Primary Malignant Anorectal Melanoma

Excerpt

A 70-year-old woman presented with periumbilical pain and increased stool frequency that had progressed over the preceding 25 days. She had no documented history. Physical examination was unremarkable, except that rectal examination showed two palpable masses protruding from the anal verge. Colonoscopy discovered two erosive and ill-defined masses of each about 1.5 cm protruding into the rectal cavity in the lower rectum (Fig.  1A and B), which was best seen on retroflexion and confirmed as the masses located in the mucosal layer by endoscopic ultrasonography. Endoscopic submucosal dissection was performed to remove the two nodules. The histopathological detection of the specimen by hematoxylin and eosin staining (Fig.  2A) and the lesion staining strongly immunoreactive for HMB45 (Fig.  2B), MelanA (Fig.  2C), and SOX10 (Fig.  2D) by immunohistochemistry were consistent with mucosal malignant melanoma, which was further characterized by a high proliferation index Ki67 around 70% in neoplastic cells (Fig.  2E). The melanomas of anorectum is a rare and highly malignant neoplasm. ^1,2 Patients eventually diagnosed with anorectal malignant melanoma were commonly misdiagnosed with polyps. ³ If melanoma is localized, endoscopic or surgical resection is the first-line therapy. ² We can only postulate about the origin of the anorectal melanoma in this patient due to obscure pathogenesis and lacking of pathologic technique for distinguishing primary from secondary melanoma. Due to its nonspecificity and aggressiveness, radical surgery should be reserved for this patient with positive resection margin. After a well-informed discussion of options for interventions with her, the decision was not made to pursue immunotherapy and surgery. Follow-up colonoscopy after 2 months confirmed no recurrence.

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