How much radiation is too much?

Its because of my interest in this aspect of cancer biology that I felt compelled to review the safety reports released on the TSA website here. However, my interest is not only professional, but also personal. My grandmother died of breast cancer in 2005 after being in remission for 20+ years. While she was never tested for either BRCA1 or BRCA2, her family history indicates that there is a strong probability of one of these mutations running in my family. Including my grandmother, at least four of her siblings developed cancer: two died of breast cancer, one developed a rare form of leukemia and another died of skin cancer. All of her female siblings had cancer, and its noteworthy that her mother died of a very young age (maybe 30's or early 40's) of an unknown (to me) cause. For these reasons, I fear that inadequate safety evaluation of these machines could unduly expose my family (and myself) to levels of radiation that might be harmful should this high familial cancer rate in fact be hereditary...