# Thyroid Dosimetry and Its Association With Radiation-Induced Hypothyroidism in Head and Neck Cancer Patients Treated With Conformal Radiotherapy: An Observational Study

## TL;DR

This study finds that higher radiation doses to the thyroid and certain cancer types increase hypothyroidism risk in head and neck cancer patients.

## Contribution

Identifies specific thyroid dosimetric thresholds and risk factors for hypothyroidism in conformal radiotherapy for head and neck cancer.

## Key findings

- 24% of patients developed hypothyroidism after conformal radiotherapy.
- Dmean > 57 Gy, V55 > 80%, and V60 > 37% are predictive of hypothyroidism.
- Pharyngeal primary and concurrent chemotherapy are significant risk factors.

## Abstract

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.