The worst part about this test is the prep, it really is! It's more or less the same prep you'd have for a colonoscopy as they need to clear the bowel as much as possible to get a good look. A thought: get some nappy rash cream in. Apply it as soon as you start getting the 'effect' of the prep working, and slather it on!
As well as the prep you might be asked to drink some Xray dye stuff maybe a day before?
The test itself usually involves the insertion of a small flexible cannula into your arm through which they inject a bowel relaxant to help the bowel to distend with the gas as painlessly as possible (the injection can also give you fuzzy vision for 20 odd minutes as the muscle in your eye is the same type as that in your bowel wall!)- they won't give that if you have angina or glaucoma; and to inject xray dye at the time of the first CT scan as it outlines the blood vessels and organs and makes lots of grey-scale pictures more black and white thus easier to 'read'. It contains iodine- you'll be asked about allergies.
It isn't nice having anything put up your tail end but the tube is as thin as a woman's little finger. They usually inflate a small balloon on it once its inside you to stop it falling out as it will be well lubed! That might make you feel like you need to go to the loo as the balloon will sit on the same muscles that tell you you need to. It won't be 'desperate', though. They then run the gas in (carbon dioxide) using a special machine that controls the pressure. Once enough is in (you may get some gripy pain and internal 'bubbling') they'll do a quick, low does scan to check enough is in, the they'll run the xray dye through the cannula in your arm and do the CT scan itself. You hold your breath for each for about 7 seconds. The xray dye has 2 side effects- possibly an odd metallicy taste in your mouth and- the feeling like your wetting yourself down below! Ladies practically always get that feeling but it's just that, it's not real and it lasts about 20 seconds. Then they'll get you to roll onto your front and will repeat the CT scan- once they're happy there's enough inflation happening and the scans are technically good, you're unplugged, and it's all over. The vast majority of patients then exclaim 'is that it?!' as you get them a cuppa!
You may be asked to hold your breath 6 odd times for the scans. That's not 6 full loads of abdominal scan CT radiation. Usually, there are 2 'scout' or planning scans- a front and sideways one though you stay in the same position- per 'full' actual diagnostic CT scan. These planning scans are low does and low resolution. They are used to let the radiographers precisely plan where to begin and end the actual CT scan itself to get everything on (the whole large bowel) but nothing more than is necessary; so 2 planning scans plus 'The CT' on your back, ditto on your front.
Be aware the bowel prep may go on working after the scan for a few hours so take the same precautions for the rest of that day, and drink loads as the prep can dehydrate you and the injected xray dye puts a bit of a load on your kidneys.
As for cancer- as I said before, the vast majority of diagnostic tests carried out are one-offs i.e. someone presents to their GP with a set of symptoms that just might be cancer, usually a tiny percentage. So Test One will be just about always be 'Let's rule out cancer', which is done; cancer is just about always ruled out then the GP or specialist can set to work looking at other possibilities. This happens a lot with persistent coughs, rectal bleeding and persistent headaches. You're put on a 2 week wait waiting list that scares the bejeezus out of people BUT IF, IF it is The Big C you need a swift diagnosis so treatment can commence. The minute cancer is ruled out, you plummet down the waiting list designation, to 'Routine' 
! As an aside, patients do like to get angry that they were 'unnecessarily frightened' by the 2 week wait designation, then outraged at the subsequent 6 week 'routine' waits... 
...
Can I finally say that the reality is, should you be unlucky enough to be in that tiny bowel cancer percentage, that the fear of the radiation of the diagnostic scans will seems trivial?!
Good luck!