What's the next step for a patient with a history of multiple myeloma and discordant CT (Computed Tomography) and MRI (Magnetic Resonance Imaging) findings, with CT showing multiple lytic lesions in the lumbosacral spine and MRI not showing any lesions?

Management of Discordant CT and MRI Findings in Multiple Myeloma

For a patient with multiple myeloma and discordant CT/MRI findings showing lytic lesions on CT but not on MRI, a PET/CT scan should be performed as the next step to clarify disease status and guide treatment decisions. 1

Understanding the Discrepancy

When faced with discordant imaging findings in multiple myeloma, it's important to understand the different capabilities of each imaging modality:

• CT scans are highly sensitive for detecting lytic bone lesions and cortical bone destruction 1
• MRI primarily detects bone marrow involvement rather than the lytic lesions themselves 1
• PET/CT combines metabolic activity assessment with anatomical detail, providing complementary information 1

Recommended Approach

1. Obtain PET/CT Scan

• PET/CT is recommended as the next step because:
  • It provides both metabolic and anatomical information 1
  • It can help determine if the lytic lesions seen on CT are metabolically active 1
  • It's particularly useful for assessing disease activity and progression 1

2. Consider Bone Marrow Assessment

• If PET/CT results remain unclear:
  • Perform bone marrow aspiration and biopsy at sites of suspected lesions 1
  • Evaluate plasma cell morphology and degree of infiltration 1
  • Determine clonal plasma cell infiltration through flow cytometry or immunohistochemistry 1

3. Evaluate Disease Status

• Assess whether the findings represent:
  • Active disease requiring treatment
  • Residual lesions from previous disease
  • False positive findings on CT

Rationale for PET/CT

The European Myeloma Network guidelines specifically state that "PET/CT may be useful for the better definition of complete or stringent complete response and for the progression of the disease (grade 2B)" 1. This makes PET/CT particularly valuable in your case where there is uncertainty about the disease status.

Clinical Implications

• Lytic lesions on CT: CT is more sensitive than conventional radiography for detecting lytic lesions and can detect lesions with less than 5% trabecular bone destruction 2
• Negative MRI: MRI primarily shows bone marrow infiltration rather than bone destruction, so early lytic lesions might not be visible 1
• Importance of clarification: Accurate assessment is crucial as it determines treatment decisions and monitoring strategies

Common Pitfalls to Avoid

1. Don't dismiss CT findings: CT is highly sensitive for lytic lesions and findings should not be dismissed even if MRI is negative 1, 3
2. Don't rely on a single imaging modality: Each modality has strengths and limitations 1
3. Don't forget subspecialty interpretation: Consider having the images reviewed by musculoskeletal radiologists experienced in myeloma imaging, as studies show this can increase detection accuracy by up to 21% 3

Following this approach will help clarify the discordant findings and ensure appropriate management of the patient's multiple myeloma.