MR Imaging of Anal Cancer.

High-resolution pelvic MR imaging is central to the diagnosis, staging, treatment planning, and posttreatment assessment of anal squamous cell carcinoma (SCCA), the most common malignancy of the anal canal. Accurate imaging interpretation requires detailed knowledge of anal canal anatomy, lymphatic drainage, and optimized MR imaging technique. MR imaging provides excellent evaluation of primary tumor extent, while nodal and distant staging are enhanced by computed tomography (CT) and FDG-PET/CT. Imaging directly informs contemporary chemoradiation strategies and surveillance. Human papillomavirus (HPV) is implicated in nearly 90% of SCCA, is vaccine-preventable, and HPV ctDNA shows promise for response assessment and relapse prediction.