Differential Diagnosis of Malignant Brenner Tumor: If a benign or borderline Brenner tumor component cannot be identified, the differentiation of malignant Brenner tumor from other high-grade ovarian carcinomas may be difficult. The so-called transitional cell carcinoma of the ovary, which is now classified as a type of high-grade serous carcinoma, closely resembles malignant Brenner tumor. If a mucinous tumor component is present, it strongly favors malignant Brenner tumor. The presence of an endometriotic cyst or a focus of endometriosis near by would favor endometrioid carcinoma. The distinction of primary transitional cell carcinoma of the ovary from metastases to the ovary from the urinary tract requires clinical correlation. The image shows malignant Brenner tumor consisting of sheets of high-grade pleomorphic cells with enlarged hyperchromatic nuclei, prominent nucleoli, and brisk mitotic activity.