Thymic epithelial tumors: Can fluorodeoxyglucose positron emission tomography help in predicting histologic type and stage?

Objectives: To study the utility of fluorodeoxyglucose (FDG) positron emission tomography (PET) in predicting (1) the World Health Organization (WHO) histologic type and differentiating low-risk from high-risk types. (2) Tumor stage and differentiate early from advanced stage disease. Materials and Methods: Patients with thymic epithelial neoplasia who underwent a pretreatment FDG-PET study were included. Tumor maximum standardized uptake value (SUVmax) was correlated with the WHO histologic type and also with the Masaoka-Koga (MK) staging system. Patients with WHO Type A, AB, and B1 were classified as low risk and those with B2 and B3 as high risk. Thymic carcinomas belonged to Type C. Patients with MK Stage I and II disease were grouped as early stage and those with Stage III and IV as an advanced stage. Differences in SUVmax between the various groups were calculated. Results: The SUVmax of thymic carcinomas was significantly higher as compared to low-risk (P = 0.001) and high-risk groups (P = 0.007). The SUVmax of high-risk group was also significantly higher than the low-risk group (P = 0.002). SUVmax cutoff of 6.5 was able to differentiate thymic carcinomas from thymomas with 100% sensitivity and 87.2% specificity. The SUVmax in patients with advanced stage disease showed a higher trend compared to those with early stage, but the difference was not significant (P = 0.167). Conclusion: PET can differentiate thymic carcinomas from rest of the thymoma subtypes by the virtue of their higher FDG uptake. It can also provide valuable information in differentiating high-risk from low-risk thymomas and in predicting disease stage.