I am so sorry to hear that it is a sarcoma. Very sorry. I would encourage you to consult with the oncologist, and ask his/her opinion as to treatments vs. sucess rates. I feel he/she will give you an answer that will lead your heart to make a decision that is in the best interest of Panther.
I know how difficult it is, as I am in the same position with my cat, Tenny. They count on us to do what is right for them. Hugs to you and Panther.