Not setters, but we lost an Irish Wolfhound to cancer in the aftermath of Chernobyl. She was 3 years old. The vet said it was because they were using meat condemned for human consumption because of radioactive contamination in tinned dog foods.
We also lost a Wolfhound X Anatolian Karabash to cancer about 10 years ago. She was 8 years old.
Do setters have the similar short bowel for size of dog problem that giant dogs have? It could be something to do with that.
Proximity to a source of radioactivity such as a power station might be involved.
Or food contaminated with unsuitable stuff such as the Canadian vet found.
On the other hand, as with humans, there might be no known cause, or genetics, or any number of things.
It's horrible when it happens