Low-dose medical radiation exposure and breast cancer risk in women under age 50 years overall and by estrogen and progesterone receptor status: results from a case-control and a case-case comparison.

Although moderate to high-dose ionizing radiation exposure is an established risk factor for breast cancer, the effect of low-dose radiation exposure has not been clarified by epidemiological data. We evaluated the effect of low-dose radiation from medical procedures on risk of breast cancer overall and by joint estrogen and progesterone receptor (ER/PR) status in 1,742 population-based case patients aged 20-49 years and 441 control subjects identified from neighbourhoods of case patients in Los Angeles County. After excluding radiation exposures in the 5 years prior to case's diagnosis or control's initial household contact date we found an elevated breast cancer risk among women who reported having had multiple chest X-rays (Ptrend=0.0007) or 7 or more mammograms (odds ratio [OR]=1.80, 95% confidence interval [CI]=0.95-3.42). Risk was also increased among women who received dental X-rays without lead apron protection before age 20 years (OR=1.81, 95% CI=1.13-2.90). Women, who had their first exposure to these medical radiation procedures during childhood, had a greater increase in risk than those who were first exposed at older ages. Although not statistically significantly different, risk estimates were somewhat stronger for nulliparous than for parous women. We found no effect modification by ER/PR status. In conclusion, our findings support the hypothesis that low-dose ionizing radiation, and particularly exposures during childhood, increase breast cancer risk.