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A radiotherapy QA program was developed as part of a national plan for the expansion of SBRT services for oligometastatic disease. The aim was to ensure consistent, high-quality SBRT was implemented across all centres introducing this technique. As part of the QA program, radiation oncologists’ delineation of target volumes and associated OARs, according to UK SABR Consortium Guidelines v6.1 and Global Harmonization Group Consensus Guidelines (Mir et al, 2020), was assessed. Results for lower abdominal and pelvic disease are reported here.

A contouring benchmark case for each of pelvic bone, abdominal node and lumbar vertebra were circulated amongst 31 centres starting their oligometastases SBRT program. Two clinicians from each hospital were invited to contour GTV, CTV (where indicated), PTV and associated OARs following guidance provided. All submissions were reviewed by at least 2 members of a team of SBRT experts comprising 10 radiation oncologists, a medical physicist and a clinical research fellow. Detailed feedback was conveyed through standardised written and pictorial reports and any unacceptable contours were asked to be resubmitted.

Consensus volumes were agreed and acceptability criteria were defined for each structure based on cranio-caudal borders, axial under or over-contouring and inclusion/exclusion of anatomical features as detailed in the instructions.

To date, a total of 131 submissions have been received for the bone, node and spine benchmark cases, of which 12 were approved following first submission. Breakdown by anatomical site is detailed in Table 1. Failed submissions had a median of 4 unacceptable structures (range: 1-10), with 74.5%, 31.4% and 48.0% of GTVs deemed unacceptable for bone, node and spine, respectively. 85.5% of bone CTVs were acceptable, however only 40% of spine CTVs were, mainly due to the caudal half of the lamina being missed. Overall, 9/15 OARs had failure rates ≥20%, with the cauda equina, spinal cord and duodenum being unacceptable in >70% of submissions. Combined failure rates for all benchmark cases and the most common unmet criteria are detailed in Table 2.

100/119 resubmissions were approved, the remaining being accepted on the third submission.

Training needs were highlighted in delineation and interpretation of guidelines for abdominal and pelvic anatomy. Educational contouring workshops have been developed and ongoing support from more experienced centres is in place. Participation in peer review and clinical trials has been recommended.

These results highlight the importance and value of a national, QA-driven approach when implementing new techniques nationwide. Only 9% of submissions were approved on first attempt, emphasising that, whilst increasing access to SBRT services for patients is desirable, the process is not straightforward from a quality and safety perspective. Funding and support for a QA program is warranted when expanding SBRT services.