Fast 2D phantom dosimetry for scanning proton beams

Sparing of the Heart Facilitates Recovery From Cardiopulmonary Side Effects After Thoracic Irradiation

PURPOSE

When irradiating thoracic tumors, dose to the heart or lung has been associated with survival. We previously showed in a rat model that in addition to known side effects such as pericarditis, pneumonitis and fibrosis, heart and/or lung irradiation also impaired diastolic function and increased pulmonary artery pressure. Simultaneous irradiation of both organs strongly intensified these effects. However, the long-term consequences of these interactions are not yet known. Therefore, here, we investigated the long-term effects of combined heart and lung irradiation.

METHODS AND MATERIALS

Different regions of the rat thorax containing the heart and/or 50% of the lungs were irradiated with protons. Respiratory rate (RR) was measured biweekly as an overall parameter for cardiopulmonary function. Echocardiography of the heart was performed at 8, 26, and 42 weeks after irradiation. Tissue remodeling and vascular changes were assessed using Masson trichrome and Verhoeff-stained lung and left ventricle tissue collected at 8 and 42 weeks after irradiation.

RESULTS

During the entire experimental period RR was consistently increased after combined heart/lung irradiation. This coincided with persistent effects on lung vasculature and reduced right-ventricle (RV) contraction. In contrast, recovery of RR, pulmonary remodeling and RV contraction was observed after sparing of the heart. These corresponding temporal patterns suggest that the reduction of RV function is related to vascular remodeling in the lung.

CONCLUSIONS

Combined irradiation of lung and heart leads to an intensified, persistent reduction of cardiopulmonary function. Recovery of the pulmonary vasculature and RV function requires heart sparing.

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.

Absolute energy-dependent scintillating screen calibration for real-time detection of laser-accelerated proton bunches

Laser-plasma accelerators (LPAs) can deliver pico- to nanosecond long proton bunches with ≳100 nC of charge dispersed over a broad energy spectrum. Increasing the repetition rates of today's LPAs is a necessity for their practical application. This, however, creates a need for real-time proton bunch diagnostics. Scintillating screens are one detector solution commonly applied in the field of electron LPAs for spatially resolved particle and radiation detection. Yet their establishment for LPA proton detection is only slowly taking off, also due to the lack of available calibrations. In this paper, we present an absolute proton number calibration for the scintillating screen type DRZ High (Mitsubishi Chemical Corporation, Düsseldorf, Germany), one of the most sensitive screens according to calibrations for relativistic electrons and x rays. The presented absolute light yield calibration shows an uncertainty of the proton number of 10% and can seamlessly be applied at other LPA facilities. For proton irradiation of the DRZ High screen, we find an increase in light yield of >60% compared to reference calibration data for relativistic electrons. Moreover, we investigate the scintillating screen light yield dependence on proton energy since many types of scintillators (e.g., plastic, liquid, and inorganic) show a reduced light yield for increased local energy deposition densities, an effect termed ionization quenching. The ionization quenching can reduce the light yield for low-energy protons by up to ∼20%. This work provides all necessary data for absolute spectral measurements of LPA protons with DRZ High scintillating screens, e.g., when used in the commonly applied Thomson parabola spectrometers.

Proton beam spot size and position measurements using a multi-strip ionization chamber

PURPOSE

To develop and characterize a large-area multi-strip ionization chamber (MSIC) for efficient measurement of proton beam spot size and position at a synchrotron-based proton therapy facility.

METHODS AND MATERIALS

A 420 mm x 320 mm MSIC was designed with 240 vertical strips and 180 horizontal strips at 1.75 mm pitch. The MSIC was characterized by irradiating a grid of proton spots across 17 energies from 73.5 MeV to 235 MeV and comparing to simultaneous measurements made with a reference Gafchromic EBT3 film. Beam profiles, spot sizes, and positions were analyzed. Short term measurement stability and sensitivity were evaluated.

RESULTS

Excellent agreement was demonstrated between the MSIC and EBT3 film for both spot size and position measurements. Spot sizes agreed within ± 0.18 mm for all energies tested. Measured beam spot positions agreed within ± 0.17 mm. The detector showed good short term measurement stability and low noise performance.

CONCLUSION

The large-area MSIC enables efficient and accurate proton beam spot characterization across the clinical energy range. The results indicate the MSIC is suitable for pencil beam scanning proton therapy commissioning and quality assurance applications requiring fast spot size and position quantification.

Two-dimensional time-resolved scintillating sheet monitoring of proton pencil beam scanning FLASH mouse irradiations

BACKGROUND

Dosimetry in pre-clinical FLASH studies is essential for understanding the beam delivery conditions that trigger the FLASH effect. Resolving the spatial and temporal characteristics of proton pencil beam scanning (PBS) irradiations with ultra-high dose rates (UHDR) requires a detector with high spatial and temporal resolution.

PURPOSE

To implement a novel camera-based system for time-resolved two-dimensional (2D) monitoring and apply it in vivo during pre-clinical proton PBS mouse irradiations.

METHODS

Time-resolved 2D beam monitoring was performed with a scintillation imaging system consisting of a 1 mm thick transparent scintillating sheet, imaged by a CMOS camera. The sheet was placed in a water bath perpendicular to a horizontal PBS proton beam axis. The scintillation light was reflected through a system of mirrors and captured by the camera with 500 frames per second (fps) for UHDR and 4 fps for conventional dose rates. The raw images were background subtracted, geometrically transformed, flat field corrected, and spatially filtered. The system was used for 2D spot and field profile measurements and compared to radiochromic films. Furthermore, spot positions were measured for UHDR irradiations. The measured spot positions were compared to the planned positions and the relative instantaneous dose rate to equivalent fiber-coupled point scintillator measurements. For in vivo application, the scintillating sheet was placed 1 cm upstream the right hind leg of non-anaesthetized mice submerged in the water bath. The mouse leg and sheet were both placed in a 5 cm wide spread-out Bragg peak formed from the mono-energetic proton beam by a 2D range modulator. The mouse leg position within the field was identified for both conventional and FLASH irradiations. For the conventional irradiations, the mouse foot position was tracked throughout the beam delivery, which took place through repainting. For FLASH irradiations, the delivered spot positions and relative instantaneous dose rate were measured.

RESULTS

The pixel size was 0.1 mm for all measurements. The spot and field profiles measured with the scintillating sheet agreed with radiochromic films within 0.4 mm. The standard deviation between measured and planned spot positions was 0.26 mm and 0.35 mm in the horizontal and vertical direction, respectively. The measured relative instantaneous dose rate showed a linear relation with the fiber-coupled scintillator measurements. For in vivo use, the leg position within the field varied between mice, and leg movement up to 3 mm was detected during the prolonged conventional irradiations.

CONCLUSIONS

The scintillation imaging system allowed for monitoring of UHDR proton PBS delivery in vivo with 0.1 mm pixel size and 2 ms temporal resolution. The feasibility of instantaneous dose rate measurements was demonstrated, and the system was used for validation of the mouse leg position within the field.

High spatiotemporal resolution scintillation imaging of pulsed pencil beam scanning proton beams produced by a gantry-mounted synchrocyclotron

BACKGROUND

A dosimeter with high spatial and temporal resolution would be of significant interest for pencil beam scanning (PBS) proton beams' characterization, especially when facing small fields and beams with high temporal dynamics. Optical imaging of scintillators has potential in providing sub-millimeter spatial resolution with pulse-by-pulse basis temporal resolution when the imaging system is capable of operating in synchrony with the beam-producing accelerator.

PURPOSE

We demonstrate the feasibility of imaging PBS proton beams as they pass through a plastic scintillator detector to simultaneously obtain multiple beam parameters, including proton range, pencil beam's widths at different depths, spot's size, and spot's position on a pulse-by-pulse basis with sub-millimeter resolution.

MATERIALS AND METHODS

A PBS synchrocyclotron was used for proton irradiation. A BC-408 plastic scintillator block with 30 × 30 × 5 cm3 size, and another block with 30 × 30 × 0.5 cm3 size, positioned in an optically sealed housing, were used sequentially to measure the proton range, and spot size/location, respectively. A high-speed complementary metal-oxide-semiconductor (CMOS) camera system synchronized with the accelerator's pulses through a gating module was used for imaging. Scintillation images, captured with the camera directly facing the 5-cm-thick scintillator, were corrected for background (BG), and ionization quenching of the scintillator to obtain the proton range. Spots' position and size were obtained from scintillation images of the 0.5-cm-thick scintillator when a 45° mirror was used to reflect the scintillation light toward the camera.

RESULTS

Scintillation images with 0.16 mm/pixel resolution corresponding to all proton pulses were captured. Pulse-by-pulse analysis showed that variations of the range, spots' position, and size were within ± 0.2% standard deviation of their average values. The absolute ranges were within ± 1 mm of their expected values. The average spot-positions were mostly within ± 0.8 mm and spots' sigma agreed within 0.2 mm of the expected values.

CONCLUSION

Scintillation-imaging PBS beams with high-spatiotemporal resolution is feasible and may help in efficient and cost-effective acceptance testing and commissioning of existing and even emerging technologies such as FLASH, grid, mini-beams, and so forth.

Development of a digital star-shot analysis system for comparing radiation and imaging isocenters of proton treatment machine

PURPOSE

To directly compare the radiation and imaging isocenters of a proton treatment machine, we developed and evaluated a real-time radiation isocenter verification system.

METHODS

The system consists of a plastic scintillator (PI-200, Mitsubishi Chemical Corporation, Tokyo, Japan), an acrylic phantom, a steel ball on the detachable plate, Raspberry Pi 4 (Raspberry Pi Foundation, London, UK) with camera module, and analysis software implemented through a Python-based graphical user interface (GUI). After kV imaging alignment of the steel ball, the imaging isocenter defined as the position of the steel ball was extracted from the optical image. The proton star-shot was obtained by optical camera because the scintillator converted proton beam into visible light. Then the software computed both the minimum circle radius and the radiation isocenter position from the star-shot. And the deviation between the imaging isocenter and radiation isocenter was calculated. We compared our results with measurements obtained by Gafchromic EBT3 film (Ashland, NJ, USA).

RESULTS

The minimum circle radii were averaged 0.29 and 0.41 mm while the position deviations from the radiation isocenter to the laser marker were averaged 0.99 and 1.07 mm, for our system and EBT3 film, respectively. Furthermore, the average position difference between the radiation isocenter and imaging isocenter was 0.27 mm for our system. Our system reduced analysis time by 10 min.

CONCLUSIONS

Our system provided automated star-shot analysis with sufficient accuracy, and it is cost-effective alternative to conventional film-based method for radiation isocenter verification.

Proton therapy induces a local microglial neuroimmune response

BACKGROUND AND PURPOSE

Although proton therapy is increasingly being used in the treatment of paediatric and adult brain tumours, there are still uncertainties surrounding the biological effect of protons on the normal brain. Microglia, the brain-resident macrophages, have been shown to play a role in the development of radiation-induced neurotoxicity. However, their molecular and hence functional response to proton irradiation remains unknown. This study investigates the effect of protons on microglia by comparing the effect of photons and protons as well as the influence of age and different irradiated volumes.

MATERIALS AND METHODS

Rats were irradiated with 14 Gy to the whole brain with photons (X-rays), plateau protons, spread-out Bragg peak (SOBP) protons or to 50 % anterior, or 50 % posterior brain sub-volumes with plateau protons. RNA sequencing, validation of microglial priming gene expression using qPCR and high-content imaging analysis of microglial morphology were performed in the cortex at 12 weeks post irradiation.

RESULTS

Photons and plateau protons induced a shared transcriptomic response associated with neuroinflammation. This response was associated with a similar microglial priming gene expression signature and distribution of microglial morphologies. Expression of the priming gene signature was less pronounced in juvenile rats compared to adults and slightly increased in rats irradiated with SOBP protons. High-precision partial brain irradiation with protons induced a local microglial priming response and morphological changes.

CONCLUSION

Overall, our data indicate that the brain responds in a similar manner to photons and plateau protons with a shared local upregulation of microglial priming-associated genes, potentially enhancing the immune response to subsequent inflammatory challenges.

A Monte-Carlo-based study of a single-2D-detector proton-radiography system

PURPOSE

To assess the feasibility of a proton radiography (pRG) system based on a single thin pixelated detector for water-equivalent path length (WEPL) and relative stopping power (RSP) measurements.

METHODS

A model of a pRG system consisting of a single pixelated detector measuring energy deposition and proton fluence was investigated in a Geant4-based Monte Carlo study. At the position directly after an object traversed by a broad proton beam, spatial 2D distributions are calculated of the energy deposition in, and the number of protons entering the detector. Their ratio relates to the 2D distribution of the average stopping power of protons in the detector. The system response is calibrated against the residual range in water of the protons to provide the 2D distribution of the WEPL of the object. The WEPL distribution is converted into the distribution of the RSP of the object. Simulations have been done, where the system has been tested on 13 samples of homogeneous materials of which the RSPs have been calculated and compared with RSPs determined from simulations of residual-range-in-water, which we refer to as reference RSPs.

RESULTS

For both human-tissue- and non-human-tissue-equivalent materials, the RSPs derived with the detector agree with the reference values within 1%.

CONCLUSION

The study shows that a pRG system based on one thin pixelated detection screen has the potential to provide RSP predictions with an accuracy of 1%.

Design, testing and characterization of a proton central axis alignment device for the dynamic collimation system

Objective. Proton therapy conformity has improved over the years by evolving from passive scattering to spot scanning delivery technologies with smaller proton beam spot sizes. Ancillary collimation devices, such the Dynamic Collimation System (DCS), further improves high dose conformity by sharpening the lateral penumbra. However, as spot sizes are reduced, collimator positional errors play a significant impact on the dose distributions and hence accurate collimator to radiation field alignment is critical.Approach. The purpose of this work was to develop a system to align and verify coincidence between the center of the DCS and the proton beam central axis. The Central Axis Alignment Device (CAAD) is composed of a camera and scintillating screen-based beam characterization system. Within a light-tight box, a 12.3-megapixel camera monitors a P43/Gadox scintillating screen via a 45° first-surface mirror. When a collimator trimmer of the DCS is placed in the uncalibrated center of the field, the proton radiation beam continuously scans a 7×7 cm2square field across the scintillator and collimator trimmer while a 7 s exposure is acquired. From the relative positioning of the trimmer to the radiation field, the true center of the radiation field can be calculated.Main results.The CAAD can calculate the offset between the proton beam radiation central axis and the DCS central axis within 0.054 mm accuracy and 0.075 mm reproducibility.Significance.Using the CAAD, the DCS is now able to be aligned accurately to the proton radiation beam central axis and no longer relies on an x-ray source in the gantry head which is only validated to within 1.0 mm of the proton beam.

Synchronized high-speed scintillation imaging of proton beams, generated by a gantry-mounted synchrocyclotron, on a pulse-by-pulse basis

BACKGROUND

With the emergence of more complex and novel proton delivery techniques, there is a need for quality assurance (QA) tools with high spatiotemporal resolution to conveniently measure the spatial and temporal properties of the beam. In this context, scintillation-based dosimeters, if synchronized with the radiation beam and corrected for ionization quenching, are appealing.

PURPOSE

To develop a synchronized high-speed scintillation imaging system for characterization and verification of the proton therapy beams on a pulse-by-pulse basis.

MATERIALS AND METHODS

A 30 cm × 30 cm × 5 cm block of BC-408 plastic scintillator placed in a light-tight housing was irradiated by proton beams generated by a Mevion S250^TM proton therapy synchrocyclotron. A high-speed camera system, placed perpendicular to the beam direction and facing the scintillator, was synchronized to the accelerator's pulses to capture images. Opening and closing of the camera's shutter was controlled by setting a proper time delay and exposure time, respectively. The scintillation signal was recorded as a set of two-dimensional (2D) images. Empirical correction factors were applied to the images to correct for the non-uniformity of the pixel sensitivity and quenching of the scintillator. Proton range and modulation were obtained from the corrected images.

RESULTS

The camera system was able to capture all data on a pulse-by-pulse basis at a rate of ∼504 frames per second. The applied empirical correction method for ionization quenching was effective and the corrected composite image provided a 2D map of dose distribution. The measured range (depth of distal 90%) through scintillation imaging agreed within 1.2 mm with that obtained from ionization chamber measurement.

CONCLUSION

A high-speed camera system capable of capturing scintillation signals from individual proton pulses was developed. The scintillation imaging system is promising for rapid proton beam characterization and verification. This article is protected by copyright. All rights reserved.

Three-dimensional dose-distribution measurement of therapeutic carbon-ion beams using a ZnS scintillator sheet

The accurate measurement of the 3D dose distribution of carbon-ion beams is essential for safe carbon-ion therapy. Although ionization chambers scanned in a water tank or air are conventionally used for this purpose, these measurement methods are time-consuming. We thus developed a rapid 3D dose-measurement tool that employs a silver-activated zinc sulfide (ZnS) scintillator with lower linear energy transfer (LET) dependence than gadolinium-based (Gd) scintillators; this tool enables the measurement of carbon-ion beams with small corrections. A ZnS scintillator sheet was placed vertical to the beam axis and installed in a shaded box. Scintillation images produced by incident carbon-ions were reflected with a mirror and captured with a charge-coupled device (CCD) camera. A 290 MeV/nucleon mono-energetic beam and spread-out Bragg peak (SOBP) carbon-ion passive beams were delivered at the Gunma University Heavy Ion Medical Center. A water tank was installed above the scintillator with the water level remotely adjusted to the measurement depth. Images were recorded at various water depths and stacked in the depth direction to create 3D scintillation images. Depth and lateral profiles were analyzed from the images. The ZnS-scintillator-measured depth profile agreed with the depth dose measured using an ionization chamber, outperforming the conventional Gd-based scintillator. Measurements were realized with smaller corrections for a carbon-ion beam with a higher LET than a proton. Lateral profiles at the entrance and the Bragg peak depths could be measured with this tool. The proposed method would make it possible to rapidly perform 3D dose-distribution measurements of carbon-ion beams with smaller quenching corrections.

A simple microscopy setup for visualizing cellular responses to DNA damage at particle accelerator facilities

Cellular responses to DNA double-strand breaks (DSBs) not only promote genomic integrity in healthy tissues, but also largely determine the efficacy of many DNA-damaging cancer treatments, including X-ray and particle therapies. A growing body of evidence suggests that activation of the mechanisms that detect, signal and repair DSBs may depend on the complexity of the initiating DNA lesions. Studies focusing on this, as well as on many other radiobiological questions, require reliable methods to induce DSBs of varying complexity, and to visualize the ensuing cellular responses. Accelerated particles of different energies and masses are exceptionally well suited for this task, due to the nature of their physical interactions with the intracellular environment, but visualizing cellular responses to particle-induced damage - especially in their early stages - at particle accelerator facilities, remains challenging. Here we describe a straightforward approach for real-time imaging of early response to particle-induced DNA damage. We rely on a transportable setup with an inverted fluorescence confocal microscope, tilted at a small angle relative to the particle beam, such that cells can be irradiated and imaged without any microscope or beamline modifications. Using this setup, we image and analyze the accumulation of fluorescently-tagged MDC1, RNF168 and 53BP1-key factors involved in DSB signalling-at DNA lesions induced by 254 MeV α-particles. Our results provide a demonstration of technical feasibility and reveal asynchronous initiation of accumulation of these proteins at different individual DSBs.

Development and characterization of optical readout well-type glass gas electron multiplier for dose imaging in clinical carbon beams

The use of carbon ion beams in cancer therapy (also known as hadron therapy) is steadily growing worldwide; therefore, the demand for more efficient dosimetry systems is also increasing because daily quality assurance (QA) measurements of hadron radiotherapy is one of the most complex and time consuming tasks. The aim of this study is to develop a two-dimensional dosimetry system that offers high spatial resolution, a large field of view, quick data response, and a linear dose-response relationship. We demonstrate the dose imaging performance of a novel digital dose imager using carbon ion beams for hadron therapy. The dose imager is based on a newly-developed gaseous detector, a well-type glass gas electron multiplier. The imager is successfully operated in a hadron therapy facility with clinical intensity beams for radiotherapy. It features a high spatial resolution of less than 1 mm and an almost linear dose-response relationship with no saturation and very low linear-energy-transfer dependence. Experimental results show that the dose imager has the potential to improve dosimetry accuracy for daily QA.

Development of a time-resolved mirrorless scintillation detector

Purpose

We developed a compact and lightweight time-resolved mirrorless scintillation detector (TRMLSD) employing image processing techniques and a convolutional neural network (CNN) for high-resolution two-dimensional (2D) dosimetry.

Methods

The TRMLSD comprises a camera and an inorganic scintillator plate without a mirror. The camera was installed at a certain angle from the horizontal plane to collect scintillation from the scintillator plate. The geometric distortion due to the absence of a mirror and camera lens was corrected using a projective transform. Variations in brightness due to the distance between the image sensor and each point on the scintillator plate and the inhomogeneity of the material constituting the scintillator were corrected using a 20.0 × 20.0 cm² radiation field. Hot pixels were removed using a frame-based noise-reduction technique. Finally, a CNN-based 2D dose distribution deconvolution model was applied to compensate for the dose error in the penumbra region and a lack of backscatter. The linearity, reproducibility, dose rate dependency, and dose profile were tested for a 6 MV X-ray beam to verify dosimeter characteristics. Gamma analysis was performed for two simple and 10 clinical intensity-modulated radiation therapy (IMRT) plans.

Results

The dose linearity with brightness ranging from 0.0 cGy to 200.0 cGy was 0.9998 (R-squared value), and the root-mean-square error value was 1.010. For five consecutive measurements, the reproducibility was within 3% error, and the dose rate dependency was within 1%. The depth dose distribution and lateral dose profile coincided with the ionization chamber data with a 1% mean error. In 2D dosimetry for IMRT plans, the mean gamma passing rates with a 3%/3 mm gamma criterion for the two simple and ten clinical IMRT plans were 96.77% and 95.75%, respectively.

Conclusion

The verified accuracy and time-resolved characteristics of the dosimeter may be useful for the quality assurance of machines and patient-specific quality assurance for clinical step-and-shoot IMRT plans.

Update on yesterday's dose-Use of delivery log-files for daily adaptive proton therapy (DAPT)

In daily adaptive proton therapy (DAPT), the treatment plan is re-optimized on a daily basis. It is a straightforward idea to incorporate information from the previous deliveries during the optimization to refine this daily proton delivery. A feedback signal was used to correct for delivery errors and errors from an inaccurate dose calculation used for plan optimization. This feedback signal consisted of a dose distribution calculated with a Monte Carlo algorithm and was based on the spot delivery information from the previous deliveries in the form of log-files. We therefore called the method Update On Yesterday's Dose (UYD). The UYD method was first tested with a simulated DAPT treatment and second with dose measurements using an anthropomorphic phantom. For both, the simulations and the measurements, a better agreement between the delivered and the intended dose distribution could be observed using UYD. Gamma pass rates (1%/1 mm) increased from around 75% to above 90%, when applying the closed-loop correction for the simulations, as well as the measurements. For a DAPT treatment, positioning errors or anatomical changes are incorporated during the optimization and therefore are less dominant in the overall dose uncertainty. Hence, the relevance of algorithm or delivery machine errors even increases compared to standard therapy. The closed-loop process described here is a method to correct for these errors, and potentially further improve DAPT.

Characterization of a new scintillation imaging system for proton pencil beam dose rate measurements

The goal of this work was to create a technique that could measure all possible spatial and temporal delivery rates used in pencil-beam scanning (PBS) proton therapy. The proposed system used a fast scintillation screen for full-field imaging to resolve temporal and spatial patterns as it was delivered. A fast intensified CMOS camera used continuous mode with 10 ms temporal frame rate and 1 × 1 mm2 spatial resolution, imaging a scintillation screen during clinical proton PBS delivery. PBS plans with varying dose, dose rate, energy, field size, and spot-spacing were generated, delivered and imaged. The captured images were post processed to provide dose and dose rate values after background subtraction, perspective transformation, uniformity correction for the camera and the scintillation screen, and calibration into dose. The linearity in scintillation response with respect to varying dose rate, dose, and field size was within 2%. The quenching corrected response with varying energy was also within 2%. Large spatio-temporal variations in dose rate were observed, even for plans delivered with similar dose distributions. Dose and dose rate histograms and maximum dose rate maps were generated for quantitative evaluations. With the fastest PBS delivery on a clinical system, dose rates up to 26.0 Gy s-1 were resolved. The scintillation imaging technique was able to quantify proton PBS dose rate profiles with spot weight as low as 2 MU, with spot-spacing of 2.5 mm, having a 1 × 1 mm2 spatial resolution. These dose rate temporal profiles, spatial maps, and cumulative dose rate histograms provide useful metrics for the potential evaluation and optimization of dose rate in treatment plans.

A scintillator-based range telescope for particle therapy

The commissioning and operation of a particle therapy centre requires an extensive set of detectors for measuring various parameters of the treatment beam. Among the key devices are detectors for beam range quality assurance. In this work, a novel range telescope based on a plastic scintillator and read out by a large-scale CMOS sensor is presented. The detector is made of a stack of 49 plastic scintillator sheets with a thickness of 2-3 mm and an active area of 100 × 100 mm², resulting in a total physical stack thickness of 124.2 mm. This compact design avoids optical artefacts that are common in other scintillation detectors. The range of a proton beam is reconstructed using a novel Bragg curve model that incorporates scintillator quenching effects. Measurements to characterise the performance of the detector were carried out at the Heidelberger Ionenstrahl-Therapiezentrum (HIT, Heidelberg, GER) and the Clatterbridge Cancer Centre (CCC, Bebington, UK). The maximum difference between the measured range and the reference range was found to be 0.41 mm at a proton beam range of 310 mm and was dominated by detector alignment uncertainties. With the new detector prototype, the water-equivalent thickness of PMMA degrader blocks has been reconstructed within ± 0.1 mm. An evaluation of the radiation hardness proves that the range reconstruction algorithm is robust following the deposition of 6,300 Gy peak dose into the detector. Furthermore, small variations in the beam spot size and transverse beam position are shown to have a negligible effect on the range reconstruction accuracy. The potential for range measurements of ion beams is also investigated.

Feasibility study of a plastic scintillating plate-based treatment beam fluence monitoring system for use in pencil beam scanning proton therapy

PURPOSE

The purpose of this study was to describe a plastic scintillating plate-based gantry-attachable dosimetry system for pencil beam scanning proton therapy to monitor entrance proton beam fluence, and to evaluate the dosimetric characteristics of this system and its feasibility for clinical use.

METHODS

The dosimetry system, consisting of a plastic scintillating plate and a CMOS camera, was attached to a dedicated scanning nozzle and scintillation during proton beam irradiation was recorded. Dose distribution was calculated from the accumulated recorded frames. The dosimetric characteristics (energy dependency, dose linearity, dose rate dependency, and reproducibility) of the gantry-attachable dosimetry system for use with therapeutic proton beams were measured, and the feasibility of this system during clinical use was evaluated by determining selected quality assurance items at our institution.

RESULTS

The scintillating plate shortened the range of the proton beam by the water-equivalent thickness of the plate and broadened the spatial profile of the single proton spot by 11% at 70 MeV. The developed system functioned independently of the beam energy (<1.3%) and showed dose linearity, and also functioned independently of the dose rate. The feasibility of the system for clinical use was evaluated by comparing the measured quality assurance dose distribution to that of the treatment planning system. The gamma passing rate with a criterion of 3%/3 mm was 97.58%.

CONCLUSIONS

This study evaluated the dosimetric characteristics of a plastic scintillating plate-based dosimetry system for use with scanning proton beams. The ability to account for the interference of the dosimetry system on the therapeutic beam enabled offline monitoring of the entrance beam fluence of the pencil beam scanning proton therapy independent of the treatment system with high resolution and in a cost-effective manner.

Feasibility of two-dimensional dose distribution deconvolution using convolution neural networks

PURPOSE

The purpose of this study was to investigate the feasibility of two-dimensional (2D) dose distribution deconvolution using convolutional neural networks (CNNs) instead of an analytical approach for an in-house scintillation detector that has a detector-interface artifact in the penumbra region.

METHODS

Datasets of 2D dose distributions were acquired from a medical linear accelerator of Novalis Tx. The datasets comprise two different sizes of square radiation fields and 13 clinical intensity-modulated radiation treatment (IMRT) plans. These datasets were divided into two datasets (training and test) to train and validate the developed network, called PenumbraNet, which is a shallow linear CNN. The PenumbraNet was trained to transform the measured dose distribution [M(x, y)] to calculated distribution [D(x, y)] by the treatment planning system. After training of the PenumbraNet was completed, the performance was evaluated using test data, which were 10 × 10 cm² open field and ten clinical IMRT cases. The corrected dose distribution [C(x, y)] was evaluated against D(x, y) with 2%/2 mm and 3%/3 mm criteria of the gamma index for each field. The M(x, y) and deconvolved dose distribution with the analytically obtained kernel using Wiener filtering [A(x, y)] were also evaluated for comparison. In addition, we compared the performance of the shallow depth of linear PenumbraNet with that of nonlinear PenumbraNet and a deep nonlinear PenumbraNet within the same training epoch.

RESULTS

The mean gamma passing rates were 84.77% and 95.81% with 3%/3 mm gamma criteria for A(x, y) and C(x, y) of the PenumbraNet, respectively. The mean gamma pass rates of nonlinear PenumbraNet and the deep depth of nonlinear PenumbraNet were 96.62%, 93.42% with 3%/3 mm gamma criteria, respectively.

CONCLUSIONS

We demonstrated the feasibility of the PenumbraNets for 2D dose distribution deconvolution. The nonlinear PenumbraNet which has the best performance improved the gamma passing rate by 11.85% from the M(x, y) at 3%/3 mm gamma criteria.

In vitro biological response of cancer and normal tissue cells to proton irradiation not affected by an added magnetic field

To optimize beam delivery and conformality of proton therapy, MRI integration has been proposed. Therefore, we investigated if proton irradiation in a magnetic field would change biological responses. Our data in cancer cell lines and stem cell-derived organoid models suggest that a magnetic field does not modify the biological response.

A benchmarking method to evaluate the accuracy of a commercial proton monte carlo pencil beam scanning treatment planning system

AcurosPT is a Monte Carlo algorithm in the Eclipse 13.7 treatment planning system, which is designed to provide rapid and accurate dose calculations for proton therapy. Computational run-time in minimized by simplifying or eliminating less significant physics processes. In this article, the accuracy of AcurosPT was benchmarked against both measurement and an independent MC calculation, TOPAS. Such a method can be applied to any new MC calculation for the detection of potential inaccuracies. To validate multiple Coulomb scattering (MCS) which affects primary beam broadening, single spot profiles in a Solidwater^® phantom were compared for beams of five selected proton energies between AcurosPT, measurement and TOPAS. The spot Gaussian sigma in AcurosPT was found to increase faster with depth than both measurement and TOPAS, suggesting that the MCS algorithm in AcurosPT overestimates the scattering effect. To validate AcurosPT modeling of the halo component beyond primary beam broadening, field size factors (FSF) were compared for multi-spot profiles measured in a water phantom. The FSF for small field sizes were found to disagree with measurement, with the disagreement increasing with depth. Conversely, TOPAS simulations of the same FSF consistently agreed with measurement to within 1.5%. The disagreement in absolute dose between AcurosPT and measurement was smaller than 2% at the mid-range depth of multi-energy beams. While AcurosPT calculates acceptable dose distributions for typical clinical beams, users are cautioned of potentially larger errors at distal depths due to overestimated MCS and halo implementation.

Sparing the region of the salivary gland containing stem cells preserves saliva production after radiotherapy for head and neck cancer

Each year, 500,000 patients are treated with radiotherapy for head and neck cancer, resulting in relatively high survival rates. However, in 40% of patients, quality of life is severely compromised because of radiation-induced impairment of salivary gland function and consequent xerostomia (dry mouth). New radiation treatment technologies enable sparing of parts of the salivary glands. We have determined the parts of the major salivary gland, the parotid gland, that need to be spared to ensure that the gland continues to produce saliva after irradiation treatment. In mice, rats, and humans, we showed that stem and progenitor cells reside in the region of the parotid gland containing the major ducts. We demonstrated in rats that inclusion of the ducts in the radiation field led to loss of regenerative capacity, resulting in long-term gland dysfunction with reduced saliva production. Then we showed in a cohort of patients with head and neck cancer that the radiation dose to the region of the salivary gland containing the stem/progenitor cells predicted the function of the salivary glands one year after radiotherapy. Finally, we showed that this region of the salivary gland could be spared during radiotherapy, thus reducing the risk of post-radiotherapy xerostomia.

Fast range measurement of spot scanning proton beams using a volumetric liquid scintillator detector

Accurate confirmation and verification of the range of spot scanning proton beams is crucial for correct dose delivery. Current methods to measure proton beam range using ionization chambers are either time-consuming or result in measurements with poor spatial resolution. The large-volume liquid scintillator detector allows real-time measurements of the entire dose profile of a spot scanning proton beam. Thus, liquid scintillator detectors are an ideal tool for measuring the proton beam range for commissioning and quality assurance. However, optical artefacts may decrease the accuracy of measuring the proton beam range within the scintillator tank. The purpose of the current study was to 1) develop a geometric calibration system to accurately calculate physical distances within the liquid scintillator detector, taking into account optical artefacts; and 2) assess the accuracy, consistency, and robustness of proton beam range measurement using the liquid scintillator detector with our geometric calibration system. The range of the proton beam was measured with the calibrated liquid scintillator system and was compared to the nominal range. Measurements were made on three different days to evaluate the setup robustness from day to day, and three sets of measurements were made for each day to evaluate the consistency from delivery to delivery. All proton beam ranges measured using the liquid scintillator system were within half a millimeter of the nominal range. The delivery-to-delivery standard deviation of the range measurement was 0.04 mm, and the day-to-day standard deviation was 0.10 mm. In addition to the accuracy and robustness demonstrated by these results when our geometric calibration system was used, the liquid scintillator system allowed the range of all 94 proton beams to be measured in just two deliveries, making the liquid scintillator detector a perfect tool for range measurement of spot scanning proton beams.

A proton therapy system in Nagoya Proton Therapy Center

The purpose of this paper is to describe an outline of a proton therapy system in Nagoya Proton Therapy Center (NPTC). The NPTC has a synchrotron with a linac injector and three treatment rooms: two rooms are equipped with a gantry and the other one is equipped with a fixed horizontal beamline. One gantry treatment room has a pencil beam scanning treatment delivery nozzle. The other two treatment rooms have a passive scattering treatment delivery nozzle. In the scanning treatment delivery nozzle, an energy absorber and an aperture system to treat head and neck cancer have been equipped. In the passive treatment delivery nozzle, a multi-leaf collimator is equipped. We employ respiratory gating to treat lung and liver cancers for passive irradiation. The proton therapy system passed all acceptance tests. The first patient was treated on February 25, 2013, using passive scattering fixed beams. Respiratory gating is commonly used to treat lung and liver cancers in the passive scattering system. The MLCs are our first choice to limit the irradiation field. The use of the aperture for scanning irradiation reduced the lateral fall off by half or less. The energy absorber and aperture system in scanning delivery is beneficial to treat head and neck cancer.

Review on the characteristics of radiation detectors for dosimetry and imaging

The enormous advances in the understanding of human anatomy, physiology and pathology in recent decades have led to ever-improving methods of disease prevention, diagnosis and treatment. Many of these achievements have been enabled, at least in part, by advances in ionizing radiation detectors. Radiology has been transformed by the implementation of multi-slice CT and digital x-ray imaging systems, with silver halide films now largely obsolete for many applications. Nuclear medicine has benefited from more sensitive, faster and higher-resolution detectors delivering ever-higher SPECT and PET image quality. PET/MR systems have been enabled by the development of gamma ray detectors that can operate in high magnetic fields. These huge advances in imaging have enabled equally impressive steps forward in radiotherapy delivery accuracy, with 4DCT, PET and MRI routinely used in treatment planning and online image guidance provided by cone-beam CT. The challenge of ensuring safe, accurate and precise delivery of highly complex radiation fields has also both driven and benefited from advances in radiation detectors. Detector systems have been developed for the measurement of electron, intensity-modulated and modulated arc x-ray, proton and ion beams, and around brachytherapy sources based on a very wide range of technologies. The types of measurement performed are equally wide, encompassing commissioning and quality assurance, reference dosimetry, in vivo dosimetry and personal and environmental monitoring. In this article, we briefly introduce the general physical characteristics and properties that are commonly used to describe the behaviour and performance of both discrete and imaging detectors. The physical principles of operation of calorimeters; ionization and charge detectors; semiconductor, luminescent, scintillating and chemical detectors; and radiochromic and radiographic films are then reviewed and their principle applications discussed. Finally, a general discussion of the application of detectors for x-ray nuclear medicine and ion beam imaging and dosimetry is presented.

3D reconstruction of scintillation light emission from proton pencil beams using limited viewing angles-a simulation study

An accurate and high-resolution quality assurance (QA) method for proton radiotherapy beams is necessary to ensure correct dose delivery to the target. Detectors based on a large volume of liquid scintillator have shown great promise in providing fast and high-resolution measurements of proton treatment fields. However, previous work with these detectors has been limited to two-dimensional measurements, and the quantitative measurement of dose distributions was lacking. The purpose of the current study is to assess the feasibility of reconstructing three-dimensional (3D) scintillation light distributions of spot scanning proton beams using a scintillation system. The proposed system consists of a tank of liquid scintillator imaged by charge-coupled device cameras at three orthogonal viewing angles. Because of the limited number of viewing angles, we developed a profile-based technique to obtain an initial estimate that can improve the quality of the 3D reconstruction. We found that our proposed scintillator system and profile-based technique can reconstruct a single energy proton beam in 3D with a gamma passing rate (3%/3 mm local) of 100.0%. For a single energy layer of an intensity modulated proton therapy prostate treatment plan, the proposed method can reconstruct the 3D light distribution with a gamma pass rate (3%/3 mm local) of 99.7%. In addition, we also found that the proposed method is effective in detecting errors in the treatment plan, indicating that it can be a very useful tool for 3D proton beam QA.

The effectiveness of combined gating and re-scanning for treating mobile targets with proton spot scanning. An experimental and simulation-based investigation

Organ motion is one of the major obstacles in radiotherapy and charged particle therapy. Even more so, the theoretical advantages of dose distributions in scanned ion beam therapy may be lost due to the interplay between organ motion and beam scanning. Several techniques for dealing with this problem have been devised. In re-scanning, the target volume is scanned several times to average out the motion effects. In gating and breath-hold, dose is only delivered if the tumour is in a narrow window of position. Experiments have been performed to verify if gating and re-scanning are effective means of motion mitigation. Dose distributions were acquired in a lateral plane of a homogeneous phantom. For a spherical target volume and regular motion gating was sufficient. However, for realistic, irregular motion or a patient target volume, gating did not reduce the interplay effect to an acceptable level. Combining gating with re-scanning recovered the dose distributions. The simplest re-scanning approach, where a treatment plan is duplicated several times and applied in sequence, was not efficient. Simulations of different combinations of gating window sizes and re-scanning schemes revealed that reducing the gating window is the most efficient approach. However, very small gating windows are not robust for irregular motion.

Experimental characterization of two-dimensional pencil beam scanning proton spot profiles

Dose calculations of pencil beam scanning treatment plans rely on the accuracy of proton spot profiles; not only the primary component but also the broad tail components. Four films are placed at several locations in air and multiple depths in Solidwater® for six selected energies. The films used for the primary components are exposed to 50-200 MU to avoid saturation; the films used for the tail components are exposed to 800, 8000 and 80,000 MU. By applying a pair/magnification method and merging these data, dose kernels down to 10(-4) of the central spot dose can be generated. From these kernels one can calculate the dose-per-MU for different field sizes and shapes. Measurements agree within 1% of dose-kernel-based calculations for output versus field size comparisons. Asymmetric, comet-shaped profile tails have a bigger impact at superficial depths and low energies: the output difference between two orientations at the surface of a rectangular field of 40 mm×200 mm is about 2% at the isocentre at 100 MeV. Integration of these dose kernels from 0 to 40 mm radius shows that the charge deficit in the Bragg peak chamber varies <2% from entrance to the end of range for energies <180 MeV, but exceeds 5% at 225 MeV.

Verification of proton range, position, and intensity in IMPT with a 3D liquid scintillator detector system

PURPOSE

Intensity-modulated proton therapy (IMPT) using spot scanned proton beams relies on the delivery of a large number of beamlets to shape the dose distribution in a highly conformal manner. The authors have developed a 3D system based on liquid scintillator to measure the spatial location, intensity, and depth of penetration (energy) of the proton beamlets in near real-time.

METHODS

The detector system consists of a 20 × 20 × 20 cc liquid scintillator (LS) material in a light tight enclosure connected to a CCD camera. This camera has a field of view of 25.7 by 19.3 cm and a pixel size of 0.4 mm. While the LS is irradiated, the camera continuously acquires images of the light distribution produced inside the LS. Irradiations were made with proton pencil beams produced with a spot-scanning nozzle. Pencil beams with nominal ranges in water between 9.5 and 17.6 cm were scanned to irradiate an area of 10 × 10 cm square on the surface of the LS phantom. Image frames were acquired at 50 ms per frame.

RESULTS

The signal to noise ratio of a typical Bragg peak was about 170. Proton range measured from the light distribution produced in the LS was accurate to within 0.3 mm on average. The largest deviation seen between the nominal and measured range was 0.6 mm. Lateral position of the measured pencil beam was accurate to within 0.4 mm on average. The largest deviation seen between the nominal and measured lateral position was 0.8 mm; however, the accuracy of this measurement could be improved by correcting light scattering artifacts. Intensity of single proton spots were measured with precision ranging from 3 % for the smallest spot intensity (0.005 MU) to 0.5 % for the largest spot (0.04 MU).

CONCLUSIONS

Our LS detector system has been shown to be capable of fast, submillimeter spatial localization of proton spots delivered in a 3D volume. This system could be used for beam range, intensity and position verification in IMPT.

Investigation of the feasibility of a simple method for verifying the motion of a binary multileaf collimator synchronized with the rotation of the gantry for helical tomotherapy

In this paper, we suggest a new method for verifying the motion of a binary multileaf collimator (MLC) in helical tomotherapy. For this we used a combination of a cylindrical scintillator and a general-purpose camcorder. The camcorder records the light from the scintillator following photon irradiation, which we use to track the motion of the binary MLC. The purpose of this study is to demonstrate the feasibility of this method as a binary MLC quality assurance (QA) tool. First, the verification was performed using a simple binary MLC pattern with a constant leaf open time; secondly, verification using the binary MLC pattern used in a clinical setting was also performed. Sinograms of simple binary MLC patterns, in which leaves that were open were detected as "open" from the measured light, define the sensitivity which, in this case, was 1.000. On the other hand, the specificity, which gives the fraction of closed leaves detected as "closed", was 0.919. The leaf open error identified by our method was -1.3 ± 7.5%. The 68.6% of observed leaves were performed within ± 3% relative error. The leaf open error was expressed by the relative errors calculated on the sinogram. In the clinical binary MLC pattern, the sensitivity and specificity were 0.994 and 0.997, respectively. The measurement could be performed with -3.4 ± 8.0% leaf open error. The 77.5% of observed leaves were performed within ± 3% relative error. With this method, we can easily verify the motion of the binary MLC, and the measurement unit developed was found to be an effective QA tool.