Carcinoid tumor arising in a tailgut cyst of the anorectal junction with distant metastasis: a case report and review of the literature.

Tailgut cyst is a rare congenital presacral lesion and is believed to arise from the persistent remnants of the postanal gut. Malignancy occurring in a tailgut cyst is extremely rare, and to our knowledge only 5 cases of carcinoid tumor arising in a tailgut cyst have been reported in the literature to date. We report a sixth case of carcinoid tumor arising in a tailgut cyst. The patient was a 41-year-old woman who presented with perianal pain. Sigmoidoscopy showed a 2-cm submucosal mass located 4 cm above the anal verge. The mass was a multilocular cyst with gray-tan solid portions. The cyst was lined by ciliated columnar, squamous, and transitional epithelia with small foci of carcinoid tumor in the cystic wall. The carcinoid tumor showed a trabecular growth pattern with uniform oval or round cells containing fine chromatin and positive immunoreactivity for chromogranin, synaptophysin, and cytokeratin. This case was unique because the tumor occurred at the anorectal junction, not in the retrorectal space, and unlike previously reported cases showed aggressive behavior and distant metastases.