Anal Adenocarcinoma Arising From a Fistula-in-Ano: A Case Report

Crohn's disease (CD) is an inflammatory disease that can affect any portion of the gastrointestinal tract (GIT). Although it can present with a number of complications, perianal fistulae are among the most common consequences in patients with CD. In very rare cases, these patients can develop fistula-associated anal adenocarcinoma (FAAA). In this case report, we discuss a 72-year-old man with a long-term history of CD complicated by perianal fistulae, which failed medical and surgical management, ultimately presenting with acute anal pain in the outpatient setting. The physical examination revealed a seton traversing through a fistula surrounded by circumferential granulation tissue suspicious for malignancy. A biopsy of the tissue confirmed grade 3 mucinous-type infiltrating adenocarcinoma of the perianal skin. The patient was diagnosed with an anal verge malignancy associated with a fistula in the context of long-standing CD, and MRI staging demonstrated a T3N1 lesion with potential left inguinal node involvement. He completed neoadjuvant chemo-radiotherapy using capecitabine for five weeks with minimal tumor response, and subsequently, an abdominoperineal resection (APR) was performed with patient discharge on the fifth post-procedure day. Post-operative chemotherapy with oxaliplatin/leucovorin/fluorouracil was administered with minimal complications. Although rare, this report demonstrates the importance of consistent follow-up and mitigation of risk factors in patients with CD, along with the significance of prompt multimodal treatment in the event of developing FAAA.