Local recurrence in rectal cancer: from detection to structured reporting.

Despite advances in total mesorectal excision and neoadjuvant therapy, locally recurrent rectal cancer remains a clinically important source of pelvic morbidity and cancer-related mortality, affecting approximately 2-10% of the treated patients. Most recurrences develop within three years of surgery and often arise near the surgical bed, where fibrosis and distorted anatomy can delay detection. Early recognition is critical because complete R0 resection offers the best chance of durable disease control and survival. Imaging provides the anatomic and functional information required to diagnose pelvic recurrence and guide multidisciplinary management. Contrast-enhanced CT of the chest, abdomen, and pelvis is typically used for routine surveillance and often provides the first radiologic suspicion of recurrence, particularly by detecting distant metastases or pelvic disease. However, CT has a limited ability to distinguish viable tumor from postoperative or post-radiation fibrosis in the pelvis. Suspicious or equivocal CT findings should therefore prompt high-resolution pelvic MRI using a recurrence protocol. MRI, including T2-weighted, diffusion-weighted, and contrast-enhanced sequences, is the preferred modality for local characterization and staging. MRI helps differentiate recurrent tumor from fibrosis, defines locoregional extent, and evaluates involvement of adjacent structures that directly influence resectability. For posterior compartment recurrence, dedicated musculoskeletal MRI of the sacrum may further delineate marrow infiltration, cortical involvement, sacral foraminal invasion, and nerve root extension, supporting surgical planning and margin assessment. FDG-PET/CT is useful when CT or MRI findings are equivocal and for excluding unsuspected metastatic disease, although mucinous tumors, chronic inflammation, and small-volume disease may reduce sensitivity. Hybrid FDG-PET/MRI has shown promising performance in detecting pelvic recurrence in small studies and may reduce equivocal interpretations; however, its incremental value in evaluating perineural spread and altering surgical management has not been established. This review summarizes recurrence patterns, optimal multimodality imaging techniques, pitfalls, structured reporting, and emerging applications of functional imaging, radiomics, and artificial intelligence in the individualized management of these patients.