Case report of squamous cell carcinoma secondary to recurrent anal fistula.

Anal fistula is a common benign perianal condition, and its malignant transformation, particularly into squamous cell carcinoma, is rare and often associated with comorbidities such as Crohn's disease and HIV infection. This article reports the case of a 66-year-old male with a history of recurrent anal fistula spanning more than 10 years who underwent 3 surgical treatments. Pathological findings from the first two surgeries showed no malignant lesions, while the third confirmed moderately to poorly differentiated squamous cell carcinoma. The patient had a history of lymphoma and small bowel resection; HIV infection was excluded upon admission. Carcinoembryonic antigen and cytokeratin 19 fragment levels were mildly elevated, and imaging revealed a high complex anal fistula. The diagnosis was ultimately confirmed by pathological biopsy. We propose that malignant transformation may be linked to chronic inflammatory stimulation, reduced immune function, and prior abdominal surgery. Diagnosis depends on pathological confirmation, and the use of multiple imaging modalities can improve diagnostic accuracy. The uniqueness of this case lies in the non-continuous recurrence of the anal fistula, offering a new perspective on the clinical characteristics of anal fistula carcinogenesis.