An amelanotic melanoma of the rectum: report of a rare aggressive primary tumor

Anorectal melanoma is a rare and highly aggressive malignancy accounting for 0.4% to 1.6% of all 0.5% anorectal cancer. About 80% cases are confused with haemorrhoids, rectal polyp, ulcer or adenocarcinoma as they commonly present bleed. We herein reported case 55-year male presenting 6-months history vague abdominal pain, constipation, tenesmus, weight loss, passage blood in stool. Clinical examination proctoscopy revealed fungating mass. CT scan showed mass involving lower rectum periserosal infiltration. The patient underwent total abdominoperineal resection specimen was sent histopathological examination. Gross growth measuring 3.5×2.5×2 cm along 8 lymph nodes. On microscopy mucosa normal, submucosa spindle shape tumor cells arranged sheets & fascicles having pleomorphic nuclei, high N:C ratio moderate amount eosinophilic cytoplasm areas necrosis no evidence lymphovascular emboli. 6/8 nodes metastatic deposits large number atypical mitotic figures. H E suggested diagnosis gastrointestinal stromal (T2N1Mx). A panel IHC markers advised definite diagnosis. were reactive HMB45, S-100, Ki67 thus rendering amelanotic melanoma. Due absence/small numbers melanin granules diversity morphology neoplastic cells, misdiagnosis lymphoma, sarcoma very common. Hence plays an indispensable role the rectum.