Is Tamoxifen Intake Necessary After A Lumpectomy?

I recently completed 16 radiation treatments for non invasive, in situ , 0 stage breast cancer.I believe it was in the tubes. Had lumpectomy. Medical oncologist now recommending I take Tamoxifen. After reading the pharma flyer, wonder if it is REALLY necessary for me to take this drug. The benefit has not been proven and there are 3 pages of serious side effects, the most frightening is the possibility of a stroke or blood cloth or uterine cancer!!! I am 72 years of age and until this, have had no health issues. Do not take any estrogen as had to stop taking that in Aug 2016 due to the discovery of this microscopic cancer. Exercise in a gym 4x a week and am not overweight. Since I am not taking any estrogen, is there a chance of recurrence? From what other source would get estrogen since my type of cancer was estrogen receptive. Your most honest opinion is greatly appreciated. THank you! Irena