Bone versus soft tissue setup in proton therapy for patients with oesophageal cancer

Background The aim of this study was to investigate the effect of patient positioning based on either bone or soft-tissue matching for PT in oesophageal cancer and its impact on plan adaptation. Materials and methods Two retrospective patient cohorts treated with radiotherapy were included in the study. Cohort A consisted of 26 consecutive patients with a planning 4DCT scan (CT1) and a surveillance 4DCT scan (CT2) at fraction ten. Cohort B consisted of 17 patients selected based on large anatomical changes identified during treatment resulting in a rescan (CT2). Mean dose to the iCTV (sum of the CTVs in all respiratory phases) was 50.4 Gy (RBE) in 28 fractions or 41.4 Gy (RBE) in 23 fractions. A nominal pencil beam scanning plan was created using two posterior beams and robust optimization (5 mm setup, 3.5% range). For each patient, two rigid registrations were made between average (avg) CT1 and CT2: a match on the vertebral column (bone match) and a match on the iCTV (soft-tissue match). Robustness towards setup (5 mm) and range (3.5%) errors was evaluated at CT2. Robustness towards respiration was evaluated by recalculation of the plan on all phases of the CT2 scan. Dose coverage <96% would trigger adaptation. The statistical significance (p-value <0.05) between dose coverage for the two registration methods was assessed using the Wilcoxon signed rank test. Results All plans fulfilled V95%(iCTV)>99% for the nominal plan and V95%(iCTV)>97% for all respiratory phases and robustness scenarios at CT1. In two (8%) and three (18%) patients, V95%(iCTV)<96% on CT2 for Cohort A and B, respectively when bone match was used. For soft-tissue match, V95%iCTV >96% for all patients. V95%(iCTV) was significantly higher (p-value = 0.0001) for soft-tissue match than bone match. Conclusion Anatomical changes during the treatment course led to target dose deterioration and a need for plan adaptation when using a bone match.