Hormone Replacement Therapy and Breast Cancer: The Risks in Perspective
by Patricia T. Kelly, Ph.D.
Janet M., a fifties-something woman, entered my office and
said as she sat down, "I've read that if I take hormones I'll
increase my breast cancer risk. I'm going crazy without sleep and
with these mood swings, but I don't want to increase my breast
cancer risk by taking hormones."
Like many women, Janet had heard that a recent study, the
Women's Health Initiative (WHI), definitively showed that hormone
replacement therapy (HRT) increases breast cancer risk. Janet,
like most people, didn't realize that this study found no
statistically significant increase in breast cancer risk to women
who took HRT.
When differences are not significant, an increase in risk
may well be due to other factors, not the one being studied, such
as HRT use. As often happens when a medical story is reported,
the emphasis was on the increase in risk, not whether the
increase was likely to be due to the agent being studied or to
the size of the risk.
The actual size of a risk is important in any woman's
decision making process. In this case the risk was exceedingly
small -- only 8 in 10,000 women a year -- which is 0.08% or eight
hundredths of one percent! Janet was amazed to learn the actual
size of the increase, and said, "You mean I was getting all
concerned for a risk that small!"
"And," I pointed out, "even this very small difference in
risk may not be due to hormone use." I explained that breast
cancers take an average of eight years to reach about half an
inch in size. This means that breast cancers started in the first
year of the study would not be detected for eight or more years.
The study followed women for only about five years, so all or
most of the breast cancers found were probably present in an
undetected state before the study began.
Janet asked if HRT use might have caused some breast cancers
to grow more rapidly and therefore be detected sooner than eight
years. This is unlikely. A number of studies find that breast
cancers in women who were using HRT were not larger and were not
dividing more rapidly than breast cancers in non hormone users.
Since breast cancers grow more slowly in older women and the
average age in this study was 63, breast cancers in this group
would tend to grow more slowly and so take even longer than the
eight year average to be detected.
Women in the WHI study used a particular type of hormone
Prempro. The results of this study therefore do not apply to
other, newer approaches in which more natural hormones are used
and a woman's menstrual cycle is more closely approximated.
Janet was surprised to learn that many studies find that
women who use HRT do not have an increase in breast cancer risk
compared to women who don't use hormones, even when hormones are
used for twenty years. Also, in another large study in which some
women were assigned to take Prempro and others not, women who
used Prempro had no significant increase in breast cancer risk.
As Janet left, she said, "I can see now that when I hear
about a study I need to know how big a risk is and not just that
it is increased. I'll also ask how long a study it was. This
discussion has given me a whole different perspective."
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Patricia T. Kelly, Ph.D. is a medical geneticist who specializes in providing information about cancer risk to
individuals and health professionals. She is affiliated with Saint Francis Memorial Hospital in San Francisco.
Information about her book, Assess Your True Risk of Breast Cancer, can be found on her web site: ptkelly.com.