From what I know -- and certainly you should check this -- cancer won't show in the lungs on an x-ray after metastisis until it's significantly invaded. That's what the vets told me about Sander, anyway. IIRC they said the x-ray wouldn't show anything unless the lungs were more than 30% affected, which is a LOT. So if I had done an x-ray (and I did a few) of the lungs that showed cancer, it would be at the stage where palliative care was about the best option.
Sander's cancer, though, was not a type likely to go to the lungs on metastasis, so I didn't x-ray more than a couple of times and I didn't spend much time researching that.
Also, as you said, I thought 'so if I see it's in his lungs, what will I do then that I'm not doing now?' and the answer was: nothing. I was already doing all the things I could to help his immune system. From there, for us, it was just how much his body could use that help to keep the cancer in check.
It's a very tough spot, dealing with cancer in a senior dog. The courses of action available may not be in the best interests of the dog, given the age. I think it's a really individual thing and the dog has a big say in how far the owner will want to go.
It's still devastating, no matter the age, though. I'm very sorry you're facing this with Ginger.