Solitary plasmacytoma

Definition and Classification

• Plasmacytomas are malignant tumors composed of clonal plasma cells. Classified as:
  • Solitary: medullary (solitary bone plasmacytoma, SBP) or extramedullary (SEP).
  • Multiple: extramedullary or cutaneous.
• Epidemiology: annual incidence ~0.15/100,000 (declining with improved diagnostic imaging); median age ~60 yr (about 10 yr younger than MM); slight male predominance.
• Can be the initial presentation or arise in patients with established myeloma (often marking progression).

Definitions and Subtypes (IMWG)

• IMWG diagnostic criteria: (1) biopsy-proven solitary lesion of clonal plasma cells; (2) normal bone marrow with no clonal plasma cells (clonal BMPC <10% = plasmacytoma with minimal marrow involvement); (3) absence of myeloma-defining events (no SLiM-CRAB); (4) no other lesions on whole-body imaging (Rajkumar, Lancet Oncol 2014; PMID 25439696).
• Solitary bone plasmacytoma (SBP): ~70% of cases; vertebral > pelvis > ribs > long bones. Median age ~55, M>F.
• Solitary extramedullary plasmacytoma (SEP): ~30%; ~80% in upper aerodigestive tract (nasopharynx, paranasal sinuses, oral cavity, larynx); GI tract, lymph nodes, and skin less common.
• Plasmacytoma with minimal marrow involvement (MMI): same imaging picture but BM clonal plasma cells >0% and <10%; much higher progression to MM (~60% at 3 yr for solitary bone plasmacytoma with minimal marrow involvement and ~20% for extramedullary with minimal marrow involvement, vs ~10% for true solitary (no clonal marrow)).

Clinical Presentation

• SBP: localized bone pain, possible pathologic fracture; cord compression with vertebral lesions.
• SEP: site-specific, i.e. nasal obstruction, epistaxis, dysphagia, hoarseness, head and neck mass.
• Paraprotein: small monoclonal protein on SPEP/UPEP in ~50% of SBP and ~25% of SEP; usually disappears after radiation. Persistence after RT predicts progression to MM.

Workup (must exclude MM)

• Labs: CBC, BMP, calcium, albumin, LDH, β2-microglobulin, SPEP/UPEP with immunofixation, serum free light chains (FLC), 24-hr urine.
• Bone marrow biopsy with flow cytometry and FISH/cytogenetics, required: no clonal plasma cells = solitary plasmacytoma; clonal plasma cells >0% and <10% = plasmacytoma with MMI; ≥10% = MM.
• Imaging: whole-body MRI (preferred for bone) or FDG-PET/CT (preferred for extramedullary) is mandatory; an adequate PET/CT can serve as the whole-body study; PET/CT detects occult lesions in up to 30% of presumed SBP and reclassifies them as MM.
• Skeletal survey alone is insufficient (too low sensitivity, per IMWG 2014/2024 updates).

Treatment

• Definitive radiation therapy: 1.8 to 2 Gy fractions; 35 to 40 Gy for solitary bone plasmacytoma under 5 cm; 40 to 50 Gy for bone lesions 5 cm or larger and for extramedullary disease (ILROG). Local control >85%. RT is curative-intent and well tolerated.
• Surgery: limited role, i.e. biopsy/cytoreduction for cord compression, structural instability, or selected SEP (e.g., laryngeal); always followed by RT.
• Adjuvant systemic therapy: not standard. Consider in MMI, refractory disease, or recurrence; typically bortezomib/IMiD-based induction.
• Bisphosphonates: if vertebral SBP with structural risk; not routine.

Prognosis and Follow-up

• Progression to MM (at 10 yr): SBP ~50 to 65%; SEP ~10 to 30% (much lower). Median time to progression ~2 to 4 yr.
• Adverse predictors of progression: MMI on marrow, persistent paraprotein after RT, abnormal FLC ratio, high-risk FISH (t(4;14), del(17p)), tumor >5 cm.
• Surveillance: H&P plus CBC, CMP, SPEP/UPEP, FLC every 3 mo for 1 to 2 yr, then every 6 mo; imaging (MRI or PET/CT) every 6 to 12 mo for 5 yr, then annually. Indefinite follow-up (late progression occurs).

High-Yield Pearls

• Persistent M-spike after RT is the strongest predictor of progression to MM.
• Abnormal FLC ratio at diagnosis predicts progression (Dingli, Blood 2006).
• Always obtain PET or MRI: skeletal survey misses ~30% of additional lesions.
• SEP of the upper aerodigestive tract has the best prognosis and lowest MM progression rate.
• Any myeloma-defining event (SLiM-CRAB) at presentation reclassifies the patient as MM, not solitary plasmacytoma.