Thyroid Dysfunction Following Exposure to Therapeutic Doses of External Beam Radiotherapy in Head and Neck Cancer

Background: Head and neck cancers are among the most common cancers worldwide and particularly prevalent in India. These malignancies often necessitate a multimodal treatment approach, with radiotherapy being a cornerstone of management. This study aims to investigate the incidence and nature of thyroid dysfunction in patients undergoing external beam radiotherapy (EBRT) for head and neck cancer, highlighting the critical need for routine thyroid function assessments in this population. Methods: A prospective observational study was conducted over two years, from April 2022 to April 2024, at the Department of Radiation Oncology, Government Medical College, Srinagar. The study enrolled 63 newly diagnosed head and neck cancer patients who met specific inclusion criteria. Comprehensive pre-treatment evaluations, including thyroid function tests (TFTs), were performed prior to initiating treatment. Patients received conformal radiotherapy, with careful planning to minimize thyroid exposure, and TFTs were monitored immediately post-treatment and at three-month intervals for six months. Results: The cohort comprised predominantly male patients (79.4%) with a median age of 60 years. The most common histological type was squamous cell carcinoma (87.3%), and advanced stages (III and IV) were prevalent at diagnosis, accounting for 63.5% of cases. A total of 29 patients (46%) developed hypothyroidism during follow-up, with 22 patients (75.9%) diagnosed with subclinical hypothyroidism and 7 patients (24.1%) with overt hypothyroidism. At three months post-radiation, 9 patients exhibited subclinical hypothyroidism, while no cases of overt hypothyroidism were observed. By six months, the numbers increased to 11 for subclinical and 5 for overt hypothyroidism, and by nine months, an additional 2 patients developed both forms. Analysis showed a significant increase in TSH levels over time (p < 0.05), indicating thyroid dysfunction. Although the mean free T4 levels exhibited a non-significant decrease, a notable trend was observed regarding thyroid volume and radiation dose. Patients who developed hypothyroidism had a lower mean thyroid volume (11.83 cm³) compared to the euthyroid group (13.80 cm³). Furthermore, while the mean radiation dose to the thyroid gland was higher in the hypothyroid group (48.40 Gy vs. 45.30 Gy), this difference did not reach statistical significance. However, a higher radiation dose (>60 Gy) correlated with an increased incidence of hypothyroidism, suggesting a potential dose-response relationship. Conclusion: This study highlights a significant incidence of hypothyroidism (46%) in patients treated for head and neck cancer with radiotherapy. The findings suggest that radiotherapy can impair thyroid function through direct tissue damage and vascular injury, necessitating regular thyroid function monitoring in affected patients. Future research should focus on larger, multi-institutional studies with extended follow-up to better understand the long-term impacts of radiotherapy on thyroid health and to develop comprehensive management strategies for this treatment-related side effect.