Risk of hypothyroidism in older breast cancer patients treated with radiation

Hypothyroidism is a potential complication after radiation therapy (RT) when treatment fields include the thyroid. The authors assessed risk of hypothyroidism in breast cancer patients who were receiving RT to a supraclavicular field, which typically includes a portion of thyroid.

The authors identified 38,255 women (aged>65) without history of hypothyroidism from the Surveillance, Epidemiology, and End Results (SEER)-Medicare cohort diagnosed with stage 0-III breast cancer from 1992-2002 and 111,944 cancer-free controls. The authors compared hypothyroidism incidence among irradiated patients with 4+positive lymph nodes (4+LN, surrogate for supraclavicular RT) and no positive nodes (0 LN, surrogate for no supraclavicular RT), nonirradiated patients, and controls. Proportional hazards models tested associations of LN, RT, and breast cancer status with hypothyroidism.

The 5-year incidence of hypothyroidism was identical (14%) in irradiated patients with 4+LN, 0 LN, and nonirradiated patients (P=.52). After adjusting for sociodemographic and clinical characteristics, hypothyroidism risk was not increased in irradiated patients with 4+LN versus 0 LN (hazard ratio(HR)=1.04; 95% confidence interval (CI), 0.89-1.23). However, all patients, regardless of RT status, were more likely to be diagnosed with hypothyroidism compared with cancer-free controls (HR=1.21; 95% CI, 1.17-1.25).

Development of hypothyroidism is fairly common in older breast cancer survivors. Although supraclavicular irradiation does not appear to amplify risks, further studies on the role of routine thyroid function monitoring in all breast cancer patients regardless of treatment status may be warranted, given the excess risks compared with the general population.