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Program Information

Expanding TOPAS Towards Biological Modeling

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J Schuemann

J Schuemann1*, D Giantsoudi1 , I Rinaldi2 , J Perl3 , B Faddegon4 , H Paganetti1 , (1) Massachusetts General Hospital & Harvard Medical School, Boston, MA (2) Deutsches Krebsforschungs Zentrum, Heidelberg, Germany (3) Stanford Linear Accelerator Center, Menlo Park, CA, (4) UC San Francisco, San Francisco, CA,


TH-A-19A-2 Thursday 7:30AM - 9:30AM Room: 19A

To expand Monte Carlo simulations of TOPAS to directly calculate radiobiological effects using various biological models.

We use the TOPAS toolkit version beta9 and expand the provided scoring system to directly include scorers for relative biological effect (RBE) modeling. A scorer is a class to record a given physical property in the Monte Carlo simulations. We developed a scorer for the dose averaged linear energy transfer (LETd). This scorer was used to compare RBE weighted dose distributions assuming a constant RBE of 1.1 with the RBE weighted dose using a variable RBE based on a model using LETd and dose as input.
Additionally, the local effect model (LEM) was implemented in TOPAS to estimate RBE weighted dose distributions. Survival curves for the LEM were simulated for a monoenergetic proton beams impinging on phantoms and for full patient treatment plans.

We have implemented radiobiological effect models in TOPAS, providing users with the possibility to directly obtain parameters related to radiobiological dose and use those as additional information to assess the quality of treatment plans. We applied the scorers to clinical patient treatments and phantom simulations. We find that for the proton therapy plans investigated the regions with the highest LETd are generally outside of the target volume, coinciding with the distal edge of treatment fields. LETd-based variable RBE weighted dose models predict higher doses at the distal edge of over 10% of the prescribed dose than when assuming a constant RBE.

By implementing radiobiological effect models into a Monte Carlo simulation package we combine accurate dose calculation with biological modeling. TOPAS thereby provides distributions of multiple physical quantities such as the dose and LETd, and estimates of biological parameters (RBE, survival) thus allowing direct comparison of biological effective dose predictions between different models.

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