Usefulness of 18F-FDG PET/CT in treatment-naive patients with thymic squamous cell carcinoma

Objective Thymic squamous cell carcinoma (TSCC) is very rare. This study aims to investigate the clinical utility of fluorine-18-fluorodeoxyglucose positron emission tomography/computed tomography (F-18-FDG PET/CT) in treatment-naive patients with TSCC. Methods The tumor metabolic parameters of F-18-FDG PET/CT, including maximum standard uptake value (SUVmax), metabolic tumor volume of primary lesion (MTV-P) and combination of primary lesion and metastases (MTV-C), and total lesion glycolysis of primary lesion (TLG-P) and combination of primary lesion and metastases (TLG-C) were collected. Age, sex, smoking, serum tumor markers, tumor size, Masaoka-Koga stage, TNM stage, contrast-enhanced CT scan, and tumor immunity were also reviewed. Moreover, progression-free survival (PFS) and overall survival (OS) of these patients were analyzed. Results Forty-two treatment-naive patients with TSCC were enrolled in this study. All primary tumors were FDG-avid with the average SUVmax of 10.0 +/- 4.5 (range, 1.5-20.4). Higher SUVmax, MTV-C, and TLG-C were observed in advanced Masaoka-Koga stage than early stage, and higher SUVmax was found in advanced TNM stage than early stage. Next, 36 out of 42 patients performed chest contrast-enhanced CT scan, which showed SUVmax associated with the enhancement degree of CT. Moreover, 27 out of 42 lesions were assessed tumor immunity, and the detective rates of PD-L1, PD-1, CD4, CD8, and Foxp3 were 59.3%, 37.0%, 59.3%, 100%, and 77.8%, respectively. Higher SUVmax was observed in lesions with lower CD4-positive tumor-infiltrating lymphocytes. Furthermore, 12- and 24-month PFS and OS rates were 62.0% vs 32.8% and 84.5% vs 68.9%, respectively. Multivariate Cox regression analysis showed that only MTV-C was an independent predictor of PFS. Conclusion F-18-FDG PET/CT is useful in evaluating tumor staging, assessing CT enhancement degree, and detecting tumor immunity of TSCC before treatment. F-18-FDG PET/CT could also be a promising tool to provide prognostic information for treatment-naive patients with TSCC.