Steve, I'm so sorry you lost Lucy.
Both my Retrievers were lost in a similar way: tumors growing without anybody being able to notice. When tey got ill it was all over very quickly. It is harder for us humans, but I believe for the dogs it is easier, because they do not suffer a protracted illness and have to undergo demanding treatments. It happens quite often with dogs of great vitality.
Run free, Lucy! And what a great picture of her as a memorial!
Susanna