Is thymoma Fluorodeoxyglucose (FDG) Positron Emission Tomography/Computed Tomography (PET/CT) positive?

FDG PET/CT Positivity in Thymoma

Yes, thymomas are typically FDG PET/CT positive, with higher-risk thymomas and thymic carcinomas showing significantly higher SUV values than low-risk thymomas. 1

Patterns of FDG Uptake in Thymic Epithelial Tumors

• Thymomas demonstrate variable FDG uptake that generally correlates with histologic subtype rather than stage 2, 3
• Higher SUVs are more frequently found in high-risk thymoma (type B3), thymic carcinoma, and thymic neuroendocrine tumors than in low-risk thymoma (types A, AB, B1, B2) 1, 4, 2
• Type B3 thymomas show significantly higher FDG uptake compared to types A, AB, B1, and B2 2, 5
• Thymic carcinomas demonstrate the highest FDG avidity, with SUVmax values typically >6.3 3, 5

Diagnostic Utility of FDG PET/CT in Thymoma

• FDG PET/CT is useful for distinguishing between thymoma subtypes and thymic carcinoma, but has limited value in determining tumor invasiveness or stage 1, 2
• With an optimal cut-off SUVmax value of 5.6, the sensitivity, specificity, and accuracy for diagnosing thymic carcinoma versus thymoma are 0.75,0.80, and 0.79, respectively 5
• FDG uptake patterns can help differentiate thymic tumors:
  • Low-risk thymomas: Lower SUV values, often with heterogeneous uptake 3, 5
  • High-risk thymomas: Moderate to high SUV values 2, 5
  • Thymic carcinomas: Highest SUV values, often with homogeneous uptake pattern 5

Factors Affecting FDG Uptake in Thymomas

• Tumor size shows significant correlation with SUVmax (r = 0.60, p < 0.001) 5
• Expression of hypoxia-inducible factor-1α (HIF-1α) shows moderate association with SUVmax 5
• Glucose transporter-1 (GLUT-1) expression increases with higher-grade thymic tumors but doesn't always directly correlate with FDG uptake 5
• Normal and hyperplastic thymus can also be FDG-avid, which can be a confounder in PET/CT assessment of the prevascular mediastinum 1

Clinical Applications and Limitations

• FDG PET/CT is more sensitive than CT alone for detection of mediastinal recurrence of thymoma 1
• A negative FDG PET/CT is helpful in excluding malignancy in prevascular mediastinal masses 1
• Benign thymic cysts can also be FDG-avid, potentially leading to false positives 1
• Combined use of FDG PET/CT and dynamic contrast-enhanced (DCE) MRI can help distinguish prevascular mediastinal solid tumors from one another 1

Alternative Imaging Considerations

• For thymic neuroendocrine tumors, 68Ga-DOTATATE PET/CT may identify more lesions than FDG PET/CT 4
• MRI provides superior tissue characterization of mediastinal masses beyond that of CT and FDG PET/CT 1
• MRI can help differentiate low-risk from high-risk thymomas, thymic carcinoma, and lymphoma by DCE pattern and diffusion-weighted imaging 1

In clinical practice, while thymomas are generally FDG-avid, the degree of uptake should be interpreted in the context of histologic subtype rather than as an indicator of invasiveness or stage.