Undifferentiated, Transitional, Mixed, and Other Epithelial Tumors of the Ovary

There are a number of less common but important benign and malignant epithelial ovarian tumors to be aware of in addition to the commonly encountered serous, endometrioid, mucinous, and clear cell tumors. These less frequent tumors include those formed of transitional cells, squamous cells, mixed epithelial cells, mixed epithelial and mesenchymal cells, and a large group in which the epithelial cells are undifferentiated. The most important of these are the undifferentiated cell group which includes the aggressive carcinomas with or without a neuroendocrine component as well as an associated malignant or benign epithelial lesion. Apart from the benign transitional tumors, which are often incidental findings, the unusual epithelial ovarian tumors make up less than 3 % of all ovarian tumors. All of these will be considered in turn with emphasis on their diagnostic features, differential diagnoses, ancillary methods for diagnosis, and common pitfalls. There will be mention of diagnostic difficulties with frozen sections in this group of tumors.