Sorry to hear that your liver met was missed by your oncologist. Is this liver met your first? If it isn't, there is not much you can do from a legal perspective as the 'mistake' has to have had an effect on your prognosis/treatment which I don't think you could argue if already had mets. In this case, I think the best thing is to save your energy, but write as many complaint letters as you can too vent.
I explored the legal perspective a bit myself as I had been seeing a physiotherapist for over 12 months with hip pain and she couldn't understand what was going on. Went fro my routine mammogram and had two large breast tumours and low and behold the PET scan revealed multiple tumours in and around my hip. I was told that the prognosis or treatment would not be any different if the bone mets had of been found initially or at the time they were found 12 months later.
It is annoying as it seems so wrong!!
Xx