Neuroendocrine Tumor Arising in a Tailgut Cyst: A Rare Presacral Tumor.

A tailgut cyst (retrorectal cystic hamartoma) is an uncommon lesion that develops in the presacral (retrorectal) space. Malignant change in a tailgut cyst is extremely rare and presents as a soft tissue (presacral) or bone (sacral) neoplasm. We report a case of tailgut cyst in which a neuroendocrine tumor developed in a 25-year-old female. Computed tomography and magnetic resonance imaging scans revealed a sacrococcygeal malformation with absent left S4 and S5 and a partly cystic lesion within the right presacral space. Histologically, the lesion contained cystic and solid elements. The cysts were lined by columnar and stratified squamous epithelial cells with underlying patchy smooth muscle. The solid element was a partly necrotic neuroendocrine tumor composed mainly of ribbons of tumor cells, which showed mitotic activity and expressed cytokeratin, chromogranin, and synaptophysin. Histologically, tailgut cysts are lined by epithelium and contain scattered smooth muscle bundles in the cyst wall. Although rare, the possibility of tailgut cyst with neuroendocrine tumor should be included in the differential diagnosis of an enlarging presacral tumor.