Answer

No problem one of the pluses of this site is people view when they have time!

If the mass is not attached to the heart and is in an operable area and the lungs are not showing signs of more tumors then you might consider surgery to remove the mass.

Or you might consider, if possible, a needle biopsy to determine what the mass is before deciding on surgical intervention or not.

Masses attached to the heart or chest wall are sometimes not operable.

If you have not consulted a veterinary oncologist that might be a good next step if your regular vet has no options to offer you beyond reducing fluid build up with lasix. A specialist reviewing the x-rays may be able to tell you more than a general vet might.

What can be done depends on several things, the location of the mass, what it is attached to, if it is a tumor, whether or not chemo or radiation are things you might consider if surgery is not an option, whether your dog is well enough to even consider surgery as an option etc. or if you just want to keep your dog comfortable for as long as possible.

I think you might do best with some more investigation perhaps with a specialist or with your vet to determine what choices are available to you for him.

Here is some info on biopsy

http://www.oncolink.org/types/article.cfm?c=22&s=69&ss=547&id=9600

and some on veterinary oncology

http://www.oncolink.org/types/article.cfm?c=22&s=69&ss=550&id=6003

and this cancer treatment center for pets has some faqs which may be useful reading for you just scroll down past the video

http://www.nevog.com/faqs/

Hope this helps you!