I agree with Ghilly. Get the bloodwork done, see how his general health and fitness is going (he sounds like a basically fit dog -right?) And whether he basically is fit for major surgery.
If so -in the vet's opinion, then get the spleen removed.
Yes, this is major surgery. And with that there are always risks. And yes, statistically, the risks do increase with age. But there are risks most likely even greater, in leaving the tumor, even if it is not cancer.
My dog is age 14. She has had 3 surgeries in 2 years. 2 of them pretty major. There were some scary moments with her last surgery, but they passed quickly, and she is now doing very well indeed. Age need not be a deterrent to surgery, so long as the dog is basically strong, and the bloodwork, heart function and breathing is good.
I wish you and Hotrod all my very very best wishes, and hope this is over very soon, and Hotrod doing OK again.
Did she do any tests that would indicate that the mass was malignant? Splenic masses are pretty common in older dogs, especially in certain breeds (Poodles, German Shepherds, Golden Retrievers and Labs). About 1/3 of those masses are benign but still necessitate removal of the spleen because if they rupture, they can cause instant death. In the 2/3 that are malignant, in about 15% of the cases it involves a spread of the tumor to the right atrium of the heart.
If your dog is in good health otherwise, as long as you have bloodwork done to assess liver and kidney values to make sure they are fully functional, I would see no reason that surgery wouldn't be an excellent option at this time. If the mass is benign, it could buy your dog who knows how many more years, and if it is not, then you need to see how many more organs are involved before deciding where to go from there. The most important thing, however, is to remove the spleen and the tumor along with it to also remove the possibility of the tumor rupturing and your dog bleeding to death. I would still get the opinion of UC Davis, it's always great if you have a veterinary college that you can go to, since their diagnostics and techniques are state of the art and much more likely to be able to help in instances where a regular neighborhood vet clinic may not be able to.
Please let us know how things go and what the vet says.
Ghilly