Solitary Plasmacytoma

Solitary plasmacytoma is a clonal plasma cell neoplasm characterised by a single focus of abnormal plasma cells, either within bone (solitary bone plasmacytoma, SBP) or in soft tissue without bone involvement (extramedullary plasmacytoma, EMP), in the absence of systemic multiple myeloma. It represents the localised end of the plasma cell dyscrasia spectrum, with multiple myeloma (MM) at the disseminated end. The critical clinical question in solitary plasmacytoma management is whether the lesion truly represents isolated disease or is an early presentation of multiple myeloma.

• Solitary bone plasmacytoma (SBP): accounts for approximately 3–5% of all plasma cell neoplasms; median age at diagnosis approximately 55–60 years; male predominance (2:1); most common sites: axial skeleton (vertebrae — thoracic most common, then lumbar and cervical), pelvis, skull, and proximal long bones (femur, humerus)
• Extramedullary plasmacytoma (EMP): more common in the upper respiratory tract (nasal cavity, paranasal sinuses, nasopharynx, tonsil); better prognosis than SBP; lower risk of progression to MM
• Progression to multiple myeloma: SBP has a 50–70% risk of progressing to MM within 10 years (median time to progression approximately 2–3 years); EMP has a lower progression risk (approximately 15–30% at 10 years); the presence of a persistent serum paraprotein after radiotherapy significantly increases the risk of progression to MM; stringent diagnostic criteria (MRI of whole spine + pelvis + PET-CT) are required to confirm true solitary disease

Diagnosis of solitary plasmacytoma requires ALL of the following (International Myeloma Working Group criteria):

• Biopsy-proven monoclonal plasma cell infiltrate in a single site (bone or soft tissue)
• Normal bone marrow biopsy (plasma cells <10%) from a distant site
• Absence of lytic bone lesions other than the solitary focus on whole-body imaging (MRI of spine and pelvis + PET-CT or CT skeletal survey)
• Absence of CRAB criteria attributable to the plasma cell disorder: CRAB — hyperCalcaemia, Renal insufficiency, Anaemia, Bone lesions (lytic); presence of any CRAB feature = MM diagnosis, not SBP
• No evidence of end-organ damage from plasma cell disease

• Pain: the most common presenting symptom — localised bone pain at the site of the plasmacytoma; vertebral SBP may present with back pain, and compression fracture can cause acute pain and neurological compromise
• Neurological symptoms: vertebral SBP causing spinal cord compression or nerve root compression; progressive myelopathy or radiculopathy; emergency decompression may be required
• Pathological fracture: SBP in long bones (femur, humerus) or pelvis may present with or without fracture
• Incidental finding: occasionally detected on imaging performed for other reasons
• Serum and urine protein electrophoresis: M-protein (paraprotein) is detectable in approximately 50–70% of SBP; Bence Jones protein (light chains) in urine in approximately 30%; a persistent M-protein after radiotherapy treatment is the most important predictor of progression to MM

• Full blood count (FBC): anaemia of chronic disease in MM; normal or near-normal in SBP
• Serum calcium, creatinine, albumin: assess CRAB criteria — hypercalcaemia and renal failure suggest MM
• Serum and urine protein electrophoresis + immunofixation: quantify M-protein and identify immunoglobulin type; serum free light chain assay (kappa/lambda ratio)
• Bone marrow trephine biopsy (from a distant site, usually the posterior iliac crest): mandatory; plasma cells <10% required for SBP diagnosis; flow cytometry and FISH for cytogenetic risk stratification if plasma cells are present
• MRI whole spine and pelvis: most sensitive imaging for occult marrow disease; should be performed in all patients with suspected SBP to exclude additional lesions not visible on plain X-ray or CT; detects marrow infiltration before cortical destruction occurs
• PET-CT or CT of the whole body (skeletal survey): assesses for additional lytic lesions outside the spine; PET-CT superior to plain skeletal survey for detecting additional lesions
• Biopsy of the suspected lesion: CT-guided core biopsy confirms the diagnosis of plasmacytoma; essential before initiating treatment

• Radiotherapy: the primary treatment for solitary plasmacytoma; dose 40–50 Gy in fractionated doses (daily fractions over 4–5 weeks); achieves local control in approximately 85–90% of patients; radiotherapy does not prevent progression to MM but achieves durable local control; the entire lesion with appropriate margins should be included in the treatment field
• Surgery: not the primary treatment unless required for specific indications — spinal cord compression requiring decompression and stabilisation (surgery + post-operative radiotherapy); pathological fracture of a long bone requiring internal fixation; vertebral collapse causing mechanical instability
• Vertebral plasmacytoma with cord compression: urgent surgical decompression + internal fixation + post-operative radiotherapy; vertebroplasty or kyphoplasty for painful vertebral collapse without cord compromise
• Systemic therapy: no routine role for chemotherapy in confirmed SBP; systemic therapy is initiated if/when progression to MM occurs; systemic bortezomib-based or lenalidomide-based regimens then apply as per MM guidelines
• Follow-up after radiotherapy: serial clinical review, protein electrophoresis (serum and urine), and imaging every 3–6 months; the disappearance of the M-protein after radiotherapy suggests true solitary disease and better prognosis; persistent M-protein after radiotherapy predicts higher progression risk to MM

• 5-year overall survival for SBP: approximately 70–80%
• Predictors of progression to MM: persistent M-protein after radiotherapy (strongest predictor); large lesion size (>5 cm); age (>60 years); presence of minor clonal plasma cells on bone marrow trephine (<10% but present vs completely negative); MRI showing minor additional marrow changes; serum free light chain ratio abnormality
• EMP prognosis: better than SBP; 5-year survival approximately 80–90%; progression to MM approximately 15–30% at 10 years; local recurrence rate approximately 15–25%
• Patients who remain free of M-protein 12–24 months post-radiotherapy have the best prognosis with approximately 20–30% progression to MM at 10 years

• Solitary plasmacytoma with minimal bone marrow involvement (SMM sub-category): the IMWG recognises a category of SBP with minor marrow involvement (clonal plasma cells 10–59% without CRAB features) — this is termed smouldering MM (SMM) and has a higher progression risk; orthopaedic surgeons should be aware that borderline marrow biopsy results require haematological input for classification and risk stratification
• Vertebroplasty and kyphoplasty in spinal plasmacytoma: cement augmentation provides immediate pain relief and structural support in painful vertebral fractures caused by plasmacytoma; kyphoplasty (balloon expansion before cement) can partially restore vertebral height; does not affect the underlying plasma cell disease; can be performed before or concurrent with radiotherapy; adequate cement fill is important to prevent further collapse
• Pathological fracture of a long bone SBP: internal fixation with intramedullary nail or plate + post-operative radiotherapy; cement augmentation to fill the lesion cavity may improve construct stability; reaming of the entire femoral canal should be performed during IM nailing of femoral lesions to prevent metachronous metastasis at the distal femur (although SBP is not truly metastatic, ensuring the entire femoral medulla is addressed is good practice)
• Distinguishing SBP from MM on orthopaedic referral: the orthopaedic surgeon often receives the biopsy result first; immediate haematological review is essential; do not proceed with local orthopaedic intervention without haematological staging; the treatment plan may change entirely if MM is confirmed rather than SBP

• SBP diagnosis requires: biopsy-proven single lesion + normal bone marrow (<10% plasma cells) + no other lesions on whole body imaging + no CRAB features
• CRAB: hyperCalcaemia, Renal failure, Anaemia, Bone lesions — any CRAB = MM, not SBP
• SBP progression to MM: 50–70% at 10 years; strongest predictor — persistent M-protein after radiotherapy
• EMP: better prognosis; upper respiratory tract most common; 15–30% progression to MM at 10 years
• Radiotherapy: primary treatment for SBP; 40–50 Gy; 85–90% local control; does not prevent MM progression
• MRI whole spine + pelvis mandatory: most sensitive for occult marrow disease; required to confirm single lesion
• Bone marrow trephine: distant site (posterior iliac crest); <10% plasma cells required for SBP diagnosis
• Surgery in SBP: for cord compression, pathological fracture, or mechanical instability — not primary treatment; combine with post-operative radiotherapy
• Vertebroplasty/kyphoplasty: for painful vertebral collapse without cord compromise; can be combined with radiotherapy
• Haematological referral mandatory before orthopaedic intervention — staging must confirm SBP vs MM before local treatment planning