Association Between Radiotherapy (±Chemotherapy) and the Severity of Low Anterior Resection Syndrome After Rectal Cancer Surgery: Does Radiotherapy Separate Risk?

Background: Low anterior resection syndrome (LARS) is a frequent survivorship problem after sphincter-preserving rectal cancer surgery. Pelvic radiotherapy (RT), often combined with chemotherapy, is frequently implicated in LARS development, but its apparent effect may be confounded by low tumor location and diversion. We evaluated whether RT (±chemotherapy) separates the risk of postoperative LARS severity-especially major LARS-beyond classical anatomic and pathway determinants. Methods: We conducted a single-centre observational cohort study of operated rectal cancer patients managed between 2013 and 2024, who completed the Romanian-validated LARS score by standardized telephone interview after restoration of bowel continuity (up to 18 months postoperatively). Outcomes were postoperative LARS score, LARS category, and major LARS. Comparisons were performed by RT status and by oncologic treatment pattern. Multivariable logistic regression assessed associations with major LARS, adjusting a priori for tumor location and diverting ileostomy; furthermore, extended sensitivity models incorporated technical/pathway variables. Discrimination was explored using 5-fold cross-validated ROC/AUC. Item-level LARS responses were analyzed to characterize symptom phenotype. Results: Overall, 182 patients were included (RT: 106; no RT: 76); 43.4% had LARS (minor 14.8%, major 28.6%). RT-treated patients had higher postoperative LARS scores (median 21 vs. 12; p = 0.002) and a higher prevalence of major LARS (35.8% vs. 18.4%; p = 0.012). Across treatment patterns, LARS severity was highest in RT + chemotherapy. Item-level analyses indicated that RT-associated differences were driven mainly by urgency and clustering domains. In adjusted models, RT was not independently associated with major LARS, whereas low tumor location and diverting ileostomy were strong predictors. Discrimination for major LARS was modest: AUC 0.561 for RT alone, 0.643 for location + ileostomy, and 0.654 for location + ileostomy + RT (5-fold cross-validation). Conclusions: RT is associated with worse unadjusted postoperative bowel dysfunction after rectal cancer surgery and is linked to urgency/clustering-dominant symptom patterns. However, in this cohort, the risk of major LARS was predominantly explained by tumor location and diversion rather than RT alone, supporting integrated risk stratification and early symptom-directed survivorship care.