My dogs are young and healthy, knock on wood...so I am not looking down this tunnel quite yet.
But anything can happen...my big dog had a lump near his spine a few months back; I haven't been monitoring it, I don't think it's obvious unless he's pretty lean. It's possible it's just a fatty deposit or a cyst or something, but it's in a bad spot if it grows. Last time I looked for it I couldn't find it, so hopefully no news is good news.
My terrier is 7 years old and will probably live to 25. I've never even had to take her to the vet, and I've had her 5 years. I give her immunizations myself. Unlike my monster, who has to go to the vet every couple of months for allergy-related issues (ears, skin).