Diffuse skeletal muscle extramedullary plasmacytomas: a rare case and review of the literature.

Extramedullary plasmacytomas are rare neoplasms arising from proliferations of monoclonal plasma cells. In primary form, these malignancies occur without other sites of plasma cell disease. Secondary extramedullary plasmacytomas occur in association with multiple myeloma and may be discovered during initial intramedullary disease or may occur during multiple myeloma relapse. In very rare instances, secondary extramedullary plasmacytomas have multifocal skeletal muscle involvement. We present a case of multifocal skeletal muscle plasmacytomas in a 58-year-old man with shoulder-reduced range of motion, pain, and a history of previously treated multiple myeloma. To our knowledge, the patient's unique relapse presentation of torso and shoulder soft tissue masses and the vast extent of skeletal muscle involvement are unique to cases in the current literature. This case also has MRI findings of a muscular plasmacytoma with internal hemorrhage which has not been previously reported. This case report will review imaging features and clinical presentations of intramuscular extramedullary plasmacytomas. Since imaging surveillance for multiple myeloma relapse is commonly performed, radiologists should be aware of these uncommon relapsing features including multifocal intramuscular masses which may contain internal hemorrhage.