A simple <scp>MRI</scp> / <scp>PET</scp> ‐derived index predicts relapse‐free survival after neoadjuvant chemoradiotherapy in locally advanced rectal cancer

Abstract Aim Reliable pretreatment biomarkers for predicting resistance to standard chemoradiotherapy (CRT) in locally advanced rectal cancer (LARC) remain lacking. This study evaluated the prognostic value of the tumour area (TA)‐to‐maximum standardized uptake value (SUVmax) [T/S] ratio, a novel imaging index derived from magnetic resonance imaging (MRI) and 18 F‐fluorodeoxyglucose positron emission tomography‐computed tomography (PET‐CT), for predicting oncological outcomes in LARC. Method We retrospectively analysed 68 patients with LARC who underwent preoperative CRT followed by curative resection between 2013 and 2024. TA was measured on axial T2‐weighted MRI, and SUVmax was obtained from PET‐CT. The T/S ratio was calculated as TA/SUVmax. Receiver operating characteristic analysis determined the optimal cut‐off for relapse‐free survival (RFS). Cox proportional hazards models identified independent prognostic factors. Immunohistochemical analysis of L‐type amino acid transporter 1 (LAT1) was performed in 33 patients without CRT to explore biological relevance. Results The T/S ratio showed superior predictive accuracy for RFS (area under the curve, 0.714) than for TA (0.616) and SUVmax (0.574). Patients with a high T/S ratio (cut‐off 55.81) demonstrated significantly poorer 3‐year RFS (51.8% vs. 84.0%, p = 0.002). Multivariate analysis confirmed high T/S ratio as an independent RFS predictor ( HR 4.59; 95% CI: 1.27–16.55; p = 0.019). Tumours with high T/S ratios tended to exhibit elevated LAT1 expression, potentially reflecting a CRT ‐resistant phenotype. Conclusions The pretreatment T/S ratio is a simple, noninvasive biomarker that independently predicts RFS in patients with LARC undergoing CRT. This index may assist in risk stratification and individualized neoadjuvant strategy selection.