SAN ANTONIO — Truly large-scale prevention of breast cancer will require the development of a “hormonal vaccine” for young women that mimics the effects of repeated childbearing and breastfeeding, according to a prominent expert in cancer epidemiology.
“It's not research that many people are doing. It's not cutting edge. It won't get into [the journals] Nature or Science. But it will get a Nobel Prize in Medicine,” Dr. Valerie Beral predicted at the San Antonio Breast Cancer Symposium.
More than 1 million new cases of breast cancer occur annually worldwide. It is known that relatively short-term exposure in early adulthood to the hormones of late pregnancy and lactation confers lifelong protection against the malignancy. No other preventive factor can approach the size of this protective effect. But it's utterly unrealistic to expect women in developed countries to revert to such childbearing patterns.
That's why a “hormonal vaccine”—something that could be given to young women for 9 months at a time, perhaps repeatedly, in order to mimic the effects of childbearing on breast tissue—is a must in order to achieve great success in breast cancer prevention. It's badly needed not only in the developed world, but also in the major urban areas of the developing world, where the incidence of breast cancer is climbing rapidly, according to Dr. Beral, director of the cancer epidemiology unit and professor of epidemiology at the University of Oxford (England).
The notion that a major cause of breast cancer is small family size and a lack of prolonged breastfeeding is not new. It recapitulates an observation made by Dr. Bernardo Ramazzini, the Italian physician known as “the father of occupational medicine,” who in the early 1700s described breast cancer as “an occupational disease of nuns.”
“We're all like nuns now,” Dr. Beral said. “Women in developed countries have had few or no children and haven't breastfed. If there were large numbers of women in the West who'd had many children and kept breastfeeding for a long time, we'd see the difference, but we're all like that now.”
Other modifiable risk factors for breast cancer draw a lot of attention, but the best estimates are that even if no women drank alcohol, were obese, or used hormone therapy, the U.S. incidence of breast cancer would drop only moderately, from 180,000 cases annually to 140,000.
“That's a lot, but it's still only about a 20% decrease,” she noted.
Dr. Beral cited World Health Organization data in support of her argument that drastically different childbearing and breastfeeding practices account for the great bulk of variation in breast cancer rates between the developed world, where the cumulative incidence to age 70 years is 6.3%, and rural areas of Asia and Africa, where the figure is just 1.0%.
Modeling studies indicate that if women in developed countries were to adopt the childbearing and breastfeeding practices that are the norm in rural Africa and Asia, their cumulative incidence of breast cancer to age 70 would plunge from 6.3% to 2.7%. Eliminating postmenopausal obesity, alcohol consumption, and hormone therapy would knock the rate down further to 1.6%, which is very close to the rate in the rural undeveloped world.
Genetic studies of breast cancer risk grab headlines. But when investigators at the University of Oxford–based Million Women Study (www.millionwomenstudy.org
“It's not a huge variation in risk. It's not as big as people perhaps might have wished to find,” she continued.
And that observation led Dr. Beral to what she stressed was the most important point of her plenary lecture: Few women in developed countries are at low risk of breast cancer.
“One in 10 women in developed countries will get breast cancer by age 80. The reason that 1 in 10 does and the other 9 don't is largely chance. The people who get it are just unlucky, and the ones that don't are lucky. There is, of course, some variation due to genes and other things, but the predominant factor is luck,” she said.
The Oxford-based Collaborative Group on Hormonal Factors in Breast Cancer, which meets every 5 years to analyze pooled data from roughly 100 epidemiologic studies conducted worldwide, has shown that a woman's breast cancer risk drops by about 10% for each live birth. Only term births count: Miscarriages and induced abortions have no impact on risk. It takes about 10 years for the preventive effect to appear, and then it persists for life.
What is it about term pregnancies and lengthy breastfeeding that confers delayed but subsequently lifelong protection against breast cancer? It's not just the elevation in estrogen and progestins. The Collaborative Group and others have shown that oral contraceptives and hormone therapy are associated with increased breast cancer risk during their use and soon after, but a few years later the increased risk is gone.