Relative risk and breast cancer

Relative risk and breast cancer

In my last post, I wrote about alcohol as a possible breast cancer risk factor, with a brief glance at the comparison tool of relative risk.

Relative risk (RR) is a score assigned to a specific factor that might (or might not) affect risk of a particular disease–in this case, breast cancer. RR compares a group who have been exposed to the factor with a group not exposed to the factor: RR = exposed group not-exposed group

The not-exposed group is given the value of 1.0. If the exposed group develops breast cancer at the same rate as the not-exposed group, it appears that the specific factor doesn’t increase risk at all. The RR is 1.0.

For breast cancer, factors with an RR of 1.0 include

  • bras
  • anti-perspirants and deodorants
  • breast implants
  • such common breast changes as cysts and fibroadenomas
  • miscarriage
  • abortion

Before going any further, it’s time to make a few points. First, RR concentrates on one specific factor, with researchers collecting information about whether, and how, large groups of people differ, based solely on their exposure to the factor.

The focus on one factor is RR’s strength–because of that intense concentration–but can also be a shortcoming. After all, individuals combine multiple factors, all with different effects on breast cancer risk. Also, an RR is the average for a group, which probably means some individual variation within the group.

Second, how accurate the RR is comes from the quality of the research behind it. The scientific question asked, the size of the group studied, the type of study, any additional studies which confirm the results, and the professionalism of the researchers all matter a lot. An RR may change over time, if new and convincing research disproves or otherwise changes earlier data. It’s also true that the further away from 1.0 an RR is, the more believable it is that the factor–rather than chance–makes the difference.

Third, it’s clear that each individual comes into this life with certain built-in factors, such as family history. Certain other factors, such as alcohol use and exercise, are called modifiable risk factors because they can be altered. A woman’s thinking about modifiable risk factors often depends on the built-in factors she faces.

Back to the RR categories…

Some factors are actually protective. The group exposed to them develops cancer less often, or later, than the group not exposed. To be considered protective, the factor’s RR must be less than 1.0, perhaps 0.7 or 0.8.

Examples of protective factors, with an RR < 1.0:

  • having a first child before age 20
  • having several children
  • breastfeeding children (the longer the better)
  • consistent exercise after menopause.

Back to the alcohol post of last week. The breast cancer RR for alcohol is greater than 1.0–it does increase risk somewhat even at low levels (even as red wine decreases heart disease risk)–but how high it is depends largely on the level of alcohol use. Thus, the group of women drinking 3-6 drinks a week gets an RR of about 1.07, barely above 1.0. On the other hand, the group of women who drink more than 3 alcoholic beverages a day for several years push into the 1.5 range and above.

Alcohol use and other factors with an RR between 1.1 and 2.0 are still considered relatively mild risks–at least compared with the real biggies. Other mild factors include

  • overweight or obesity after menopause
  • biopsy-proven breast hyperplasia (too many cells, but the cells themselves look normal)
  • Ashkenazi Jewish heritage without altered BRCA1 or BRCA2
  • a close relative with postmenopausal breast cancer (1.8 if it’s a 1st degree relative, such as a mother, sister or daughter, 1.5 if it’s 2nd degree relative, such as a grandparent or aunt)
  • early start of menstrual periods or late menopause
  • no children or first child born after age 30
  • hormone replacement therapy combining estrogen and progesterone.

(Obviously, none of these lists is complete, and more factors are being added all the time.)

Moving along to groups of women at higher risk…

Factors with an RR between 2.1 and 4 are considered to increase risk moderately and include having a history of high-dose chest radiation before age 30 or one 1st degree relative with premenopausal breast cancer

Finally, the high risk factors, with RRs above 4, begin with one all women have: female gender. Compared to men, who are not exposed to this female gender factor, all women face an astronomically-high breast cancer risk. (On the other hand, when it comes to RRs for testicular or prostate cancer….)

Other high risk factors:

  • age over 65
  • altered BRCA1 or BRCA2 genes
  • biopsy-confirmed breast hyperplasia with atypia or DCIS
  • 2 or more 1st degree relatives with premenopausal breast cancer
  • personal history of breast cancer
  • highest level of breast density seen on mammograms after menopause

More research on pesticides and other environmental hazards, dietary factors, and stress is ongoing. The results can’t come soon enough.

Kerry Anne McGinn