Variation in patient position and impact on carbon-ion scanning beam distribution during prostate treatment

Objective:We assessed the impact of changes in patient position on carbon-ion scanning beam distribution during treatment for prostate cancer.Methods:68 patients were selected. Carbon-ion scanning dose was calculated. Two different planning target volumes (PTVs) were defined: PTV1 was the clinical target volume plus a set-up margin for the anterior/lateral sides and posterior side, while PTV2 was the same as PTV1 minus the posterior side. Total prescribed doses of 34.4 Gy [relative biological effectiveness (RBE)] and 17.2 Gy (RBE) were given to PTV1 and PTV2, respectively. To estimate the influence of geometric variations on dose distribution, the dose was recalculated on the rigidly shifted single planning CT based on two dimensional–three dimensional rigid registration of the orthogonal radiographs before and after treatment for the fraction of maximum positional changes.Results:Intrafractional patient positional change values averaged over all patients throughout the treatment course were less than the...