Compact Proton Therapy System Research Articles

Operational Performance of a Compact Proton Therapy System: A 5-Year Experience.

PurposeWe present an analysis of various operational metrics for a novel compact proton therapy system, including clinical case mix, subsystems utilization, and quality assurance trends in beam delivery parameters over a period of 5 years.Materials and MethodsPatient-specific data from a total of 850 patients (25,567 fractions) have been collected and analyzed. The patient mix include a variety of simple, intermediate, and complex cases. Beam-specific delivery parameters for a total of 3585 beams were analyzed. In-room imaging system usage for off-line adaptive purpose is reported. We also report key machine performances metrics based on routine quality assurance in addition to uptime.ResultsOur analysis shows that system subcomponents including gantry and patient positioning system have maintained a tight mechanical tolerance over the 5-year period. Various beam parameters were all within acceptable tolerances with no clear trends. Utilization frequency histograms of gantry and patient positioning system show that only a small fraction of all available angles was used for patient deliveries with cardinal angels as the most usable. Similarly, beam-specific metrics, such as range, modulation, and air gaps, were clustered unevenly over the available range indicating that this compact system was more than capable to treat the complex variety of tumors of our patient mix.ConclusionOur data show that this compact system is versatile, robust, and capable of delivering complex treatments like a large full-gantry system. Utilization data show that a fraction of all subcomponents range of angular motion has been used. Compilation of beam-specific metrics, such as range and modulation, show uneven distributions with specific clustering over the entire usable range. Our findings could be used to further optimize the performance and cost-effectiveness of future compact proton systems.

Technical note: Does the greater power of pencil beam scanning reduce the need for a proton gantry? A study of head‐and‐neck and brain tumors

Proton therapy systems without a gantry can be more compact and less expensive in terms of capital cost and therefore more available to a larger patient population. Would the advances in pencil beam scanning (PBS) and robotics make gantry-less treatment possible? In this study, we explore if the high-quality treatment plans can be obtained without a gantry. We recently showed that proton treatments with the patient in an upright position may be feasible with a new soft robotic immobilization device and imaging which enables multiple possible patient orientations during a treatment. In this study, we evaluate if this new treatment geometry could enable high quality treatment plans without a gantry. We created PBS treatment plans for seven patients with head-and-neck or brain tumors. Each patient was planned with two scenarios: one with a gantry with the patient in supine position and the other with a gantry-less fixed horizontal beam-line with the patient sitting upright. For the treatment plans, dose-volume-histograms (DVHs), target homogeneity index (HI), mean dose, , and are reported. A robustness analysis of one plan was performed with 2.5-mm setup errors and 3.5% range uncertainties with nine scenarios. Most of the PBS-gantry-less plans had similar target HI and organs-at-risk mean dose as compared to PBS-gantry plans and similar robustness with respect to range uncertainties and setup errors. PBS provides sufficient power to deliver high quality treatment plans without requiring a gantry for head-and-neck or brain tumors. In combination with the development of the new positioning and immobilization methods required to support this treatment geometry, this work suggests the feasibility of further development of a compact proton therapy system with a fixed horizontal beam-line to treat patients in sitting and reclined positions.

Beam commissioning of the first compact proton therapy system with spot scanning and dynamic field collimation.

To describe the measurements and to present the results of the beam commissioning and the beam model validation of a compact, gantry-mounted, spot scanning proton accelerator system with dynamic layer-by-layer field collimation. We performed measurements of depth dose distributions in water, spot and scanned field size in air at different positions from the isocenter plane, spot position over the 20 × 20 cm2 scanned area, beam monitor calibration in terms of absorbed dose to water and specific field collimation measurements at different gantry angles to commission the system. To validate the beam model in the treatment planning system (TPS), we measured spot profiles in water at different depths, absolute dose in water of single energy layers of different field sizes and inversely optimised spread-out Bragg peaks (SOBP) under normal and oblique beam incidence, field size and penumbra in water of SOBPs, and patient treatment specific quality assurance in homogeneous and heterogeneous phantoms. Energy range, spot size, spot position and dose output were consistent at all gantry angles with 0.3 mm, 0.4 mm, 0.6 mm and 0.5% maximum deviations, respectively. Uncollimated spot size (one sigma) in air with an air-gap of 10 cm ranged from 4.1 to 16.4 mm covering a range from 32.2 to 1.9 cm in water, respectively. Absolute dose measurements were within 3% when comparing TPS and experimental data. Gamma pass rates >98% and >96% at 3%/3 mm were obtained when performing 2D dose measurements in homogeneous and in heterogeneous media, respectively. Leaf position was within ±1 mm at all gantry angles and nozzle positions. Beam characterisation and machine commissioning results, and the exhaustive end-to-end tests performed to assess the proper functionality of the system, confirm that it is safe and accurate to treat patients. This is the first paper addressing the beam commissioning and the beam validation of a compact, gantry-mounted, pencil beam scanning proton accelerator system with dynamic layer-by-layer multileaf collimation.

Technical Note: An approach to building a Monte Carlo simulation model for a double scattering proton beam system.

The purpose of this study was to demonstrate and develop a Monte Carlo (MC) simulation model for a passive double scattering compact proton therapy system based on limited information of the mechanical components. We built a virtual machine source model (VMSM) which included a detailed definition of each beam-modifying component in the nozzle. Conceptually, it is similar to the conventional virtual analytical source model (VASM), except that the numerical machine nozzle or beamline is constructed in the VMSM, whereas in the VASM analytical parameters characterizing the energy spectrum and source fluence distribution are sought. All major beam shaping components were included in the VMSM and the model simulates interactions of the beam with a rotating range modulation wheel (RMW) combined with the beam current modulation. The RMWs, the first and second scatterer in the system were generated and tuned to reproduce measurement data as closely as possible. To validate the model, we compared the percent depth dose curves, spread out Bragg peaks (SOBPs) and lateral profiles against measured commissioning beam data. The agreement of beam range between the MC calculation and measurement was within 1 mm for all beam options. The distal-falloff length was in good agreement as well (<1 mm for the large and deep groups, <1.5 mm for the small group). Agreement to within 2.5 mm of measured SOBP widths was obtained for all MC calculations. For lateral profiles, differences were found to be less than 2 mm. We demonstrated that with limited geometrical information it is possible to build an acceptable source model for MC simulations of a passive double scattering compact proton therapy system. The agreement between the measurements and the MC model provides validation for use of the model for further studies of the dosimetric effects in patient treatments.

Effectiveness of base-of-skull immobilization system in a compact proton therapy setting.

PurposeThe purpose of this study was to investigate daily repositioning accuracy by analyzing inter‐ and intra‐fractional uncertainties associated with patients treated for intracranial or base of skull tumors in a compact proton therapy system with 6 degrees of freedom (DOF) robotic couch and a thermoplastic head mask indexed to a base of skull (BoS) frame.Materials and methodsDaily orthogonal kV alignment images at setup position before and after daily treatments were analyzed for 33 patients. The system was composed of a new type of thermoplastic mask, a bite block, and carbon‐fiber BoS couch‐top insert specifically designed for proton therapy treatments. The correctional shifts in robotic treatment table with 6 DOF were evaluated and recorded based on over 1500 planar kV image pairs. Correctional shifts for patients with and without bite blocks were compared.ResultsSystematic and random errors were evaluated for all 6 DOF coordinates available for daily vector corrections. Uncertainties associated with geometrical errors and their sources, in addition to robustness analysis of various combinations of immobilization components were presented.ConclusionsAnalysis of 644 fractions including patients with and without a bite block shows that the BoS immobilization system is capable of maintaining intra‐fraction localization with submillimeter accuracy (in nearly 83%, 86%, 95% of cases along SI, LAT, and PA, respectively) in translational coordinates and subdegree precision (in 98.85%, 98.85%, and 96.4% of cases for roll, pitch, and yaw respectively) in rotational coordinates. The system overall fares better in intra‐fraction localization precision compared to previously reported particle therapy immobilization systems. The use of a mask‐attached type bite block has marginal impact on inter‐ or intra‐fraction uncertainties compared to no bite block.

SU-D-BRE-07: Neutron Shielding Assessment for a Compact Proton Therapy Vault

Purpose: To perform a neutron shielding assessment of a commercially available compact proton therapy system. Methods: TOPAS (TOol for PArticle Simulation) beta release was used to model beam line components for Mevion S250 proton treatment system the design of which is that the cyclotron is present in the treatment room. Three neutron production sources were taken into account in the simulation. These are the cyclotron, the treatment nozzle and the patient itself, respectively. The cyclotron was modeled as a cylindrical iron target (r =5 cm, length = 8 cm). A water phantom (10 cm ×10 cm ×60 cm) was used to model the patient and various structures (scattering foils, range modulator wheel, applicator and compensator) defaulted in TOPAS were used to model the passive scattering treatment nozzle. Neutron fluences and energy spectra were counted in a spherical scoring geometry per incident proton in 18 angular bins (10 degree each). Fluence to dose conversion factors from ICRU publication 74 were used to acquire neutron ambient dose equivalent H*(10). A point source line of sight model was then used to calculate neutron dose at eight locations beyond shielding barriers. Results: The neutron ambient dose equivalent was calculated at the 8 points of interest around the proton treatment vault. The highest dose was found to be less than 0.781 mSv/year outside south barrier wall. However, the dose is less than 0.05 mSv/year at the control room area of the proton vault. Conclusion: All Points of interest were well under annual dose limits. This suggests that the shielding design of this compact proton therapy system is sufficient for radiation protection purpose. However, it is important to note that the workload and the occupancy factors are direct multipliers for dose calculations beyond the barrier and must be accurately estimated for validation of our results.

Secondary neutron doses in a compact proton therapy system

Proton therapy offers several advantages compared with classical radiotherapy owing to a better dose conformity to the tumour volume. However, proton interactions with beam transport elements and the human tissues lead to the production of secondary neutrons, resulting in an extra whole-body dose with some carcinogenic potential. In this study, the secondary neutron doses generated with an active beam scanning system and with two compact proton therapy systems recently appeared on the market are compared.

EP-1782: Performance of Double-Decker compact proton therapy system

EP-1782: Performance of Double-Decker compact proton therapy system

SU-E-T-526: Modeling of a Compact Proton Therapy System Using TOPAS Monte Carlo Code

Purpose: To demonstrate the potential of using a newly innovated Monte Carlo platform TOPAS based on GEANT4 toolkits for proton therapy simulation is the primary purpose of this study. Methods: Based on preliminary geometry and material information provided by Mevion Medical system for a compact 250 MeV proton therapy machine planned to be delivered at our institution, a scattering foil, a secondary scatter filter, a range modulator wheel and a treatment nozzle were created using TOPAS. Beams of 250 MeV incident protons with energy spread of 0.75 MeV, shaped as point sources were used to interact in the phantom for the simulation. The beam angular spread in the X and Y directions were 0.0032 radians. Six hundred scoring detectors each with 1 mm thickness were placed in a water phantom (15 cm × 15 cm × 60 cm) created in the beam path. A default modular physics list was used in the simulation that included standard electromagnetic, nuclear elastic (G4Elastic) and inelastic (binary cascade) processes for protons and all secondary particles. Range modulator wheel in the simulation was rotating with a constant speed (using time feature function) to produce spread out Bragg peak (SOBP). Total 1 million incident particles were used in the simulation. Results: A complete model of beam components from a compact proton therapy system planned to be used in our facility has been achieved. A normalized single Bragg peak and a modulated spread out Bragg peak of 10 cm width for 250 MeV incident proton beam generated in this study will be presented. Conclusion: A compact double scattered proton therapy system was effectively modeled with TOPAS where the build-in features dedicated for particle therapy were used. Further validation on beam parameters and physics settings are needed before utilizing this to patient specific applications.