Optimising delivery of Stereotactic Ablative Radiotherapy (SABR) in the pelvis

Introduction Multiple considerations exist concerning Stereotactic Ablative Radiotherapy (SABR) in the pelvis, including impact of teaching on target volume/organ at risk (OAR) delineation, the most appropriate target volume and dose fractionation schedules and how these relate to treatment planning, management of pelvic organ motion and optimum practice of pelvic SABR re-irradiation in the absence of high-level evidence. Materials and Methods An evaluation of target volume/OAR delineation variation during a national contouring workshop was performed before and after teaching. A planning study was performed to develop a class solution for ultra hypofractionated Extended Nodal Irradiation (ENI). A prospective feasibility study was performed to determine the impact of hyposcine butylbromide (buscopan) on bowel motion artefacts on cone beam computed tomography (CBCT). An international Delphi study was performed to establish consensus for statements to guide practice for pelvic SABR re-irradiation. Results Teaching was associated with modest improvements in delineation variation for multiple target volume/OAR structures. Ultra hypofractionated ENI planning appeared feasible. A schedule of 25 Gy in 5 fractions with simultaneous integrated boost to involved node(s) of 30 Gy provided encouraging rates of tumour control probability and low rates of normal tissue complication probability. Trends to improved CBCT overall image quality and reduced bowel motion artefact were observed with administration of both intramuscular and intravenous buscopan. Delivery of buscopan was feasible and well tolerated by participants. Consensus was established for most statements relating to patient selection, pre-treatment investigations, treatment planning and delivery for pelvic SABR re-irradiation. Absence of consensus remained for statements relating to minimum time to re-irradiation, limits on lesion size/number and the most appropriate cumulative constraints for most OARs. Conclusions These findings have provided preliminary evidence and the justification for further investigation into several aspects of pelvic SABR in larger confirmatory studies to determine the clinical impact of these interventions.