Is endometriosis a risk factor for ovarian cancer?

I often get asked to what degree endometriosis is causing ovarian cancer.

While women with endometriosis appear to be more likely to develop gynaecological cancer, this risk is limited to certain types of ovarian cancer only.

What studies have been conducted?

In a study published in 1997 the risk of cancer after hospitalization for endometriosis was investigated in a sample of 20,686 women. After an average follow-up of 11.4 years, the risk for cancer among these women was elevated by 90% for ovarian cancer, 40% for hematopoietic cancer (primarily non-Hodgkin's lymphoma), and 30% for breast cancer. Being diagnosed at a young age was associated with increased ovarian cancer risk. The study also found women whose site of origin of endometriosis was the ovary had a high risk for ovarian cancer.

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A more recent 2017 study, of over 18 years of follow-up among 102,025 women found that self-reported cases of endometriosis were associated with an 81% increased risk of ovarian cancer, compared to women with no history of this condition. There was no clear association between endometriosis and endometrial cancer.

Why the connection between diseases?

Endometriosis and cancer are two entirely separate diseases and unfortunately, the relationship between the two is not fully understood. Both depend on estrogen for their growth and both seem to be affected by inflammation. In addition, women being treated for endometriosis may also be more likely to experience infertility and either infertility or medication to treat infertility may increase the risk of ovarian cancer. However, we need to be careful not to overstate those risks, because links between the two diseases don't prove that one causes the other. 

The lifetime risk of ovarian cancer of women in Australia is 1.3% and we estimate that 1,580 cases of ovarian cancer were diagnosed in 2017. Endometriosis is mainly connected to endometrioid and clear cell epithelial ovarian cancers – which are less common than other subtypes. 

Unfortunately, as I talked about in my previous blog, screening programs for ovarian cancer are not currently effective. Hormonal treatment of endometriosis can help reduce symptoms of endometriosis and there is a possibility that hormonal treatment, such as the oral contraceptive pill, could also help reduce the risk of ovarian cancer.

In summary, endometriosis only increases the risk of clear-cell and endometrioid cell types of ovarian cancer. The majority of ovarian cancers are not related to endometriosis.

These cell types account for approximately 15% of ovarian cancers (200 of 1500 ovarian cancer patients) per year.

The majority of endometriosis sufferers will never develop ovarian cancer.

The vast majority of ovarian cancer develops after menopause.

The current tools are too imprecise to allow us predict who the sufferers of endometriosis are who are at a high risk of developing ovarian cancer. I therefore, do not advocate for all women diagnosed with endometriosis to proactively reduce their risk of ovarian cancer. If surgical removal of tubes, ovaries +/- uterus is requested to protect from gynaecological cancer, it should be timed after menopause.

Further studies with molecular samples of women diagnosed with endometriosis are required to examine the association of risk further.

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