One of the first things I did once the shock of my breast cancer diagnosis wore off was start to worry about what my diagnosis meant for my two sisters and my daughter. My doctor told me that their risk had increased by 50%.
How could I tell them that they had a 50% chance of getting cancer? With three first degree relatives to worry about and a fifty-fifty chance for breast cancer, it seemed inevitable that we would soon be a family with two or more breast cancer patients.
Fortunately, before I called them, I did some more reading and learned that I had made a common statistical mistake. I had confused absolute risk with relative risk. To know my relatives’ real risk of breast cancer, I needed to understand their initial risk and add 50% to that. (The 50% figure my doctors gave me was based on my type of cancer and my medical history. Only your own doctor can tell you how your diagnosis affects your family members’ risk.)
In the United States about one in eight women will have breast cancer sometime during her lifetime, so a better way to think about my daughter’s and sisters’ risk would be to take their lifetime risk of about 12% and add 50% of 12% to get a risk factor of about 18%. That’s a huge difference between thinking that the odds were even for getting breast cancer or staying well, and realizing that there was a less than 20% chance they would get it. I could imagine a room with five women with the same risk as my relatives and see that four of them would be just fine.
Dr. Susan Love in Dr. Susan Love’s Breast Book points out that I made another statistical error in my estimate. The one in eight number is the life-time risk for all women, and it includes women who have many risk factors. Dr. Love says that what I should have been starting with is the 3.3% risk for women with no risk factors and adding 50% of that. Since my family members do have some other risk factors, I’m comfortable with my down and dirty stats. The important thing to me was that I went from assuming that it was almost inevitable that at least one of them would get breast cancer to realizing they had a better than 80% chance of staying well.
I had to learn to do the same kind of calculations when making treatment decisions. If the doctor tells you that a certain treatment reduces your risk of recurrence by 50%, that number is meaningless unless you know what your original risk of recurrence was. If your original recurrence risk was 80%, a 50% reduction means that your risk of recurrence is now 40%. That is a huge difference, and it would be worth taking a difficult course of treatment for those results.
However, suppose your risk of recurrence without that treatment was 10%. A 50% reduction would be 5%. Your risk of recurrence would go from 10% to 5%. That’s significant too, but if the course of treatment is difficult and has potentially dangerous side effects, you might decide you’ll stick with your 10% risk and deal with recurrence if it happens.
Statistics are complicated. Here are some suggestions for understanding the stats that affect you.
- When doctors or studies start throwing around percentages, make sure you know what the starting number is. It’s easy to get confused if you don’t know the beginning point. My local hospital had a 100% mortality rate among all the inflammatory breast cancer patients they had ever treated. That’s scary stuff, but knowing that there had only been two previous patients with my diagnosis at that hospital makes a huge difference in how to interpret that information. Have the doctor restate the information in more than one way until you are sure you understand it.
- Know how your individual case may be different from the overall statistics. For example, Dr. Love reports that a 2003-04 American Cancer Society shows 141 age-adjusted cases of breast cancer per 100,000 women per year in Caucasian women, but only 58 per 100,000 for American Indians and Alaska natives. You need to consider your age as well. There is a one in 2,152 chance of getting breast cancer in the next ten years if you are in your twenties and a one in 23 chance of getting breast cancer in the next ten years if you are in your seventies. If you are considering a specific treatment, be sure that the people in the studies that state its risks and benefits are as similar to you as possible in type of cancer, age, and other demographic factors.
- Understand that statistics don’t predict what will happen to you. They are a snapshot of a group of people in the past. You are unique. There is no study with people exactly like you. Use statistics to give you some information about ways you can reduce your risk for breast cancer such as exercising and avoiding alcohol. Use them to help you make treatment decisions. But don’t let statistics scare you.