I'm so sorry

I have lost two very loved cats to tumours. My experience was that the vets were very happy to help me treat them and keep them comfortable for as long as possible. My guess would be that in this case they cannot offer any rteatment to prolong life or reduce the tumour, as I am sure they would have discussed this with you if it had been an option.
You are doing the bravest and kindest thing to not allow any more suffering.
Once again, I am so sorry, I know how much it hurts.