Plasmacytoma

Plasmacytoma is a malignant neoplasm composed of monoclonal plasma cells arising as a single mass without evidence of systemic multiple myeloma.[1][2] In the jaws, it most commonly presents as a solitary osteolytic lesion of the mandible, producing a well-defined radiolucency with cortical destruction and frequent soft-tissue extension.[3][4] The lesion arises from clonal expansion of terminally differentiated B lymphocytes and is classified within the spectrum of plasma cell neoplasms.[2][5] Diagnosis requires biopsy confirmation of clonal plasma cells and exclusion of systemic disease through bone marrow evaluation and whole-body imaging.[1][6] Management centers on definitive radiation therapy, with long-term surveillance mandated by a substantial risk of progression to multiple myeloma.[1][2]

The 2022 International Consensus Classification (ICC) retains solitary plasmacytoma of bone and primary extraosseous (extramedullary) plasmacytoma as distinct entities within the plasma cell neoplasm category.[5] The International Myeloma Working Group (IMWG) further recognizes solitary plasmacytoma with minimal marrow involvement.[7] The term "plasma cell myeloma," used in the revised 4th edition WHO classification, has been replaced by "multiple myeloma" in both the WHO 5th edition and the ICC.[5]

• Accepted names: solitary plasmacytoma of bone (SPB); solitary osseous plasmacytoma; extramedullary plasmacytoma (EMP); solitary extraosseous plasmacytoma
• Obsolete or discouraged: "solitary myeloma" (imprecise; should not be used interchangeably with SPB)

When a jaw lesion is the initial presentation, the term "plasmacytoma of the jaw" is used descriptively; definitive classification requires systemic workup to distinguish SPB from a jaw manifestation of multiple myeloma.[4]

• Solitary plasmacytoma accounts for approximately 3% of all plasma cell neoplasms; SPB represents 1–2%. The incidence of SPB is approximately 0.45 per 100,000 persons.[2][8]
• Median age at diagnosis is approximately 55 years, with a male-to-female ratio of roughly 2:1.[2][9]
• SPB most commonly involves the axial skeleton (vertebrae, ribs, skull, pelvis); jaw involvement is uncommon, reported in approximately 4% of SPB cases.[10]
• Within the jaws, the mandible is affected far more frequently than the maxilla, with the posterior mandible (angle, ramus, molar region) being the characteristic site. One clinicopathologic series of 33 jaw myeloma cases found the maxilla involved in 71% of SPB patients, though this distribution is atypical of most published series.[3][4][11][12]
• Extramedullary plasmacytoma of the head and neck shows a predilection for the sinonasal tract; gingival involvement is rare.[8][13]

Presentation and symptoms

• The most common presenting symptom is a painless or mildly painful swelling of the jaw, often of prolonged duration (weeks to months).[4][11]
• Swelling is reported in the vast majority of cases; additional symptoms include tooth mobility, infected teeth, and nasal obstruction (particularly with maxillary lesions).[4]
• Paresthesia or numbness of the lower lip (inferior alveolar nerve involvement) may occur with mandibular lesions but is not consistently reported.[14]
• Pathologic fracture is an occasional complication.[4]
• Extramedullary plasmacytoma of the gingiva may mimic benign hyperplasia, presenting as a painless soft-tissue mass.[13]

Tempo and red flags

• Symptom duration is variable; median duration in one jaw series was 10 months for SPB.[4]
• A jaw plasmacytoma may be the first clinical manifestation of occult multiple myeloma; systemic workup is mandatory in every case.[4][15]
• Monoclonal immunoglobulin (M-protein) is detectable in the serum at diagnosis in a minority of SPB patients but its persistence after treatment is a risk factor for progression.[11][16]

Location and distribution

• Plasmacytoma of the jaw characteristically arises in the posterior mandible, particularly the angle, ramus, and molar region, corresponding to areas of active hematopoietic marrow. Maxillary involvement is less common and tends to affect the posterior alveolar process and tuberosity.[3][4][10][11][12]
• Lesions are solitary by definition; the presence of multiple jaw lesions should raise suspicion for multiple myeloma.[4]

Periphery and shape

• The lesion typically presents as a well-defined, round or ovoid radiolucency with a smooth, non-corticated margin. A corticated (sclerotic) border is characteristically absent.[3]
• Three radiographic patterns have been described in the jaws:[4]
  • Unilocular radiolucency with a cyst-like appearance — common
  • Multilocular "soap-bubble" radiolucency — occasional
  • Ill-defined, destructive osteolysis — occasional, more suggestive of aggressive behavior or higher histologic grade
• Periosteal reaction is characteristically absent.[3]

Internal architecture

• The internal density is uniformly radiolucent in most cases, without internal calcification, septation, or ground-glass opacity.[3]
• Residual bony trabeculae within the lesion may produce a coarse trabecular or multilocular appearance on panoramic radiographs.[4]
• True internal mineralization is not a feature; any radiopacity within the lesion should prompt consideration of alternative diagnoses.

Effects on teeth and adjacent structures

• Tooth displacement may occur with larger lesions. Root resorption is rare in association with plasmacytoma or myeloma of the jaws and has been described only in isolated case reports.[17]
• The inferior alveolar canal may be involved or obliterated by mandibular lesions, correlating with clinical paresthesia.[14]
• Lamina dura loss adjacent to the lesion may be observed but is nonspecific.

Extent and cortical involvement

• Cortical destruction is characteristic, present in the vast majority of cases. Despite well-defined margins within the medullary bone, the lesion commonly perforates the cortex and extends into adjacent soft tissues.[3][14]
• Soft-tissue mass formation is a hallmark feature; cross-sectional imaging consistently demonstrates an associated soft-tissue component.[3]
• Invasion of adjacent anatomic structures (masticator space, maxillary sinus, parotid region, infratemporal fossa) is common with larger lesions.[3][14]
• On cross-sectional imaging, residual bony islands or fragments within the destructive zone are a characteristic finding, distinguishing plasmacytoma from purely lytic metastatic disease.[3]

Variants

• Rare cases present with a predominantly radiopaque or mixed-density appearance due to reactive osteoblastic activity, particularly during or after chemotherapy.[18]
• Extramedullary plasmacytoma of the gingiva may show underlying alveolar bone erosion on cross-sectional imaging without a primary intraosseous component.[13]

Medical CT

• On contrast-enhanced CT, plasmacytoma demonstrates marked, homogeneous enhancement, reflecting the lesion's high vascularity. The soft-tissue component is typically of uniform density on unenhanced images.[3]
• CT is superior to panoramic radiography for delineating cortical destruction, residual bone fragments, and the extent of soft-tissue invasion into adjacent spaces.[3][14]

MRI

• Plasmacytoma appears hypointense on T1-weighted and iso- to hyperintense on T2-weighted sequences relative to muscle, with avid contrast enhancement.[19][20]
• MRI is the preferred modality for evaluating bone marrow infiltration and is recommended by the IMWG as the first-choice whole-body imaging technique for staging solitary osseous plasmacytoma to exclude additional marrow lesions.[20]

PET/CT

• ¹⁸F-FDG PET/CT demonstrates hypermetabolic uptake within the lesion and is the imaging modality with the highest reliability for detecting occult sites of clonal plasma cell involvement.[21]
• PET/CT is the first-choice modality for initial and continued evaluation of extramedullary plasmacytoma and serves as an alternative to whole-body MRI for osseous plasmacytoma staging.[1][20]

A solitary, well-defined osteolytic lesion of the posterior mandible in a middle-aged or older adult without internal mineralization generates a broad differential. The absence of a sclerotic border, the presence of cortical perforation with soft-tissue mass, and the lack of root resorption help narrow the possibilities.

The highest-risk diagnostic pitfall is failing to distinguish a solitary jaw plasmacytoma from a jaw manifestation of multiple myeloma, which requires systemic workup including serum/urine protein electrophoresis, free light chain assay, bone marrow biopsy, and whole-body imaging.[1][6]

Microscopic features

• Plasmacytoma is composed of sheets of monoclonal plasma cells with varying degrees of maturation. Well-differentiated tumors show cells with eccentric round nuclei, clock-face chromatin, perinuclear hof, and abundant amphophilic cytoplasm. Less mature (plasmablastic) variants exhibit higher nuclear-to-cytoplasmic ratio, prominent nucleoli, and increased mitotic activity.[26][27]
• Binucleated and multinucleated plasma cells are common; Dutcher bodies (intranuclear immunoglobulin inclusions) and Russell bodies (cytoplasmic immunoglobulin aggregates) may be identified.[27]
• A branching thin-walled vascular pattern (hemangiopericytomatous pattern) may be present.[26]

Immunohistochemistry

• Neoplastic plasma cells are characteristically positive for CD138 (syndecan-1), CD38, MUM1/IRF4, and show monotypic cytoplasmic light chain restriction (kappa or lambda), establishing clonality.[13][26][28]
• CD56 expression is variable; it is more frequently positive in myeloma-associated plasmacytoma than in primary extramedullary plasmacytoma.[27][29]
• CD45 is typically negative or weakly expressed; CD20 is usually negative. Cyclin D1 expression is characteristic of myeloma but is typically absent in primary extramedullary plasmacytoma.[26][28][29]
• Aberrant expression of cytokeratin or myeloid markers may occur rarely and can lead to diagnostic confusion with carcinoma or other neoplasms.[28][30]

Radiologic-pathologic correlation

• The well-defined osteolytic pattern corresponds to the expansile growth of monoclonal plasma cells within the medullary cavity, with osteoclast-activating factors driving bone resorption. Residual bony trabeculae seen on imaging correlate with incomplete bone destruction at the tumor periphery. The marked contrast enhancement reflects the dense cellularity and rich vascularity of the tumor substrate.[3][31]

Molecular considerations

• The 2022 ICC emphasizes the importance of excluding minimal bone marrow infiltration by flow cytometry, as detection of clonal plasma cells in the marrow significantly predicts progression to multiple myeloma.[5][32]
• High-risk FISH abnormalities — particularly del(17p) and gain of 1q21 — in clonal plasma cells from the plasmacytoma tissue or staging bone marrow are independently associated with shorter time to progression to myeloma.[33]

Workup and referral

• Any jaw lesion suspected of being a plasmacytoma requires incisional biopsy for histopathologic confirmation with immunohistochemistry.[6][12]
• Once confirmed, referral to hematology/oncology is essential for systemic staging, which includes serum and urine protein electrophoresis with immunofixation, serum free light chain assay, bone marrow aspirate and biopsy (with flow cytometry and FISH), and whole-body imaging (PET/CT or MRI).[1][6]

Definitive treatment

• Radiation therapy (40–50 Gy in 1.8–2.0 Gy fractions) is the standard of care, achieving local control rates exceeding 90%. For tumors [1][16][34]
• Surgery is considered when structural instability or neurologic compromise from mass effect is present. Combined radiation and surgery has been associated with improved survival in large database analyses.[1][35]
• Systemic therapy is not standard for solitary plasmacytoma but may be considered for patients at high risk of progression.[1][34]

Prognosis and surveillance

• The principal long-term concern is progression to multiple myeloma, which occurs in approximately 50–76% of SPB patients over extended follow-up. The risk is 14–38% within the first 3 years. Extramedullary plasmacytoma carries a substantially lower progression risk.[1][2][16][20][36][37]
• Risk factors for progression include osseous (vs. extramedullary) location, persistence of M-protein after treatment, tumor size >5 cm, minimal marrow involvement detected by flow cytometry, and high-risk cytogenetic abnormalities.[2][16][32][33][36]
• Surveillance every 3–6 months with serum protein studies and free light chain assay is recommended, with yearly imaging using the same modality as at diagnosis for at least 5 years.[1][20]
• Median overall survival for SPB is approximately 106 months; skull and mandible lesions have been associated with relatively favorable outcomes among osseous sites.[35]

Distinction from multiple myeloma at presentation

• A jaw plasmacytoma may be the initial presentation of systemic multiple myeloma in a substantial minority of cases; in one jaw series, 43% of SPB patients eventually converted to myeloma. Solitary plasmacytoma with ≥10% clonal bone marrow plasma cells is reclassified as active multiple myeloma and requires systemic therapy.[1][4][7]

Flow cytometry for risk stratification

• Multiparameter flow cytometry of the bone marrow detects occult clonal plasma cells in approximately half of patients with apparent SPB; flow-positive patients have a markedly higher rate of progression to myeloma compared with flow-negative patients. This finding has been incorporated into the 2022 ICC as a key prognostic consideration.[5][32]