Just got back from the vet. Remington is sleeping next to me at the moment but will be going back in the a.m. for an operation.
Hope I get all the terminology right--I'm a bit zonked from lack of sleep.
The short story is that there is a large mass of tissue around and in the right atrium of his heart--hemangiosarcoma--which is a blood-fed, rapidly growing aggressive tumor. Can't be 100% positive because apparently if you cut into one of these, e.g. for a bioposy, patient will bleed to death. However, there apparently isn't much else that it could be.
As it grows and/or is disturbed, it tends to bleed into the sack around the heart. Eventually it ruptures or pressure on the heart causes cardiac failure. Untreated, best estimate for survival is maybe a month.
Operation tomorrow--thoracoscopy, where they use slight incisions and a scope on a rope (my terminology)--will cut a window in the sack, which will be left open to allow the fluids to drain continuously into his chest, where they should be reabsorbed by the body.
We'll start chemotherapy at the same time.
Best guess at prognosis with this double-pronged treatment is 5-6 months of survival, but a good portion of that should be at a decent quality of life and activity.
Read the excruciating medical details if you're interested.