Just guessing, but I should think internal radiotherapy penetrates the
tissue quite far enough to take care of anything of that nature that
external radiation would deal with.

Fluid filled cysts are more likely to be benign, but if in doubt a fine
needle aspiration would make sure.

To your diagnosis questions, I think we have to rely on the consultants'
judgements.  Any of your remaining breast tissue could "harbour cancer cells
waiting to become tumours", as could most of the rest of your body.  No-one
in their right mind would try to second-guess this by email.  If you don't
feel confident in their recommendations, or they don't make any, then get a
second opinion.

Tim Jackson