Hi Kathryn, You are wise to keep an open mind about things as it can change once the full pathology comes in after your mastectomy, Not always for the worst though, Mine actually went in my favour, initial scans showed 10mm of DCIS and invasive BC but after my op I was told it was just 4mm of invasive, there was no DCIS.
I had radiotheraphy which was fine, painless and caused me no problems so it's really nothing to worry about if you were to need it.
I think on the whole they get things pretty much right but of course they can't give you 100% certainty and I know it's hard not to worry but all you can do is dig deep and try not to over think, So many of us here have been through the same wait and stress and can tell you it gets easier once you know the full story Xx Jo