Fistula-associated Mucinous anal Adenocarcinoma, a Surprise Diagnosis in a Benign Chronic Peri-Anal Fistula Resection

Abstract

Abstract Introduction/Objective Mucinous anal adenocarcinoma arising in a long standing, chronic peri-anal fistula is rare, accounting for 2-3% of the total anal adenocarcinomas with only few cases reported in a literature. These slow growing, locally aggressive neoplasms with a low-grade histologic appearance, clinically manifest late in a disease course. This entity’s pathogenesis and origin remains controversial. As it arises within a chronic fistula, these tumors present a late stage. Hence, this is generally a surprise finding upon fistula excision. Mucinous carcinomas from other sites should be ruled out with ancillary studies before making this diagnosis. Awareness of this rare cancer is crucial. Methods/Case Report 67-year-old male with a known history of fistula, presented with reports of recent onset of pain and swelling near his anus. He had a history of seton placement, perirectal abscess drainage and past fistulotomy in 2006 and 2008. On physical exam, he had a firm, tender mass with an ostium just right and posterior to his anus. Pre- operative diagnosis was a chronic, recurrent fistula. Immunohistochemical stains showed mucinous adenocarcinoma with anal gland phenotype. Hence, by WHO criteria, this was diagnosed as Fistula-associated mucinous adenocarcinoma. After the diagnosis of cancer, imaging studies for staging did not reveal any metastatic disease, nor was there any residual lesion or fistulous tract as evaluated by the surgeon. Hence evaluation of the margins on the resection specimen was crucial for post-operative radiotherapy. Results (if a Case Study enter NA) NA. Conclusion 1. Mucinous adenocarcinoma arising in a chronic, benign fistula is a rare entity with unsuspected diagnosis, and since the presentation is often delayed, a high degree of clinical suspicion is required for early diagnosis and management. Early detection is crucial as prognosis is worse if size is greater than 5 cm, and/or if there is lymphatic or vascular invasion. Acellular mucinous pool in excision of a benign fistula, should raise the suspicion of this entity. Excision specimen from the long standing chronic, fistulas should be submitted in its entirety for microscopic evaluation to avoid the possibility of missing this underlying malignancy. Most patients can be cured with aggressive surgical and adjuvant chemoradiotherapy, hence, when possible, this specimen should be inked to enable the evaluation of the margins.