ABSTRACT
INTRODUCTION: Malignant melanoma of the gastrointestinal tract is an extremely rare event of which 50 % occurs in anorectal region. The lesion can easily be misdiagnosed as rectal-carcinoma, which comprises >90 % of rectal tumors and has a different treatment. The behavior of the anorectal melanoma is very aggressive and has very poor prognosis with fatal outcome. PRESENTATION OF CASE: A 48-year-old man presented complaining of rectal bleeding of two months' duration, without any other significant history. Colonoscopy showed a polypoidal mass in the rectum that was in favor of adenocarcinoma. The microscope examination of biopsy tissue showed sheets of poorly differentiated malignant neoplasm. Immuno-histochemical (IHC) staining showed negativity of pan Cytokeratin and CD31. IHC for HMB45 showed diffuse and strong positivity in neoplastic cells, confirming the diagnosis of malignant melanoma. CLINICAL DISCUSSION: According to a report by the National Cancer Database of the United States, primary rectal melanoma is very rare. Mucosal surface of the body is third most common site for primary melanoma after skin and eye. The first case of anorectal melanoma was reported in 1857. Histopathological examinations are gold standard for diagnosis, but histopathology examination without immunohistochemistry will misdiagnose some cases as poorly differentiated adenocarcinoma which has completely different treatment. Surgical resection has been reported as the most useful treatment option. CONCLUSION: Malignant melanoma of the rectum is extremely rare and difficult to diagnose in low resources settings. Histopathologic examination with IHC stains can differentiate poorly differentiated adenocarcinoma from melanoma and other rare tumors of anorectal region.
