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Monte Carlo Dose Calculation of Small Field Electron Beams

Q Wu

Q Wu1,2*, A Rodrigues1,2, D Sawkey3 F Yin1,2, (1) Duke University Medical Center, Durham, NC, (2) Medical Physics Graduate Program, Duke University Medical Center, Durham, NC, (3) Varian Medical Systems, Palo Alto, CA


SU-E-T-358 Sunday 3:00PM - 6:00PM Room: Exhibit Hall

Purpose: Dynamic radiotherapy involving electron beams such as Dynamic Electron Arc Radiotherapy (DEAR) requires accurate dose modelling of small field sizes, similar to the requirement of IMRT field on the small photon field. The current commercial electron Monte Carlo algorithms such as eMC v11 in Eclipse were developed for standard field sizes and do not support the planning of dynamic therapy yet. The purpose of this study is to develop a method to accurately model small field electron beam dosimetry using Monte Carlo simulations.

Methods: Comparison between eMC, phantom measurements (diode), and Monte Carlo (MC) simulations (BEAMnrc/DOSYZnrc) were performed for a Varian TrueBeam linac. MC simulations utilized Varian TrueBeam phase space files which had been validated in another study. Static single small field was assessed by comparing dose distributions in water for a 16 MeV beam for circular (2 cm diameter) and rectangular (1x10 cm²) cut-out. MC was performed with a resolution of 2.5x2.5x2 mm² and statistical uncertainty <4%. The dose distribution was averaged over adjacent bins to improve precision. Depth dose and orthogonal profiles were evaluated.

Results: Small field PDDs differ from those with standard cones. For both circular and rectangular cut-outs, the difference in range R₈₀–R₁₀ is less than 2 mm and in dose within 2%. For the orthogonal profiles, field size and penumbra differences were within 1 mm at depth of maximum dose. The eMC displayed a distinctive “step” in the out-field dose profile in disagreement with both measurement and MC results and needs further investigation.

Conclusion: MC was able to characterize the small field dosimetry with good agreement with the measurement data, and thus offers the opportunity for treatment planning of dynamic radiotherapy. Analyses for all other electron energies and cut-out sizes are under way and results will be included in the presentation.

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