Canine Bladder Question and Help Needed

I have an 8 year old mixed breed neutered dog. He is asymptomatic and a routine urinalysis last week... 9 months after oxalate stone removal,  showed abnormal cells. Cytoligist diagnosed "probable transitional cell carcinoma". I elected to have the V-BTA test which came up negative. I understand this test with a negative means it is highly unlikely the dog has TCC. An ultrasound and x-ray came back normal. His prostrate is slightly enlarged.  He is happy, energetic and eats like a horse. No blood in urine, no straining, no pain...etc.  The vet wants to do more tests but as far as I know a biopsy is what is left to do but there is no observed mass to biopsy. She is thinking about putting him on Piroxcam "just in case".

Has anyone had this same experience and if so can you shed any light? I am lost and losing it even more.  

My cocker is a 1 year bc survivor. Get the cystoscopy. Its a relatively minor procedure and will be more definitive than ultrasound. Our Rachael had 1 tumor on ultrasound, 2 tumors at cystoscopy, and ultimately 3 tumors were removed with surgery. She is still cancer free 1 year after surgery. Have the cystoscopy done at an omncologist. We had the cysto done at a general vet and it was a mistake. The oncologist could have gotten much more testing and info out of the procedure than we got. Ultimately it didn't make a difference in survival or treatment for her but it could have.