Lots of research that shows a lumpectomy with radiotherapy has as good results as mastectomy in removing tbe cancer.
I chose mastectomy due to my tumor not being visible in a mammogram only 5 weeks before I felt a palpable lump due to breast density. I felt I could never feel confident a cancer couldn't be regrowing and remain undetected. I also live remotely and wanted to avoid a 5 week stint away from my family which I would have needed for radiotherapy. My lumpectomy would have resulted in quite a disfigured breast with nipple loss due to location. So I chose a mastectomy with reconstruction. If I had a detectable cancer via mammogram, a different pathology with a different breast location, I may have chosen differently.
We all worry about our decisions. It's a normal part of this darn diagnosis as well. If your surgeon believed it was necessary for mastectomy, he would have recomnended it.
I hope your treatment goes well. Kath x