Understanding risk

Experts, researchers and the media sometimes use statistics to describe the chance, or risk, a woman has of getting breast cancer or reducing her chance of getting breast cancer.

All women are at risk for developing breast cancer. As you get older, your risk increases. Assuming you live to age 85, your estimated risk of getting breast cancer over your lifetime is 1 in 8, or about 12.5%. You can also look at it another way: a 12.5% risk means there's an 87.5% chance that you won't get breast cancer.

Risk can be described in two different ways, relative risk and absolute risk. Absolute risk refers to your chance of developing a specific disease over a specified time period. For example, the absolute risk of a woman developing breast cancer by the age of 85 years is one in eight.

Absolute risk is of most value to women trying to determine how some risk numbers might apply to them personally. However, most scientific or medical reports use relative risk.

Relative risk compares two absolute risks and emphasises the difference between the two, rather than the significance of the risk itself.

Relative risk is the number that tells you how much something you do, such as taking hormone replacement therapy (HRT), can change your risk compared to your risk without taking HRT.

As an example, women in their 50s taking hormone replacement therapy (HRT) for five or more years for the management of menopause symptoms could be said to be at 35% increased risk of breast cancer over women who do not use HRT. In other words, the risk has increased from 11 in 1000 women to 15 in 1000 women. This is an increase of 35%, but in real terms it is only 4 extra women in 1000 - or 0.4% absolute risk.*

Over five years, of 1000 women in their 50s who DO NOT take HRT, 11 women may get breast cancer.

Over five years, of 1000 women in their 50s who DO take HRT, 15 women may get breast cancer. In other words, there will be 4 extra cases of breast cancer in HRT users compared to non-HRT users.

* Source: Making decisions: Should I use hormone replacement therapy (HRT)? National Health & Medical Research Council 2005