Inter-fraction motion robustness in a prospective phase II trial on dose-escalated proton reirradiation for locally recurrent rectal cancer

Background and purpose: Intensity modulated proton therapy (IMPT) enables generation of conformal dose plans with organ at risk (OAR) sparing potential.However, pelvic IMPT robustness is challenged by inter-fraction motion caused by constant anatomical variations.In this study, the dosimetric impact of inter-fraction motion on target coverage and dose to OAR was quantified in the prospective phase II study ReRad-II on dose-escalated proton reirradiation for locally recurrent rectal cancer (LRRC). Materials and methods:The inter-fraction motion robustness was assessed for the initial twelve patients enrolled in the ReRad-II study.Patients with resectable LRRC were assessed for neoadjuvant IMPT (55 Gy(RBE)/44Fx) and unresectable recurrences for definitive IMPT (57.5-65Gy(RBE)/ 46-52Fx).Target coverage and dose to OAR were assessed for robustly optimised three-field IMPT, on 12 plan computerized tomography (CT) scans (pCT)and 47 repetitive control CT scans (cCTs) during the treatment.The target coverage and doses to OAR were recalculated on each cCT and the mean dose ratio (pCT/cCT-ratio) and target coverage (V 95% ) was evaluated.Results: The target coverage was robust with a mean dose pCT/cCT-ratio of 1.00 (+/-1%).The V 95% target coverage for every cCT were above the accepted worst-case scenario in the robust evaluation.Considerable variation in bladder-, bowel bag-, and bowel loop volume was observed.The OAR with the largest variation in ratio was the bladder (pCT/cCT-ratio: 1.3 (range: 0.5-4.7).Conclusions: IMPT for dose-escalated reirradiation of LRRC provided anatomically robust target coverage despite OAR changes.Inter-fraction motion resulted in OAR doses varying within clinically acceptable range.