Role of Combined 18F-FDG-PET/CT for Predicting the WHO Malignancy Grade of Thymic Epithelial Tumors: a Multicenter Analysis

Introduction: To investigate the performance of combined (18)F-FDG-PET/CT as a predictor of the WHO-classification based malignancy grade in thymic epithelial tumors.

Methods: From 05/06 to 02/12, the data of 47 patients with thymic epithelial tumors assessed by (18)F-FDG-PET/CT before being surgically treated were collected in 3 centers and retrospectively reviewed for the purposes of this study. The SUVmax and the SUVmax/T index (the ratio tumor-SUVmax to tumor-size) have been matched with specific subgroups of the WHO-classification: low-risk thymomas (types A-AB-B1), high-risk thymomas (types B2-B3) and thymic carcinomas (type C).

Results: There were 22 men and 25 women (age range: 31-84 yrs). Mean tumor size was 44.7 ± 19.0 mm. The WHO-classification was: type-A #2, type-AB #11, type-B1 #9, type-B2 #9, type-B3 #9 and type-C #7. The SUVmax and the SUVmax/T were found to be predictive factors useful to distinguish thymomas from thymic carcinomas (SUVmax: area under ROC-curve: 0.955, p = 0.0045; SUVmax/T-size: area under ROC-curve: 0.927, p = 0.0022). Moreover, both parameters were found to be correlated with the WHO malignancy grade (low-risk thymomas; high-risk thymomas; thymic carcinoma), Spearman correlation coefficients being 0.56 (p < 0.0001) and 0.76 (p < 0.0001), respectively for the SUVmax and for the SUVmax/T index. In addition, the SUVmax is also significantly correlated with Masaoka stage (Spearman correlation coefficient: 0.30, p = 0.0436) CONCLUSIONS: A significant relationship was observed between (18)F-FDG-PET/CT findings and histologic WHO-classification for this cohort of thymic epithelial tumors. Thus, on the basis of these evidences, we infer that (18)F-FDG-PET/CT may be useful to predict histology and the WHO classes of risk.