Why are serum protein electrophoresis (SPEP) and urine protein electrophoresis (UPEP) important in the work‑up of FDG‑avid thyroid, gastric, and iliac‑bone lesions?

SPEP and UPEP Are Critical for Detecting Plasma Cell Disorders in FDG-Avid Lesions

When FDG-avid lesions are identified in the thyroid, gastric region, and iliac bone, SPEP and UPEP are essential to rule out multiple myeloma or solitary plasmacytoma, as these lesions may represent plasma cell neoplasms that require fundamentally different management than other malignancies.

Why These Tests Matter in Your Clinical Scenario

Diagnostic Imperative for Plasma Cell Disorders

The combination of FDG-avid bone lesions (iliac bone) plus soft tissue involvement (thyroid, gastric) raises immediate concern for:

• Multiple myeloma with extramedullary disease - which carries poor prognosis 1
• Solitary plasmacytoma - which may progress to myeloma in 65-84% of cases at 10 years 1
• Systemic plasma cell disorder requiring immediate hematologic workup

FDG-PET/CT identifies additional plasma cell lesions in 33-55% of patients initially thought to have solitary disease 1. Your patient's multiple FDG-avid sites across different anatomic regions (bone + soft tissue) makes plasma cell disorder a leading differential diagnosis.

The Diagnostic Algorithm

When confronted with multiple FDG-avid lesions:

1. Order SPEP and UPEP immediately - before biopsy if possible

2. Add serum free light chains to increase sensitivity 2

3. Recognize SPEP limitations:

   • Sensitivity only 71% for plasma cell neoplasms in lytic bone lesions 2
   • However, negative predictive value is 94% - excellent for ruling out disease 2

4. UPEP detects monoclonal proteins in 5% of cases where SPEP is negative 3, particularly in:

   • Light chain disease
   • Early/minimal disease
   • Light chain amyloidosis 3

Critical Clinical Pitfalls

Do not rely on SPEP alone - the positive predictive value is only 47% in lytic bone lesions 2. This means:

• Nearly half of positive SPEP results in bone lesions may be false positives
• Always combine SPEP with UPEP and immunofixation 2
• Consider serum free light chain assay as part of the initial panel

FDG-avid lesions correlate with aggressive disease: Larger FDG-avid plasmacytomas (mean 41.4 mm) have higher progression risk than FDG-negative lesions (23.5 mm) 1. Your patient's multiple FDG-avid sites suggest either:

• Multiple myeloma (already systemic)
• High-risk solitary plasmacytoma about to progress

Why This Changes Management Completely

If plasma cell disorder is confirmed:

• Bone marrow biopsy becomes mandatory 1
• Treatment shifts from surgical resection to systemic therapy (chemotherapy/immunotherapy)
• Prognosis assessment requires R-ISS staging, cytogenetics 4
• Radiation therapy may be primary treatment for true solitary plasmacytoma 1

Patients with normal PET/CT at baseline who have solitary plasmacytoma do not develop multiple myeloma 1 - but your patient has multiple lesions, making systemic disease more likely.

The Bottom Line for Your Patient

With FDG-avid thyroid, gastric, and iliac bone lesions:

• Order SPEP, UPEP, serum free light chains, and immunofixation electrophoresis now
• If any are positive → proceed to bone marrow biopsy
• If all negative (NPV 94%) → plasma cell disorder effectively ruled out, pursue other diagnoses
• Do not biopsy without these tests - you need to know if this is systemic disease before planning local therapy

The 10-15 minute delay to order these blood and urine tests could prevent inappropriate surgical intervention on what may be systemic myeloma requiring chemotherapy instead.