Is histopathological evidence really essential for making a surgical decision about mucinous carcinoma arising in a perianal fistula? Report of a case.

We report an unusual case of mucinous adenocarcinoma of the anus associated with a chronic anal fistula, treated successfully by abdominoperineal resection (APR). Although multiple biopsies failed to reveal any histological evidence of malignancy, cancer was diagnosed from the mucin obtained for cytology. Subsequent histological examination of the resected specimen revealed clusters of cancer cells floating in a mucous lake, suggesting that it would have been difficult to acquire the cells in a biopsy sample. Conversely, the presence of mucin lakes and globules in specimens drained from the region of perianal sepsis may have been histologically informative for diagnosis. Thus, although biopsy of the lesion is undoubtedly essential for diagnosis, it often fails to provide enough information to make a definite diagnosis of mucinous carcinoma. This case illustrates that clinicians should base their decision on whether to perform surgery on clinical manifestations, imaging findings, and cytology of mucin obtained by drainage when it is difficult to obtain malignant cells by biopsy.