Hypothyroidism after 3-dimensional conformal radiotherapy and intensity-modulated radiotherapy for head and neck cancers: prospective data from 2 randomized controlled trials.

BACKGROUND The purpose of this study was to determine the incidence of hypothyroidism after chemoradiation in head and neck squamous cell cancer (HNSCC). METHODS One hundred twenty-two patients treated with 3-dimensional conformal radiotherapy (3DCRT; 70 Gy/35#) or intensity-modulated radiation therapy (IMRT; 66 Gy/30#) in 2 identical simultaneous randomized trials were studied. Thyroid function was assessed at baseline and every 3 to 6 monthly thereafter. Development of subclinical (thyroid-stimulating hormone [TSH] >4.67 μIU/mL) or biochemical (T4 <4.5 μg/dL) hypothyroidism was noted. Multivariate analyses were done to determine the factors associated with hypothyroidism. RESULTS At a median of 41 months, 55.1% of the patients developed hypothyroidism (39.3% subclinical, 15.7% biochemical). The IMRT arm had higher subclinical hypothyroidism (51.1% vs 27.3%; p = .021) peaking around 1 year postradiotherapy in both arms. Younger age, hypopharynx/larynx primary, node positivity, higher dose/fraction (IMRT arm), and D100 were statistically significant factors for developing hypothyroidism. CONCLUSION Postradiotherapy hypothyroidism peaks at 1 year. Higher dose per fraction possibly led to greater incidence of hypothyroidism in the IMRT cohort.