EP316 - Fistula-Associated Anal Cancer in Absence of Crohn's Disease: An Uncommon Phenomenon

Purpose/Background: Fistula-associated anal cancer (FAAC) is an uncommonly diagnosed entity, most frequently seen in patients with Crohn’s disease. Here, we present a case of FAAC in the absence of Crohn’s disease and our successful management. Our patient is a 58-year-old male with a chronic perianal fistula presented to the colorectal surgery outpatient clinic with complaints of perianal pain and fecal smearing. The patient had undergone fistula surgery 7 years prior at an outside institution.

Methods/Interventions: Anorectal exam was notable for an exophytic mass emerging from the anal canal. Examination under anesthesia demonstrated a fungating mass in the left lateral position extending posteriorly, with purulent drainage. The mass was firm and adherent to the sphincter complex, extending proximally to the dentate line.
Incisional biopsies were obtained, and pathology confirmed moderately differentiated adenocarcinoma with mucinous differentiation, positive for CK7, CK20 and CDX2, consistent with anal adenocarcinoma. There was loss of DNA mismatch repair protein, indicating microsatellite instability (MSI). Colonoscopy revealed a non-obstructive frond-like anal mass extending proximally to the dentate line, with no evidence of inflammatory bowel disease. Carcinoembryonic antigen (CEA) was 3.7. Staging CT chest, abdomen and pelvis showed no evidence of metastatic disease. Pelvic MRI demonstrated perianal inflammatory thickening due to large intersphincteric/ trans-sphincteric fistula with involvement of the left levator ani, radiographically staged as T1/2N0 disease. Our multidisciplinary rectal cancer tumor board recommended upfront abdominoperineal resection (APR).

Results/Outcomes: The patient underwent a robotic APR and had an uncomplicated postoperative course. He was discharged on post operative day 2. Surgical pathology confirmed a 4.5 cm fistula-associated adenocarcinoma of the anal canal extending into the sphincter muscle, with negative margins and 0/17 lymph nodes involved. Immunostaining showed tumor cells positive for CK7, CK20 and CDX2, and negative for SABT2, consistent with anal adenocarcinoma of a rectal phenotype. Final staging was pT2N0. The patient continues to recover well without the need for adjuvant treatment. He will undergo surveillance according to routine anal cancer guidelines.

Conclusion/Discussion: Fistula-associated anal adenocarcinoma (FAAC) is a rare entity associated with chronic fistulas, typically in the setting of Crohn's disease. This case highlights the occurrence of FAAC in a patient without a history of inflammatory bowel disease, underscoring the importance of recognizing non-Crohn’s disease associated FAAC. Surgeons should maintain a high index of suspicion for malignancy in patients with chronic perianal fistulas to facilitate prompt diagnosis and treatment, as these cancers typically present at an advanced stage. Non-Crohn's disease FAACs are known to respond poorly to chemoradiation; therefore APR remains the mainstay of treatment.

Figure 1.
Specimen cross section with a cystically dilated cavity associated with an ill-defined mass. The cavity communicates with the anorectal lumen, consistent with fistula tract.

Figure 2.
H&E micrograph showing adenocarcinoma (bottom left) arising from the mucosal lining of a pre-existing sinus tract. Overlying squamous mucosa (top) is benign.