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Towards Evaluating the Biological Impact of Increased Scattered Radiation in Highly Modulated Radiation Beams

H Nusrat

H Nusrat1*, G Pang1,2,3, A Medrek1, S Ahmad2,3, B Keller2,3, A Sarfehnia1,2,3 (1) Ryerson University, Toronto, ON, (2) Sunnybrook Health Sciences Centre, Toronto, ON, (3) University of Toronto, Toronto, ON


SU-I-GPD-T-250 (Sunday, July 30, 2017) 3:00 PM - 6:00 PM Room: Exhibit Hall

Purpose: Scattered radiation seen at the edges of a treatment field consists of low energy electrons and photons. Lower energy particles have a higher linear energy transfer (LET), making them more biologically damaging. With a single beam of a large field size (FS), the amount of these highly damaging electrons is relatively low compared to the electron fluence within the treatment beam. When multiple beam angles and smaller FS are used (as done in IMRT/VMAT), low energy scattered radiation increases significantly and possible biological consequences may vary.

Methods: Using Monte Carlo Geant4.10.2, various LINAC phase-spaces (representing different beams) were set incident upon a water tank. Phase-spaces included: Varian 6MV photon (1x1cm²; 10x10cm²) and 6MeV electron (10x10cm²) beams, and a 250 kVp orthovoltage beam (1x1cm²) modelled using SpekCalc®. Additionally, a composite 6MV 10x10cm² beam was created by summating one hundred 1x1cm² beamlets in order to analyze increased scatter of a modulated yet uniform distribution (i.e. IMRT). The energy fluence of electrons (Φ(E,e-)) generated by each beam was scored (using a 3D voxelized scoring plane; voxel size=8mm³) and normalized to dose. Φ(E,e-) was converted to LET fluence, Φ(LET∞,e-) using ICRU Report 16.

Results: Variation in LET fluence was evaluated by computing the area under the fluence vs. LET∞ curves. For the 6MV photon beam, the edge LET fluence was significantly higher compared to the central axis (+22% for 1x1cm², +27% for the 10x10cm²). The greatest differential was seen at LET∞ values of 6keV/µm in both cases. LET differences between the edge and the central axis for the 6MeV electron beam and the 250kV orthovoltage beam were insignificant.

Conclusion: Scattered, low energy radiation at the edges of the beam significantly increase LET in high energy photon beams. Future work involves calculating α/β ratios to estimate the biological impact.

Funding Support, Disclosures, and Conflict of Interest: Funding provided by Natural Sciences and Engineering Research Council (NSERC) RGPIN 435608

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