RESUMO
Perianal fistulae are commonly seen clinical entity. Development of malignancy within a perianal fistula is rare. Even rarer is the development of mucinous adenocarcinoma in a chronic fistula-in-ano. Only a handful of such cases have been reported in the past. A case of mucinous adenocarcinoma arising in chronic perianal fistula in a 34-year-old woman is being described. She presented with complaints of perineal fullness, pain and recurrent pus discharge from perianal fistula for 4 years. On radiological workup, a large solid-cystic pelvic mass was seen in relation to the fistula. On MRI, the lesion was mimicking a large horseshoe abscess. Transrectal ultrasound-guided biopsy and subsequent histopathological examination confirmed the presence of mucinous adenocarcinoma with tumour cells immunopositive for CK7 and CK20.