Solitary Bone Plasmacytoma of Left Wrist Joint - A Case Report

Solitary Plasmacytoma is an infrequent form of plasma cell (PC) dyscrasia which presents as a single mass of monoclonal PCs, located either intraosseous or extramedullary. Solitary bone plasmacytoma (SBP) is characterized by a mass of neoplastic monoclonal PCs primarily occurring in the axial skeleton without systemic involvement. This case delineated an unusual appendicular-osteoarticular (wrist joint) involvement, which was diagnosed through an exhaustive set of investigations and managed by radiotherapy (RT), which is the standard of care. The rarity of the clinico-radiological presentation makes this case noteworthy.

The present case highlights an unusual clinico-radiological finding of SBP involving the left wrist joint in a 45-year-old male patient. The patient presented with non-traumatic, chronic pain and swelling of the left wrist joint with restricted range of motion. Radiological investigations showed multiple osteoarticular lytic lesions, and histopathology revealed binucleated sheets of PCs, "Mott cells" and Russell bodies in bony trabeculae, which clinched the diagnosis as PC dyscrasias. Interestingly, multiple intracytoplasmic crystalline inclusions were found in this case, which is a quite rare entity as per SBP and warrants further investigations to rule out other PC dyscrasias, such as multiple myeloma (MM). Immunofixation electrophoresis was done to confirm the monoclonality, that is, light chain restriction (increased immunoglobulin G kappa) of the tumor. Immunohistochemical markers evaluation was positive for CD138 and CD38. Further investigations were done to rule out MM (details tabulated in the report). The patient was managed with RT as per International Myeloma Working Group (IMWG) guidelines a. Being notorious for a higher risk for progression to MM for SBP patients (65-84% at 10 years), we did continuous monitoring of the patient post-radiation therapy as per IMWG guidelines.

SBP of the appendicular skeleton, especially distal to the elbow, as seen in this case, is very rare in adults. In such scenarios, a multidisciplinatory approach of extensive radiological, biochemical, histopathological, and immunohistochemical investigations to rule out the differential diagnosis is the key to eschew misdiagnosis. Prompt treatment and strict follow-up are the pre-requisites for a favorable outcome and to prevent progression into MM which is a more aggressive entity and lies at the other end of the spectrum of PC dyscrasias.