Incidental discovery of mucinous adenocarcinoma from a suspected inflammatory perianal mass.

Mucinous adenocarcinoma is a rare tumor that may be associated with longstanding perianal fistulas. It represents 6.9% of all anal cancers. The diagnosis of this malignancy can be delayed because the symptoms experienced by people with the disease are usually attributed to the fistula and not to malignant transformation. As a result, the diagnosis of perianal mucinous adenocarcinoma in the setting of chronic perianal fistulas requires a high index of clinical suspicion and specific histological features.