Brass Aperture Research Articles

Modelling small block aperture in an in-house developed GPU-accelerated Monte Carlo-based dose engine for pencil beam scanning proton therapy

Purpose. To enhance an in-house graphic-processing-unit accelerated virtual particle (VP)-based Monte Carlo (MC) proton dose engine (VPMC) to model aperture blocks in both dose calculation and optimization for pencil beam scanning proton therapy (PBSPT)-based stereotactic radiosurgery (SRS). Methods and materials. A module to simulate VPs passing through patient-specific aperture blocks was developed and integrated in VPMC based on simulation results of realistic particles (primary protons and their secondaries). To validate the aperture block module, VPMC was first validated by an opensource MC code, MCsquare, in eight water phantom simulations with 3 cm thick brass apertures: four were with aperture openings of 1, 2, 3, and 4 cm without a range shifter, while the other four were with same aperture opening configurations with a range shifter of 45 mm water equivalent thickness. Then, VPMC was benchmarked with MCsquare and RayStation MC for 10 patients with small targets (average volume 8.4 c.c. with range of 0.4–43.3 c.c.). Finally, 3 typical patients were selected for robust optimization with aperture blocks using VPMC. Results. In the water phantoms, 3D gamma passing rate (2%/2 mm/10%) between VPMC and MCsquare was 99.71 ± 0.23%. In the patient geometries, 3D gamma passing rates (3%/2 mm/10%) between VPMC/MCsquare and RayStation MC were 97.79 ± 2.21%/97.78 ± 1.97%, respectively. Meanwhile, the calculation time was drastically decreased from 112.45 ± 114.08 s (MCsquare) to 8.20 ± 6.42 s (VPMC) with the same statistical uncertainties of ~0.5%. The robustly optimized plans met all the dose–volume-constraints (DVCs) for the targets and OARs per our institutional protocols. The mean calculation time for 13 influence matrices in robust optimization by VPMC was 41.6 s and the subsequent on-the-fly ‘trial-and-error’ optimization procedure took only 71.4 s on average for the selected three patients. Conclusion. VPMC has been successfully enhanced to model aperture blocks in dose calculation and optimization for the PBSPT-based SRS.

Design and characterization of an aperture system and spot configuration for ocular treatments with a gantry-based spot scanning proton beam.

Proton therapy is a key modality used in the treatment of ocular melanoma. Traditionally ocular sites are treated using a dedicated eyeline with a passively scattered proton beam and a brass aperture. This work aims to design and characterize a beam-collimating aperture to treat ocular targets with a gantry-based spot scanning proton beam. A plastic aperture system that slides into the gantry nozzle of a spot scanning proton beam was designed and constructed. It consists of an intermediate scraper layer to attenuate stray protons and a 3D-printed patient-specific aperture positioned 5.7cm from the surface of the eye. The aperture system was modeled in TOPAS and Monte Carlo simulations were validated with film measurements. Two different spot configurations were investigated for treatment planning and characterized based on lateral penumbra, central axis (CAX) dose and relative efficiency. Alignment and leakage were investigated through experimental film measurements. Range was verified using a multi-layer ionization chamber. Reference dose measurements were made with a PinPoint 3D ion chamber. Neutron dose was evaluated through Monte Carlo simulations. Aperture alignment with radiation isocenter was determined to be within 0.31mm at a gantry angle of 0°. A single-spot configuration with a 10mm diameter aperture yielded film-measured lateral penumbras of 1mm to 1.25mm, depending on depth in the spread-out Bragg peak. TOPAS simulations found that a single spot configuration results in a flat dose distribution for a 10mm diameter aperture and provides a CAX dose of less than 106% for apertures less than 14mm in diameter. For larger targets, adding four corner spots to fill in the dose distribution is beneficial. Trade-offs between lateral penumbra, CAX dose and relative efficiency were characterized for different spot configurations and can be used for future clinical decision-making. The aperture was experimentally determined to not affect proton beam range, and no concerning leakage radiation or neutron dose was identified. Reference dose measurements with a PinPoint ion chamber were within 2.1% of Monte Carlo calculated doses. The aperture system developed in this work provides a method of treating ocular sites on a gantry-based spot scanning proton system. Additional work to develop compatible gaze tracking and gating infrastructure is ongoing.

Commissioning a 250 MeV research beamline for proton FLASH radiotherapy preclinical experiments.

The potential reduction of normal tissue toxicities during FLASH radiotherapy (FLASH-RT) has inspired many efforts to investigate its underlying mechanism and to translate it into the clinic. Such investigations require experimental platforms of FLASH-RT capabilities. To commission and characterize a 250 MeV proton research beamline with a saturated nozzle monitor ionization chamber for proton FLASH-RT small animal experiments. A 2D strip ionization chamber array (SICA) with high spatiotemporal resolution was used to measure spot dwell times under various beam currents and to quantify dose rates for various field sizes. An Advanced Markus chamber and a Faraday cup were irradiated with spot-scanned uniform fields and nozzle currents from 50 to 215 nA to investigate dose scaling relations. The SICA detector was set up upstream to establish a correlation between SICA signal and delivered dose at isocenter to serve as an in vivo dosimeter and monitor the delivered dose rate. Two off-the-shelf brass blocks were used as apertures to shape the dose laterally. Dose profiles in 2D were measured with an amorphous silicon detector array at a low current of 2 nA and validated with Gafchromic films EBT-XD at high currents of up to 215 nA. Spot dwell times become asymptotically constant as a function of the requested beam current at the nozzle of greater than 30 nA due to the saturation of monitor ionization chamber (MIC). With a saturated nozzle MIC, the delivered dose is always greater than the planned dose, but the desired dose can be achieved by scaling the MU of the field. The delivered doses exhibit excellent linearity with with respect to MU, beam current, and the product of MU and beam current. If the total number of spots is less than 100 at a nozzle current of 215 nA, a field-averaged dose rate greater than 40 Gy/s can be achieved. The SICA-based in vivo dosimetry system achieved excellent estimates of the delivered dose with an average (maximum) deviation of 0.02 Gy (0.05 Gy) over a range of delivered doses from 3 to 44 Gy. Using brass aperture blocks reduced the 80%-20% penumbra by 64% from 7.55 to 2.75 mm. The 2D dose profiles measured by the Phoenix detector at 2 nA and the EBT-XD film at 215 nA showed great agreement, with a gamma passing rate of 95.99% using 1 mm/2% criterion. A 250 MeV proton research beamline was successfully commissioned and characterized. Challenges due to the saturated monitor ionization chamber were mitigated by scaling MU and using an in vivo dosimetry system. A simple aperture system was designed and validated to provide sharp dose fall-off for small animal experiments. This experience can serve as a foundation for other centers interested in implementing FLASH radiotherapy preclinical research, especially those equipped with a similar saturated MIC.

Feasibility study of utilizing Sphinx Compact for quality assurance in uniform scanning proton therapy

PurposeTo investigate the feasibility of utilizing the Sphinx Compact detector for quality assurance in a uniform scanning proton therapy system. MethodThe Sphinx Compact detector was used to measure various dosimetric parameters of uniform scanning proton beam at the Oklahoma Proton Center: distal range, distal-fall-off, collinearity, field symmetry, flatness, and field size for four different beams. A specially designed brass aperture was used to perform the required measurements. The Sphinx Compact measurement results were validated against the measurement results from the well-established detectors in proton therapy: IBA Zebra, IBA MatriXX-PT, EBT3 films, and Logos XRV-124. The data collected using the Sphinx Compact was analyzed in myQA software. ResultsBased on the data analysis performed, the Sphinx Compact measurements were within acceptable accuracy to the results from the detectors mentioned in the Method section. Specifically, the lateral penumbra was within ±0.4 mm, collinearity was within ± 0.5 mm, flatness was within ±0.6 %, symmetry within ±1.6 %, distal range was within ±0.5 mm, distal-fall-off was <0.9 mm, and field size was within ±1 mm. The reproducibility of the Sphinx Compact was tested for range and collinearity, and the results were within ±0.1 mm. ConclusionThe sphinx Compact detector could potentially replace multiple detectors utilized for monthly QA in uniform scanning proton therapy. In a multi-room center, performing the QA with one detector compared to using multiple detectors dramatically reduces total QA time and the complexity of the QA process.

Technical note: Evaluation and second check of a commercial Monte Carlo dose engine for small-field apertures in pencil beam scanning proton therapy.

To evaluate the accuracy of the RayStation Monte Carlo dose engine (RayStation MC) in modeling small-field block apertures in proton pencil beam scanning. Furthermore, we evaluate the suitability of MCsquare as a second check for RayStation MC. We have enhanced MCsquare to model block apertures. To test the accuracy of both RayStation MC and the newly enhanced MCsquare, we compare the dose predictions of each to in-water dose measurements obtained using diode detectors and radiochromic film. Nine brass apertures with openings of 1, 2, 3, 4, and 5cm and either 2cm or 4cm thickness were used in the irradiation of a water phantom. Two measurement setups were used, one with a range shifter and 119.7 MeV proton beam energy and the other with no range shifter and 147 MeV proton beam energy. To further test the validity of RayStation MC and MCsquare in modeling block apertures and to evaluate MCsquare as a second check tool, 10 small-field (average target volume 8.3 cm3 ) patient treatment plans were calculated by each dose engine followed by a statistical comparison. Comparing to the absolute dose measurements in water, RayStation MC differed by 1.2% ± 1.0% while MCsquare differed by -1.8% ± 3.7% in the plateau region of a pristine Bragg peak. Compared to the in-water film measurements, RayStation MC and MCsquare both performed well with an average 2D-3D gamma passing rate of 99.4% and 99.7% (3%/3mm), respectively. A t-test comparing the agreement with the film measurements between RayStation MC and MCsquare suggested that the relative spatial dose distributions calculated by MCsquare and RayStation MC were statistically indistinguishable. Directly comparing the dose calculations between MCsquare and RayStation MC over 10 patients resulted in an average 3D-3D gamma passing rates of 98.5% (3%/3mm) and 94.1% (2%/2mm), respectively. The validity of RayStation MC algorithm for use with patient-specific apertures has been expanded to include small apertures. MCsquare has been enhanced to model apertures and was found to be an adequate second check of RayStation MC in this scenario.

Spread-out Bragg peak proton FLASH irradiation using a clinical synchrocyclotron: Proof of concept and ion chamber characterization.

The purpose of this work is to (a) demonstrate the feasibility of delivering a spread-out Bragg peak (SOBP) proton beam in ultra-high dose rate (FLASH) using a proton therapy synchrocyclotron as a major step toward realizing an experimental platform for preclinical studies, and (b) evaluate the response of four models of ionization chambers in such a radiation field. A clinical Mevion HYPERSCAN® synchrocyclotron was adjusted for ultra-high dose rate proton delivery. Protons with nominal energy of 230MeV were delivered in pulses with temporal width ranging from 12.5μs to 24μs spanning from conventional to FLASH dose rates. A boron carbide absorber and a range modulator block were placed in the beam path for range modulation and creating an SOBP dose profile. The radiation field was defined by a brass aperture with 11mm diameter. Two Faraday cups were used to determine the number of protons per pulse at various dose rates. The dosimetric response of two cylindrical (IBA CC04 and CC13) and two plane-parallel (IBA PPC05 and PTW Advanced Markus® ) ionization chambers were evaluated. The dose rate was measured using the plane-parallel ionization chambers. The integral depth dose (IDD) was measured with a PTW Bragg Peak® ionization chamber. The lateral beam profile was measured with EBT-XD radiochromic film. Monte Carlo simulation was performed in TOPAS as the secondary check for the measurements and as a tool for further optimization of the range modulators' design. Faraday cups measurement showed that the maximum protons per pulse is 39.9 pC at 24μs pulse width. A good agreement between the measured and simulated IDD and lateral beam profiles was observed. The cylindrical ionization chambers showed very high ion recombination and deemed not suitable for absolute dosimetry at ultra-high dose rates. The average dose rate measured using the PPC05 ionization chamber was 163Gy/s at the pristine Bragg peak and 126Gy/s at 1cm depth for the SOBP beam. The SOBP beam range and modulation were measured 24.4mm and 19mm, respectively. The pristine Bragg peak beam had 25.6mm range. Simulation results showed that the IDD and profile flatness can be improved by the cavity diameter of the range modulator and the number of scanned spots, respectively. Feasibility of delivering protons in an SOBP pattern with >100Gy/s average dose rate using a clinical synchrocyclotron was demonstrated. The dose heterogeneity can be improved through optimization of the range modulator and number of delivered spots. Plane-parallel chambers with smaller gap between electrodes are more suitable for FLASH dosimetry compared to the other ion chambers used in this work.

Design and characterization of a prototype tertiary device for proton beam stereotactic radiosurgery

Though potentially beneficial, proton beam stereotactic radiosurgery has not been adopted widely secondary to the technical challenge of safely delivering multiple focused beams of proton radiation. In this study, we describe the design and characterization of a proton beam stereotactic radiosurgery system that can be adopted by existing passive scattering systems. This system utilizes a helmet-like device in which patient-specific brass apertures required for final beam collimation are positioned on a scaffold that is separate from the treatment gantry. The proton snout is then fitted with a generic aperture to focus the primary proton beam onto the patient specific apertures that are in the helmet-like device. The patient-specific apertures can all be placed at the start of the treatment, thus treatment with multiple beams can be accomplished without the delay of switching the apertures. In this report we describe a prototype design of this collimation system and dosimetric testing to verify efficacy. Subsequently, we describe a custom 3D printing of a prototype device and report on overall localization accuracy using Winston-Lutz tests. Our results show that it is possible to develop an add-on device for proton beam radiosurgery that is safe and efficient and capable of wide adoption on existing proton delivery systems.

Clinical Implementation of Proton Therapy Using Pencil-Beam Scanning Delivery Combined With Static Apertures.

Proton therapy makes use of the favorable depth-dose distribution with its characteristic Bragg peak to spare normal tissue distal of the target volume. A steep dose gradient would be desired in lateral dimensions, too. The widespread spot scanning delivery technique is based, however, on pencil-beams with in-air spot full-widths-at-half-maximum of typically 1 cm or more. This hampers the sparing of organs-at-risk if small-scale structures adjacent to the target volume are concerned. The trimming of spot scanning fields with collimating apertures constitutes a simple measure to increase the transversal dose gradient. The current study describes the clinical implementation of brass apertures in conjunction with the pencil-beam scanning delivery mode at a horizontal, clinical treatment head based on commercial hardware and software components. Furthermore, clinical cases, which comprised craniopharyngiomas, re-irradiations and ocular tumors, were evaluated. The dosimetric benefits of 31 treatment plans using apertures were compared to the corresponding plans without aperture. Furthermore, an overview of the radiation protection aspects is given. Regarding the results, robust optimization considering range and setup uncertainties was combined with apertures. The treatment plan optimizations followed a single-field uniform dose or a restricted multi-field optimization approach. Robustness evaluation was expanded to account for possible deviations of the center of the pencil-beam delivery and the mechanical center of the aperture holder. Supplementary apertures improved the conformity index on average by 15.3%. The volume of the dose gradient surrounding the PTV (evaluated between 80 and 20% dose levels) was decreased on average by 17.6%. The mean dose of the hippocampi could be reduced on average by 2.9 GyRBE. In particular cases the apertures facilitated a sparing of an organ-at-risk, e.g. the eye lens or the brainstem. For six craniopharyngioma cases the inclusion of apertures led to a reduction of the mean dose of 1.5 GyRBE (13%) for the brain and 3.1 GyRBE (16%) for the hippocampi.

FLASH Proton Irradiation Using a Synchrocyclotron: Proof of Concept

Recent studies have shown that radiotherapy at ultra-high dose rates (>40 Gy/s, FLASH vs. 0.5 Gy/s, Conventional) has a potential advantage in sparing healthy tissues compared to that at conventional dose rates. The purpose of this work is to demonstrate the feasibility of proton beam delivery in FLASH mode using a clinical synchrocyclotron. A unique feature associated with a synchrocyclotron system is that it provides a platform for further investigation with the intention to clarify whether instantaneous dose rate vs. average dose rate is the main driver of the FLASH-irradiation protective effect in proton therapy. A clinical synchrocyclotron was engineered for ultra-high dose rate beam delivery. The pulse structure of the protons was varied between 1 μs to 20 μs to deliver in conventional and ultra-high dose rates. A boron carbide block was used to reduce the 230 MeV energy of the protons to 70 MeV corresponding to 4 cm range in water. A brass aperture was used for beam shaping in longitudinal plane. A Faraday cup was utilized to measure the number of protons per pulse at various dose rates. A plane-parallel ionization chamber with sensitive volume of 1cm in diameter was used to measure the dose rate and integral depth dose. Radiochromic film dosimetry was performed for verification of the ion chamber measurements. Monte Carlo simulation was performed in TOPAS to calculate the IDD and dose-rate for comparison with the experimental measurements. Average dose rate of 100 Gy/s at the entrance and 216 Gy/s at the Bragg peak was measured. The field size measured with radiochromic film had 1.2 cm full-width at half-maximum (FWHM). A reasonable agreement was observed among the ion chamber, film measurements and the MC simulation calculations. Feasibility of proton FLASH irradiation with a synchrocyclotron was demonstrated to achieve average dose rate between 100 Gy/s and 216 Gy/s along the depth dose curve.

Evaluation of the activation of brass apertures in proton therapy using gamma-ray spectrometry and Monte Carlo simulations

Collimating apertures are used in proton therapy to laterally conform treatment fields to the target volume. While this is a standard technique in passive spreading treatment heads, patient-specific apertures can supplement pencil-beam scanning (PBS) techniques to sharpen the lateral dose fall-off. A radiation protection issue is that proton-induced nuclear reactions can lead to the formation of radionuclides in the apertures. In the experiments of the current study, cylindrical, thick brass targets were irradiated with quasi-monoenergetic proton fields of 100.0 MeV and of 226.7 MeV in PBS mode. The radioactivation of these two brass samples was characterised with a low-level gamma-ray spectrometer. The activation products were scored in a Monte Carlo simulation, too, and compared with the experimental activities. For the high-energy field, 63Zn, 60Cu, and 61Cu were the most important short-lived isotopes regarding the measured specific activity. After irradiation with the 100.0 MeV field, 62Cu, 63Zn, and 60Cu had the highest activity. Regarding long-lived isotopes, which determine the storage time of the used apertures, the isotopes 57Co, 65Zn, 54Mn, 56Co had the largest contribution to the activity. The relative difference of activities between simulation and experiment was typically between 10%–20% for short-lived nuclides and were up to a factor of five larger for long-lived nuclides. Summarising experiments and simulations for both incident proton energies, 62Cu was the most important detected residual nucleus regardless if specific activity or equivalent dose is considered.

An optimized approach for robust spot placement in proton pencil beam scanning

Maintaining a sharp lateral dose falloff in pencil beam scanning (PBS) proton therapy is crucial for sparing organs at risk (OARs), especially when they are in close proximity to the target volume. The most common approach to improve lateral dose falloff is through the use of physical beam shaping devices, such as brass apertures or collimator based systems. A recently proposed approach focuses on proton beam spot placements, moving away from traditional grid-based placements to concentric-contours based schemes. This improves lateral dose falloff in two ways: (1) by better conforming all spots to the tumor boundary and (2) allowing for ‘edge enhancement’, where boundary spots deliver higher fluence than more central spots, thereby creating a steeper lateral dose falloff. However, these benefits come at the expense of maintaining uniformity of spot distribution inside the target volume. In this work we have developed a new optimized spot placement scheme that provides robust spot distributions inside the target. This approach achieves the boundary conformity of a concentric-contours based approach and uses a fast-iterative method to distribute the interior spots in a highly uniform fashion in an attempt to improve both the lateral dose falloff and uniformity. Furthermore, we quantified the impact of this new approach through direct comparison with grid, contour, and hybrid spot placements schemes, showing improvements for this new approach. The results were validated in homogeneous medium for two different target shapes having concave and convex geometry.

Dose impact of a brass aperture on low dose tails for a proton wobbling nozzle with a multileaf collimator — A Monte Carlo study

At the Chang Gung Memorial Hospital, a wobbled beam together with a set of single scatterers produces a uniform proton field at the first and second gantries for proton radiotherapy. Conformal field shaping was accomplished by using only a multileaf collimator (MLC) or a combination of an MLC and a brass aperture (downstream of the MLC). The MLC-only technique typically delivered extra low dose tails in penumbra compared to the dose delivered using both the MLC and brass aperture/block. This study attempted to provide a physical explanation of this phenomenon by Monte Carlo simulation and phase space analysis. The Particle Therapy System Simulation Framework was used as the Monte Carlo tool for beam propagation simulation. This research team investigated four possible causes for low dose tails including, (a) same-side MLC scattering; (b) opposite-side MLC scattering; (c) MLC transmission; and (d) proton angular distribution broadened by the wobbling magnets. The results indicated that the proton angular distribution broadened by the wobbling magnets is the single cause of low dose tails and can be stopped by inserting a downstream brass aperture.

Secondary Neutron Dose From a Dynamic Collimation System During Intracranial Pencil Beam Scanning Proton Therapy: A Monte Carlo Investigation

Patients receiving pencil beam scanning (PBS) proton therapy with the addition of a dynamic collimation system (DCS) are potentially subject to an additional neutron dose from interactions between the incident proton beam and the trimmer blades. This study investigates the secondary neutron dose rates for both single-field uniform dose (SFUD) and intensity modulated proton therapy treatments. Secondary neutron dose distributions were calculated for both a dynamically collimated and an uncollimated, dual-field chordoma treatment plan and compared with previously published neutron dose rates from other contemporary scanning treatment modalities. Monte Carlo N-Particle transport code was used to track all primary and secondary particles generated from nuclear reactions within the DCS during treatment through a model of the patient geometry acquired from the computed tomography planning data set. Secondary neutron ambient dose equivalent distributions were calculated throughout the patient using a meshgrid with a tally resolution equivalent to that of the treatment planning computed tomography. The median healthy-brain neutron ambient dose equivalent for a dynamically collimated intracranial chordoma treatment plan using a DCS was found to be 0.97mSv/Gy for the right lateral SFUD field, 1.37mSv/Gy for the apex SFUD field, and 1.24mSv/Gy for the composite intensity modulated proton therapy distribution from 2 fields. These results were at least 55% lower than what has been reported for uniform scanning modalities with brass apertures. However, they still reflect an increase in the excess relative risk of secondary cancer incidence compared with an uncollimated PBS treatment using only a graphite range shifter. Regardless, the secondary neutron dose expected from the DCS for these PBS proton therapy treatments appears to be on the order of, or below, what is expected for alternative collimated proton therapy techniques.

SU‐E‐T‐637: Proton Aperture Quality Assurance Using Computed Tomography

Purpose:To develop an automated algorithm for aperture quality assurance for double scattering proton radiotherapy.Methods:Anterior‐Posterior scout images were acquired on a CT scanner for five brass apertures (2 large, 3 small) with 120 kVp and 10mA. A semi‐automated algorithm was developed for the QA process. The input includes a user selecting the scout image and the associated patient plan, field and aperture size. The program automatically thresholds the scout image. Next, a connectivity algorithm is used to determine the points associated with the central shape to obtain the contour. To compare with the plan contour, the program converts the detected and plan contours into polar coordinates, interpolates the data to a 1 degree spaced grid, and determines the differences in radial distance at each grid point.Results:The mean and maximum difference and the percentage of points with distance differences less than 1.25 mm (due to the divergent cut of the aperture) between the detected and plan field aperture contours were obtained. A repeatability coefficient was derived based on repeated scanning and processing of three of the apertures. The mean difference for five apertures was 0.44 +/−0.08 mm. The maximum difference in distance was 1.2 +/−0.23 mm for all apertures. The Repeatability Coefficient was +/−0.038 mm, indicating the technique is highly repeatable. The mean percent of points with distance less than 1.25 mm was 97.41 +/−0.35%.Conclusion:Automated CT scout image based proton aperture QA is feasible, saves time, and provides a quantitative metric for proton patient specific aperture QA.

Development of Manual Multi-Leaf Collimator for Proton Therapy in National Cancer Center

Multi-leaf collimator (MLC) systems are frequently used to deliver photon-based radiation, and allow conformal shaping of treatment beams. Many proton beam centers currently make use of aperture and snout systems, which involve use of a snout to shape and focus the proton beam, a brass aperture to modify field shape, and an acrylic compensator to modulate depth. However, it needs a lot of time and cost of preparing treatment, therefore, we developed the manual MLC for solving this problem. This study was carried out with the intent of designing an MLC system as an alternative to an aperture block system. Radio-activation and dose due to primary proton beam leakage and the presence of secondary neutrons were taken into account during these iterations. Analytical calculations were used to study the effects of leaf material on activation. We have fabricated tray model for adoption with a wobbling snout (30×40 cm2) system which used uniform scanning beam. We designed the manual MLC and tray and can reduce the cost and time for treatment. After leakage test of new tray, we upgrade the tray with brass and made the safety tool. First, we have tested the radio-activation with usually brass and new brass for new manual MLC. It shows similar behavior and decay trend. In addition, we have measured the leakage test of a gantry with new tray and MLC tray, while we exposed the high energy with full modulation process on film dosimetry. The radiation leakage is less than 1%. From these results, we have developed the design of the tray and upgrade for safety. Through the radio-activation behavior, we figure out the proton beam leakage level of safety, where there detects the secondary particle, including neutron. After developing new design of the tray, it will be able to reduce the time and cost of proton treatment. Finally, we have applied in clinic test with original brass aperture and manual MLC and calculated the gamma index, 99.74% between them.

Dosimetric evaluation of hybrid brass/stainless-steel apertures for proton therapy

In passive scattering proton therapy, patient specific collimators (apertures) are used to laterally shape the proton beam, and compensators are employed to distally conform proton dose to the target. Brass is a commonly used material for apertures and recently a hybrid brass/stainless-steel (BR/SST) aperture design has been introduced to reduce treatment cost without clinical flow change. We measured stopping power and leakage dose for apertures made of stainless steel and brass in the Proton Therapy system. The linear stopping power ratios for stainless steel (type 304) and brass to water were calculated to be 5.46 and 5.51, respectively. Measured stopping power ratios of SST and BR were 5.51 ± 0.04 and 5.56 ± 0.08, respectively, which agrees with the calculated values within 1%. Leakage dose on the downstream surface of two slabs of Ø18 cm stainless steel apertures (total thickness of 6.5 cm) for the maximum available proton energy (235 MeV) was 1.283% ± 0.004% of the prescription dose, and was smaller compared to the 1.358% ± 0.005% leakage dose measured for existing brass apertures of identical physical dimensions. Therefore, the existing beam range limits for brass aperture slabs used at our institution with safety margin allowances for material composition and delivered beam range uncertainties can be safely applied for the new BR/SST aperture design. Potential range differences in the brass and stainless steel interface regions of the hybrid design were further investigated using EBT3 GafChromic film. Film dosimetry revealed no discernible range variations across the brass and stainless steel interface regions. Neutron dose to the patient from brass and stainless steel apertures was simulated using the Monte Carlo method. The results indicate that stainless steel produces similar patient neutron dose compared to brass. Material activation dose rates of stainless steel were measured over a period of 7 d after irradiation. The measurements showed that the proton induced SST activity is initially lower and also decays at a faster rate than that induced in brass, therefore requires no changes in radiation protection requirements on material disposals. The Monte Carlo simulation confirmed higher initial activity of brass than stainless steel shortly after irradiation. The hybrid BR/SST aperture design is suitable for clinical use to replace the current brass apertures for all clinically used proton ranges. The existing aperture disposal procedures also satisfy radiation protection requirements for the new hybrid type apertures.

SU-E-T-590: An Activation Study of Materials and Devices Present in a Proton Treatment Room

Purpose: The use of protons for radiation therapy is growing rapidly. One consequence of protons interacting with different media is activation. These nuclear reactions induced by the protons, scattered neutrons, and gamma rays, activate different materials encountered, particularly by the therapists. The purpose of this study was to examine the derived nuclides from the activation, and also the decay rate. Methods: The study was conducted in our proton therapy facility. Protons are derived from a synchrocyclotron and pass through field shipping systems, apertures, and range compensators to define the beam within the patient.Included materials of concerns measured; the patient support couch, sheet rock in the wall, solid plastics used for quality assurance and dosimetry, and the passive scattering system itself, which includes brass apertures, and Lucite or blue wax compensators. All devices were studied post irradiation using gamma spectroscopy to determine the nuclides, and a sodium iodine scintillation detector to measure decay, particularly when the dose rate fell to background levels. Results: We have also determined from the measurements we will maintain brass apertures for three months before sending them for scrap. Some of the radionuclides arrived from these measurements included Na-22 for the blue wax compensator, C1-34m for themore » sheetrock, and Sc-44 and Co-60 for the brass apertures. We found compensators made out of Lucite or wax decayed to background in 2 hours. The patient support couch decayed to background in approximately 40 minutes, and sheet rock decayed in 80 minutes. In terms of the aperture layers, the most proximal aperture slab had much higher activity than the distal slab. Also the proximal sides of the slabs were much more activate than the distal. Conclusion: We have given proper instruction to therapists performing quality assurance in terms of the handled plastics, and to handle apertures rapidly as possible.« less

Radiation Safety Considerations in Proton Aperture Disposal

Beam shaping in scattered and uniform scanned proton beam therapy (PBT) is made commonly by brass apertures. Due to proton interactions, these devices become radioactive and could pose safety issues and radiation hazards. Nearly 2,000 patient-specific devices per year are used at Indiana University Cyclotron Operations (IUCO) and IU Health Proton Therapy Center (IUHPTC); these devices require proper guidelines for disposal. IUCO practice has been to store these apertures for at least 4 mo to allow for safe transfer to recycling contractors. The devices require decay in two staged secure locations, including at least 4 mo in a separate building, at which point half are ready for disposal. At 6 mo, 20-30% of apertures require further storage. This process requires significant space and manpower and should be considered in the design process for new clinical facilities. More widespread adoption of pencil beam or spot scanning nozzles may obviate this issue, as apertures then will no longer be necessary.

Reduced Secondary Neutrons and Occupational Exposure With Optimization of Beam Shaping Device Material in Proton Therapy

In proton therapy, beam shaping is achieved by patient-specific apertures (blocks), spot scanning, or a multi-leaf collimator (MLC). The primary collimators and apertures are the main sources of neutrons in scanning proton beams. Spot scanning is currently in limited clinical use, and brass apertures are used in most proton centers, while tungsten MLCs have been implement in one center. Neither brass nor tungsten are optimal materials for beam shaping due to activation and generation of secondary neutrons.

SU-E-T-287: Measured Neutron Levels at the Washington University Proton Therapy Facility

Purpose: To measure neutron dose levels at various locations in the proton radiation therapy treatment room. Methods: Proton beam was delivered to a phantom from the Mevion S250 superconducting synchro-cyclotron that uses passive scattering. The proton beam had a range of 17.7 g/cm2 and was modulated to give uniform dose over the full range. The beam was collimated to 10×10 cm2 using a brass aperture. The beam current/proton dose rate was varied to ensure the neutron detector response was within its linear dose rate measurement range. Dose to the phantom was measured using an IBA FC65-P cylindrical ionization chamber and an IBA Dose 1 electrometer. Neutrons were measured with a Thermo Scientific FHT 762 WENDI-2 neutron dose rate detector including a quality factor of 10. Neutron dose rates were measured at 4 room locations: 1m lateral from isocenter, 4m from isocenter, near the opening to the treatment room maze, and within the maze. Results: Pile-up from prompt neutrons limited the WENDI-2 to a dose rate of ∼2mSv/hr. Beyond that, neutron readings would not scale with proton dose, indicating a reduced efficiency for measuring neutron dose. The highest neutron dose observed was at the location 1m lateral to isocenter. At that location a dose of 0.3mSv per Gray of dose delivered to the phantom was measured. Four meters from isocenter the neutron dose rate reduced to 0.1 mSv per Gray of proton dose. At the opening to the maze we measured 0.07 mSv/Gy. Immediately inside the maze this dose dropped to 0.003 mSv/Gy. Conclusion: The WENDI-2 gave reproducible results as long as the dose rate from prompt neutrons was kept below 2mSv/hr. The Mevion system neutron levels were measured to be well below the 0.1% leakage limit set for x-ray therapy units.