What is the role of PET (Positron Emission Tomography) CT in the diagnosis and management of inflammatory myositis?

PET-CT in Inflammatory Myositis: Evidence-Based Recommendations

Primary Role: Cancer Screening in High-Risk Patients

PET-CT should be considered specifically for cancer screening in adult-onset inflammatory myositis patients at high risk for malignancy when initial comprehensive screening has not identified an underlying cancer. 1

When to Use PET-CT for Cancer Screening

High-Risk Patients After Negative Initial Screening:

• Consider 18F-FDG PET-CT for patients classified as "high risk" for IIM-related cancer when basic and enhanced cancer screening panels have not detected malignancy at diagnosis 1
• This is particularly valuable when lymphoma is suspected, as PET-CT has demonstrated effectiveness in detecting hematologic malignancies 1

Anti-TIF1γ Antibody-Positive Dermatomyositis:

• PET-CT can be used as a single screening investigation for patients with anti-TIF1γ antibody-positive dermatomyositis who are >40 years old with ≥1 additional high-risk clinical feature 1
• This approach may facilitate earlier cancer diagnosis and reduce the need for multiple separate investigations 1

Risk Stratification Framework

The 2023 International Myositis Assessment and Clinical Studies Group (IMACS) guidelines provide a structured approach 1:

High-Risk Category (requiring PET-CT consideration):

• Patients with ≥2 high-risk factors (IIM subtype, autoantibody profile, or clinical features) 1
• Dermatomyositis carries 25% cancer risk within 0-5 years of disease onset 1

Red Flag Symptoms (consider screening regardless of risk category):

• Unintentional weight loss
• Unexplained fever
• Night sweats
• Family history of cancer
• Smoking history 1

Important Caveats and Limitations

Radiation Risk vs. Benefit:

• Clinicians must balance the increased cancer risk from PET-CT radiation exposure against the potential benefit of cancer detection 1
• This consideration is particularly important in younger patients or those requiring serial imaging

Availability and Cost:

• PET-CT may not be available in all healthcare systems 1
• Cost-effectiveness should be considered, though studies suggest PET-CT is suitable and cost-effective for cancer screening in myositis patients 2

Timing Considerations:

• These recommendations apply only to patients diagnosed within 3 years of IIM symptom onset 1
• Cancer screening should occur at diagnosis for high-risk patients, with basic screening repeated annually for 3 years 1

Secondary Role: Limited Utility for Myositis Diagnosis

PET-CT has limited usefulness for evaluating muscle inflammation in inflammatory myositis due to low sensitivity (33%) and should NOT be used as a primary diagnostic tool for myositis itself. 3

Why PET-CT is Not Recommended for Myositis Diagnosis

Poor Sensitivity for Muscle Inflammation:

• FDG-PET detected increased muscle uptake in only 33% of patients with active inflammatory myopathy 3
• Sensitivity is significantly lower than EMG, MRI, and muscle biopsy for detecting myositis 3
• No significant clinical differences were found between patients with and without FDG muscle uptake 3

Superior Alternatives for Muscle Assessment:

• MRI remains the gold standard for evaluating muscle inflammation, with T2-weighted/STIR sequences being the preferred protocol 4, 5, 6
• MRI provides excellent characterization of muscle edema and fat replacement and is recommended for initial diagnostic workup 4, 5
• Ultrasound may serve as an alternative when MRI is unavailable, though it is less sensitive 4

Potential Role: Interstitial Lung Disease Detection

PET-CT may have emerging utility for detecting and monitoring inflammatory lung activity in IIM-associated interstitial lung disease, though this requires validation in well-designed studies. 2, 3

• FDG uptake in lungs was detected in 7 of 18 patients (39%) with interstitial lung disease 3
• Preliminary data suggest PET-CT could diagnose and measure disease activity in the lung 2
• This application remains investigational and should not replace standard pulmonary function tests and high-resolution CT for ILD screening 1

Practical Algorithm for PET-CT Use

1. Perform risk stratification at IIM diagnosis using subtype, autoantibody profile, and clinical features 1

2. For high-risk patients:

   • Complete basic and enhanced cancer screening panels first 1
   • If no cancer detected, consider 18F-FDG PET-CT 1

3. For anti-TIF1γ-positive dermatomyositis patients >40 years:

   • If ≥1 additional high-risk feature present, PET-CT can be used as single screening investigation 1

4. Do NOT use PET-CT:

   • As a primary tool for diagnosing muscle inflammation 3
   • In children with juvenile dermatomyositis (malignancy is rare) 1
   • Beyond 3 years from IIM symptom onset for cancer screening 1