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Session: Clinical Dosimetry, Calibration, Shielding [Return to Session]

Investigating Dosimetry for Individual Proton Beamlets

N Nelson1*, W Culberson1, D Hyer2, L Bennett2, T Geoghegan2, K Patwardhan2, B Smith2, R Flynn2, J Yu3, A Gutierrez3, P Hill4, (1) University of Wisconsin - Madison, School of Medicine and Public Health, Department of Medical Physics, Madison, WI, USA, (2) University of Iowa, Department of Radiation Oncology, Iowa City, IA, USA, (3) Miami Cancer Institute, Department of Radiation Oncology, Miami, FL, USA, (4) University of Wisconsin - Madison, School of Medicine and Public Health, Department of Human Oncology, Madison, WI, USA


TH-B-BRA-5 (Thursday, 7/14/2022) 8:30 AM - 9:30 AM [Eastern Time (GMT-4)]

Ballroom A

Purpose: To investigate the validity of IAEA TRS-398-derived output factors for Monte Carlo (MC) dose predictions of individual uncollimated and collimated proton beamlets. In addition, validate the collimation-induced dose rate reductions through Monte Carlo simulations and identical radiochromic film and integral ionization chamber measurements.

Methods: A TOPAS MC model of the dynamic collimation system (DCS) integrated with IBA dedicated nozzle was developed and validated using relative depth dose and lateral profile measurements. The MC model’s output was determined by relating the simulated dose to water (Gy/proton) under the TRS-398 reference conditions to the measured dose to water (Gy/MU) to determine the number of simulated protons per MU (protons/MU). This output factor was then applied to single beamlet simulations of uncollimated, single-trimmed (one DCS trimmer), and double-trimmed (two orthogonal DCS trimmers), 125 MeV proton beamlets to simulate the dose in Gy/MU. EBT3 films were irradiated to MC-predicted max doses of 1, 2.5, and 5 Gy in 4 cm of SolidWater. The Stingray ionization chamber (12 cm diameter) was used as an additional dosimeter to estimate collimation-induced dose rate reductions through a comparison of the ionization signals between the uncollimated and collimated configurations.

Results: Absolute 2D gamma pass rates between Monte Carlo and film at the 1%/1mm criterion were 96.5%, 99.5%, and 96.5% for the uncollimated, single trimmed, and double trimmed cases, respectively. The simulated dose rate reductions were 40% and 64% for the single and double trimmed cases, respectively. Stingray ionization chamber measurements of the collimation-induced dose rate reduction agreed with Monte Carlo to within 0.5% for both cases while film differed by 2% and 4% for the single and double trimmed cases, respectively.

Conclusion: Excellent agreement was observed between film and Monte Carlo dose simulations. Collimation-induced dose rate reductions were very similar between Monte Carlo and the Stingray ionization chamber.

Funding Support, Disclosures, and Conflict of Interest: Research reported in this publication was supported by the National Cancer Institute of the National Institutes of Health under Award Number R37CA226518. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health.


Dose, Protons, Collimation


TH- External Beam- Particle/high LET therapy: Proton therapy – computational dosimetry-Monte Carlo

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