ESTRO 2024 - Abstract Book

S1873

Clinical - Mixed sites, palliation

ESTRO 2024

1 The Christie Hospital NHS Foundation Trust, Clinical Oncology, Manchester, United Kingdom. 2 University of St Andrews, School of Medicine, St Andrews, United Kingdom

Purpose/Objective:

The use of Stereotactic Ablative Body Radiotherapy (SABR) to treat oligometastatic disease has increasingly well established evidence. However, treating lesions in the abdomen and pelvis can be challenging due to the proximity of mobile organs at risk (OARs) in particular the stomach, duodenum, and small bowel. When these OARs are close it is often necessary to compromise on Planning Target Volume (PTV) coverage to meet mandatory dose constraints. Even with compromise the steep dose gradient characteristic of SABR means relatively small OAR movements can prevent treatment delivery. To help overcome these challenges our department introduced an alternative ‘isotoxic’ 30-35Gy in 5-fraction SABR (5#iSABR) protocol for use when GI tract is in immediate proximity to the target. We aimed to evaluate patient outcomes to establish if this regime achieved satisfactory results.

Material/Methods:

We performed a single-centre retrospective review of all patients treated with SABR to oligometastatic abdominal or pelvic lymph nodes from February 2019 to February 2023. Patient eligibility was in line with NHS commissioning criteria[1]. Patients were assigned to 3 fraction SABR (3#SABR) or 5#iSABR at the discretion of the treating clinician and planning team. In general, if the GI tract +5mm overlapped with GTV this was felt to represent a high probability of clinically significant PTV compromise and 5#iSABR was selected. 3#SABR was prescribed as 30-45Gy and 5#iSABR was prescribed as 30-35Gy in 5 fractions. Increasing fractionation allowed greater OAR tolerance e.g Duodenum DMax 22.2Gy in three-fractions vs 35Gy in five-fractions. Both regimes were planned as conventional SABR but for 5#iSABR the peak plan dose was kept below proximal OAR tolerance, often resulting in a lower peak and more homogenous dose. Treatment set-up, image guidance and delivery were the same for both groups.

Statistics were evaluated using R (version 4.2.2). All doses were converted to BED10, whilst α:β 10 is not representative of all tumours in our group it allowed for comparison of doses delivered in different fractionations.

Results:

Between February 2019 and February 2023 110 patients and 134 lesions were treated with a median follow up of 18 months. Median age was 67 (33-90), the most common primary tumours were lower gastro-intestinal (31%), prostate (25%), renal (10%) and melanoma (9%). Both groups were well balanced with 55 patients receiving each fractionation schedule, however the five-fraction patients had larger targets (mean PTV 30.7cm3 vs 22.8cm3) and more frequently had multiple lesions (20% vs 15%). Patients treated in five fractions received a significantly lower BED10 with a median mean GTV dose of 56Gy vs 74Gy (figure 1). It is important to note that our five-fraction group represents the patients with proximal OARs, and if their treatment was delivered in 3 fractions doses lower than our three-fraction cohort achieved would be expected.