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Anterior Proton Beams for Prostate Treatments Lead to Substantial Elevations in Modeled RBE-Weighted Rectal Dose

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T Underwood

T Underwood(1)(2), D Giantsoudi(1) , M Moteabbed(1) , A Zietman(1) , J Efstathiou(1) , H Paganetti(1) , HM Lu(1) , (1) Department of Radiation Oncology, Massachusetts General Hospital & Harvard Medical School, Boston, Massachusetts, USA (2) Dept. Medical Physics and Bioengineering, University College London, UK


TU-EF-304-5 (Tuesday, July 14, 2015) 1:45 PM - 3:45 PM Room: 304

Purpose: If a constant RBE of 1.1 is assumed, range-verified anterior oblique (AO) proton beams appear to offer the potential to reduce the mean dose delivered to the rectum, anterior rectal wall and penile bulb by a factor of ~2 relative to standard bilateral (SB) proton beam arrangements. Additionally, AO proton beams targeted at the prostate avoid the femoral heads and so form a particularly appealing option for patients with hip prostheses. This study investigates LET enhancement at the distal edge of AO SOBPs, applying RBE models to consider the extent to which AO beams lead to hotspots in rectum biological dose.

Methods: Eight patients were selected, all treated with passively scattered, SB proton beams to 79.2Gy(E) within the prostate and 50.4Gy(E) within the proximal 5-15mm of seminal vesicles. Additional plans utilizing AO beams (±35°) were created in XiO (Elekta) assuming a fixed RBE of 1.1. Voxelised dose and LET distributions were calculated using Monte Carlo (TOPAS). Three different LET/RBE models were applied (Carabe 2012, Wedenberg 2013, McNamara 2015).

Results: Across the eight patients, the mean LET within the rectal wall was found to be 3.5(3.2-4.0)keV/μm for the SB plans compared to 10.5(8.6-13.0)keV/μm for the AO plans. Application of the median LET/RBE model to the AO plans, rather than a fixed RBE of 1.1, resulted in an increase of 13.6(11.9-14.7)GyRBE in maximum rectal wall (1cc) RBEw dose, leading to values of 90.4(90.0-91.3)GyRBE.

Conclusion: relative to SB proton beams, AO proton beams exhibit substantially increased mean LET values within the rectal wall. For the passively scattered AO beams, modeling indicates that this enhancement is likely to translate into unacceptable RBEw dose hotspots. Whilst no RBE-LET model has yet been fully validated in-vivo, caution must be applied if AO beams are to be considered for prostate patients.

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