Thyroid V40 is a good predictor for subclinical hypothyroidism in patients with nasopharyngeal carcinoma after intensity modulated radiation therapy: a randomized clinical trial

BACKGROUND: Hypothyroidism (HT) and subclinical HT after radiotherapy is frequent in nasopharyngeal carcinoma (NPC) patients, results in negative impact on patients' quality of life. The percentage of thyroid volume receiving more than 40 Gy (V40) ≤ 85% was reported to be a useful dose constraint to adopt during intensity-modulated radiation therapy (IMRT) planning. This study aims to verify whether V40 ≤ 85% can be used as an effective dose constraint in IMRT planning in a randomized clinical trial. METHODS: This single-center 1:1 randomized clinical trial was conducted in Fujian province hospital between March 2018 and September 2022. All patients were treated with IMRT and randomized to induction chemo followed by concurrent chemo-IMRT or concurrent chemo-IMRT alone. Ninety-two clinically NPC patients were included in this study. The thyroid function tests were performed for all patients before and after radiation at regular intervals. Thyroid dose-constraint was defined as V40 ≤ 85%. The primary outcome in this study was subclinical HT. RESULTS: Median follow up was 34 months. Significant difference in the incidence of subclinical HT between the thyroid dose-constraint group and unrestricted group was observed (P = 0.023). The risk of subclinical HT in the thyroid dose-constraint group was lower than that in the unrestricted group (P = 0.022). Univariate and multivariate cox regression analysis indicated that thyroid dose-constraint was a protective effect of subclinical HT (HR = 0.408, 95% CI 0.184–0.904; HR(adjusted) = 0.361, 95% CI 0.155–0.841). CONCLUSION: V40 ≤ 85% can be used as an effective dose constraint in IMRT planning to prevent radiation-induced subclinical HT.