ESTRO 2023 - Abstract Book

S1172

Digital Posters

ESTRO 2023

Belfast, Patrick G. Johnston Centre for Cancer Research, Belfast, United Kingdom; 4 Northern Ireland Cancer Centre, Belfast Health and Social Care Trust, Radiotherapy Medical Physics, Belfast, United Kingdom; 5 Belfast City Hospital, Northern Ireland Cancer Trials Network, Belfast, United Kingdom Purpose or Objective Emerging evidence suggests citrulline levels decrease during pelvic RT in response to bowel toxicity. This study explored the potential for citrulline as a predictive biomarker of late bowel toxicity in a randomised trial of SABR ± elective nodal irradiation in high risk prostate cancer (SPORT trial). Materials and Methods 30 men were randomised 1:1 to SABR to prostate-only (P-SABR) or to prostate plus pelvic lymph nodes (PPN-SABR). P-SABR patients received 36.25Gy/5 fractions/29 days. PPN-SABR patients also received 25Gy/5 fractions to pelvic nodes with the final cohort receiving a boost to the dominant intraprostatic lesion of 45-50Gy. Patient-reported EPIC summary scores and serum citrulline levels prior to first (#1), second (#2) and third (#3) fractions were quantified. A minimally clinically important change (MCIC) in EPIC score was defined as a decrease of 5 points for GI toxicity. Time to MCIC was stratified by median citrulline levels taken at #2 relative to levels at #1 and by treatment group. Results Median citrulline levels at #2 and #3 relative to #1 were 0.92 and 0.86 respectively. At 36 months, 33% of patients with relative citrulline levels ≤ median at #2 and 40% with relative citrulline levels ≤ median at #3 had experienced late bowel toxicity versus 20% with relative citrulline levels > median at #2 and 13% with relative citrulline levels > median at #3 (HR 1.685, P = 0.4508 and HR 3.099, P = 0.1271 respectively) (Fig. 1a,1b) When stratified by treatment group, 33% of patients in the PPN-SABR group had experienced late bowel toxicity versus 20% in the P-SABR group (HR 1.785. P = 0.4008) (Fig. 1c). At 36 months, 57% of patients with relative citrulline levels ≤ median had experienced 1 MCIC for late bowel toxicity versus 40% of patients with relative citrulline levels > median (HR 1.659, P = 0.3217) (Fig. 2a). 64% of patients in the PPN-SABR group had experienced 1 MCIC for late bowel toxicity compared to 33% in the P-SABR group (HR 2.283, P = 0.1098) (Fig. 2b). 86% of patients with relative citrulline levels ≤ median had experienced 1 MCIC for late diarrhoea compared to 60% of patients with relative citrulline levels > median (HR 2.035, P = 0.0636) (Fig. 2c). 79% of patients in the PPN-SABR group had experienced 1 MCIC for late bowel toxicity compared to 67% in the P-SABR group (HR 1.328, P = 0.4690) (Fig. 2d).