Like Hanna said sugar is the one thing that cancer cells feed on, so you would want to make sure there is not alot of sugar. If your questions was "did the food cause the cancer" probably not, there are things that link sugar and grains to cancer, but to my knowledge there is no hard proof of that, most of the things I have seen have been published by other dog food companies....
There are plenty of mass found that are not cancer and hopefully that is what you are dealing with...
I have seen plenty of tumor/mass shrink with antibiotics...
But, I do know that keeping the immunity up is the important thing and Fish oil is a great source at around 1000 mg per 20 lbs of dog each day, this is a recommended supplement for dogs with cancer
There are plenty of mass found that are not cancer and hopefully that is what you are dealing with...
But, I do know that keeping the immunity up is the important thing and Fish oil is a great source at around 1000 mg per 20 lbs of dog each day, this is a recommended supplement for dogs with cancer