Malignancy in Long-Standing Perianal Fistulizing Crohn's Disease: Too Little, Too Late.

Perianal fistulizing Crohn's disease (PFCD) is a chronic, aggressive phenotype associated with an elevated risk of malignancy arising within long-standing fistula tracts. Although rare, PFCD-associated cancers carry a poor prognosis and are often diagnosed late due to overlapping symptoms with active inflammatory disease and the absence of established screening guidelines. We present a case of a 71-year-old man with nearly 50 years of untreated PFCD, who developed mucinous adenocarcinoma arising from a chronic perianal fistula. Despite multiple hospitalizations for recurrent abscesses, sepsis, and fistulizing disease, he initially declined definitive surgical and medical therapy, and later was unable to be initiated on advanced Crohn's treatments due to ongoing infections. Progressive enlargement of a complex perianal abscess and new pelvic collections raised concern for malignant transformation, and exam under anesthesia (EUA) with drainage ultimately revealed mucinous adenocarcinoma. His clinical course rapidly deteriorated, preventing oncologic therapy initiation, after which he was transitioned to hospice. This case highlights key diagnostic challenges in PFCD-associated cancer, including the difficulty distinguishing malignancy from active inflammation with imaging and the need for biopsy via EUA for confirmation. Persistent or new perianal symptoms in patients with long-standing PFCD-particularly beyond 10 years-should prompt urgent evaluation. Although formal guidelines are lacking, early involvement of a multidisciplinary team is essential to optimize diagnosis and management. Greater awareness of this rare complication is needed to support earlier recognition and to inform future strategies for screening and treatment.