Imaging Features of Anal Carcinoma after Chemoradiation.

Anal cancer is a rare malignancy that is primarily treated with chemoradiation therapy (CRT). Clinical examination of the anal canal after CRT is often limited owing to the patient's discomfort. Therefore, radiologic surveillance plays a fundamental role in treatment response assessment. Currently recommended imaging modalities for posttreatment follow-up include pelvic MRI for local response evaluation and CT for evaluation of possible distant metastases. Patients who demonstrate a complete treatment response undergo regular clinical and imaging surveillance. Cases demonstrating an equivocal treatment response, an incomplete response, or disease progression should be streamlined for biopsy confirmation of the suspicious site and considered for salvage abdominoperineal resection. Radiologic differentiation of post-CRT inflammatory changes versus residual tumor, particularly in the early post-CRT period, can be challenging. However, careful interrogation of T2-weighted MR images correlated with matching diffusion-weighted and apparent diffusion coefficient images can increase reader confidence. The role of fluorine 18-fluorodeoxyglucose (FDG) PET/CT in assessing the response to anal cancer treatment is a debated topic. However, emerging research suggests that FDG PET/CT is complementary to pelvic MRI for accurate treatment response assessment, providing additional metabolic information. In this article, the authors provide a comprehensive review of the post-CRT imaging appearances of anal cancer, including examples from the spectrum of disease responses and therapy-related complications, and describe the strengths and limitations of pelvic MRI and FDG PET/CT. The authors also share the pearls and pitfalls in differentiating residual tumor from posttreatment inflammatory mimics. ^©RSNA, 2025 Supplemental material is available for this article.