Abnormal cells are found in tissue lining the inside of the bladder. These abnormal cells may become cancer and spread into nearby normal tissue. Stage 0 is divided into stage 0a (papillary carcinoma) and stage 0is (carcinoma in situ), depending on the type of tumor. Stage 0a may look like tiny mushrooms growing from the lining of the bladder. Stage 0is is a flat tumor in the tissue lining the inside of the bladder.