Complete pathological response to neoadjuvant chemoradiotherapy in Crohn's disease-associated fistula adenocarcinoma: a case report.

Fistula-associated adenocarcinoma (FAA) in Crohn's disease is an exceptionally rare and aggressive complication, often diagnosed late and historically managed with immediate radical surgery. Critically, the role of neoadjuvant chemoradiotherapy has remained undefined, with no reported cases of complete pathological response to date. We report the case of a 52-year-old man with longstanding Crohn's disease who developed a mucinous adenocarcinoma arising from a chronic perianal fistula, initially revealed by a perineal abscess. Histopathological analysis confirmed the diagnosis. Staging work-up demonstrated no evidence of distant metastasis. The patient underwent neoadjuvant chemoradiotherapy according to the PRODIGE-23 protocol, followed by abdominoperineal resection. Pathological assessment of the resection specimen demonstrated a complete pathological response (ypT0N0). This case highlights the importance of maintaining high clinical suspicion for malignant transformation in patients with chronic perianal fistulas in Crohn's disease and provides the first evidence that complete pathological response is achievable in fistula-associated adenocarcinoma, suggesting that neoadjuvant multimodal therapy may allow exploration of organ-preserving approaches in this rare condition.