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Will the Reduction of Margins in Proton Therapy Increase the Impact of RBE Variations?

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M Marteinsdottir

M Marteinsdottir1*, J Schuemann2 , H Paganetti3 , (1) Massachusetts General Hospital & University of Iceland, Boston, MA, (2) Massachusetts General Hospital, Boston, MA, (3) Massachusetts General Hospital, Boston, MA


SU-H2-GePD-T-1 (Sunday, July 30, 2017) 3:30 PM - 4:00 PM Room: Therapy ePoster Lounge

Purpose: To evaluate the clinical impact of dose calculation as well as biological uncertainties in proton radiosurgery as the assumption of using variable relative biological effectiveness (RBE-weighted) compared to constant RBE of 1.1 (RBE-fixed) as well as the effects of uncertainties of applied range margins around target volumes are presumably most significant for small target volumes.

Methods: Eight patients with arteriovenous malformation (AVM) treated with radiosurgery were selected. Dose distributions were compared for RBE-weighted and RBE-fixed MC dose calculations for prescription doses between 13-16Gy(RBE) and for scaled doses of 2Gy(RBE). Three different (α/β)x ratios were investigated. In addition, dose distributions were recalculated using MC simulations with reduced range margins. Differences were estimated using dosimetric indices based on dose-volume histogram analyses.

Results: For prescribed doses the dosimetric indices for RBE-weighted having (α/β)x=2Gy were up to 2.5% larger than for RBE-fixed but around 2% smaller for (α/β)x=6.4Gy. Assuming a dose per fraction of 2Gy(RBE) increased the difference between RBE-weighted and RBE-fixed for all indices; the indices were up to 19.4% larger for (α/β)x=2Gy than for RBE-fixed and up to 9.0% larger for (α/β)x=6.4Gy. The difference in mean value of RBE in the target due to different (α/β)x ratios was up to 2.6% for prescribed doses compared 9.4% for scaled doses. Reducing range margins from 3.5% to 2.5% resulted in small difference of up to 1.8% for all dosimetric indices for both prescribed and scaled doses. The largest difference in the mean value of RBE by decreasing the range was around 0.5%.

Conclusion: The biological dose distribution is clearly dependent on the (α/β)x ratio; the dependence is greater when decreasing the dose per fraction. Surprisingly, reducing the range margins from 3.5%+1mm to 2.5%+1mm exposed only small RBE variation, suggesting that range margin uncertainty is less important than (α/β)x uncertainty.

Funding Support, Disclosures, and Conflict of Interest: NIH grant U19 CA021239. PI: Delaney

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