What causes breast cancer? Who’s at risk, and why?
These seem like simple questions. But the answers remain vexingly elusive.
Talk to any credible researcher and you’ll come up with a list of risk factors that can be counted on one hand — indicating not only the extent of our knowledge, but also our lack of it.
“Less than 30 to 50 percent of breast cancer can be explained by the known risk factors,” noted Joan Reinhardt Reiss, public policy advocate for San Francisco’s nonprofit Breast Cancer Fund. “So, what about the other 50 to 70 percent — where is it coming from?”
The most commonly suspected source of risk is from environmental exposures to toxins and pollutants — perhaps from industrial dumping or pesticides accumulating in the food chain.
These concerns have spurred vigorous public debate and calls for pre-emptive bans on potentially harmful chemicals.
But meticulously documented scientific evidence of chemical cancer threats to humans is scarce, in part due to the overwhelming scope and depth required for any effective study of environmental exposures.
“Environmental epidemiology is very difficult to do,” said Dr. Angela Prehn, an epidemiologist with the Northern California Cancer Center in Union City, California. “Ideally what you want is from the day a child is born to measure the air, water and food that they intake every day for their entire life. That would be the ideal study. Well, you can’t do that.”
Prehn said it’s only in the past few years that breast cancer research of any sort has received major funding. Some notable efforts looking at environmental exposure include the U.S. government-funded Long Island Breast Cancer Study Project, the research at the National Institute of Environmental Health Sciences (nih.gov/) and Nancy Rubin’s “linear plan” taking shape up in Marin County.
Meanwhile, said Prehn, “you could pull a million things out of a hat.” But she noted that while some chemical risks and associations have turned up in the laboratory, “they are not well-established in human epidemiological studies.”
The risk factors most widely recognized by the scientific community are limited to:
White, non-Hispanic women have the highest risk among the different ethnic groups, followed by African-Americans, then non-white Hispanics and finally Asian women.
Family & health history
A woman with a mother or sister with a history of pre-menopausal breast cancer has an increased risk.
A previous history of breast, ovarian or endometrial cancer indicates a higher risk.
Older age is a risk factor for almost any kind of cancer. Women over the age of 60 are at a higher risk, “and the risk goes up as age goes up. Most women are diagnosed with breast cancer in their 70s. It is not a young-woman cancer, contrary to what you might believe,” said Prehn.
Women who are obese and post-menopausal have an increased risk, but pre-menopausal obesity seems to decrease risk.
There are four major hormonal rushes during a woman’s life — in utero, pre-puberty, during pregnancy and pre-menopause — when exposure to potential environmental risk factors may be decisive.
Stress weakens the human immune system, so “you can certainly hypothesize that stress could be related to any cancer risk … [but] it hasn’t been talked about as being a known risk factor for breast cancer,” said Prehn.
Height is a factor. “Did you know that taller women are at a higher risk?” Prehn asked. There’s a possible connection with estrogen exposure and growth during puberty, but “they don’t know why.”
Early menarche — before age 12 or 13 — and late age at first full-term pregnancy (usually over 30 or 35 years) means a higher risk, as does late age at menopause.
Women who have their first child at a very young age — generally before age 20 — have the lowest risk.
Women who have not breast-fed their children seem to be at an increased risk, said Prehn, which “may have to do with development of breast tissue and cells dividing.”
Women who have not had any children are at a higher risk.
Perversely, if the first pregnancy occurs after age 35, that seems to increase the risk compared with not having children at all, said Dr. Timothy Crowley, director of Marin General Hospital’s Cancer Institute.
One current hypothesis connects a woman’s hormonal cycles and estrogen exposures. “If you’re not getting pregnant, you’re going through more cycles,” Prehn said, also noting that other recognized risk factors like early menarche and late menopause also mean more cycles and more exposures.
“Probably what [these] all have in common is an exposure to estrogen,” Prehn said. “I need to qualify this: There’s nobody out there that thinks estrogen itself causes breast cancer, but there are certain types of breast cancer cells that respond to estrogen. So if there’s a cell that has turned malignant, and there’s estrogen out there, it may promote the growth of the tumor. It didn’t cause the cancer in the first place but it may cause its growing and spreading.”
Childhood radiation exposure
Getting high doses of chest radiation at a young age is a proven risk factor, but is only relevant to medical x-rays, not mammograms. “This is when children were being x-rayed in the ’50s and ’60s,” Prehn said, when much higher radiation doses were the norm.
Breast cancer seems to be a disease of well-educated, affluent women, particularly white non-hispanic women.
“Certainly money doesn’t give you breast cancer,” said Prehn, “and I would hope school doesn’t give you breast cancer, so what is it? … The one thing we know is that women who are well-educated or affluent are generally the women who tend to delay childbearing, and have fewer children also, and we know those are risk factors.”
But whether that is a definitive answer, she said, has yet to be established.