Absolute Risk and Relative Risk of Developing Breast Cancer

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Breast Cancer

Having several risk factors doesn’t mean that you are likely to develop breast cancer and having low risk factors doesn’t mean that you will never develop it. The term risk describes the number or percentage of the possibility for a certain event to occur. Understanding the risk factors can help you take certain preventive strategies.

Absolute risk describes the probability of an individual to develop breast cancer. When you say that 12% of U.S women or 1 in 8 U.S women will develop breast cancer over the course of a lifetime, it shows the absolute risk. Over an 80-year lifespan, on average, a woman has a 1 in 8 chance of developing breast cancer. On the other hand, the chance of not developing breast cancer is 87.2%, or about 7 in 8.

The younger you are, the lower the risk. For instance:

  • According to breastcancer.org, if your present age is 20, for the next 10 years, the chance of developing invasive breast cancer, is .06% or 1 in 1,732 which means that, 1 in 1732 women in the same age group can expect to develop breast cancer. It can also be said that, if you are in this age group, your chance of developing breast cancer is 1 in 1732.
  • If your present age is 30, for the next 10 years, your chance of developing invasive breast cancer is .44% or 1 in 228.
  • If your present age is 40, for the next 10 years, your chance of developing invasive breast cancer is 1.45% or 1 in 69.
  • If your present age is 50, for the next 10 years, your chance of developing invasive breast cancer is 2.31 or 1 in 43.
  • If your present age is 60, for the next 10 years, your chance of developing invasive breast cancer is 3.49% or 1 in 29.
  • If your present age is 70, for the next 10 years, your chance of developing invasive breast cancer is 3.84% i.e. 1 in 26.

So, this means that when you are younger, the risk of developing breast cancer is low and when you get older, the probability of developing breast cancer increases. That means the older you are, the higher your absolute risk of breast cancer. Remember that these numbers and percentages are averages for the whole population.

Your individual breast cancer risk can be higher or lower based on family history, gender, race, weight, reproductive history, and other factors.

For instance, considering family history, the absolute risk of breast cancer for women who have inherited mutations in the genes known as BRCA1 or BRCA2 is much higher. For women with a BRCA1 mutation, the risk of developing breast cancer is 72% by age 80. This shows that, out of 100 women who have this mutation, 72 of them can expect the chance of developing breast cancer should they live to age 80. The risk is a bit lower for the women who have BRCA2 mutation, it is 69%.

If you have breast cancer, absolute risk can also describe the chance of the outcome of a certain treatment, or the course of the disease. For instance, based on the characteristics of the breast cancer like stage, grade, other test results, your age and medical history, and the treatments you have, your doctor will tell you that your chance of disease-free survival or being alive with no evidence of breast cancer within 5 years is 90%. That means that your absolute risk of having breast cancer come back within 5 years is 10% or 1 in10.

In other words, 1 out of 10 women with similar characteristics and the same treatment plan have the chance of having breast cancer to come back within 5 years, and 9 out of 10 have no chance of recurrence.

Relative Risk provides the percentage or number by comparing one group’s risk of developing breast cancer to another group’s risk. Relative risk is the type of risk that research studies normally use to compare groups of women with different characteristics or behaviours to find out whether one group has a higher or lower risk of breast cancer than the other (either as a first-time diagnosis or recurrence).

Higher Breast Cancer Risk

Studies have shown that many women consuming two or more alcoholic drinks (means 12 ounces of regular beer, 5 ounces of wine, or 1.5 ounces of 80-proof liquor) per day have a higher risk of developing breast cancer.

  • Women who consume two or more alcoholic drinks per day have 50 % higher risk of developing breast cancer when compared to women who do not drink. This doesn’t mean that their risk of developing breast cancer is 50% for the lifetime, it means that the risk is 50 % higher when compared to non-drinkers. This percentage is how you are likely to see relative risk reported by the internet, television, and newspapers.
  • Women who consume two or more alcoholic drinks per day have a relative risk of 1.5 when compared to women who do not drink. This is the number that researchers and scientific papers would usually talk about relative risk. The number “1” is the baseline group i.e. the women who do not drink, since the risk remains the same for them. The .5 is the relative increase in the risk for the other group, it is expressing the 50% higher lifetime risk in another way (50%= .5).

    In other words, the women who have alcoholic drinks per day for two or more times, the risk of developing breast cancer is 1.5 times than women who don’t drink.

Here 50% risk is the risk relative to the people who do not drink. However, it does not tell a women about her lifetime risk if she drinks two or more drinks per day for the rest of her life. It is necessary to multiply the absolute risk of breast cancer (12%) for women in the general population by relative risk (1.5), since women in this group have 1.5 times the risk of developing breast cancer.

  • 12*1.5 = 18. This means that the absolute lifetime risk of developing breast cancer for a women, if she drinks two or more alcoholic drinks per day is about 18%, versus 12% for women who do not drink.

Lower Breast Cancer Risk

  • If you had early breast cancer and lumpectomy, the absolute risk of having the recurrence of breast cancer within 10 years is 35%.
  • According to a study that reviewed 17 clinical trials of radiation therapy after lumpectomy, if you have radiation therapy to the remaining breast tissue, the risk is reduced by about 46%.
  • Compared to a woman who had a lumpectomy alone, within 10 years, your risk of developing cancer is 46% lower if you have radiation therapy after lumpectomy.
  • Medical researchers might say, when compared to the women who do not have radiation therapy, your relative risk of developing breast cancer is 1 – .46=.54. The number “1” remains in the baseline group. Just because it shows the decrease in risk, .46 is subtracted from 1. That means, when compared to the women who have lumpectomy alone, you have 54% of the risk of developing breast cancer again in the same breast as they do within 10 years.
  • To know the difference what radiation therapy really make you to reduce the absolute risk of cancer recurrence in the same breast, you have to multiply the 10-year risk of recurrence without radiation by the relative risk i.e. .35*.54 = 19.
  • This means 19% of absolute risk or just under 1 in 5 of cancer recurrence in the same breast, if you have radiation therapy versus about 35% or just over 1 in 3, if you don’t have radiation therapy. So, fewer than 1 in 9 women can expect the recurrence of breast cancer in the same breast versus more than 1 in 3 women who do not have radiation therapy.

A relative risk of .5 means that your risk goes down by half or 50%, 1.88 means your risk goes up by 88% and 3.0 means your risk has become 3 times or goes up by 300%.

From this, it is obvious that lifestyle factors and treatment options can increase or decrease the risk of breast cancer. As mentioned earlier, when you get older, the risk of developing breast cancer goes up. So, it is better to understand the risk factors and monitor yourself even if your risk factors are low to prevent the occurrence of breast cancer.