Track: Case Reports

Abstract:

Primary anorectal melanoma is a rare subtype of mucosal melanoma that often mimics both benign and malignant lesions such as hemorrhoids, abscesses, and squamous cell carcinoma, leading to diagnostic delays. Its variable morphology and nonspecific clinical features present significant challenges. An 80-year-old male presented with a necrotic anorectal mass, clinically diagnosed as a gangrenous hemorrhoid. Surgical excision revealed a 2.5 cm ulcerated tumor mass composed of sheets of large atypical cells resembling atypical lymphoid cells with frequent mitoses, apoptosis, and, focally, larger regions of necrosis. Atypical cells were also focally identified within the basilar epithelium of preserved mucosa, supporting intraepithelial involvement. By immunohistochemistry, the tumor cells were negative for epithelial and myoid markers, as well as CD3, CD20, CD30, CD45, and PAX5. However, they were focally positive for CD4, CD10, and ALK, with a high Ki-67 proliferation index. Although ALK positivity raised the consideration of ALK+ large B-cell lymphoma, additional immunohistochemistry demonstrated strong, diffuse positivity for SOX10, Mel-C (multiplex stain for melanocytes), and PRAME, with focal staining for S100. There was no history of cutaneous or ocular melanoma. While considered a diagnosis of exclusion with respect to primary or metastatic disease, the intraepithelial component would favor primary anorectal melanoma, specifically the lymphoma-like variant. This case highlights an unusual presentation of anorectal melanoma, closely mimicking a gangrenous hemorrhoid clinically and aggressive lymphoma pathologically. Extensive histologic sampling and a broad immunohistochemical panel are essential for accurate diagnosis and timely management in such unusual cases.

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