tumor in retriever

[hope hierlmeier]

I have a 5 year old golden retriever who I took to the Veterinarian 8 days ago for
heavy panting and problems breathing. My Veterinarian referred me to a specialty clinic who based on her x rays determined she had a mast cell tumor in her stomach which had progressed to her chest cavity which was filling up with fluid. They drained her lungs and kept her for 2 days. A week ago today they performed an ultrasound which pretty much substantiated their prognosis.
..but did not do a biopsy because they said I did not request one. i wasn't informed I needed to do that.
They gave her predisone and told me it would be a matter of time.
I took her back in Friday for a recheck. They could not feel the tumor nor pick it up on the x rays. Her lungs looked ok with the exception of a little swelling. She is, as she has always been doing from the beginning of this nightmare, eating well, drinking, & going to the bathroom regularly.
I had to take her back in this morning to try to get an ultrasound to see if anything is in there and if so maybe they can get a biopsy.
I have been advised that it could possibly be lymphoma but again from there are no other symptoms to substantiate this. No lumps anywhere.
At this point I really don't know what to do.
Anyone seen other cases similar to this?
Your help is greatly appreciated.
Thank you, Hope