I personally have always tried to help my pets but only if the vet tells me it’s possible. Having said that, one of my cats had lymphoma and we tried chemo, but it didn’t work. All I would say is try it, but if it doesn’t work recognise it and act quickly to end things if need be.
My dog has cancer in her liver. They tried to operate but couldn’t remove it. It’s slow growing and she’s checked every few months and currently has no pain, but eventually this won’t be the case. As soon as we see her deteriorate we will take action.