Primary malignant melanoma of the gall bladder masquerading as xanthogranulomatous cholecystitis.

Malignant melanoma of the gall bladder is rare. Most cases are metastatic and primary gall bladder melanoma is even more rare. We report a case of primary malignant melanoma of the gall bladder which illustrates the diagnostic challenge posed by this condition. Histopathology and immunohistochemistry play a pivotal role in making a diagnosis and ruling out conditions which mimic it such as xanthogranulomatous cholecystitis and other relatively common epithelial malignancies. We tested for prognostic and predictive markers including BRAF and PD-L1 and immunohistochemistry showed positive staining for BRAF. The tumour cells expressed HMB-45 and were negative for cytokeratin and CD68, favouring a diagnosis of malignant melanoma and excluding the possibility of xanthogranulomatous cholecystitis and carcinoma. On follow-up at 3 months there was no evidence of recurrence of metastasis.