Session Item

Proton therapy (PT) may reduce dose to organs-at-risk compared to photon therapy (RT) which makes PT favourable for oesophageal tumours where anatomical changes and respiratory motion risk imposing target dose deterioration. The aim of this study was to investigate the effect of patient (pt) positioning based on bone versus soft tissue setup for PT in oesophageal cancer and the impact on plan adaptation in preparation for the European randomized phase-III trial (PROTECT).

This retrospective study included two consecutive patient cohorts (A and B) with mid-thoracic, distal, or gastro-oesophageal junction (GEJ) cancer treated with RT. Cohort A consisted of 26 pts who had a planning CT scan (pCT) and a surveillance scan (sCT) acquired at fraction (fx) ten. Cohort B consisted of 180 pts who had a pCT and were treated according to an adaptive strategy prompting a rescan CT (sCT) in case of target under dosage due to anatomical changes observed on daily CBCT. In cohort B 17 pts had sCT and were included in this study. 

All CT scans were acquired as 10-phase 4DCT scans. The average scan created from all phases was used for delineation and treatment planning. The iCTV (sum of CTV in all respiratory phases) was used for optimization, total dose 50.4Gy (RBE)/28 fx or 41.4Gy (RBE)/23 fx. Maximum dose to spinal cord was <45Gy (RBE). A pencil beam scanning nominal plan was created using two posterior beams with robust optimization using setup (5mm) and range (3.5%) errors. Robustness towards respiration was evaluated by recalculating the plan on all respiratory phases. Robustness towards setup (5mm) and range (3.5%) errors were evaluated. For each patient, two rigid registrations were made between pCT and sCT: a match on the vertebral column (4D bone match) and a match on the iCTV (4D soft tissue). The treatment plan was recalculated on all phases of the delineated sCT scan with each registration to evaluate the combined effect of respiration and inter-fractional anatomical changes. Dose coverage of iCTV<97% triggered adaptation. The statistical significance (p-value <0.05) of iCTV dose coverage was assessed using the Wilcoxon signed-rank test.

In all plans, V95%iCTV>99% for the nominal plan and V95%iCTVtotal>97% for all respiratory and robustness scenarios. Dose to the spinal cord was <45Gy (RBE) for all plans at sCT. In cohort A, two (8%) and one (4%) pts needed adaptation when bone and soft tissue registration was used.  In cohort B selected based on plan adaptation when treated with RT, five and three pts needed adaptation for PT (see Fig 1). Dose coverage was significantly higher for soft tissue setup. For patient 18 with a large shift between bones and tumour along the cranio-caudal direction, target dose degradation was seen for the bone match (Fig 2).

Anatomical changes during the treatment course led to larger target dose deterioration and required more adaptation of proton plans having bone registration than soft tissue match.