Contribution of preoperative 18F-FDG PET/BT in characterization of thymic epithelial tumor

Purpose: Thymic epithelial tumors (TET), which are extremely rare tumors, are the most common pathology of the anterior mediastinum. According to the WHO histopathological classification system, TET is divided into three groups: low-grade thymoma, high-grade thymoma, and thymic carcinoma (TC). Insufficient evidence has been demonstrated to be included in clinical guidelines regarding the routine use of 18F-FDG PET/CT imaging for staging in TETs. However, it is stated in current guidelines that it can be used in advanced disease, since high FDG uptake can be observed in high-grade thymoma and TCs, and metastases can be found in unexpected areas. In this study, our aim is to evaluate the metabolic information that 18F-FDG PET/CT will provide in thymus originated lesions that are planned to be operated, in the light of the histopathological information obtained after the operation, and thus to determine its contribution to patient management. Material-Method: Between December 2010 and October 2021, a total of 100 patients who underwent preoperative 18F-FDG PET/CT imaging for the purpose of metabolic characterization/staging with the suspicion/diagnosis of benign/malignant tumor originating from the thymus were included in the study. After the benign pathologies originating from the thymus were excluded, there were 73 patients in the TET group. By using visual evaluation, semi-quantitative PET and heterogeneity index (HI-1 and HI-2) parameters, the predictive power of 18F-FDG PET/CT to predict tumor grade according to WHO histological classification system and advanced stage disease according to routinely used staging systems were investigated. In addition, extrathoracic metastases and extrathymic findings obtained by 18F-FDG PET/CT imaging were examined. Results: There were 42 male (58%) and 31 female patients (42%) in the TET group (median age: 52 years). According to the WHO histopathological classification system, 26 patients (36%) had low-grade thymoma, 36 patients (49%) had high-grade thymoma, and 11 patients (15%) had thymic carcinoma. The mean SUVmax value was 5.14 in low-grade thymoma, 6.58 in high-grade thymoma, and 15.41 in TC. SUVmax, MTV and TLG values were statistically significantly higher in the TC group than both thymomas and high grade thymomas in subgroup analysis. When the cut-off value of 7.25 was taken for SUVmax, TC was differentiated from thymomas with 91% sensitivity and 74% specificity. Although lower SUVmax, MTV and TLG values were observed in low grade thymomas compared to high grade thymomas, there was no statistically significant difference. According to the Masaoka-Koga and TNM staging systems, which are the most frequently used for staging purposes, SUVmax, MTV, TLG values were higher in advanced disease than in the early stage disease, both in the whole TET group and within the subgroups defined according to the WHO histopathological classification system. When the cut-off value of 6.34 was taken for SUVmax, the sensitivity and specificity in differentiating advanced disease according to the Masaoka-Koga staging system were 75% and 84%, respectively. Although the discriminative power of HI-1 and HI-2 parameters is higher than other semi-quantitative PET parameters, especially SUVmax, in distinguishing the TC group from thymoma, HI-1 and HI-2 parameters have not been thought to be significantly superior to SUVmax, considering its easy accessibility and widespread use. Conclusion: Visual evaluation of 18F-FDG PET/CT images, when used together with quantitative information, primarily SUVmax, can reliably predict both advanced disease according to routine staging systems and the grade of primary mass according to WHO histopathological classification system in TETs. Therefore, as a promising metabolic imaging method in terms of patient management and prognosis, it makes important contributions to preoperative evaluation in routine clinical practice. Key Words: 18F-FDG PET/CT, Thymic Epithelial Tumors