Greetings! question about lymphoma treatment

Hello everyone! My cat is 18 years old and two weeks ago I brought him into the vet for a check up. They found a lump in his abdominal area and after an Xray, it turned out to be a cancerous tumor, Lymphoma. They recommeneded to do nothing since 18 years old is too old for the stress of surgery and cancer treatment..
My question is, is there any drug or food I can give my cat to extend his life and slow the growth of the tumor?
I read DCA may be good for this??

Any help is greatly appreciated!!