What is the role of Positron Emission Tomography (PET) scan in diagnosing suspected multiple myeloma?

Role of PET Scan in Suspected Multiple Myeloma

[18F]FDG PET/CT is recommended as a first-line imaging modality for patients with suspected multiple myeloma, especially when extramedullary disease is suspected or when other imaging modalities are inconclusive. 1

Diagnostic Value of PET/CT in Multiple Myeloma

Initial Diagnosis and Staging

• PET/CT provides both functional and anatomical information by combining metabolic activity assessment with CT imaging of bone lesions 1
• Superior to conventional skeletal survey in detecting bone lesions, particularly in areas difficult to visualize such as spine and pelvis 1
• The International Myeloma Working Group recommends either whole-body low-dose CT or FDG PET/CT for initial diagnostic workup of suspected multiple myeloma 1, 2
• PET/CT is particularly valuable for:
  • Detection of extramedullary disease (sensitivity 96.0%) 3
  • Assessment of intramedullary lesions (specificity 94.1%) 3
  • Distinguishing between metabolically active and inactive disease 2

Specific Clinical Scenarios

• Suspected active multiple myeloma: PET/CT is indicated in all patients regardless of results from other imaging procedures 1
• Non-secretory multiple myeloma: PET/CT is particularly useful due to the inability to monitor disease through serum markers 4
• Solitary plasmacytoma: PET/CT is mandatory to confirm diagnosis and rule out additional lesions when whole-body MRI cannot be performed 2
• Smoldering multiple myeloma: PET/CT is not routinely recommended unless there are equivocal findings on other imaging modalities 1
• MGUS (Monoclonal Gammopathy of Undetermined Significance): PET/CT is not recommended 1

Advantages Over Other Imaging Modalities

• Compared to skeletal survey: PET/CT identifies more lesions and can detect lesions in patients with negative skeletal surveys 1
• Compared to MRI: PET/CT can provide earlier evaluation of response to therapy and better predict outcomes 2
• Unique capability: Can assess both bone involvement and extramedullary disease in a single examination 5

Standardized Reporting Recommendations

A comprehensive PET/CT report for multiple myeloma should include:

• Number and size of lytic lesions on CT
• Presence of fractures
• Exact number of PET-positive focal lesions (grouped as 0,1-3, or >3)
• Maximum standardized uptake value (SUVmax) of the hottest focal lesion
• Assessment of diffuse uptake in bone marrow 1

Prognostic Value

• Baseline PET parameters (presence of extramedullary disease, number of focal bone lesions, and maximum SUV values) have independent prognostic value for progression-free survival and overall survival 5
• Persistence of significant FDG uptake after treatment is an independent negative prognostic factor 5
• PET/CT can be used to assess minimal residual disease outside the bone marrow 2

Limitations and Pitfalls

• False negatives can occur with small lytic lesions (< 5mm) 6
• Recent radiation therapy (within 3 weeks) can cause false positive results 6
• The CT component of PET/CT must be of sufficient quality (equivalent to whole-body low-dose CT) to properly assess bone disease 1
• Standardization of what constitutes a "metabolic complete response" is still needed 5

Algorithm for PET/CT Use in Suspected Multiple Myeloma

1. For suspected active multiple myeloma:

   • PET/CT recommended as first-line imaging along with whole-body low-dose CT 1
   • Particularly valuable when extramedullary disease is suspected 1

2. For inconclusive findings on other imaging:

   • If whole-body low-dose CT or MRI shows equivocal lesions or single small lesions, PET/CT should be performed for clarification 1

3. For treatment response assessment:

   • PET/CT is the preferred functional imaging modality to evaluate therapy effects on myeloma-cell metabolism 2
   • Should be performed at diagnosis, after induction therapy, and before maintenance therapy 5

4. For suspected relapse:

   • PET/CT is valuable for differentiating between post-therapeutic changes and residual/recurrent tumor 6

By providing both anatomical and functional information in a single examination, [18F]FDG PET/CT has become an essential tool in the diagnostic workup, staging, and treatment response assessment of multiple myeloma.