Endometriosis Does Not Increase Overall Risk of Breast Cancer, Study Finds

Researchers analyzed data from more than 100,000 women over 24 years.

Key Points

Highlight:

• Endometriosis is not associated with higher risk of overall breast cancer.
• The association varies according to the hormone receptor expression status of breast tumors.

Importance:

• These results could instigate further research into the relationship between different types of breast tumors and endometriosis. 

What's done here:

• Researchers analyzed data from the Nurse’s Health Study to investigate the potential relationship between endometriosis and the risk of breast cancer.

Key Results:

• Endometriosis is not associated with overall risk of breast cancer 
• Endometriosis may be associated with breast tumors expressing estrogen receptor (ER) but not expressing progesterone receptor (PR) on their surface.
• The menopausal status of the women at the time of breast cancer diagnosis does not influence the findings.

Limitations:

• Some women grouped as not having endometriosis may be asymptomatic and not know that they have endometriosis. However, the large sample size would mean that the grouping/misgrouping has a minimal effect on the results.

• All women included in the study are registered nurses and not a random sample of all women.

• Only a small proportion of women had tumor cells expressing ER and not expressing PR, so the results should be interpreted with caution.

Lay Summary

Endometriosis is not associated with the overall risk of breast cancer, according to a study published in the journal Obstetrics & Gynecology. 

The condition may, however, be significantly associated with an increased risk of breast tumors that express estrogen receptors (ER) but do not express progesterone receptors (PR) on their surface. But according to the researchers, “given the small sample size among the ER+/PR- group, these results should be interpreted cautiously”.

The relationship between endometriosis and breast cancer has been investigated in many studies with conflicting results; some studies suggested that the condition may increase the risk of breast cancer while others showed no clear association and others still showed that endometriosis may actually decrease the risk of breast cancer. 

But none of the studies investigated whether the relationship between endometriosis and breast cancer is influenced by the different hormone receptors expressed on the surface of tumor cells.

A group of researchers led by Dr. Stacey Missmer wanted to clarify the relationship between endometriosis and breast cancer taking into account hormone receptor expression status and menopausal status of women at the time when they were diagnosed with breast cancer.

In order to do this, they used data from the Nurses' Health Study, one of the largest studies into the risk factors for major chronic diseases in women. The study began in 1989 and includes information about 116,430 registered nurses, ages 25 to 42 about whether or not they have been diagnosed with endometriosis and/or breast cancer as well as general information about environmental, dietary, and lifestyle risk factors associated with both diseases.

During the 24 year period from the study start to 2016, 5,389 women (five percent of all women included in the study) were diagnosed with endometriosis and 4,979 women (three percent of all women) were diagnosed with breast cancer. 

Using statistical analyses the researchers found that women with endometriosis were not at higher risk for overall breast cancer compared to women without endometriosis, before or after menopause. 

When they looked in more detail into the hormone receptors expressed by the tumors of women diagnosed with breast cancer, the researchers found that there was no association between endometriosis and breast tumors expressing both ER and PR on their surface, or breast tumors not expressing either receptor on their surface. 

However, they found that women with breast tumors expressing ER but not expressing PR on their surface were twice as likely to also have endometriosis.

The authors concluded that endometriosis is not associated with risk of overall breast cancer, but that tumors expressing ER but not PR may be associated with endometriosis.