Our understanding of how to prevent breast cancer is about to get much more precise. Here’s why.
We now know more than we ever have about breast cancer. Screening methods and treatment options have improved at a pace that was unimaginable just a decade ago. Such advances have a real world effect: today, millions of survivors are living longer, healthier lives.
Until very recently, however, the science of breast cancer prevention lagged behind. For years, studies tracking diets and breast cancer rates in large populations yielded inconsistent results. Now, at last, scientists think they might have figured out a major reason why: Breast cancer is not a single disease, but studies have historically treated it like one.
Different Types with Different Causes
That there are different types of breast cancer is a fact that doctors who treat the disease know very well. Breast cancer that occurs in pre-menopausal women has surprisingly little in common with the breast cancer that occurs in post-menopausal women, so the treatment options are different.
But this has important implications for prevention as well: it’s now clear that these two types of breast cancer have different root causes that are influenced by factors like diet, physical activity and body weight in different ways.
Similarly, breast tumors that contain receptors for estrogen (ER-positive tumors) are different from tumors that don’t have those receptors (ER-negative tumors). The kinds of things that can encourage ER-positive tumors to grow, such as circulating hormones in the blood, do not affect ER-negative tumors.
Accounting for these differences, along with several others, is crucial to the treatment of breast cancer. And now it’s becoming clear that they must be taken into account to develop a meaningful understanding of how to prevent breast cancer in the first place.
Cohorts Bear Traces of 20th-Century Thinking
To appreciate how these differences influence what we know about prevention, think about the kind of breast cancer study that often appears on the news. Most likely it’s a cohort study – one that tracks the diets and breast cancer rates of a large group (or cohort) of women over several years.
It turns out, however, that most of the results on diet and breast cancer that make headlines come from cohort studies that began years, even decades ago – before scientists understood the distinctions between types of breast cancer.
So when you see a study that analyzes, for example, the link between a certain food and breast cancer, researchers may simply be grouping all the women in the study who developed breast cancer together: pre-menopausal, post-menopausal, ER-positive and ER-negative.
High Numbers Needed to Find Diet-Cancer Links
Why does that matter? Simply put, the more cases of breast cancer that occur in a cohort, the more certain scientists can be about any links that turn up between those cases and diet.
That certainty is one reason large cohort studies that have been going on for years routinely make the news. If you read about a cohort study in which there are, say, 1,000 cases of breast cancer after 10 years of follow-up, the researchers can be reasonably certain that a given link did or did not exist.
But wait: that large number of breast cancer cases is misleading. How many of those breast cancer cases were ER-positive, and how many ER-negative? How many of those women were diagnosed before menopause, and how many after? Knowing these numbers would help sort the different types into discrete groups: X number of ER-positive, X number of ER-negative, etc.
As we now know, this focus is very useful, because it means the findings for one group won’t get watered-down or lost completely by mixing them with all the other types. But focusing on one kind or another means we’re not looking at 1000 breast cancer cases anymore, we’re looking at, say, 150 ER-negative, pre-menopausal breast cancers. And that smaller number of cases means less certainty.
So we need large cohort studies that are specifically designed to take the different types of breast cancer into account by asking subjects more detailed questions from the very beginning. And we need to let them run long enough for each of the different categories of breast cancer to show up in significant numbers.
Today’s Scientists Are Retooling the Cohort
Does this mean that all those studies on diet and breast cancer that you’ve been reading about for years are somehow invalid? Absolutely not.
But the fact that they weren’t designed to account for emerging distinctions between breast cancer types does mean that their focus may not be precise enough to find important connections. And when you’re trying to study something as complex and ever-changing as the human diet, that’s not a good thing.
That’s why the scientists who are constructing today’s breast cancer cohort studies – the ones that will select groups of women and follow them into the future – are reengineering their methods by building on what they’ve learned. They start by asking the women in their cohorts questions about their diet and physical activity, just as the older studies did.
But today’s scientists are also collecting detailed information about things like diet during adolescence (because research indicates breast tissues are particularly sensitive to dietary influence during breast development), physical activity (type, intensity and duration) and a host of other factors.
And they are doing so with a focus older cohorts simply could not match, utilizing new technologies (such as blood samples which provide meaningful snapshots of hormonal status) that did not exist before.
In the Meantime…
That’s the good news. The bad news is that these studies are only beginning, and it may take years for these studies to achieve the kind of statistical power necessary to draw conclusions.
In the meantime, the most important thing to keep in mind is this: don’t regard any single study as definitive, no matter how large it is and how much statistical power it packs.
Even in the future, as we continue to refine the study of breast cancer prevention, it will still be necessary to look for agreement among different studies and examine any new results in the context of findings that have come before.
The American Institute for Cancer Research (AICR) is the only major cancer charity focusing exclusively on the link between diet, nutrition and cancer. The Institute provides a wide range of consumer education programs that help millions of Americans learn to make changes for lower cancer risk. AICR also supports innovative research in cancer prevention and treatment at universities, hospitals, and research centers across the U.S. The Institute has provided more than $70 million in funding for research in diet, nutrition and cancer. Visit AICR’s Web site. AICR is a member of the World Cancer Research Fund International.