Answer: Carcinoid tumor arising in teratoma

Histology: Most of the lesion has the classic histological appearance of a carcinoid tumor. The tumor consists of ribbons and cords of relatively bland cells. The nuclei have a “salt and pepper” appearance. Cytoplasm is slightly amphophilic. Separating the ribbons and nests is a prominent thin vascular network, typical of endocrine tumors. Focally within this lesion are glandular structures, lined by cuboidal epithelium, which in some areas is ciliated.

Discussion: This tumor was keratin positive and S-100 negative, ruling out paraganglioma and melanoma. The presence of these tubular structures suggests that this lesion either arose within a cystic teratoma or within a tailgut cyst (retrorectal cystic hamartoma). The distinction between these two lesions cannot be made based on the small residual amount of the glandular epithelium present. As only a few of such lesions have been reported, one cannot predict the clinical behavior of this lesion.