Science has proved that mothers and babies benefit from
breast-feeding in all sorts of ways, and now researchers at the
Fred Hutchinson Cancer Research Center have shown that this
practice can reduce the risk of a particularly aggressive form
of breast cancer.
"It's a form more common among younger women
and African-American women," said Amanda Phipps, a scientist in
the public health division of the Seattle cancer center and lead
author of a report published Monday in the journal Cancer. "We
wanted to find out what puts women at risk for this."
The aggressive cancer type is called "triple negative" breast
cancer because the tumor doesn't respond to a critical
regulatory protein known as "HER2" or two key hormones, estrogen
and progestin, which also are fundamental to many hormone-based
treatments for breast cancer.
"That's why we can't use hormonal treatments on these
tumors," Phipps said.
Triple-negative breast cancer is an aggressive cancer
estimated to be involved in anywhere from 15 percent to 30
percent of all breast cancers, she said. Little is known about
why some women are at higher risk of developing this form of
cancer.
Phipps and her colleagues at Fred Hutchinson examined two
groups of women from 55 to 79 years old with and without a
diagnosis of breast cancer. They compared 1,476 women without
the cancer with 1,140 women with several different forms of
breast cancer including the "triple negative," the most common
"luminal" form and another subtype associated only with the HER2
protein.
The researchers then examined these women with respect to
their reproductive health histories including breast-feeding
practices, onset of menstruation and menopause. All of these,
Phipps said, provide indicators of the hormone levels over time
for these women.
"We've known for a long time that breast-feeding reduces your
risk of breast cancer, and that it was probably related to
hormones," she said. But there has been little specific
research, she said, on which hormones might affect the risk of
these more aggressive forms of breast cancer.
By comparing the molecular nature of these breast cancer
types with each woman's reproductive history, Phipps and her
colleagues found that breast-feeding for at least six months
corresponded with a lower risk of both the common luminal form
as well as the triple-negative form of breast cancer. They also
found that early onset of menstruation (before age 13) was
associated with a higher risk of the HER2 breast cancer.
Late onset of menopause and the use of hormone treatments
were found to increase the risk of luminal breast cancer. They
found no risk differences associated with the number of children
or the mother's age at first birth.
While it is common knowledge that female hormone levels
change with the onset of menstruation or menopause, and so may
be related to hormonally influenced cancer risks, it is not
fully understood why breast-feeding would be correlated with
this cancer-hormone link.
"One possible explanation is that while women are
breast-feeding, they aren't menstruating and so their hormones
aren't cycling," Phipps said.
So the more women breast-feed, she said, the less chance
their hormones may have to trigger a cancer. Another theory, she
said, is that breast-feeding alters breast cell structures in
ways that make them less prone to develop into cancer cells.
The findings support the overall view that breast-feeding can
reduce a woman's risk of cancer, Phipps said. More importantly
for scientists, she said, it shows that different reproductive
health behaviors have different effects on various forms of
breast cancer.
"It helps explain why some women are at higher risk and also
why certain therapies are not effective against these more
aggressive forms of breast cancer," she said.
