I’m sorry for your diagnosis of your cat. I had a patient recently that I diagnosed with the same based on cytology of pleural effusion. He had a visible mass on radiographs. Owners opted to go to oncologist. The oncologist put patient on prednisone to see if that helped his quality of life before starting chemo. Unfortunately, he died a few short days after seeing the oncologist. He was 10 years old I believe and completely healthy until one day he started to breath heavy. He presented like an ashma cat and owners want to try medical therapy before radiographs, so we did a short acting steroid shot and he was okay for awhile but within 2 weeks went downhill again and within 2 weeks of that he had passed.
I hope your boy sticks around longer.