In this study we evaluated the association between endometriosis and MBD, and the risk factors of endometriosis in the case and control groups. We found that women with endometriosis had a lower MBD than those without endometriosis. Age and progesterone usage were the other predictors of MBD.
According to the studies around the world, the rate of diagnosing endometriosis is rising due to the increased awareness of women about the disease, changing social patterns like late marriage, and the widespread use of laparoscopy .
On the other hand, MBD is a potential risk factor for breast cancer. There are several studies that confirm the association between this cancer and MBD [10–13]. The risk of breast cancer according to MBD category varies by studies, a study reported that women with high MBD have two times a higher risk for this cancer . Another study reported a four-to-six fold risk of breast cancer in women with high MBD . What stands out from these reports is that MBD has a major impact on breast malignancy. Thus, investigating the influential factors on MBD can play a major role in prevention and control of breast cancer, also evaluation of a possible association between endometriosis and MBD may pave the way to revealing the pathway from endometriosis to breast cancer.
To evaluate the role of endometriosis on MBD, we conducted Univariate and Multivariate linear regression analysis. In addition to endometriosis, age, BMI, OCP, progesterone use, and family history of breast cancer were expected to impact MBD based on previous knowledge; and were considered in the analysis. The result of Univariate analysis revealed that age and endometriosis were independently associated with MBD. Consequently, these factors were included in the multivariate analysis, and result showed that age, progesterone use and endometriosis were independently associated with MBD.
According to the findings, endometriosis is a significant predictor for MBD; however in contrast with our expectation, MBD was lower in women with endometriosis. The mechanism for this reverse association is not clear to us, but this shows that the association of endometriosis and breast cancer is not through MBD. It also infers that sex hormones alone are not implicated in female cancers after endometriosis. To the best of our knowledge, the only study which evaluated the relationship between endometriosis and MBD was that of Farland et al . According to this study, endometriosis was not found to be associated with mammographic density, which was in contrast with our finding. However, our sample size was higher, and Farland et al did not consider the use of steroid hormones as a confounding factor.
Age and progesterone use were the other variables that showed significant relationship with MBD. We found that a history of progesterone consumption was associated with a higher MBD. There are studies that are in agreement with our finding about the role of progesterone in MBD [16–19]. Those studies also reported that higher levels of progesterone were associated with greater MBD. This finding is not unexpected, as progesterone has a key role in regulation of tissue development and maturation in the young breast, and atrophy and involution of the lobules and ducts during and after menopause .
Among variables that were evaluated as influential factors for MBD, BMI and OCP usage were not significantly associated with MBD. These variables have been reported as associated with MBD in some studies. For instance, in a study conducted by Yang et al, BMI was negatively correlated with MBD . In a study conducted on Chinese women, Shang et al identified BMI as an independent influential factor on MBD .
In conclusion, our study showed that endometriosis was inversely associated with BMD. Considering the increased risk of breast cancer in women with higher BMD, our findings show that were there a positive association between endometriosis and breast cancer, this is not mediated via MBD. Further studies are warranted to define the complex relations among endometriosis, MBD and breast cancer.