What is the most likely diagnosis for an asymptomatic adult patient with normal blood workups and a solitary lytic lesion in the inferior right occipital bone?

Most Likely Diagnosis: Solitary Plasmacytoma

For an asymptomatic adult patient with normal blood workups and a solitary lytic lesion in the inferior right occipital bone, the most likely diagnosis is solitary plasmacytoma of bone. 1, 2, 3

Diagnostic Reasoning

Why Plasmacytoma is Most Likely

• Solitary plasmacytoma characteristically presents as a lytic skull lesion in asymptomatic patients with normal laboratory studies, distinguishing it from multiple myeloma which requires end-organ damage (hypercalcemia, renal failure, anemia, or multiple lytic lesions). 4, 1

• The occipital bone location is a recognized site for solitary plasmacytoma, with documented cases presenting as painless, solitary lytic lesions in otherwise asymptomatic patients. 5, 3

• Normal blood workups effectively exclude symptomatic multiple myeloma, which requires either serum M-protein ≥3 g/dL and/or bone marrow plasma cells ≥10% plus clinical manifestations (anemia, hypercalcemia, renal failure, or multiple lytic lesions). 4

Key Distinguishing Features from Multiple Myeloma

• A single asymptomatic lytic bone lesion with normal labs does NOT meet criteria for symptomatic multiple myeloma, which requires evidence of end-organ damage attributable to plasma cell disorder. 4

• The International Myeloma Working Group specifically notes that in patients with a single asymptomatic lytic bone lesion, benign conditions including bone cysts or angiomas should be considered, and advanced imaging (CT or MRI) is helpful for differential diagnosis. 4

• Patients with apparent solitary plasmacytoma who have bone marrow plasmacytosis >10% are reclassified as having multiple myeloma, but this requires bone marrow biopsy for confirmation. 2

Alternative Diagnoses to Consider (Less Likely Given Clinical Context)

• Giant cell tumor of bone can present as an expansile lytic occipital lesion in young adults (typically ages 20-40), but these are usually symptomatic with localized tenderness and palpable swelling. 6

• Metastatic disease is less likely in a truly asymptomatic patient with normal blood workups and no known primary malignancy, though breast, lung, renal, and thyroid cancers characteristically produce lytic skull metastases. 7

• Ewing's sarcoma typically presents in children and adolescents with systemic symptoms (intermittent high fever) and local pain, making it unlikely in an asymptomatic adult. 8

• Intraosseous meningioma is rare and usually exhibits osteoblastic (not purely lytic) activity, though purely lytic variants have been documented. 9

Mandatory Next Steps for Definitive Diagnosis

Immediate Imaging Requirements

• Obtain MRI with gadolinium contrast immediately to assess bone marrow involvement, soft tissue extension, and intracranial complications—this is the gold standard for skull base lesions. 1, 2

• Perform CT with bone windows concurrently to evaluate cortical bone detail and identify the precise extent of osseous destruction. 1, 2

• Complete whole-body low-dose CT (WBLD-CT) to exclude additional lytic lesions that would indicate multiple myeloma rather than solitary plasmacytoma (Grade 1A recommendation). 1, 2

Essential Laboratory Workup

• Serum protein electrophoresis with immunofixation to detect monoclonal protein. 1, 2

• Serum free light chain assay to identify abnormal kappa/lambda ratios. 1, 2

• Complete blood count, serum calcium, creatinine, and albumin to exclude end-organ damage from multiple myeloma. 2

Tissue Diagnosis (Mandatory Before Treatment)

• Unilateral bone marrow aspiration and trephine biopsy with flow cytometry is mandatory to detect occult bone marrow involvement—flow cytometry detects occult disease in 49-68% of apparent solitary plasmacytoma cases, and these patients have dramatically higher progression rates to multiple myeloma (71-72% versus 8-12.5%). 2

• Image-guided biopsy of the occipital lesion (CT-guided) is essential for definitive histologic diagnosis before treatment planning. 1

Critical Pitfalls to Avoid

• Do not assume this is benign without excluding plasma cell neoplasms and metastatic disease through appropriate laboratory and imaging studies—solitary lytic skull lesions require systematic exclusion of malignancy. 1

• Do not rely solely on normal blood workups to exclude plasmacytoma—up to 30% of solitary plasmacytomas have no detectable serum M-protein, and bone marrow evaluation with flow cytometry is mandatory. 2

• Do not skip bone marrow biopsy—occult marrow involvement detected by flow cytometry dramatically changes prognosis and treatment approach, converting apparent solitary plasmacytoma to multiple myeloma. 2

• Do not perform any surgical intervention until malignancy is excluded—this can complicate definitive surgical management if the lesion proves malignant. 1