Mucinous borderline-like tumor of the gastrointestinal type arising from mature cystic teratoma of the ovary and its immunohistochemical cytokeratin and mucin phenotype.

Malignant transformation is rarely seen in the disease course of mature cystic teratoma (MCT) of the ovary. Adenocarcinoma arising from MCT is especially rare. We herein present the case of a premenopausal woman with a mucinous borderline-like tumor arising from a MCT. Based on the histological transition between the borderline-like tumor and gastrointestinal elements of the MCT, we consider that the tumor derived from teratomatous gastrointestinal epithelium. Immunohistochemistry showed that the proliferating mucinous cells were diffusely positive for cytokeratin 20 and partially positive for cytokeratin 7. MUC5AC was partially positive, whereas MUC2 and MUC6 were positive in a small number of tumor cells. The immunophenotype of cytokeratins and mucins in the present case was compatible with malignant transformation of the teratomatous gastrointestinal epithelium.