LAUSR

Abstract Purpose In craniospinal irradiation, two or three isocenter groups along the craniocaudal axis are required to cover the long treatment target. Adapting the isocenter distance according to daily deviations in patient position is challenging because dosimetric hot or cold spots may occur in the field junction. The aim of this study was to quantify the effect of adapting the isocenter distance to patient position on the dose distribution of the field overlap region in craniospinal irradiation using partial‐arc volumetric modulated arc therapy. Methods The magnitude of isocenter distance deviations in craniocaudal direction was quantified by registering the setup images of 204 fractions of 12 patients to the planning images. The dosimetric effect of these deviations was determined by shifting the isocenters of the original treatment plan and calculating the resulting dose distribution. Results On fraction‐level, deviations larger than 3 mm caused more than 5 percentage point changes in the doses covering 2% (D 2%) and 98% (D 98%) of the junction volume in several patients. On treatment course‐level, the changes in D 2% and D 98% of the junction volume were less than 5 percentage points in all cases except for one patient. Conclusions Craniocaudal isocenter distance adaptation can be conducted provided that the mean isocenter distance deviation over the treatment course is within 3 mm.