Thanks for getting back to me.
That is a pretty nasty looking x-ray to say the least. There are a couple of things that have have me worried. As you can tell and your vet has probably told you, the biggest concern is that she sheer size of this mass is literally compressing everything, decreasing the functional area for the lungs to work and could potentially cause her to suffocate. From the views you posted, though its hard to tell without seeing other views, this does not look like a heart based mass and it actually does not really look like a lung-based mass but again, I would need at least one other view to determine it. there is also fluid in the lungs as well. In that area, it could be a lymph node or thymic tumor, abscess, etc. Here is how I would proceed:
I would have your vet take an aspirate of this. With the size and location, this actually would be quite easy and isn't very expensive. Send it off to the lab and see what it is. There is a possibility that, if it is a thymic tumor, surgery would be curative. If it does turn out to be a lung mass, and is primary, then surgery could be curative as well. Aspirates usually take 1-3 days to get back so the sooner the better.
Its hard for me to send you into surgery not knowing what this is or to tell you a treatment plan. This is not an easy surgery, I will tell you that but a dog who is 7 years old, may have a long life to live if the surgery is successful.
I hope that helps a little bit. It isn't an easy decision to make, but you may found that by figuring out what it is first, you can then have a clearer decision to make.
Please let me know if you have any other questions!