Role of FDG-PET/CT in Lymphoma, TB Osteomyelitis, Thyroiditis, and Sarcoidosis
FDG-PET/CT is essential for staging and response assessment in FDG-avid lymphomas, while it serves primarily as a problem-solving tool in TB osteomyelitis, thyroiditis, and sarcoidosis due to its ability to detect inflammatory activity but limited specificity in these conditions.
Role in Lymphoma
Staging and Initial Assessment
- FDG-PET/CT is strongly recommended before treatment for routinely FDG-avid, potentially curable lymphomas (e.g., diffuse large B-cell lymphoma [DLBCL], Hodgkin's lymphoma) 1
- Nearly universal FDG avidity at diagnosis in:
- Hodgkin lymphoma
- DLBCL
- Follicular lymphoma (FL)
- Approximately 90% in T-cell lymphoma and nodal marginal zone lymphoma 1
- Advantages over conventional CT:
- Ability to distinguish between viable tumor and necrosis/fibrosis in residual masses
- Can detect additional disease sites, modifying clinical stage in 15-20% of patients 1
Response Assessment
- Visual assessment alone is adequate for interpreting PET findings as positive or negative when assessing treatment response 1
- Timing recommendations for post-therapy assessment:
- At least 3 weeks, preferably 6-8 weeks after chemotherapy/chemoimmunotherapy
- 8-12 weeks after radiation or chemoradiotherapy 1
- Definition of positive scan:
- For residual mass ≥2 cm: activity above mediastinal blood pool
- For smaller residual mass or normal-sized lymph node: activity above surrounding background 1
- Complete response requires:
Pitfalls and Limitations
- False-positive findings can occur due to:
- Rebound thymic hyperplasia
- Infection
- Inflammation
- Sarcoidosis
- Brown fat 1
- False-negative results may occur due to:
- Resolution limitations of equipment
- Technical issues
- Variable FDG avidity among histologic subtypes 1
Role in TB Osteomyelitis
- FDG-PET/CT is not recommended as a first-line imaging modality for TB osteomyelitis
- Can be useful when conventional imaging is inconclusive
- Limitations:
- Cannot reliably differentiate between infectious and non-infectious inflammatory processes
- Cannot distinguish TB from other types of osteomyelitis or malignancy
Role in Thyroiditis
- FDG-PET/CT is not routinely recommended for diagnosis of thyroiditis
- Incidental thyroid uptake on FDG-PET/CT performed for other indications may represent:
- Thyroiditis
- Benign nodules
- Malignancy
- Diffuse uptake typically suggests thyroiditis, while focal uptake warrants further investigation
Role in Sarcoidosis
- FDG-PET/CT can detect metabolic disease activity in clinically active sarcoidosis 2
- Useful for:
- Assessing disease extent, including clinically inapparent sites
- Monitoring response to treatment
- Guiding biopsy site selection 2
- Substantial agreement exists between HRCT and metabolic parameters of disease activity 2
- Limitations:
- Cannot definitively distinguish sarcoidosis from malignancy or other inflammatory conditions
- Cost considerations in developing countries 2
Technical Considerations
- Integrated PET-CT has largely replaced dedicated CT scans in the United States 1
- Use of attenuation-corrected PET is strongly encouraged 1
- For lymphoma response assessment, the 5-point Deauville scale is recommended:
- No uptake above background
- Uptake ≤ mediastinum
- Uptake > mediastinum but ≤ liver
- Uptake moderately > liver
- Uptake markedly higher than liver and/or new lesions 1
Common Pitfalls to Avoid
- Performing PET too soon after therapy (can lead to false-positive results)
- Misinterpreting physiologic uptake (e.g., brown fat, muscle, bowel)
- Failing to consider benign causes of FDG uptake (infection, inflammation)
- Not correlating PET findings with clinical context and other imaging modalities
- Relying solely on SUV values without visual assessment
FDG-PET/CT has revolutionized the management of lymphoma but should be used judiciously in inflammatory conditions like TB osteomyelitis, thyroiditis, and sarcoidosis, where its findings must be interpreted in the appropriate clinical context.