Doctors in Sweden have shown for the first time that although endometriosis is associated with an increased risk of various cancers, this risk does not depend on the number of times women with the condition have given birth.
Dr Anna-Sofia Melin, told the 23rd annual meeting of the European Society of Human Reproduction and Embryology today (Monday 2 July): "Several epidemiological studies have shown an increased cancer risk among women with endometriosis, especially ovarian cancer. Infertility and never having given birth (nulliparity) are also known risk factors for different types of cancer, such as breast and endometrial cancer. However, as far as we know, this is the first study to investigate cancer risk among women with endometriosis that also stratifies for parity."
Dr Melin, a specialist doctor in the department of obstetrics and gynaecology at the Karolinska University Hospital in Stockholm, Sweden, and epidemiologists at the Karolinska Institute looked at data from 63,630 women who had been discharged from hospital with a diagnosis of endometriosis between 1969 and 2002. They were identified and followed up via the National Swedish Inpatient Register, the National Swedish Cancer Register and the Swedish Multi-Generation Register.
The researchers identified 3,822 cases of cancer amongst the women with endometriosis. While they found no overall increased risk of cancer, they did find that the women had an elevated risk of certain types of cancer, but that there was no significant difference in risk between women who had never given birth and those who had.
"We found that, contrary to what one might expect, endometriosis and nulliparity did not combine to give a higher risk of cancer," said Dr Melin.
The researchers found that endometriosis increased the risk of developing ovarian cancer by more than a third (37%) above the risk for the normal population of women without endometriosis. There were similar increases in risk for endocrine tumours (38%), kidney cancer (36%) and thyroid cancer (33%). Slightly lower increases were found for brain tumours (27%) and malignant melanoma (23%), and there was a small increased risk of breast cancer (8%). Interestingly, women with endometriosis had a reduced risk of cervical cancer of just under a third (29%).
"There was no significant difference between nulliparous and parous women with endometriosis regarding cancer risk for any of the cancer types. We found a non-significant decrease in ovarian cancer risk the more children a woman had had," said Dr Melin.
Little is known about the possible mechanisms involved in the increased risk of cancer from endometriosis (or decreased risk, in the case of cervical cancer).
Dr Melin said: "The fact that our study did not show an association between cancer risk and parity increases the possibility that it is the endometriosis disease in itself that causes the cancer and not the infertility issue.
"Various theories have been suggested. For ovarian cancer it might be the exposure of the ovaries to the hostile endometrium cells that 'invade' the ovary during the course of the endometriosis disease. Or it could be defects in the immune system that allow the endometriosis to grow and also might allow cancer cells to grow in different parts of the body. Maybe the treatment of endometriosis, hormonal or surgical, can influence cancer development. We do not know yet."
Dr Melin plans to investigate whether hormonal or surgical treatment of endometriosis might be involved in the increased cancer risk, and she also wants to identify which women are more at risk of developing cancer than others.
But she said that it was too early to use the results of her study to give advice to doctors about improved treatments for women with endometriosis.
"Our hope is that doctors in general start to view the endometriosis disease as a serious disease that causes a lot of suffering to the patient and also may lead to cancer. We hope that in the future we will be able to identify those women with endometriosis that may have a more aggressive form of disease with more atypical cells, for instance, and that this may lead to better care for the patient and, hopefully, to a early diagnosis if cancer should occur," she concluded.
Materials provided by European Society for Human Reproduction and Embryology. Note: Content may be edited for style and length.
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