Advice on ceasing treatment

It's a tough subject and one that is in the back (or front) of my mind constantly since diagnosis. My sister was diagnosed at 32 - very early breast cancer. The lump she found was actually a cyst and there was a very small cancer behind it.  Lumpectomies were only just becoming a thing at that stage.  Her surgeon did a mastectomy because he wanted to be sure that there would be no recurrence.  She passed her 5 year all clear.  It came back with a vengeance a few years later.  She died at 45 - way too bloody young.  I know treatments have improved since then but it haunts me on so many levels.