[A case of metastatic melanoma in the gallbladder]

Malignant melanoma is an aggressive tumor with a high potential for distant metastases. Autopsy studies have shown that gallbladder metastases are found in 15% of patients. However, metastatic melanoma of the gallbladder is rarely discovered in living patients. A 73-year-old man was reported. The patient underwent surgical removal of malignant melanoma on his back and lymphadenectomy of the axillary lymph nodes. In addition, the patient developed cutaneous metastases to the right axillary and the middle of the chest 1.5 years after the surgery. Consequently, nivolumab chemotherapy was started. A computed tomography (CT) scan showed a well-enhanced mass in the gallbladder 4 months after. Abdominal ultrasonography revealed a 13-mm hypoechoic heterogeneous mass in the gallbladder with a hyperechoic layer on the mass surface. Magnetic resonance imaging demonstrated that the gallbladder tumor showed high signal intensity on T1-weighted images, low signal intensity on T2-weighted images, and high signal intensity on diffusion-weighted images. Positron emission tomography-CT revealed the slight uptake of fluorodeoxyglucose at the tumor. Endoscopic ultrasonography showed a hypoechoic tumor infiltrating the submucosal layer. The patient underwent open cholecystectomy. Examination of the resected specimens revealed a black, nodular-type tumor in the gallbladder body. The histopathological diagnosis was malignant melanoma. It was judged as metastatic melanoma of the gallbladder.