My condolences.
When our little shitzu died at 13 from cancer in 2007 our vet said he was seeing much more cancer in dogs - up from one maybe a month to several a week. He really felt it was something to do with dog food as so much of it is produced in China and other countries as opposed to the US. He said even if made in US a lot of the ingredients came from other countries. He said it was just his opinion but I have a feeling he was really onto something.
My vet and I had a conversation on this subject within the last year or so. His thoughts were that dogs and cats are living MUCH longer than they had in the past. With that increasing longevity, comes a myriad of diseases relatively unheard of in veterinary circles until recently: cancer, high blood pressure, strokes, arthritis, kidney issues, diabetes, senility, and the list goes on.
We have taken better care of them, allowing them to live longer. With that increased life span, the diseases long thought to be the sole purview of man, rear their monstrous heads in our beloved canine and feline companions.
NO, NOW we VACCINE them to death! We kill their immune system