urinary bladder polyps

I have had bladder polyps first removed around 6 months ago. The biopsy turned out as there was not enough material. Doctor tald me that as soon as it is a polyp it is always malignant. (Is that true?). Couple of months later they found some "cancerous?" cells in the urine, but they were not so sure and specific about it. And on my 3 months after  surgery cytoscopy they did not see anything. But now after 6 months they found the polyps again, so they scheduled me for another surgery. How may I be sure about definite diagnosis? ( if the would  not have anough this time also?) Is there some other diagnostic tests available? Could the new growth be prevented? What is the prognosis? Thank you.