[Development of an extramedullary plasmacytoma despite disappearing M protein in multiple myeloma by bortezomib treatment].

A 65-year-old male with IgG-kappa multiple myeloma was treated with melphalan-prednisolone (MP) and obtained a minimal response. Five months after the initiation of MP, he developed back pain, renal failure, hypercalcemia and increased plasma cells in the bone marrow. He was treated with bortezomib. After 2 cycles, he developed a peripheral neuropathy, and the dose of bortezomib was decreased to 1.0 mg/m(2). After 5 cycles, serum monoclonal protein was not detected by immunofixation, and the percentage of bone marrow plasma cells decreased to less than 5%. In March 2007, he developed lumbago again, and MRI of the lumbar vertebrae showed a tumor at the para pediculus arcus vertebrae. Immunohistochemistry of the biopsied tumor demonstrated monoclonal plasma cell infiltration. The patient was treated with local radiation therapy. Bortezomib is a new and effective agent for refractory/relapsed multiple myeloma. It has also been reported that bortezomib is effective for solitary extramedullary plasmacytoma (EMP). However, in the patient reported here, although bortezomib induced a complete response with regard to the serum monoclonal protein and the percentage of bone marrow plasma cells, EMP developed in the parapediculus arcus vertebrae. Herein, we document a case of EMP development during successful bortezomib therapy.