Actually, the Natural Balance foods are good. I have a Grey that is having protein problems and needed a food low in phosphorus. I switched him to the Natural Balance Fish and Sweet Potato. He was diagnosed in May with Lymphoma. :-( I am still feeding him the Natural Balance Fish and Sweet Potato with a high protein *stew* I cook up plus cottage cheese and about 2000 units of Omega 3 daily. If the tumor is cancer, theory is you want to feed a high quality protein and no or very little carbs. You can also feed Salmon, canned sardines, etc.
You don't really have to cook, per se. You have a crock pot? I just throw everything into a crock pot (mainly lean ground beef, I just brown lightly, chicken breasts, liver, vegetables such as canned green beans, squash, some garlic and some brown rice.) To that I add some low sodium chicken or beef stock and water. I've been adding this to his Natural Balance alone with low fat cottage cheese twice a day.
Actually, there are a couple of Yahoo groups with tons of information, if you are dealing with cancer. There is a canine cancer group and another group that is treating with artemisinin treatment.
Due to my dog's age and pre-existing health issues, I have opted to do palliative care only. You might want to look at those groups.
Did not have the same heath condition the dog is doing good just did not know what kind of dry food to give. I hope your dog is doing better.