Thymic epithelial tumors: Can FDG PET help in predicting histologic type and stage?

Objectives 1)To study the utility of FDG PET in predicting histology and differentiating low risk from high risk types. 2)To study the utility of FDG PET in predicting tumor stage and differentiating early from advanced stage disease.

Methods Patients with thymic epithelial neoplasia who underwent a baseline FDG PET/CT study were included in this retrospective study.Standardised uptake value (SUV)max of the thymic neoplasms was correlated with the histologic type according to the World Health Organisation (WHO) classification and with the Masoka-Koga staging system(1,2).Patients with WHO type A1,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 Masoka Koga Stage I and II disease were grouped as early stage and those with stage III and IV as advanced stage.Mann Whitney U test was used to compare the differences in SUVmax between the three groups (low risk,high risk and thymic carcinoma) and also between the early and advanced stages.

Results The study included 40 patients (M/F- 27/13),median age-49 yrs(range 28-83).Histologically 7 type A, 5 type AB,11 type B1,8 type B2,1 type B3 and 8 type C tumors were seen.13 patients were diagnosed as stage I,7 as stage II,9 as stage III and 5 as stage IV.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 of tumors with advanced stage disease showed a higher trend compared to those with early stage but the difference was not significant (p=0.167).

Conclusions FDG PET can differentiate thymic carcinomas from rest of the thymoma subtypes (3). It can also provide valuable information in differentiating high risk from low risk thymomas. Advanced stage disease shows a definite trend towards higher metabolism as compared to early stage disease.