2165 - Clinical Course and Dose-Volume Analysis of Radiation-Induced Hypothyroidism after IMRT for Cervical Esophageal Cancer

Purpose/Objective(s): Radiation-induced hypothyroidism (RIHT) is a well-recognized late adverse event when the thyroid gland is included in the irradiation field. Although most previous studies have focused on head and neck cancers, the thyroid gland is often in closer proximity to the target volume in cervical esophageal cancer. However, detailed analyses of RIHT specifically in cervical esophageal cancer are limited. This study aimed to evaluate the clinical course and dosimetric predictors of RIHT following intensity-modulated radiation therapy (IMRT) for cervical esophageal cancer.

Materials/Methods: We retrospectively analyzed 51 patients with cervical esophageal cancer who underwent definitive IMRT at our institution between 2011 and 2023 and had post-treatment thyroid function tests available. Patients who underwent thyroidectomy for local recurrence before the onset of RIHT were excluded. The date of the last thyroid function test was defined as the censoring date. In patients who received immune checkpoint inhibitors (ICIs) for recurrence, the initiation date of ICI therapy was considered the censoring date. RIHT was graded according to CTCAE version 5.0. The cumulative incidence of RIHT was estimated using the Kaplan–Meier method. Dosimetric parameters were compared between patients with grade 2 RIHT and those without using the Mann–Whitney U test. For dose–volume analysis, patients with grade 0–1 RIHT were limited to those with a thyroid function follow-up period of =18 months.

Results: The median age was 68 years (range, 23–84 years), and clinical stages I/II/III/IV were observed in 13/8/14/16 patients, respectively. The median prescribed dose was 60 Gy in 30 fractions (range, 60–70 Gy). The median thyroid function follow-up duration was 31.4 months (range, 2.3–114.9 months). Grade 1 and grade 2 RIHT occurred in 15 and 26 patients, respectively. The median time to the development of grade 2 RIHT was 13.5 months (range, 3.0–48.7 months), and the median thyroid-stimulating hormone (TSH) level at the initiation of levothyroxine was 18.5 mIU/L (range, 5.5–147.6 mIU/L). The cumulative incidence of RIHT at 2 and 3 years was 71% and 81% for grade 1 and 46% and 49% for grade 2, respectively. The mean thyroid dose (Dmean), V50, and V60 were significantly higher in patients who developed grade 2 RIHT (median Dmean: 56 Gy vs 59 Gy; median V50: 85% vs 98%; median V60: 29% vs 54%).

Conclusion: RIHT frequently occurs after IMRT for cervical esophageal cancer and may develop relatively early after treatment, highlighting the importance of regular thyroid function monitoring. Thyroid Dmean, V50, and V60 may serve as predictive dosimetric parameters for grade 2 RIHT.