[anonymously]

I lost a dog to hemangiosarcoma a few years ago, by the time the dog had any symptoms it was too late. A veterinarian told me the only way to possibly find some of these rare conditions would be to have x-rays, ultrasounds and bloodwork done at routine appointments. This is not recommended for a dog that appears to be healthy and has no symptoms, obviously, due to the cost.
Plus, sometimes these things just happen without any warning. Don’t underestimate genetic factors, my dog had a good diet, exercise and was fit.

Sorry for your loss. Take care of yourself. Remember, your other dogs need you now.