An Extremely Rare Case of Primary Malignant Melanoma of the Kidney.

Malignant melanoma (MM) is a tumor that usually occurs in the skin, but this malignant tumor can also develop in extracutaneous tissues, including urogenital tissues. In regard to MM occurring in urogenital tissues, bladder origin is common but renal primary MM is extremely rare. In the Department of Emergency and Urology at Gifu Municipal Hospital, a tumor of the right kidney was detected in a computed tomography scan to determine the cause of severe pain in the lower extremities of a 45-year-old Japanese woman. With the clinical diagnosis of renal cell carcinoma, resection of the right kidney was performed under laparoscopy. The cut surface of the tumor encapsulated by a thick fibrous capsule was dark brown, and the tumor cells with large nuclei, large nucleoli, acidophil cytoplasm, and numerous melanin granules showed papillary, solid, or alveolar growth. Immunohistochemically, the tumor cells were positive for Melan A and human melanoma black 45 (HMG45) but negative for transcription factor E3 (TFE3), transcription factor EB (TFEB), cytokeratin 7 (CK7), carbonic anhydrase 9 (CA9), and AEl/AE3. We conducted careful and detailed examinations, including an association of the patient's medical history, but there were no indications for tumors, particularly MM, in any organs. Therefore, she was ultimately diagnosed with primary kidney MM.