Tamoxifen

I just had a double mastectomy Sept 12th without reconstruction surgery. And I was er positive and progesterone positive,however since having the mastectomy and all the dcis was removed the oncologist said I don't need che9,radiation or Tamoxifen. I am a bit concerned because my cancer was er positive. She said all the cancer wa s removed,but should I still be on that medication to prevent it coming back? My MD said yes because no one can guarantee that every single cell was removed. I dont understand and not sure what to do.

At the time of surgery, my diagnosis was DCIS. The bilateral mastectomy meant tamoxifen wouldn't be a part of my treatment as DCIS ALMOST always just recurs locally and that's the benefit of tamoxifen treatment use in DCIS. Although not 100% it is close enough that I would have taken those odds and skipped off merrily. Unfortunately, invasive cancer was found in the final pathology.