Case Series of Metastatic Cutaneous Malignant Melanoma to the Gallbladder and the First Reported Case of Metachronous Adenocarcinoma of the Colon

Two female patients in their fifties with a previous history of cutaneous malignant melanoma were found during follow-up to have a 'hot' lesion in the gallbladder on a Positron Emission Tomography scan. Imaging showed a gallbladder polyp. Histology revealed infiltration of the polyp mucosa by metastatic malignant melanoma. One case had a BRAF mutation. A male in his 70s was found on a staging computed tomography scan to have a suspicious intraluminal lesion in the gallbladder and thickening of the sigmoid colon. Subsequent histology confirmed metastatic malignant melanoma in the chest wall and to the gallbladder and adenocarcinoma in the colon. Molecular testing showed BRAF mutation. The metachronous adenocarcinoma in the colon was mismatch repair protein proficient and had no KRAS mutation. Histology from all cases showed that metastatic malignant melanoma to the gallbladder is superficial. Discussion: Reports from autopsy examinations revealed that metastasis from malignant melanoma to the gallbladder can be up to 15% - 20%. Most patients have mild symptoms or are asymptomatic which explains the paucity of cases reported in living patients within the published literature. Most of the previous reports showed the metastatic malignant melanoma to the gallbladder presented macroscopically as a polyploidal lesion. Conclusion: Our histological observation and previous reports showed that metastatic malignant melanoma in the gallbladder tends to be superficial. All our cases show no lymphatic or vascular invasion in the histological examination as previously published reports, however, the hematological spread is the most commonly suggested mechanism of spread.