Hi Kathy, welcome! We're glad Annette steered you here for support and jump in to any thread you want...
You can have a reduction of the other breast when you have the mast, there are laws that protect us and our rights for symmetry. Not that you would achieve symmetry, but to be much less lopsided and have an opportunity for a good match with a prosthesis or for reconstruction. You may want to inquire about that.
I had a lumpectomy at age 30, did chemo and rads and finishing up on Tamoxifen. For me, they said mast was the same as lumpectomy plus rads, just depends on the individual situation and tumor pathology.
Annette will be a great resource for you, and we'll try to fill in the gaps-