January 28, 2008
Ovarian Cancer
Cancer of the ovaries occurs in about 1 in 50 women over the age of 50 or so. While not that common, the disease is troublesome because it is difficult to diagnosis early and most forms of ovarian cancer are highly malignant, spreading quickly. Thus there has been an interest in detecting the causes of ovarian cancer and perhaps preventing it.
January 28, 2008
Most of these studies are case-control designs. Here women with diagnosed ovarian cancer are interviewed about possible risk factors. Their answers are compared with answers to the same questions by women without ovarian cancer. These types of studies provide the most of the evidence we have, but are fraught with difficulties: For example, people with a particular condition may remember past events they think might have caused their illness more frequently than people without the condition or illness. A bias (recall bias) results, distorting the true picture and creating a false association and a misleading risk factor.
In this week’s issue of The Lancet evidence is summarized from 48 different case-control and prospective studies that included questions about past or present use of oral contraceptives. The evidence is coherent and consistent across all studies and in the two research designs (case control and prospective): Women who reported using oral contraceptives had a lower risk of developing ovarian cancer. The risk ratio was 0.42 (about a 60% reduction in risk) for women who had reported using oral contraceptives for 15 years or more. Of great interest, the protective effect of taking oral contraceptives early in life, persists for years after a woman has stopped taking them. A podcast by one of the authors and a Lancet editor is available free.
Implications
For an individual woman:
As ovarian cancer is relatively rare, the chances of benefit for any one woman are small. For example if 250 women took oral contraceptives for 10 years, 1 case of ovarian cancer would be prevented.
There are hazards of oral contraceptives - mainly deep vein thrombosis (clots in the veins in the legs and abdomen) which may lead to pulmonary emboli and death, but these events are extremely rare especially with the newer low estrogen dose contraceptive pills. Oral contraceptive use has also been linked, but not strongly, with other cancers, particularly breast and cervical cancer - although the latter is most likely confounded (misleading) because we now know that it is caused by several papilloma viruses, themselves associated with more frequent sex (and now potentially preventable with the new vaccines).
And there are benefits. Oral contraceptives are the most effective form of birth control and prevention of unwanted pregnancy and even more unwanted abortions, (both of which carry substantially higher health risks than oral contraceptives).
For society:
The widespread use of oral contraceptives in high and middle-income countries means that millions of women are taking them. Thus even if the benefits of risk reduction for a single woman are small, for all women, as many as 300,000 cases of ovarian cancer will be prevented by use of oral contraceptives. This is an argument for making oral contraceptives more easily available throughout the world. Indeed, The Lancet editors have called for making the pill available over the counter, without a physicians prescription.
Cautions
This research was carefully done by experienced epidemiologists. Nonetheless, the fundamental problem with cancer prevention research on humans, is that we have to rely on non-experimental information. Thus, it remains possible that some other factor, unknown to the women or to the researchers, may have led women who were unlikely to ever get ovarian cancer to be more likely to use oral contraceptives, and women, more likely to get ovarian cancer, less likely to use them. We have no way of knowing if this bias exists.
Warnings
Women with previous deep vein thrombi, migraines, heart disease or liver disease should probably not take oral contraceptives, or at least should discuss this with their physicians.
http://en.wikipedia.org/wiki/Ovarian_cancer
http://www.nlm.nih.gov/medlineplus/ovariancancer.html
http://podcast.thelancet.com/audio/lancet/2008/9609_26january.mp3
