Clinical history: 61 year old male presented with lower abdominal pain. Examination showed a large polynodular enhancing mass filling and distending the mid and lower rectum. Biopsies were taken.
What is your diagnosis?
Discussion and answer
This is a very poorly differentiated large cell malignancy. Considering the location possibility of a poorly differentiated adenocarcinoma was considered. However, immunohistochemical workup showed diffuse and strong staining of the tumor cells with antibodies to vimentin, CD56, and microphthalmic transcription factor, focal positive staining for HMB45, and negative staining with S100, chromogranin, LCA, epithelial markers pancytokeratin (AE1/3), low molecular weight keratin (CAM5.2), CK 20, and CK 903. These features were consistent with a malignant melanoma.
Primary rectal malignant melanoma
Prmary malignant melanoma from the anorectal region represents 0.4-1.6% of all melanomas and 1% of anal canal tumors. The tumors arise from benign melanocytic proliferations in the rectal mucosa and are located more than 4 cm from the anal verge. Patients usually present late with rectal bleeding, pain and a mass. The tumor cells have similar features as skin melanomas but are usually of the acrolentiginous type. Staging criteria for cutaneous melanomas have limited applicability to primary rectal melanomas. Differential diagnoses include poorly differentiated adenocarcinoma, lymphoma and even GIST. Diagnosis is often based on high index of suspicion and positive staining for melanoma markers HMB45, Melan A and S100. The tumors are also commonly positive when stained for c-kit, that may lead to confusion versus anorectal GISTs.
Abdominoperineal resection with or without pelvic lymphadenectomy is the mainstay of treatment. However there is survival benefit in only a small number of patients with superficial tumors 2 mm or less in thickness (as measured from the top of the overlying intact mucosa or the top of the ulcerated tumor).