The surgeon told you he took out all the breast tissue and was absolutely correct when referring to all the tissue that makes up the external breast.
However there is breast tissue that is attached to the chest wall (deeper down) and they try to get most of that breast tissue as well, but there is no guarantee that all of that remaining breast tissue attached to the chest wall was removed. And if there were a few cancer or precancer cells in that tissue it would not be detected by the original mammo or MRI's and you would not know until years later when those cells might grow and become cancerous.
So the meds are prescribed to take care of that possibility - even if it is very remote. Kind of an insurance policy that goes where the surgeon's knife cannot go.
You have taken Anastrazole for 3 months and now it is your choice whether or not you want to take Tamoxifen. Women on Tamoxifen usually have uterine ultrasound done every 6 mos or once a year to check for uterine cancer.
The choice is yours as to whether or not to continue with the Tamoxifen. Many women opt not to take Tamoxifen and are very comfortable with their decision. So that decision is yours to make. For the doctors there is only one choice, but for you there are two choices and there is no right or wrong choice - only the choice that gives you peace of mind.
Soyou should decide what is the best choice for you in this case.
Good luck and let us know what you decide.
Judy