18F-FDG PET/CT imaging of primary malignant melanoma of rectum with liver metastases mimicking rectum cancer: case report and literature review.

Malignant melanoma (MM) is an aggressive malignant tumor, which mostly occurs on the skin, uvea, etc. The mucosal MM accounts for a small proportion of all MM and can occur in the digestive tract. Primary MM of the digestive tract is rare and can be found in the middle and lower third of the esophagus and the rectum containing melanocytes. Primary rectal MM often occurs in middle-aged and elderly women, with rapid progress and strong invasion. We report a case of a 61-year-old man diagnosed with primary malignant melanoma of the rectum with liver metastases mimicking rectum cancer. ¹⁸F-FDG PET/CT showed the rectal wall was markedly thickened with a high metabolic level (SUVmax 10.6) and the boundary between the lesions and the prostate was unclear. In addition, increased FDG uptake were found in multiple lymph nodes, lung, liver, and bones, suggesting metastasis. In this case, ¹⁸F-FDG PET/CT shows the advantage of evaluating the whole-body situation and provides valuable information for the diagnosis, tumor stage, evaluation of treatment efficacy, and prognosis of MM.