Breast Cancer - Tamoxifen - Endometriosis relationship

Background:
I was diagnosed with an Er+/Pr+ invasive ductal carcinoma in January 2001. I am 51 and was perimenapausal when diagnosed.  In addition, I have had a history of endometriosis including endometrial cysts.  I've had two laparotomies 25-30 years ago.  Since my breast cancer diagnosis, I've had a lumpectomy, radiation, and have been on Tamoxifen for 8 months.  My periods stopped immediately.  However, three months into the Tamoxifen, I developed pelvic pain.  Tests revealed that I had a 4.5cm ovarian cyst and my estradiol level was 380.  In September, the test was repeated and the one ovarian cyst had reduced to 2cm but the other ovary had a 5cm cyst.  My estradiol  level was 431!  My oncologist recommended I take Lupron to bring down my estrogen levels.  After 2 months, my cysts have disappeared and my estradiol level was 20.  Luckily I have experienced almost no side effects of the Lupron.

Question: I can't seem to find any research on the phenomenon that I experienced.  Has there been any research on the effect of Tamoxifen on patients with endometriosis?  As I understand it, Tamoxifen usually lowers one's estrogen level.  Is there any research why it would raise it?

[CCF-RN,MSN-JS]

Dear Ritavv, Research continues as to the mechanisms of action of tamoxifen and other hormone therapies for breast cancer.  There is quite a bit of documentation of the effects of tamoxifen on the endometrium.  Basically tamoxifen has both estrogen blocking as well as estrogen mimicking properties.  This may explain why there are good and bad effects associated with its use.  For instance the estrogen blocking properties are the properties which reduce the risk of relapse after definitive breast cancer therapy, or reduce the incidence of breast cancer in healthy women who are at high risk of developing the disease.  The estrogen mimicking properties are believed to be responsible for certain side effects such as venous blood clotting or endometrial hyperplasia (overgrowth of normal cells in the normal tissue), or endometrial carcinoma.  On the other hand these estrogen-mimicking properties of tamoxifen are believed to be responsible for its beneficial effects on bones and lipids.

Tamoxifen is not known to increase estrodiol levels.  Rather the enlarging ovarian cysts were probably the cause of the increasing estradiol levels.  What triggered the growth of the ovarian cysts would be difficult to determine.  Tamoxifen is not associated with an increase in benign or malignant ovarian tumors.