Fistula-Associated Perianal Mucinous Adenocarcinoma.

Perianal mucinous adenocarcinoma is an uncommon entity that can be mistaken for perianal fistulous lesions due to its association with fistulas and its confinement to the perianal region. It is a diagnosis of exclusion of secondary anorectal, cutaneous, urogenital, and metastatic carcinomas. Its etiology is controversial with many speculated risk factors. Previous case reports and small case series have mainly focused on clinical and radiologic diagnoses and treatment options. We performed a retrospective review study of perianal carcinomas over 15 years focusing on the histopathologic and immunohistochemical features of perianal mucinous adenocarcinomas. We found 3 adult male patients (average age: 55 years) with primary fistula-associated perianal mucinous adenocarcinoma. The patients had a long history of recurrent discharging sinuses and fistulas involving the perianal, perineum, and gluteal regions. They had nonhealing perianal fistulas related to hidradenitis suppurativa and Crohn's disease, that antedate mass lesions by more than 5 years, despite surgical and medical treatments. The patients were smokers with diabetes. The tumors were low-grade well-differentiated carcinomas characterized by mucin pools with floating and lining neoplastic epithelium and intraluminal villoglandular growths. Characteristically, they showed a direct continuity between the neoplastic epithelium and perianal epidermis in sinus openings, abrupt transitions to squamous epithelium in epithelized fistulous tracts, and foci of intestinal metaplasia. They expressed CDX2, keratin7, keratin20, MUC1, MUC2, and MUC5AC, showed membranous staining for beta-catenin, and were negative for AMACR, GCDFP15, and MUC6, indicating an extramucosal, intestinal-type, rectal-immunophenotype mucinous carcinoma. Primary fistula-associated perianal mucinous adenocarcinomas have distinct diagnostic clinicopathologic criteria and potential precursor lesions.