Amelanotic Melanoma of The Rectum Presenting As A Polypoid Lesion: A Case Report

Abstract

Background: Primary anorectal melanoma is a rare and aggressive malignancy, accounting for a small fraction of all melanomas and often posing a diagnostic challenge when amelanotic, as it can mimic benign or epithelial lesions both endoscopically and histologically. Case Presentation: We report the case of a 75-year-old man who presented with rectal bleeding. Colonoscopy revealed a polypoid mass with ill-defined, ulcerated borders; the pedicle was macroscopically non-infiltrated. The lesion was removed by submucosal (endoscopic) polypectomy. Histopathological examination showed a poorly differentiated malignant neoplasm without visible melanin pigment. Immunohistochemistry demonstrated positivity for SOX10, vimentin, HMB-45, and CD10, with a high Ki-67 proliferation index, and negativity for pancytokeratin and CD45, confirming the diagnosis of amelanotic melanoma. Staging CT showed no distant lesions, and the patient underwent wide surgical excision of the rectum, with no metastatic involvement of the regional lymph nodes. The patient subsequently traveled abroad for further postoperative treatment, limiting the availability of long-term follow-up data.Conclusion: This case underscores the importance of maintaining a high index of suspicion for melanoma in anorectal polypoid lesions, particularly when routine histology is inconclusive, and highlights the essential role of a targeted immunohistochemical panel in reaching the correct diagnosis.