Solitary Plasmacytoma of Bone

Solitary plasmacytoma is a rare neoplasm defined by a single, localized mass of clonal (monoclonal) plasma cells with no evidence of systemic disease. It is part of the plasma-cell neoplasm spectrum - the great majority (~95%) of which present as multiple myeloma. When the lesion is in bone it is a solitary bone plasmacytoma (SBP); when in soft tissue/extraosseous sites (commonly the head and neck) it is a solitary extramedullary plasmacytoma (SEP). SBP is more common than SEP and has a poorer prognosis because of its higher rate of progression to multiple myeloma, even though both respond well to treatment.

Diagnosis of a solitary bone plasmacytoma requires all of:

1. A single lesion of biopsy-proven clonal plasma cells.
2. A normal bone marrow (no, or only minimal/limited, clonal plasma cells - more than a defined low threshold suggests myeloma).
3. No other lesions on whole-body imaging (MRI/PET).
4. No myeloma-related end-organ damage - i.e. no CRAB (hyperCalcaemia, Renal impairment, Anaemia, other Bone lesions). A small clonal marrow population with an otherwise solitary lesion is sometimes termed plasmacytoma with minimal marrow involvement and behaves more aggressively.

Definitive radiotherapy to the lesion is the standard treatment for solitary plasmacytoma - it provides durable long-term local control in solitary bone plasmacytoma and is potentially curative in the extramedullary form (which has a lower tendency to disseminate). Treatment volumes and dose follow specialist (e.g. ILROG) guidance. Surgery is not the primary treatment of the tumour itself; it is reserved for structural indications - impending or actual pathological fracture, spinal instability, or neurological compromise requiring decompression/stabilisation - usually combined with RT. The role of systemic chemotherapy is debated and not routine for true solitary disease.