Cohort Study of X-Ray Fluoroscopy and Breast Cancer: An Example

The data in Table 4-6 (see Chapter 4) are taken from a cohort study of radiation exposure and breast cancer. As part of their treatment for tuberculosis, many of the women received substantial doses of x-rays for fluoroscopic monitoring of their lungs. Because the women were followed for highly variable lengths of time, it would not have been reasonable to calculate directly the risk of breast cancer; to do so requires a fixed length of follow-up or at least a minimum follow-up time for all the women in the cohort. (They could have calculated the risk of breast cancer for segments of the follow-up time using the life-table method described in Chapter 4.) Instead, the investigators measured the incidence rate of breast cancer among these women with x-ray exposure. They compared this rate with the rate of breast cancer among women treated during the same period for tuberculosis but not with x-rays. The data in Table 4-6 show that the women who received x-ray exposure had nearly twice the incidence rate of breast cancer as the women who did not receive x-ray exposure.