Pancreatic metastasis of malignant melanoma presenting as a tumor occluding the main pancreatic duct

Metastasis to the pancreas is rare, especially for malignant melanoma. In these scarce cases, tumors normally present in the parenchyma of the pancreas. A 70-year-old woman was referred to our hospital for an asymptomatic pancreatic tumor detected by contrast-enhanced computed tomography (CE-CT). She had undergone a pneumonectomy for primary malignant melanoma (MM) of the lung approximately 3 years prior and was receiving nivolumab for recurrent intrapulmonary metastasis. CE-CT revealed a hypo-vascular lesion occluding the main pancreatic duct (MPD) which was consequently dilated. On magnetic resonance imaging, the MPD lesion had high signal intensity on T1-weighted and diffusion-weighted images. Contrast-enhanced endoscopic ultrasonography revealed that the low echoic tumor had heterogenous enhancement in the early phase. Endoscopic retrograde pancreatography (ERP) was performed for evaluation of tumor area and histological diagnosis. A contrast agent defected lesion was observed in the dilated MPD from the pancreas body to tail suggesting the tumor totally occluded the MPD. Biopsy was performed and the specimens obtained from the defect lesion were black in color. Histopathological examination of the black specimens revealed substantial growth of tumor cells with eosinophilic cytoplasm and unequal nuclei size. The tumor cells had a brownish pigmentation of melanin. From these findings, the tumor was diagnosed as pancreatic metastasis of MM. The patient underwent chemotherapy with ipilimumab and nivolumab. The final diagnosis was pancreatic metastasis of MM occurring as a tumor occluding the MPD. ERP was useful for histological diagnosis and could be useful for future cases.