Solitary Lumbar Extradural Plasmacytoma With Minimal Vertebral Involvement: A Rare Presentation.

Solitary plasmacytoma of the spine is an uncommon plasma cell neoplasm that typically arises from the vertebral body and is associated with osteolytic changes. Presentation as a predominantly extradural mass with minimal osseous involvement is rare and poses a diagnostic challenge due to nonspecific imaging features. We present the case of a 52-year-old female who presented with severe low back pain and bilateral radiculopathy without motor deficit or sphincter dysfunction. MRI revealed a well-defined, homogeneously enhancing anterior extradural mass at the L5 level, causing significant thecal sac compression. Whole-body ¹⁸F-fluorodeoxyglucose PET-CT demonstrated a small focal lytic lesion involving the superior endplate of L5, without evidence of additional skeletal or extramedullary disease. The patient underwent surgical decompression with gross total excision. Histopathology and immunohistochemistry confirmed plasmacytoma. Systemic evaluation revealed monoclonal gammopathy with minimal marrow plasma cell infiltration and absence of CRAB features, consistent with a solitary spinal plasmacytoma with minimal vertebral involvement. This case highlights the importance of considering plasmacytoma in the differential diagnosis of enhancing epidural spinal lesions and emphasizes the role of surgery for diagnosis and decompression, followed by long-term surveillance due to the risk of progression to multiple myeloma.