[¹⁸F]-FDG Uptake as a Marker of Residual Anaplastic and Poorly Differentiated Thyroid Carcinoma following BRAF-Targeted Therapy

BACKGROUND AND PURPOSE: Neoadjuvant BRAF-directed therapy and immunotherapy followed by surgery improves survival in patients with BRAF^V600E-mutant anaplastic thyroid carcinoma (ATC), more so in those who have complete ATC pathologic response. This study assesses the ability of FDG-PET to noninvasively detect residual high-risk pathologies including ATC and poorly differentiated thyroid carcinoma (PDTC) in the preoperative setting.

MATERIALS AND METHODS: This retrospective, single-center study included consecutive BRAF^V600E-mutant patients with ATC treated with at least 30 days of neoadjuvant BRAF-directed therapy and who underwent FDG-PET/CT within 30 days before surgery. The highest pathologic grade observed for every head and neck lesion resected was recorded. Each lesion on preoperative PET/CT was retrospectively characterized. The primary end point was to contrast the standardized uptake normalized by lean body mass (SULmax) for lesions with residual high-risk (ATC, PDTC) versus low-risk pathologies (papillary thyroid carcinoma, negative). An optimal SULmax threshold was then identified by using a receiver operating characteristic analysis, and the ability of this threshold to noninvasively and preoperatively risk-stratify patients by overall survival was then evaluated with a Kaplan-Meier plot.

RESULTS: Thirty patients (mean age 66.5 ± 9.0; 17 men) were included in this study, with 94 surgically sampled lesions. Of these lesions, 57 (60.6%) were low-risk (39 negative, 18 papillary thyroid carcinoma) and 37 (39.4%) were high-risk (29 ATC, 8 PDTC). FDG uptake was higher for high-risk compared with low-risk pathologies: median SULmax 5.01 (interquartile range [IQR] 2.81–10.95) versus 1.29 (IQR 1.06–3.1) (P < .001, Mann-Whitney U test). The sensitivity, specificity, and accuracy for detecting high-risk pathologies at the optimal threshold of SULmax ≥2.75 were 0.784 [95% CI, 0.628–0.886], 0.702 [95% CI, 0.573–0.805], and 0.734 [95% CI, 0.637–0.813], respectively. Patients with at least 1 high-risk lesion identified with the aforementioned cutoff had a worse prognosis compared with patients without high-risk lesions in the head and neck: median overall survival for the former group was 259 days and was not attained for the latter (P = .038, log-rank test).

CONCLUSIONS: Preoperative FDG-PET noninvasively identifies lesions with residual high-risk pathologies following neoadjuvant BRAF-directed targeted therapy and immunotherapy for BRAF-mutated ATC. FDG-PET avidity may serve as an early prognostic marker that correlates with residual high-risk pathology in BRAF-mutated ATC after neoadjuvant therapy.