Postradiotherapy Response Assessment Using 18F-FDG PET/CT in Salivary Gland Carcinoma—A Multicenter Study

Purpose This multicenter study investigates the efficacy of 18 F-FDG PET/CT in postradiotherapy (post-RT) response evaluation in salivary gland carcinoma (SGC). Methods We retrospectively reviewed 115 SGC patients who underwent definitive or adjuvant RT followed by 18 F-FDG PET/CT between 2004 and 2021. Most tumors were parotid gland malignancies (50%). The most common histological subtypes were adenoid cystic (29%) and mucoepidermoid carcinomas (18%). Results The median follow-up was 65 months. Post-RT anatomic images (CT/MRI) revealed complete response (CR) in 51 patients (44%). Among 53 patients with partial response or stable disease, only 17 (32%) patients experienced locoregional recurrence, with a 5-year locoregional control rate of 69%. Post-RT 18 F-FDG PET/CT documented metabolic CR in 81 patients (70%). Metabolic complete responders had significantly higher 5-year locoregional control (90% vs 43%), distant metastasis–free survival (80% vs 48%), progression-free survival (76% vs 24%), and overall survival rates (89% vs 42%) compared with non–complete responders (all P < 0.001), as confirmed in both univariate and multivariate analyses. It identified additional viable tumors in 18 cases (16%) and facilitated salvage local therapies in 7 patients (6%). The sensitivity, specificity, positive predictive value, negative predictive value, and accuracy of post-RT 18 F-FDG PET/CT were 63%, 91%, 70%, 88%, and 84%, respectively, in predicting locoregional recurrence. 18 F-FDG PET/CT showed significantly higher sensitivity (88% vs 36%, P = 0.011) in tumors with pre-RT SUV max ≥7.39 compared with those with SUV max <7.39. Conclusions Post-RT 18 F-FDG PET/CT demonstrates high negative predictive value and specificity, with metabolic CR predicting excellent outcomes. Additionally, it exhibits higher sensitivity for high-SUV max SGC, facilitating early detection of viable tumors.