The news last week that ASA could reduce the risk of breast cancer either recurring or of causing death grabbed plenty of headlines, including here at CTV.ca. 

But before you rush out to stock up on Aspirin or other forms of acetylsalicylic acid, it's worth noting that the study had limitations, and they’re limitations that could make all the difference.

The data for the study came out of the Nurses Health Study II. This is a huge study that began in 1989 to continue the work of the first Nurses Health Study, started in 1976. The studies are pretty amazing in their scope, really. They’ve tracked the health of tens of thousands of female nurses for decades, asking them a huge list of questions about their lifestyle habits and health issues every two years. Perhaps because the participants have been nurses, most have stuck with the study for years (Its participation rate is an impressive 90 per cent). So the NHS is considered the most definitive long-term epidemiological study conducted to date on older women's health.

While the original study was meant to track the effects of birth control pills and smoking on women’s health, the study has continued and the data collected has been used for more than 265 published scientific papers to date.

This paper looked at 4,164 nurses aged 30 to 55 who were diagnosed with stage I, II, or III breast cancer between 1976 and 2002, and included some women who regularly took ASA.

In statistician speak, the study is called a prospective cohort epidemiological (or observational) study. By that, they mean that the study followed over time a group of similar individuals (in this case: women who were nurses, living in the U.S., who were aged 30 to 55 when they were recruited) who differed by a certain characteristic (for example, nurses who smoke and those who do not  or those who regularly take ASA or who don’t) and compared them for a particular outcome (in this case: breast cancer).

This kind of study is often considered the best kind of observational study. They're superior to retrospective cohorts (in which people are asked to try to recall their intake of ASA over time, for example) and they are great for studying how a wide range of different behaviours are linked to different diseases.

But epidemiology studies are not controlled experiments, which are really the gold standard of studies.

If this study were a randomized controlled trial, it would involve recruiting two groups of women, randomly assigning one half to receive ASA and the other half to receive a placebo, but keeping the two groups otherwise as identical as possible.

That would be great for looking for short-term effects of the ASA; if there were difference in the two groups, the researchers could be pretty sure that it was the ASA that was the game-changer. But of course, a controlled study is not feasible when tracking health effects that can take years or decades to develop, such as breast cancer; one can’t control behaviours for decades.

So the problem with observational studies is that there are often other factors that could be affecting the results.

Dr. John Glaspy, a breast cancer specialist at UCLA's Jonsson Comprehensive Cancer Center, noted in a comment to the Los Angeles Times last week that researchers have been “tricked” by epidemiology studies before. He cited a study that showed a clear statistical link between drinking coffee and developing lung cancer. The problem was that heavy coffee drinkers also turned out to be heavy smokers. When you removed the data from the heavy smokers, the coffee-cancer link disappeared.

So is it fair to conclude that this latest study found that regular ASA intake cuts the risk of breast cancer recurrence? No, because we can’t be sure. What the researchers found more accurately was a LINK between ASA and reduced breast cancer recurrence and death.
 
It could be that the type of patient who would regularly take doses of ASA each day is also the kind of patient who would take their post-surgery breast cancer hormonal treatments, which could be the real reason for the lowered death risk.

"We know in the treatment of breast cancer, only 60 per cent will take their medication long-term, as directed by their oncologist," Dr. Mark Clemons, a medical oncologist with the Ottawa Hospital noted to CTV News.

Or it may be that women regularly taking ASA are more likely to go for regular checkups with their doctors. Or maybe, the link is due to something no one has yet thought of.

Clemons notes the study's findings are "possibly intriguing" but need to be considered against other studies that have been done on this topic.  He notes some retrospective studies have shown that ASA may reduce breast cancer while many others show that it doesn't seem to.

That’s why more study is needed. Doctors want to better understand the mechanism of how ASA could be affecting cancer. (The prevailing theory is that it's ASA's anti-inflammatory effects at play, since chronic inflammation has been linked to cancer.)

They also need to understand how much ASA, taken for how long, might bring benefits. And of course, they need to know if there are drawbacks to regular ASA intake. (Plenty of people can’t take ASA because it increases the risk for bleeding. Others can’t take it because it interferes with other medications they're on.)

According to Eric Jacobs, the American Cancer Society strategic director in “pharmacoepidemiology”: "It would be premature for breast cancer survivors to use Aspirin in order to reduce risk of breast cancer recurrence or of dying from their disease. Cancer survivors, as well as other adults, should discuss the risks and benefits of Aspirin use with their doctor."

So stay tuned.