Injection site sarcoma

My 13-year-old cat was diagnosed with an injection site sarcoma in May 2021.  A biopsy was taken and was told that nothing could be done, except a very invasive procedure that would affect his back muscles and would be likely to be unsuccessful in any case. 15 months on and the cat's behaviour is utterly unchanged.  The lump has got a lot bigger and occasionally bleeds (3 times ever) and he has lost weight.  But he continues to eat, groom, run up trees, jump up and down, purr etc. Lately it seems to me that the lump is not actually attached to his back.  Is this possible?  The weight of it makes it hang to one side, and I can easily feel there is now nothing between his "shoulder blades", whereas this is where the lump was first felt. I trust my vet very highly, but I know this condition is rare.  He does agree that it is growing more slowly than usual.  Is it at all possible that the lump can be easily removed?  Should I get a second opinion? Thank you so much

Hello and what an interesting case.  I wonder whether it is possible that the lump is in the skin - and of course, skin is mobile - which is what makes it seem to move around independently of the structures beneath?  Misdiagnoses are of course possible, but I am in no position to suggest this either way for your cat, not being the vet in charge of his care and never having examined him.  Rather, I think it is important to pin your vet down with the questions you are asking me:  Is a soft tissue sarcoma still the most likely differential (possible diagnosis) in this case?  Has anything happened to change your opinion?  Would it be possible for you call the lab that diagnosed the sarcoma, and ask whether it is behaving as expected and then, if not, what to do next regarding the diagnosis?  Could the lump be removed?  Would you consent to my getting a second opinion on this case as I am interested in finding out more about it?  And so on. It is relatively common for a primary vet to organize second opinions on your behalf, with someone more experienced than themselves if need be.