Cancer cells do slough off into the urine and are expelled this way. This is the basis for the urine cytology tests which are often used, along with cystoscopy, to monitor the return of bladder cancer. It is unlikely that they would "reattach" and cause cancer to reform.
Bladder cancer tumors form on the lining of the bladder partly because the immune system in the bladder lining fails to destroy them. Bladder cancer tends to come back because of the failure of the immune system. BCG is used to stimulate the immune system in the bladder lining so that it does react appropriately to any new formation of tumor cells. I admit that this is a very simple summary of what is probably going on!!
If the initial diagnosis is that the tumor is small and low grade, BCG is normally not recommended. Rather a "watch and wait" approach is used. With a little luck the tumor does not return. However, if it does, then BCG is considered. It tends to be a bit more effective with high grade tumors which are more rapidly dividing but if the cancer is returning, there is activity in the cancerous areas and it may be effective.
Hope this "explanation" helps
Sara Anne
