Plasma cell myeloma is a mature B-cell neoplasia characterized by clonal plasma cell proliferation. At diagnosis, extramedullary plasmacytoma (EMP) accounts for only 4% of cases, but it is more frequently detected in relapses as the disease progresses, it is about 3–14%.
The investigation of a 50-year-old woman started in 2019 due to weight loss, weakness, and dyspnoe. Laboratory tests revealed anemia, hypercalcemia, and impaired renal function. Peripheral blood and bone marrow examination in March 2019 revealed plasma cell leukemia with diffuse bone marrow involvement, 1q21 amplification, and hyperdiploid chromosome state. After four cycles of VTD-PACE (bortezomib, thalidomide, dexamethasone, cisplatin, doxorubicin, cyclophosphamid, etoposid) induction treatment, she achieved MRD-negative complete remission. She received two additional cycles of VTD treatment after a successful stem cell collection. Due to unfavorable prognostic markers, autologous stem cell transplantation was performed after bendamustine-melphalan conditioning treatment. At 100 days post-transplantation, MRD-negative complete remission was commented. After 22 months of maintenance lenalidomide treatment, palpable tissue proliferation was observed in both lower limbs. Bone marrow flow cytometry was negative. 18FDG-PET/CT and contrast MR imaging techniques suggested relapse of the underlying disease. Histopathological examination of core biopsy performed in our clinic confirmed multiplex extramedullary plasmacytoma with muscle involvement. DRD (daratumumab, lenalidomide, dexamethasone) therapy was initiated as 2nd line treatment, but there was an inadequate therapeutic response, so we continued with KPD (carfilzomib, pomalidomide, dexamethasone) protocol. Repeated progression was confirmed after an excellent therapeutic response to KPD treatment. That's why local irradiation was performed. Despite this fact, extensive systematic relapse appeared. For rescue treatment, the EDAP (etoposide, cisplatin, citarabin, dexamethasone) protocol was initiated with venetoclax supplementation. As the patient responded well clinically, a referral for allogeneic stem cell transplantation was accepted.
In plasma cell myeloma, secondary tissue infiltration adjacent to the muscle is relatively standard, but plasmacytomas arising in the muscle tissue are rare. We considered presenting our case because of the rare EMP localization confirmed by biopsy and imaging studies.