Radiation-induced hypothyroidism after IMRT for nasopharyngeal carcinoma: Clinical and dosimetric predictors in a prospective cohort study.

OBJECTIVES

To investigate the rate and risk factors for developing hypothyroidism (HT) in nasopharyngeal carcinoma (NPC) patients treated with intensity-modulated radiotherapy (IMRT).

MATERIALS AND METHODS

A total of 135 consecutive patients treated with IMRT for NPC were prospectively evaluated during a median follow up of 34.1months. Serum thyroid function assessments before and after IMRT were periodically monitored. To identify risk factors for HT occurrence, univariate and multivariate Cox regression analyses were performed.

RESULTS

Thirty-nine patients (28.9%) developed primary HT. The 2- and 3-year incidences of primary HT were 29.6% and 43.9%, respectively. The median clinical latency for primary HT was 15.1months (3.2-33.8months). No cases of central HT were observed. Univariate and multivariate analyses revealed that the risk increased with younger age and decreased with higher pretreatment thyroid volume. Patients with thyroid mean dose ≥45Gy had a 4.9 times increased risk of HT than those receiving lower mean dose. Alternatively, the thyroid V45 below 0.5 and V50 below 0.35 were found to significantly lower the incidence rate of HT.

CONCLUSION

The incidence of primary HT after IMRT for NPC continued to increase with time. The thyroid mean dose constraint was approximately 45Gy. We recommended plan optimization objectives to reduce thyroid V45 to 0.5 and V50 to 0.35.