You will probably have a CT scan. They will put a couple of tiny tattoo dots on you, where they will be depends on the area they are treating. That scan is for mapping, it's not diagnostic so you don't have any contrast in (no needles)
Try to make yourself as comfortable as possible before the scan -they will accommodate you having a bit of a wriggle around to set yourself up--being mindful that the position you are in during that CT is the one you will be in for the rest of your treatment.
If, for example, you have real trouble lying with your arms above your head (which seems to be the default position for BC treatment) ask if another position is an option. It turned out that I could have my arms down. Thank &%@# for that.
Good luck, Marg x