Odds of Cancer

This discussion isn’t as easy as being able to say “here’s a dose and here’s the effect of that dose.” Our current medical knowledge does not allow us to identify what causes a cancer, so a radiation-induced cancer doesn’t look any different than the same cancer caused by other possible agents. We do know that radiation-induced cancers do not appear until at least 10 years after exposure (for tumors) or 2 years after exposure (for leukemia). This time after exposure to possible cancer formation is called the “latent period.” The risk of cancer after exposure can extend beyond this latent period for the rest of your life for tumors or about 30 years for leukemia. With cancer, it’s about the risk or odds (probability) of getting cancer. We make probability decisions every day based on how we feel about something—“What are the chances I’ll get a raise if I go ask the boss for one?” “I wonder if I can fit one more stop in on the way home and still be on time.” “If I have another piece of candy, will I gain weight?” Based on how you’re feeling and what the benefits might be, you weigh the odds and make a decision.

With radiation and cancer, it is much the same type of question, but in this case it looks like “What is the chance of cancer from this radiation dose?” Whether the chance is “big” or not is a personal decision—based on your thinking about radiation and whether you think there is a benefit to you. Using the doses on our original chart, here is the percentage increase in the chance of getting cancer (keeping in mind that our everyday, ordinary risk is about 42 percent or odds of 1 in 2.4 without radiation exposure):

  Number of Cancers That Occur over a Lifetime in a Population of One Million People Odds of Cancer (natural occurrence)
Cancer Baseline 420,000* 1 in 2.4
POSSIBLE INCREASE IN CANCER CAUSED BY EXPOSURE TO RADIATION ABOVE THE NATURALLY OCCURRING BACKGROUND
Dose # Possible Cancers if One Million People Receive That Dose Combined Odds of Cancer (natural occurrence + the additional risk if you receive the dose in column 1**)
1 rem 421,700 1 in 2.4
10 rem 437,000 1 in 2.3
100 rem 590,000 1 in 1.7
1,000 rem A person would die before cancer could occur  

*Average male plus female lifetime incidence (http://srab.cancer.gov/devcan)

**Adapted from ICRP 2007, Appendix A, Table A14 (17 percent/Sv [or 17 percent/100 rem] incidence)