Thyroid Dosimetry and Its Association With Radiation-Induced Hypothyroidism in Head and Neck Cancer Patients Treated...

Introduction Hypothyroidism is a common side effect in head and neck cancer (HNC) patients treated with radiotherapy (RT). Conformal RT reduces the dose to normal tissues while delivering higher doses to tumors. However, hypothyroidism remains a significant toxicity in these patients. This study evaluates the incidence of hypothyroidism, thyroid dosimetric parameters, and their correlation with radiation-induced hypothyroidism in HNC patients treated with conformal RT at a regional cancer center in India. Methods Fifty patients with histologically confirmed squamous cell carcinoma of the head and neck, treated with conformal RT, and who underwent pre- and post-treatment thyroid function tests were included in the study. All patients were euthyroid before treatment. The thyroid gland was contoured retrospectively in the approved RT plan. The volume of the contoured thyroid gland and the thyroid dosimetric parameters (Dmean, Dmax, Dmin, and V30-V60) were recorded. Pre- and post-treatment thyroid function test results were compared to look for the incidence of hypothyroidism, and dosimetric data were compared to establish a relation with the incidence of hypothyroidism. Results The median age was 53 years. Most patients had oral cavity primaries (60%), stage III/IVA disease (86%), and received definitive (64%) or adjuvant (36%) RT. Two-thirds underwent concurrent chemotherapy. After a median follow-up of four months, 24% developed hypothyroidism, with a median onset of 16 weeks post-treatment. Pharyngeal primary and concurrent chemotherapy were significant risk factors. Dosimetric analysis revealed Dmean > 57 Gy, V55 > 80%, and V60 > 37% as predictors of hypothyroidism. Conclusion Pharyngeal cancers, concurrent chemoradiotherapy, and higher thyroid doses significantly increase the risk of hypothyroidism. Optimizing RT planning is essential to minimize thyroid toxicity, particularly in high-risk patients.