Isolated Gallbladder Melanoma: A Diagnostic Conundrum of Primary vs. Metastatic Disease

Melanoma is a malignant proliferation of melanocytes originating from neural crest cells. Gallbladder melanoma is typically reported as a metastatic disease involving the adjacent liver. Thus, isolated gallbladder melanoma must be either a primary tumor originating within the gallbladder without evidence of metastasis or an isolated metastatic lesion from an unknown or regressed primary site. Regardless of its classification as primary or metastatic, isolated gallbladder melanoma is an extremely rare diagnosis. We present a case of incidentally discovered isolated gallbladder melanoma in a resected specimen from a patient undergoing surgery for symptomatic cholelithiasis. An exhaustive workup failed to identify a primary site outside the gallbladder. The case was reviewed at a multidisciplinary tumor board, and the patient was started on immunotherapy. This case report aims to highlight this rare entity, discuss potential management strategies, and contribute to the limited body of literature on the subject.