Relationship between the maximum standardized uptake value of ¹⁸F-FDG PET/CT and WHO pathological classification and Masaoka stage of thymic epithelial tumors

Objective To investigate the relationship between the maximum standardized uptake value (SUV_max) of preoperative ¹⁸F-FDG PET/CT and the World Health Organization (WHO) pathological classification and Masaoka stage of thymic epithelial tumors.

Methods A total of 40 patients (14 males and 26 females ranging in age from 32 years to 79 years) was retrospectively reviewed with histologically proven thymic epithelial tumors who underwent ¹⁸F-FDG PET/CT before surgical resection at the First Affiliated Hospital of Nanjing Medical University from September 2007 to March 2019. SUV_max was measured. The patients were divided into three groups in accordance with a simplified pathological classification scheme WHO (2015): low-risk thymomas (types A, AB, and B1), high-risk thymomas (types B2 and B3), and thymic carcinomas (type C). In addition, all tumors were divided into three groups on the basis of the Masaoka stage: stages Ⅰ, Ⅱ, and Ⅲ. The area under the curve (AUC) calculated via receiver operating characteristic (ROC) curve analysis was used to estimate the best value of SUV_max that was capable of discriminating thymomas from thymic carcinomas. Groups were compared by using the Mann-Whitney test or Kruskal-Wallis test.

Results A total of 11 low-risk thymomas (1 type A, 4 type AB, and 6 type B1), 15 high-risk thymomas (10 type B2 and 5 type B3), and 14 thymic carcinomas were identified. Eight, 17, and 15 patients were in Masaoka stages Ⅰ, Ⅱ, and Ⅲ, respectively. The median SUV_max value was 3.78 for low-risk thymomas, 5.21 for high-risk thymomas, and 10.44 for thymic carcinomas and was significantly different among groups (Z=3.088, −3.928, 4.106; all P<0.01), the difference in SUV_max between the 3 groups is statistically significant (χ²=26.716, P<0.01). The values for Masaoka stages Ⅰ, Ⅱ, and Ⅲ were 3.74, 5.14, and 10.08, respectively, and showed significant differences when compared with each other (Z=2.680, 3.679, −3.644; all P<0.01), the difference in SUV_max between the 3 groups is statistically significant (χ²=22.295, P<0.01) . The results of ROC curve analysis showed that the AUC of SUV_max was 0.953 (95% confidence interval: 0.891–1.000, P<0.01). SUV_max=6.81 was the best threshold for the differential diagnosis of thymomas and thymic carcinomas.

Conclusions SUV_max measured by ¹⁸F-FDG PET/CT had a good correlation with the pathological classification and Masaoka stage of thymic epithelial tumors. Therefore, it can provide reference value for planning clinical treatment.