I think it depends of the reason for the CT scan.
You may want to ask if you need to hydrate before the CT scan. This expands your bladder. But, depending on your condition, guidelines about what to eat and drink before your CT scan. Also check if they are going to use dye to enhance the image of the tumor as some people are allergic to the dye.
In my case, I had a CT scan first with dye because of gross hematuria and found a 2 cm x 1 cm mass, which was later confirmed as a tumor by cystoscope. I was told to wait after the CT scan and the radiologist came and showed me the picture of the mass and told me he was concerned that it could be tumor.
At that time, I was not requested to hydrate. Also, I am going to have a CT scan for my kidneys soon. For this, I have been told to drink 5 glasses of water the day before and on the day of the CT scan.
best luck with the CT scan