple readout electronics. Experimental determination of absorbed-dose energy dependence of detectors relative to water is prevalent, but it can be prone to high detector positioning uncertainties and does not allow for decoupling of absorbed-dose energy dependence from other factors affecting detector response.

PURPOSE

To investigate which measurement conditions and detector properties could affect their absorbed-dose energy dependence in BT in vivo dosimetry.

METHODS

We used a general-purpose MC code penelope for the characterization of high-Z inorganic scintillators with the focus on ZnSe (Z¯=32). Two other promising media CsI (Z¯=54) and Al₂ O₃ (Z¯=11) were included for comparison in selected scenarios. We determined absorbed-dose energy dependence of crystals relative to water under different scatter conditions (calibration phantom 12 × 12 × 30 cm³ , characterization phantoms 20 × 20 × 20 cm³ , 30 × 30 × 30 cm³ , 40 × 40 × 40 cm³ , and patient-like elliptic phantom 40 × 30 × 25 cm³ ). To mimic irradiation conditions during prostate treatments, we evaluated whether the presence of pelvic bones and calcifications affect ZnSe response. ZnSe detector design influence was also investigated.

RESULTS

In contrast to low-Z organic and medium-Z inorganic scintillators, ZnSe and CsI media have substantially greater absorbed-dose energy dependence relative to water. The response was phantom-size dependent and changed by 11 % between limited- and full-scatter conditions for ZnSe, but not for Al₂ O₃ . For a given phantom size, a part of the absorbed-dose energy dependence of ZnSe is caused not due to in-phantom scatter but due to source anisotropy. Thus, the absorbed-dose energy dependence of high-Z scintillators is a function of not only the radial distance but also the polar angle. Pelvic bones did not affect ZnSe response, whereas large and intermediate size calcifications reduced it by 9 % and 5 %, respectively, when placed midway between the source and the detector.

CONCLUSIONS

Unlike currently prevalent low- and medium-Z scintillators, high-Z crystals are sensitive to characterization and in vivo measurement conditions. However, good agreement between MC data for ZnSe in the present study and experimental data for ZnSe:O by Jørgensen et al (2021) suggest that detector signal is proportional to the average absorbed dose to the detector cavity. This enables an easy correction for non-TG43-like scenarios (e.g., patient sizes and calcifications) through MC simulations. Information that should be provided to the clinic by the detector vendors. This article is protected by copyright. All rights reserved.

On the potential of 2D ion chamber arrays for high-dose rate remote afterloading brachytherapy quality assurance

Objective. To investigate the potential of 2D ion chamber arrays to serve as a standalone tool for the verification of source strength, positioning and dwell time, within the framework of¹⁹²Ir high-dose rate brachytherapy device quality assurance (QA).Approach.A commercially available ion chamber array was used. Fitting of a 2D Lorentzian peak function to experimental data from a multiple source dwell position irradiation on a frame-by-frame basis, facilitated tracking of the source center orthogonal projection on the array plane. For source air kerma strength verification, Monte Carlo simulation was employed to obtain a chamber array- and source-specific correction factor of calibration with a 6 MV photon beam. This factor converted the signal measured by each ion chamber element to air kerma in free space. A source positioning correction was also applied to lift potential geometry mismatch between experiment and Monte Carlo simulation.Main results.Spatial and temporal accuracy of source movement was verified within 0.5 mm and 0.02 s, respectively, in compliance with the test endpoints recommended by international professional societies. The source air kerma strength was verified experimentally within method uncertainties estimated as 1.44% (k = 1). The source positioning correction method employed did not introduce bias to experimental results of irradiations where source positioning was accurate. Development of a custom jig attachable to the chamber array for accurate and reproducible experimental set up would improve testing accuracy and obviate the need for source positioning correction in air kerma strength verification.Significance.Delivery of a single irradiation plan, optimized based on results of this work, to a 2D ion chamber array can be used for concurrent testing of source position, dwell time and air kerma strength, and the procedure can be expedited through automation. Chamber arrays merit further study in treatment planning QA and real time,in vivodose verification.

3D dose reconstruction based on in vivo dosimetry for determining the dosimetric impact of geometric variations in high-dose-rate prostate brachytherapy

INTRODUCTION

In vivo dosimetry (IVD) can be used for source tracking (ST), i.e., estimating source positions, during brachytherapy. The aim of this study was to exploit IVD-based ST to perform 3D dose reconstruction for high-dose-rate prostate brachytherapy and to evaluate the robustness of the treatments against observed geometric variations.

MATERIALS AND METHODS

Twenty-three fractions of high-dose-rate prostate brachytherapy were analysed. The treatment planning was based on MRI. Time-resolved IVD was performed using a fibre-coupled scintillator. ST was retrospectively performed using the IVD measurements. The ST identified 2D positional shifts of each treatment catheter and thereby inferred updated source positions. For each fraction, the dose was recalculated based on the source-tracked catheter positions and compared with the original plan dose using differences in dose volume histogram indices.

RESULTS

Of 352 treatment catheters, 344 had shifts of less than 5 mm. Shifts between 5 and 10 mm were observed for 3 catheters, and shifts greater than 10 mm for 2 catheters. The ST failed for 3 catheters. The maximum relative difference in clinical target volume (prostate + 3 mm isotropic margin) D_90% was 5%. In one fraction, the bladder D_2cm³ dose increased by 18% (1.4Gy) due to a single source position being inside the bladder rather than nearby as planned. The max increase in urethra dose was 1.5Gy (15%).

CONCLUSION

IVD-based 3D dose reconstruction for high-dose-rate prostate brachytherapy is feasible. The dosimetric impact of the observed catheter shifts was limited. Dose reconstruction can therefore aid in determining the dosimetric impact of geometric variations and errors in brachytherapy.

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.

A high-Z inorganic scintillator-based detector for time-resolved in vivo dosimetry during brachytherapy

PURPOSE

High-dose rate (HDR) and pulsed-dose rate (PDR) brachytherapy would benefit from an independent treatment verification system to monitor treatment delivery and to detect errors in real time. This paper characterizes and provides an uncertainty budget for a detector based on a fiber-coupled high-Z inorganic scintillator capable of performing time-resolved in vivo dosimetry during HDR and PDR brachytherapy.

METHOD

The detector was composed of a detector probe and an optical reader. The detector probe consisted of either a 0.5 × 0.4 × 0.4 mm³ (HDR) or a 1.0 × 0.4 × 0.4 mm³ (PDR) cuboid ZnSe:O crystal glued onto an optical-fiber cable. The outer diameter of the detector probes was 1 mm, and fit inside standard brachytherapy catheters. The signal from the detector probe was read out at 20 Hz by a photodiode and a data acquisition device inside the optical reader. In order to construct an uncertainty budget for the detector, six characteristics were determined: (1) temperature dependence of the detector probe, (2) energy dependence as a function of the probe-to-source position in 2D (determined with 2 mm resolution using a robotic arm), (3) the signal-to-noise ratio (SNR), (4) short-term stability over 8 h, and (5) long-term stability of three optical readers and four probes used for in vivo monitoring in HDR and PDR treatments over 21 months (196 treatments and 189 detector calibrations, and (6) dose-rate dependence.

RESULTS

The total uncertainty of the detector at a 20 mm probe-to-source distance was < 5.1% and < 5.8% for the HDR and PDR versions, respectively. Regarding the above characteristics, (1) the sensitivity of the detector decreased by an average of 1.4%/°C for detector probe temperatures varying from 22 to 37°C; (2) the energy dependence of the detector was nonlinear and depended on both probe-to-source distance and the angle between the probe and the brachytherapy source; (3) the median SNRs were 187 and 34 at a 20 mm probe-to-source distance for the HDR and PDR versions, respectively (corresponding median source activities of 4.8 and 0.56 Ci, respectively); (4) the detector response varied by 0.6% in 11 identical irradiations over 8 h; (5) the sensitivity of the four detector probes decreased systematically by 0-1.2%/100 Gy of dose delivered to the probes, and random fluctuations of 4.8% in the sensitivity were observed for the three probes used in PDR and 1.9% for the probe used in HDR; and (6) the detector response was linear with dose rate.

CONCLUSION

ZnSe:O detectors can be used effectively for in vivo dosimetry and with high accuracy for HDR and PDR brachytherapy applications.

Quantifying clinical severity of physics errors in high-dose rate prostate brachytherapy using simulations

PURPOSE

To quantitatively evaluate through automated simulations the clinical significance of potential high-dose rate (HDR) prostate brachytherapy (HDRPB) physics errors selected from our internal failure-modes and effect analysis (FMEA).

METHODS AND MATERIALS

A list of failure modes was compiled and scored independently by 8 brachytherapy physicists on a one-to-ten scale for severity (S), occurrence (O), and detectability (D), with risk priority number (RPN) = SxOxD. Variability of RPNs across observers (standard deviation/average) was calculated. Six idealized HDRPB plans were generated, and error simulations were performed: single (N = 1722) and systematic (N = 126) catheter shifts (craniocaudal; -1cm:1 cm); single catheter digitization errors (tip and connector needle-tips displaced independently in random directions; 0.1 cm:0.5 cm; N = 44,318); and swaps (two catheters swapped during digitization or connection; N = 528). The deviations due to each error in prostate D90%, urethra D20%, and rectum D1cm³ were analyzed using two thresholds: 5-20% (possible clinical impact) and >20% (potentially reportable events).

RESULTS

Twenty-nine relevant failure modes were described. Overall, RPNs ranged from 6 to 108 (average ± 1 standard deviation, 46 ± 23), with responder variability ranging from 19% to 184% (average 75% ± 30%). Potentially reportable events were observed in the simulations for systematic shifts >0.4 cm for prostate and digitization errors >0.3 cm for the urethra and >0.4 cm for rectum. Possible clinical impact was observed for catheter swaps (all organs), systematic shifts >0.2 cm for prostate and >0.4 cm for rectum, and digitization errors >0.2 cm for prostate and >0.1 cm for urethra and rectum.

CONCLUSIONS

A high variability in RPN scores was observed. Systematic simulations can provide insight in the severity scoring of multiple failure modes, supplementing typical FMEA approaches.

Brachytherapy treatment verification using gamma radiation from the internal treatment source combined with an imaging panel-a phantom study

Brachytherapy has an excellent clinical outcome for different treatment sites. However,in vivotreatment verification is not performed in the majority of hospitals due to the lack of proper monitoring systems. This study investigates the use of an imaging panel (IP) and the photons emitted by a high dose rate (HDR)¹⁹²Ir source to track source motion and obtain some information related to the patient anatomy. The feasibility of this approach was studied by monitoring the treatment delivery to a 3D printed phantom that mimicks a prostate patient. A 3D printed phantom was designed with a template for needle insertion, a cavity ('rectum') to insert an ultrasound probe, and lateral cavities used to place tissue-equivalent materials. CT images were acquired to create HDR¹⁹²Ir treatment plans with a range of dwell times, interdwell distances and needle arrangements. Treatment delivery was verified with an IP placed at several positions around the phantom using radiopaque markers on the outer surface to register acquired IP images with the planning CT. All dwell positions were identified using acquisition times ≤0.11 s (frame rates ≥ 9 fps). Interdwell distances and dwell positions (in relation to the IP) were verified with accuracy better than 0.1 cm. Radiopaque markers were visible in the acquired images and could be used for registration with CT images. Uncertainties for image registration (IP and planning CT) between 0.1 and 0.4 cm. The IP is sensitive to tissue-mimicking insert composition and showed phantom boundaries that could be used to improve treatment verification. The IP provided sufficient time and spatial resolution for real-time source tracking and allows for the registration of the planning CT and IP images. The results obtained in this study indicate that several treatment errors could be detected including swapped catheters, incorrect dwell times and dwell positions.

3D source tracking and error detection in HDR using two independent scintillator dosimetry systems

PURPOSE

The aim of this study is to perform three-dimensional (3D) source position reconstruction by combining in vivo dosimetry measurements from two independent detector systems.

METHODS

Time-resolved dosimetry was performed in a water phantom during HDR brachytherapy irradiation with 192 Ir source using two detector systems. The first was based on three plastic scintillator detectors and the second on a single inorganic crystal (CsI:Tl). Brachytherapy treatments were simulated in water under TG-43U1 conditions, including a HDR prostate plan. Treatment needles were placed in distances covering a range of source movement of 120 mm around the detectors. The distance from each dwell position to each scintillator was determined based on the measured dose rates. The three distances given by the mPSD were recalculated to a position along the catheter (z) and a distance radially away from the mPSD (xy) for each dwell position (a circumference around the mPSD). The source x, y, and z coordinates were derived from the intersection of the mPSD's circumference with the sphere around the ISD based on the distance to this detector. We evaluated the accuracy of the source position reconstruction as a function of the distance to the source, the most likely location for detector positioning within a prostate volume, as well as the capacity to detect positioning errors.

RESULTS

Approximately 4000 source dwell positions were tracked for eight different HDR plans. An intersection of the mPSD torus and the ISD sphere was observed in 77.2% of the dwell positions, assuming no uncertainty in the dose rate determined distance. This increased to 100% if 1σ search regions were added. However, only 73(96)% of the expected dwell positions were found within the intersection band for 1(2) σ uncertainties. The agreement between the source's reconstructed and expected positions was within 3 mm for a range of distances to the source up to 50 mm. The experiments on a HDR prostate plan, showed that by having at least one of the detectors located in the middle of the prostate volume, reduces the measurement deviations considerably compared to scenarios where the detectors were located outside of the prostate volume. The analysis showed a detection probability that, in most cases, is far from the random detection threshold. Errors of 1(2) mm can be detected in ranges of 5-25 (25-50) mm from the source, with a true detection probability rate higher than 80%, while the false probability rate is kept below 20%.

CONCLUSIONS

By combining two detector responses, we enabled the determination of the absolute source coordinates. The combination of the mPSD and the ISD in vivo dosimetry constitutes a promising alternative for real-time 3D source tracking in HDR brachytherapy.

Accuracy of an in vivo dosimetry-based source tracking method for afterloading brachytherapy - A phantom study

PURPOSE

To report on the accuracy of an in vivo dosimetry (IVD)-based source tracking (ST) method for high dose rate (HDR) prostate brachytherapy (BT).

METHODS

The ST was performed on a needle-by-needle basis. A least square fit of the expected to the measured dose rate was performed using the active dwell positions in the given needle. Two fitting parameters were used to determine the position of each needle relative to the IVD detector: radial (away or toward the detector) and longitudinal (along the axis of the treatment needle). The accuracy of the ST was assessed in a phantom where the geometries of five HDR prostate BT treatments previously treated at our clinic were reproduced. For each of the five treatment geometries, one irradiation was performed with the detector placed in the middle of the implant. Furthermore, four additional irradiations were performed for one of the geometries where the detector was retracted caudally in four steps of 10-15 mm and up to 12 mm inferior of the most inferior active dwell position, which represented the prostate apex. The time resolved dose measurements were retrieved at a rate of 20 Hz using a detector based on an Al₂ O₃ :C radioluminescence crystal, which was placed inside a standard BT needle. Individual calibrations of the detector were performed prior to each of the nine irradiations.

RESULTS

Source tracking could be applied in all needles across all nine irradiations. For irradiations with the detector located in the middle region of the implant (a total of 89 needles), the mean ± standard deviation (SD, k = 1) accuracy was -0.01 mm ± 0.38 mm and 0.30 mm ± 0.38 mm in the radial and longitudinal directions, respectively. Caudal retraction of the detector did not lead to reduced accuracy as long as the detector was located superior to the most inferior active dwell positions in all needles. However, reduced accuracy was observed for detector positions inferior to the most inferior active dwell positions which corresponded to detector positions in and inferior to the prostate apex region. Detector positions in the prostate apex and 12 mm inferior to the prostate resulted in mean ± SD (k = 1) ST accuracy of 0.7 mm ± 1 mm and 2.8 mm ± 1.6 mm, respectively, in radial direction, and -1.7 mm ± 1 mm and -2.1 mm ± 1.1 mm, respectively, in longitudinal direction. The largest deviations for the configurations with those detector positions were 2.6 and 5.4 mm, respectively, in the radial direction and -3.5 and -3.8 mm, respectively, in the longitudinal direction.

CONCLUSION

This phantom study demonstrates that ST based on IVD during prostate BT is adequately accurate for clinical use. The ST yields submillimeter accuracy on needle positions as long as the IVD detector is positioned superior to at least one active dwell position in all needles. Locations of the detector inferior to the prostate apex result in decreased ST accuracy while detector locations in the apex region and above are advantageous for clinical applications.

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.

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.

In vivo dosimetry in brachytherapy: Requirements and future directions for research, development, and clinical practice

Brachytherapy can deliver high doses to the target while sparing healthy tissues due to its steep dose gradient leading to excellent clinical outcome. Treatment accuracy depends on several manual steps making brachytherapy susceptible to operational mistakes. Currently, treatment delivery verification is not routinely available and has led, in some cases, to systematic errors going unnoticed for years. The brachytherapy community promoted developments in in vivo dosimetry (IVD) through research groups and small companies. Although very few of the systems have been used clinically, it was demonstrated that the likelihood of detecting deviations from the treatment plan increases significantly with time-resolved methods. Time–resolved methods could interrupt a treatment avoiding gross errors which is not possible with time-integrated dosimetry. In addition, lower experimental uncertainties can be achieved by using source-tracking instead of direct dose measurements. However, the detector position in relation to the patient anatomy remains a main source of uncertainty. The next steps towards clinical implementation will require clinical trials and systematic reporting of errors and near-misses. It is of utmost importance for each IVD system that its sensitivity to different types of errors is well understood, so that end-users can select the most suitable method for their needs. This report aims to formulate requirements for the stakeholders (clinics, vendors, and researchers) to facilitate increased clinical use of IVD in brachytherapy. The report focuses on high dose-rate IVD in brachytherapy providing an overview and outlining the need for further development and research.

Inorganic scintillation detectors for 192Ir brachytherapy

Many brachytherapy (BT) errors could be detected with real-time in vivo dosimetry technology. Inorganic scintillation detectors (ISDs) have demonstrated promising capabilities for BT, because some ISD materials can generate scintillation signals large enough that (a) the background signal emitted in the fiber-optic cable (stem signal) is insignificant, and (b) small detector volumes can be used to avoid volume averaging effects in steep dose gradients near BT sources. We investigated the characteristics of five ISD materials to identify one that is appropriate for BT. ISDs consisting of a 0.26 to 1.0 mm³ volume of ruby (Al₂O₃:Cr), a mixture of Y₂O₃:Eu and YVO⁴:Eu, ZnSe:O, or CsI:Tl coupled to a fiber-optic cable were irradiated in a water-equivalent phantom using a high-dose-rate ¹⁹²Ir BT source. Detectors based on plastic scintillators BCF-12 and BCF-60 (0.8 mm³ volume) were used as a reference. Measurements demonstrated that the ruby, Y₂O₃:Eu+YVO₄:Eu, ZnSe:O, and CsI:Tl ISDs emitted scintillation signals that were up to 19, 19, 250, and 880 times greater, respectively, than that of the BCF-12 detector. While the total signals of the plastic scintillation detectors were dominated by the stem signal for source positions 0.5 cm from the fiber-optic cable and  >3.5 cm from the scintillator volume, the stem signal for the ruby and Y₂O₃:Eu+YVO₄:Eu ISDs were  <1% of the total signal for source positions  <3.4 and  <4.4 cm from the scintillator, respectively, and  <0.7% and  <0.5% for the ZnSe:O and CsI:Tl ISDs, respectively, for positions  ⩽8.0 cm. In contrast to the other ISDs, the Y₂O₃:Eu+YVO₄:Eu ISD exhibited unstable scintillation and significant afterglow. All ISDs exhibited significant energy dependence, i.e. their dose response to distance-dependent ¹⁹²Ir energy spectra differed significantly from the absorbed dose in water. Provided that energy dependence is accounted for, ZnSe:O ISDs are promising for use in error detection and patient safety monitoring during BT.

Dosimetric impact of placement errors in optically stimulated luminescent in vivo dosimetry in radiotherapy

Introduction

Studies have suggested that optically stimulated luminescent dosimeters (OSLDs) can be used for in vivo dosimetry of intensity-modulated radiation therapy (IMRT) and volumetric-modulated arc therapy (VMAT). Clinical uncertainties such as placement error have not been thoroughly investigated. The purpose of this work was to measure OSLD placement error in a clinical sample and analyze its dosimetric impact.

Methods and materials

The analysis consisted of three parts: first, quantification of placement error in a clinical sample of 128 patients yielding 293 cone-beam CT (CBCT) with visible OSLDs registered to the treatment plan; Second, correlation of placement error and clinical OSLD measurements; third, simulation of dosimeter placement in the treatment plan and correlation of recalculated dose with placement error.

Results

In the first analysis, average placement error was 9.7 ± 9.5 mm. In the second analysis, placement error and measured-to-planned dose agreement yielded no correlation (R² = 0.02) for a subsample of 77 CBCTs of 55 head-and-neck patients. Average placement error was 7.0 ± 6.0 mm. Several factors, including image-guided shifts, introduced uncharacterized uncertainty to the measured-to-planned dose agreement. The third analysis isolated placement error from these other effects. Average dosimetric error was -2.4 ± 19.3%. Simulated dosimetric impact was weakly correlated with placement error (R² = 0.39). Removing outliers reduced the average dosimetric error to -2.1 ± 10.9%, marginally improving the correlation (R² = 0.44).

Conclusion

Placement error can substantially impact measured-to-planned dose agreement of OSLDs in high gradient regions, demonstrating the criticality of accurate dosimeter placement for IMRT and VMAT treatments.

Experience of using MOSFET detectors for dose verification measurements in an end-to-end 192Ir brachytherapy quality assurance system

PURPOSE

Establishment of an end-to-end system for the brachytherapy (BT) dosimetric chain could be valuable in clinical quality assurance. Here, the development of such a system using MOSFET (metal oxide semiconductor field effect transistor) detectors and experience gained during 2 years of use are reported with focus on the performance of the MOSFET detectors.

METHODS AND MATERIALS

A bolus phantom was constructed with two implants, mimicking prostate and head & neck treatments, using steel needles and plastic catheters to guide the ¹⁹²Ir source and house the MOSFET detectors. The phantom was taken through the BT treatment chain from image acquisition to dose evaluation. During the 2-year evaluation-period, delivered doses were verified a total of 56 times using MOSFET detectors which had been calibrated in an external ⁶⁰Co beam. An initial experimental investigation on beam quality differences between ¹⁹²Ir and ⁶⁰Co is reported.

RESULTS

The standard deviation in repeated MOSFET measurements was below 3% in the six measurement points with dose levels above 2 Gy. MOSFET measurements overestimated treatment planning system doses by 2-7%. Distance-dependent experimental beam quality correction factors derived in a phantom of similar size as that used for end-to-end tests applied on a time-resolved measurement improved the agreement.

CONCLUSIONS

MOSFET detectors provide values stable over time and function well for use as detectors for end-to-end quality assurance purposes in ¹⁹²Ir BT. Beam quality correction factors should address not only distance from source but also phantom dimensions.

3D catheter reconstruction in HDR prostate brachytherapy for pre-treatment verification using a flat panel detector

PURPOSE

High dose rate prostate brachytherapy is a widely-practiced treatment, delivering large conformal doses in relatively few treatment fractions. Inter- and intra-fraction catheter displacements have been reported. Unrecognized displacement can have a significant impact on dosimetry. Knowledge of the implant geometry at the time of treatment is important for ensuring safe and effective treatment. In this work we demonstrate a method to reconstruct the catheter positions pre-treatment, using a 'shift' imaging technique, and perform registration with the treatment plan for verification relative to the prostate.

METHODS

Two oblique 'shift' images were acquired of a phantom containing brachytherapy catheters, representing the patient immediately pre-treatment. Using a back projection approach, the catheter paths were reconstructed in 3D and registered with the planned catheter paths. The robustness of the reconstruction and registration process was investigated as a function of phantom rotation. Catheter displacement detection was performed and compared to known applied displacements.

RESULTS

Reconstruction of the implant geometry in 3D immediately prior to treatment was achieved. A mean reconstruction uncertainty of 0.8mm was determined for all catheters with a mean registration uncertainty of 0.5mm. A catheter displacement detection threshold of 2.2mm was demonstrated. Catheter displacements were all detected to within 0.5mm of the applied displacements.

CONCLUSION

This technique is robust and sensitive to assess catheter displacements throughout the implant volume. This approach provides a method to detect, in 3D, changes in catheter positions relative to the prostate. The method has sufficient sensitivity to enable clinically significant decisions immediately prior to treatment delivery.

Ruby-based inorganic scintillation detectors for 192Ir brachytherapy

We tested the potential of ruby inorganic scintillation detectors (ISDs) for use in brachytherapy and investigated various unwanted luminescence properties that may compromise their accuracy. The ISDs were composed of a ruby crystal coupled to a poly(methyl methacrylate) fiber-optic cable and a charge-coupled device camera. The ISD also included a long-pass filter that was sandwiched between the ruby crystal and the fiber-optic cable. The long-pass filter prevented the Cerenkov and fluorescence background light (stem signal) induced in the fiber-optic cable from striking the ruby crystal, which generates unwanted photoluminescence rather than the desired radioluminescence. The relative contributions of the radioluminescence signal and the stem signal were quantified by exposing the ruby detectors to a high-dose-rate brachytherapy source. The photoluminescence signal was quantified by irradiating the fiber-optic cable with the detector volume shielded. Other experiments addressed time-dependent luminescence properties and compared the ISDs to commonly used organic scintillator detectors (BCF-12, BCF-60). When the brachytherapy source dwelled 0.5 cm away from the fiber-optic cable, the unwanted photoluminescence was reduced from  >5% to  <1% of the total signal as long as the ISD incorporated the long-pass filter. The stem signal was suppressed with a band-pass filter and was  <3% as long as the source distance from the scintillator was  <7 cm. Some ruby crystals exhibited time-dependent luminescence properties that altered the ruby signal by  >5% within 10 s from the onset of irradiation and after the source had retracted. The ruby-based ISDs generated signals of up to 20 times that of BCF-12-based detectors. The study presents solutions to unwanted luminescence properties of ruby-based ISDs for high-dose-rate brachytherapy. An optic filter should be sandwiched between the ruby crystal and the fiber-optic cable to suppress the photoluminescence. Furthermore, we recommend avoiding ruby crystals that exhibit significant time-dependent luminescence.

In vivo dosimetry: trends and prospects for brachytherapy

The error types during brachytherapy (BT) treatments and their occurrence rates are not well known. The limited knowledge is partly attributed to the lack of independent verification systems of the treatment progression in the clinical workflow routine. Within the field of in vivo dosimetry (IVD), it is established that real-time IVD can provide efficient error detection and treatment verification. However, it is also recognized that widespread implementations are hampered by the lack of available high-accuracy IVD systems that are straightforward for the clinical staff to use. This article highlights the capabilities of the state-of-the-art IVD technology in the context of error detection and quality assurance (QA) and discusses related prospects of the latest developments within the field. The article emphasizes the main challenges responsible for the limited practice of IVD and provides descriptions on how they can be overcome. Finally, the article suggests a framework for collaborations between BT clinics that implemented IVD on a routine basis and postulates that such collaborations could improve BT QA measures and the knowledge about BT error types and their occurrence rates.