@OhDeDoh- I am sorry to hear about your dad’s condition. I hope he stays happy and healthy. I know it isn’t quite the same, but we had a dog that had a tumor in her eye. Canine eye tumors are usually benign. Hers was benign, thankfully. We got lucky. She didn’t have to have any surgery. She lived a long life, (especially in dog years).
From what I read, some of the procedures for treating tumors in a dog's eye are similar to treatment for humans. That made sense to me. Humans and ‘fur people’ like dogs have similar eye structures.