A Case of Amelanotic Plasmacytoid Melanoma with Strong CD138 Expression, Mimicking Multiple Myeloma - A Case Report

Melanomas have diverse pathological features that mimic other tumors, such as lymphoma, sarcoma, and even poorly differentiated carcinoma. The most recently identified variant, the plasmacytoid variant, is an uncommon variant that can appear as a solitary tumor or a metastatic disease. Due to its rarity, the epidemiologic profile of this variant is not well characterized. This case report illustrates a diagnostic challenge of plasmacytoid cell mimicker, which is rarely found in daily practice.

A 49-year-old man presented with multiple subcutaneous soft tissue nodules in the thoracic area and multiple pathological fractures in the left distal humerus and distal ulna. Clinical and radiological findings were suggestive of metastatic bone disease with differential diagnosis of multiple myeloma. Fine needle aspiration biopsy and histopathological findings were suggestive of multiple myeloma with differential diagnoses of metastatic carcinoma, rhabdomyosarcoma, and amelanotic melanoma. Thus, immunostaining for CD138, CK, desmin, vimentin, S-100, and HMB45 were requested and the results were compatible with the final diagnosis of amelanotic plasma melanoma.

It is crucial to consider melanoma as one of the differential diagnoses of a tumor with plasmacytoid feature and CD138 positive staining as it can mimic multiple myeloma as demonstrated in this case report.