Utility of 18F-fluorodeoxyglucose positron emission tomography for distinguishing between the histological types of early stage thymic epithelial tumours.

OBJECTIVES Recent studies have shown the usefulness of (18)F-fluorodeoxyglucose positron emission tomography (FDG-PET) in differentiating between World Health Organization (WHO) histological subgroups of thymic epithelial tumours. However, these reports may have included many advanced cases of these lesions (Masaoka stage III or IV) of high-risk subtypes. The objective of our present study was to assess the usefulness of FDG-PET for distinguishing the histological subtypes of early stage thymic epithelial tumours (Masaoka stage I or II). METHODS Twenty patients who had undergone an FDG-PET examination prior to treatment and had been diagnosed with an early stage thymic epithelial tumour between July 2005 and July 2011 were enrolled in the present study. All patients underwent a total thymectomy. This cohort was divided into two groups according to the WHO histological classification of their lesion, i.e. low-risk tumours (types A, AB or B1) in one group and high-risk tumours (types B2, B3 or thymic carcinoma) in the other. Focal FDG accumulation was evaluated by determining the maximum standardized uptake value (SUV-max). RESULTS The patient cohort for this study included 13 men and 7 women ranging in age from 26 to 70 years (mean 55 years). The low-risk group included seven cases (type A, 0; type AB, 7; type B1, 0), and the high-risk group comprised 13 cases (type B2, 7; type B3, 3; thymic carcinoma, 3). The SUV-max values of the low-risk and high-risk tumours were 3.09 ± 0.51 and 6.19 ± 3.13, respectively, and this was a significant difference. For the differential diagnosis of low-risk and high-risk tumours, sensitivity and specificity were 92.3% and 83.3%, respectively, when an SUV-max of 3.5 was used as a cutoff. CONCLUSIONS FDG-PET is a useful method for distinguishing histological types of early stage thymic epithelial tumours.