A simple MRI/PET-derived index predicts relapse-free survival after neoadjuvant chemoradiotherapy in locally advanced rectal cancer.

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 18F-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.