Mucinous adenocarcinoma arising from complex perianal fistula: a diagnostic and therapeutic challenge.

We present a case of a 64-year-old male with a history of perianal abscesses that have been surgically treated on 10 occasions. Eight months after the last drainage procedure, he presented with a new abscess. Drainage was performed, revealing a cavity with smooth walls, a chronic appearance, filled with mucoid material. An internal fistulous opening was identified at the 6 o'clock position above the anorectal line, which communicated with the described cavity, forming a trans-sphincteric fistula to the mid-anal canal. Biopsy with pathological anatomy showed a mucinous adenocarcinoma with possible intestinal origin (CK20+, CDX2+, TTF1-, CK7+). After completing the evaluation, he was diagnosed with T4N1M0 rectal neoplasia. A diverting colostomy was performed, followed by neoadjuvant therapy, and subsequently, a laparoscopic abdominoperineal amputation. Pathological anatomy revealed residual adenocarcinoma ypT2N0 N0V0L0, R0. This case is notable for both the rarity of a mucinous adenocarcinoma originating in a perianal fistula and the nonspecific clinical presentation of such tumors. Clinical suspicion is crucial, especially in cases of recurrent abscesses with the discharge of mucoid material through fistulous openings, prompting the need for biopsies to ensure proper diagnosis and subsequent optimal treatment.