See my post on the diet postings
It's pure genetics as the main underlying factor. I thought my Daisy 1/2 Golden-1/2 Yellow Lab would have the genetic hybrid vigor to get around such problems. Unfortunately both the Goldens and Labs have an extremely high incidence of Lymphoma- which she got at the age of 10 yrs.
Rotties also have a high incidence of hemangiosarcoma, and lymphoma. Sad to say but until cancer is recognized as a genetic problem its incidence is only going to increase. It's my personal belief the economics of breeding will get in the way of decreasing the genetic load of oncogenes. If you are tied to Rotties (and I prefer Labs), then the only realistic long range hope is better treatment for cancer when it strikes- because it will in those breeds.