A rare case of primary rhabdoid melanoma of the urinary bladder treated with ipilimumab, an anti-CTLA 4 monoclonal antibody.

Primary melanoma of the urinary bladder is a rare subentity of melanoma. The same applies for melanoma of the rhabdoid histopathologic phenotype. A female patient was initially diagnosed with melanoma of unknown origin caused by macroscopic lymph node metastasis in the left inguinal and parailiacal regions. Because of the extent of the disease, radical surgery could not be performed. The patient underwent systemic chemotherapy with dacarbazine, followed by the experimental compound tasisulam. Upon sudden macrohematuria, cystoscopy showed a large infiltrating tumorous structure located on the left side of the urinary bladder. Clinically, the amelanotic tumor showed endophytic growth into the lumen; on the histopathological specimen, the growth pattern was partially undermining the urothelium, which is commonly observed in primary melanoma of the urinary bladder. Cytologically, the tumor cells were classified as rhabdoid melanoma, a very rare variant of melanoma, which is commonly amelanotic and expresses S100, vimentin and Ncam. Mutational analysis showed positive results for BRAF V600E. After detecting the primary melanoma, the patient received anti-CTLA4 antibody treatment with 3 mg/kg ipilimumab, through which a partial response was achieved. Past computed tomography scans should be re-evaluated for suspicious lesions, and cystoscopy should be included in the clinical workup if the pattern of metastasis is congruent with the drainage sites of the urinary bladder.