Early post-treatment (18)F-FDG PET/CT for predicting radiation-induced hypothyroidism in head and neck cancer

BACKGROUND: Radiation-induced hypothyroidism (RIHT) is a common, but underestimated, late adverse effect in head and neck cancer. We investigated the value of early post-treatment (18)F-fluorodeoxyglucose (FDG) positron emission tomography/computed tomography (PET/CT) for predicting RIHT. METHODS: We searched our institutional database for patients aged ≥ 20 years who had undergone definitive radiotherapy for nasopharyngeal or oropharyngeal cancer between 2005 and 2017, followed by (18)F-FDG PET/CT within 180 days of radiotherapy completion. We visually assessed and compared PET/CT and baseline characteristics in patients with and without RIHT using the chi-square test for categorical variables and the t-test for continuous variables. Variable predictive ability was evaluated by measuring the area under receiver operating characteristic curves. RESULTS: Fifty-two patients were included; 22 (42%) developed RIHT and 30 (58%) did not. Two patients presented with diffuse thyroid uptake on PET/CT via visual assessment, and both developed RIHT later. Among the PET/CT variables, thyroid functioning volume was significantly higher in patients without RIHT than in patients with RIHT (16.30 ± 6.03 cm(3) vs. 10.61 ± 3.81 cm(3), p < 0.001). The maximum standard uptake values of the thyroid and pituitary glands did not differ significantly between the groups. Two patient characteristics, pretreatment thyroid volume and mean radiotherapy dose to the thyroid, also showed significant differences between the groups. An algorithmic approach combining visual grading of thyroid (18)F-FDG uptake and thyroid functioning volume cutoff of 14.01 yielded an area under curve of 0.89 (95% confidence interval, 0.80–0.98); the sensitivity, specificity, positive predictive value, and negative predictive value were 87.0%, 82.3%, 80.0%, and 88.9%, respectively. CONCLUSION: Early post-treatment PET/CT-derived thyroid functioning volume was a good predictor of RIHT development. Diffusely increased thyroid (18)F-FDG uptake on PET/CT may indicate impending RIHT. Routine surveillance of thyroid function is warranted in patients at high risk of developing RIHT.