Plasmacytomas in Extramedullary Manifestation of Multiple Myeloma: A Case Report.

Extramedullary plasmacytomas are an uncommon manifestation of multiple myeloma (MM) and may involve a wide range of soft-tissue sites. Superficial involvement of the back is particularly rare and may be mistaken for benign soft-tissue lesions. We report the case of a 59-year-old man with MM diagnosed in April 2024 who was treated with four cycles of bortezomib, cyclophosphamide, and dexamethasone (VCD), achieving remission with disappearance of serum and urine M-protein on immunofixation. In October 2025, the disease relapsed, with early progression despite treatment with daratumumab, prompting initiation of teclistamab therapy. During the course of his disease, he developed two progressively enlarging, painless indurations over the left upper and lower back. Ultrasound demonstrated well-circumscribed heterogeneous hypoechoic subcutaneous masses with internal arterial vascularity on Doppler imaging. Ultrasound-guided core biopsy revealed diffuse infiltration by monoclonal plasma cells expressing CD138 with lambda light-chain restriction, confirming secondary extramedullary plasmacytoma. This case highlights the importance of considering extramedullary plasmacytoma in the differential diagnosis of new superficial soft-tissue masses in patients with MM and underscores the complementary role of ultrasound and histopathological examination in establishing the diagnosis.