I don't have any information on using aspirin and the like, must be some information "out there" and the vet can prescribe a pain relief.
I read it must be time to consider putting the pain to an end. We decided to "put down" a lovely Golden Retriever (see my picture album) when he was 12 years old because of cancerous tumors. He had surgery a couple of years earlier to remove a "fat pocket" that had a slow growth cancer. He was mostly pain free, as far as we know, for a couple of years when the cancer came back in a surge. We opted to end it rather than pay the cost is pain an dollars to fight the cancer.
I still mist the dog and his picture remains in my memory and photo album on the community.
Can your vet can prescribe something stronger than over the counter meds?