Estrogen Receptor

I'm in the final stages of chemo, lumpectomy and radiation. (Done in that order)  My doctor has recommended I start tamoxifen when I am finished with radiation.  I had right breast Invasive Ductal Carcinoma.  Approx 4.5cm.  2.5cm after chemo (taxol / FAC).  I also have LCIS in the left breast. (I'm 41, never had kids and have aunts on both side with breast cancer)  I have concerns about tamoxifen and want to make an informed decision.  I've done extensive research, but am having trouble finding one piece of info.  My pathology report states estrogen receptor positive 20% and the same for progesterone. I have a very good understanding as to what estrogen receptors do and how tamoxifin (tamoxifen) blocks them.  My question is..WHAT DOES THE 20% REFER TO and should it play a role in my decision to take tamoxifen?
Thanks.  I appreciate you're time.  - Stacey

[CCF-RN,MSN-JS]

Dear Stacey, Pathologists use several techniques to test hormone receptors on tissue, and determine if the breast cancer cells respond, or attach, to estrogen receptors and progesterone receptors on the cell. Breast cancer is termed ER (or PR)-positive if its cells have more than a pre-specified baseline

[renea500]

thank you so much.  that's exactly what i was looking for.