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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. 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 &lt;96% would trigger adaptation. The statistical significance (p-value &lt;0.05) between dose coverage for the two registration methods was assessed using the Wilcoxon signed rank test. All plans fulfilled V95% iCTV &gt;99% for the nominal plan and V95% iCTV &gt;97% for all respiratory phases and robustness scenarios at CT1. In two (8%) and three (18%) patients, V95% iCTV &lt;96% on CT2 for Cohort A and B, respectively when bone match was used. For soft-tissue match, V95%iCTV &gt;96% for all patients. V95% iCTV  was significantly higher (p-value = 0.0001) for soft-tissue match than bone match. Anatomical changes during the treatment course led to target dose deterioration and a need for plan adaptation when using a bone match.

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