Dose-volumetric parameters for predicting hypothyroidism after radiotherapy for head and neck cancer. 
To investigate predictors affecting the development of hypothyroidism after radiotherapy for head and neck cancer, focusing on radiation dose-volumetric parameters, and to determine the appropriate radiation dose-volumetric threshold of radiation-induced hypothyroidism.  A total of 114 patients with head and neck cancer whose radiotherapy fields included the thyroid gland were analysed.  The purpose of the radiotherapy was either definitive (n = 81) or post-operative (n = 33).  Thyroid function was monitored before starting radiotherapy and after completion of radiotherapy at 1 month, 6 months, 1 year and 2 years.  A diagnosis of hypothyroidism was based on a thyroid stimulating hormone value greater than the maximum value of laboratory range, regardless of symptoms.  In all patients, dose volumetric parameters were analysed.  Median follow-up duration was 25 months (range; 6-38).  Forty-six percent of the patients were diagnosed as hypothyroidism after a median time of 8 months (range; 1-24).  There were no significant differences in the distribution of age, gender, surgery, radiotherapy technique and chemotherapy between the euthyroid group and the hypothyroid group.  In univariate analysis, the mean dose and V35-V50 results were significantly associated with hypothyroidism.  The V45 is the only variable that independently contributes to the prediction of hypothyroidism in multivariate analysis and V45 of 50% was a threshold value.  If V45 was <50%, the cumulative incidence of hypothyroidism at 1 year was 22.8%, whereas the incidence was 56.1% if V45 was ≥50% (P = 0.034).  The V45 may predict risk of developing hypothyroidism after radiotherapy for head and neck cancer, and a V45 of 50% can be a useful dose-volumetric threshold of radiation-induced hypothyroidism.