Ovarian cancer includes cancers of diverse origin that appear in the ovaries.
Ovarian cancer causes more deaths than any other cancer of the female reproductive system. Evidence now points to different types of ovarian cancer with distinct origins, but they remain grouped together because they first become evident in the ovaries.
The risk of a woman developing each type seems to be affected by her reproductive history. Experts have known for some time that the number of children a woman has and her use of contraception can influence her risk of ovarian cancer.
A team led by Dr. Kezia Gaitskell, a pathologist based at the Cancer Epidemiology Unit of the University of Oxford, set out to find out more about different types of the disease.
They used data from the UK Million Women Study - a longitudinal study of 1,146,985 women aged over 50 at recruitment. There were 7,570 incident cases of ovarian cancer recorded after an average of 13 years of follow-up.
Having one child reduces risk by 20%
The risk of the four most common types of ovarian cancer was examined in women with different childbearing patterns: serous, mucinous, endometrioid and clear cell tumors.
Women with one child had a 20% reduction in risk of ovarian cancer overall, compared with women without children, and about a 40% reduction of risk for endometrioid and clear cell tumors. Each additional birth decreased the overall risk of ovarian cancer by an estimated 8%.
Whether or not they had ever breastfed seems to make no difference, although longer durations of breastfeeding appear to mitigate the risk.
- Ovarian cancer is the eighth most common cancer for women
- It is the fifth leading cause of cancer death
- Over 20,000 women a year are diagnosed with ovarian cancer in the US.
Researchers also assessed the risk for women who had undergone surgery to cut or clip their fallopian tubes, a surgical procedure for permanent contraception known as tubal ligation, or sterilization.
The surgery appears to reduce the overall risk of ovarian cancer by 20%. It also decreases the risk of high-grade serous tumors by 20%, the most common type of ovarian cancer, and the risk of endometrioid and clear cell tumors by 50%.
Dr. Gaitskell speculates that tubal ligation may lower the risk by acting as a barrier that prevents the abnormal cells from passing through the fallopian tubes to the ovaries.
She explains that since the discovery that ovarian cancer does not always originate in the ovaries, understanding of the disease has changed radically.
For example, many of the most common, high-grade serous tumors seem to start in the fallopian tubes, while some endometrioid and clear cell tumors may develop from endometriosis.
Conditions that cause infertility may be risk factors
The fact that the significant reduction in risk among women with one child compared with women without children implies an association with certain conditions that cause infertility. Endometriosis, among other conditions, makes it harder for a woman to conceive; these same conditions may also increase the risk of specific types of ovarian cancer.
Dr. Gaitskell comments:
"Our results are really interesting, because they show that the associations with known risk factors for ovarian cancer, such as childbirth and fertility, vary between the different tumor types."
Prof. Charlie Swanton, chair of the 2015 National Cancer Research Institute Cancer Conference, notes that it is important to know what affects the risk of different types of ovarian cancer and the factors that impact it.
He adds that the next step is to understand the mechanisms behind the findings "to develop some way to extend this lower risk to all women, regardless of how many children they have."
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