Anal carcinoma and its differential diagnoses

Anal carcinomas are uncommon and account for 1–3% of gastrointestinal carcinomas in the USA. The commonest of these carcinomas is squamous cell carcinoma and its morphological variants, but the anatomy and mixed histology of the anal canal allows for a variety of other neoplasms, including adenocarcinoma, neuroendocrine carcinomas, malignant melanoma, lymphoma and several rarer tumours. Chemoradiation is the main treatment for squamous cell carcinomas, while other carcinomas may be treated by a combination of chemoradiation and surgery. Biopsy material is often small and/or crushed, which may render accurate diagnosis difficult. In select clinical settings, surgical excision may follow chemoradiation therapy, which may present further challenges to diagnosis. This contribution reviews the differential diagnosis of anal canal carcinoma and discusses practical, pertinent features to distinguish between these carcinomas.