My senior dog leaks urine, thickened bladder wall, no infection. What's wrong?

Buster is one handsome guy! It sounds as though you and Buster have had a frustrating few months! In most cases, I would also expect cancerous masses in the bladder to grow more rapidly and cause a worsening of his incontinence issues, but some masses grow slowly or do not metastasize (spread) quickly. Some possible causes of Buster's urine leakage include infection, cancer (transitional cell carcinoma is the most common bladder cancer), hormonal imbalances, or neurologic deficits.

At this point, it may be worth considering a urine culture to definitively determine if infection is present. A urinalysis will occasionally miss bacteria or white blood cells, particularly if the urine is dilute. This is a non-invasive test that could allow for a relatively simple fix with a course of antibiotics. If a urine culture is negative, it may be worth discussing trials with other medications with Buster's vet. Some dogs can benefit from phenylpropanolamine or testosterone if this is a true incontinence issue. If there is a suspicion that cancer may be present, it may be worth a trial with piroxicam (an NSAID that can be quite beneficial for bladder cancer) to see if his signs improve. All of these medications are prescription based and need to be prescribed by his vet, if they are indicated in his given case. I hope that all goes well for Buster!