Skin-Associated Secondary Plasmacytomas in Multiple Myeloma: A Rare Entity With Prognostic Relevance

Extramedullary plasmacytomas rarely involve the skin, and secondary cutaneous plasmacytomas (SCPs) are typically observed in advanced multiple myeloma (MM). We report a case of a 58-year-old man with known MM who developed multiple violaceous nodules and right-sided exophthalmos. Skin biopsy showed a kappa-restricted plasmacytic infiltrate, and imaging revealed extensive soft-tissue masses, widespread osteolytic lesions, and an intraorbital tumor. These findings established a diagnosis of metastatic SCP associated with progressive MM. Despite initiation of VDT-PACE chemotherapy (bortezomib, dexamethasone, thalidomide, cisplatin, doxorubicin, cyclophosphamide, and etoposide), the patient's condition rapidly deteriorated, leading to death after four cycles. This case highlights the rarity and severity of SCP and underscores the importance of recognizing cutaneous involvement as a marker of aggressive, treatment-refractory MM and imminent systemic progression.