Abstract
Purpose: Traditionally, dose in proton radiotherapy is prescribed as Gy(RBE) by scaling up the physical dose by 10%. This not only increases the absolute dose but also translates in a forward shift of the falloff of the beam and consequently its range. However, if the relative biological effectiveness (RBE) of protons is considered to vary with linear energy transfer (LET), dose and (alpha/beta)_x-rays, this shift will depend on these three parameters. We aim to show how much the range changes as a function of these three parametersMethods: Three beams of different ranges incident on a computational phantom consisting of different regions of interest (ROIs) were used. Each ROI was assigned with (alpha/beta)_x-rays values between 0.5Gy and 20Gy. The distribution of the dose-averaged LET within each ROI was obtained from a Monte Carlo simulation. The RBE values within the ROIs were calculated for doses between 1 and 15Gy using an in-house developed biophysical model. DVHs of the RBE weighted doses were extracted for each ROI for a ‘fixed-RBE’ (RBE=1.1) and a ‘variable-RBE’ (RBE=f(d,alpha/beta,LET)), and the percentage difference in range was obtained from the difference of the percentage volumes at 80% of the doseResults: Range shifts in normal tissue of the order of 3mm to 7mm were measured for a shallow and a deep beam respectively, when a 1Gy dose normalized to the mid-SOBP was delivered. As dose increased to 15Gy, the shifts decreased to 1mm and 5mm respectively for the same tissues and depthsConclusions: When the distal edge of the SOBP lays on normal tissue, the range shift increases with physical range but decreases with increasing dose or (alpha/beta)_x-rays. The results of our study allow a quantitative consideration of RBE-caused range uncertainties as a function of treatment site and dose in treatment planning
Published Version
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