Fig. 1

Workflow schematic for dose summation of two plans with disparate dose and fractionation schedules. a Workflow for image deformation for the original radiotherapy (CT #1) and reirradiation (CT #2) dataset plans. Both CT datasets were rigidly registered to bone followed by deformable image registration using 2.5 mm³ voxels. Since both CT sets belonged to the same patient, but at different time points, the choice of which acted as the reference CT was arbitrary. Once anatomically validated, individual dose distribution was overlaid on each CT #1 and #2 dataset. To validate that the final registration was accurate, a manual inspection was performed. In addition, we quantified organ at risk (OAR) doses for the registered/deformed image set and assured that the doses did not change. This indicated that registration preserved the OAR structure and distribution. Thus, after the initial registration process, two plans were superimposed but not yet converted to BED or summed but were anatomically aligned. b Workflow for the registered image sets then included conversion of physical dose of each individual plan into a corresponding BED plan. This was done using the automated algorithm for each data set (CT #1 and #2) at each 2.5 mm³ voxel. After BED conversion for each plan, the BED isodose lines for each plan were then summed to generate the composite BED dose exposed by each OAR. Thus, a composite BED isodose map was auto-generated for each patient who had received initial SABR followed by reirradiation. Composite BED exposure for each OAR was then assessed in terms of subsequent re-RT toxicity with the aim of developing potential dose thresholds