Secondary Cutaneous Plasmacytoma as a Manifestation of Extramedullary Multiple Myeloma: A Case Report.

Multiple myeloma (MM) is a clonal plasma cell malignancy that typically remains confined to the bone marrow; however, a subset of patients develops extramedullary disease (EMD), which is associated with advanced disease stage and poor prognosis. Cutaneous involvement represents a particularly rare manifestation of EMD, accounting for a small proportion of cases and usually indicating aggressive systemic progression. Due to its infrequency and heterogeneous clinical presentation, cutaneous plasmacytoma is often underrecognized, leading to diagnostic delays. We report the case of a 54-year-old Mexican woman with a history of MM who developed a painful, violaceous, ulcerated plaque on the left upper extremity following interruption of systemic therapy due to treatment-related neuropathy. Histopathological examination revealed a dense dermal infiltrate of mature plasma cells characterized by eccentric nuclei, basophilic cytoplasm, and clock-face chromatin. Immunohistochemical analysis demonstrated strong positivity for CD138 and kappa light-chain restriction, confirming the diagnosis of secondary cutaneous plasmacytoma associated with MM. This case highlights the importance of maintaining a high index of suspicion for atypical cutaneous lesions in patients with MM, particularly in the context of disease relapse or treatment interruption. Prompt skin biopsy with immunophenotypic confirmation is essential for accurate diagnosis and timely management. Cutaneous involvement reflects aggressive disease biology and underscores the limitations of localized therapies, emphasizing the need for systemic treatment strategies. Increased awareness and reporting of such cases are crucial to improving diagnostic recognition, refining prognostic assessment, and optimizing therapeutic decision-making in MM patients with extramedullary cutaneous manifestations.