Laz,

Keep in mind that I am not a doctor and you should check with a real pro before making any decisions.
It was my understanding that in IMRT they make an assumption regarding how the tumor will react to the radiation and periodically take CT�s throughout the radiation schedule to check and make sure the radiation is getting the desired effect on the tumor.
In IGRT the machine takes a CT at the start of every session. The RO checks that each time you come in and can make any minor adjustments on the spot before you get the radiation dose for that day. IGRT sessions take a few minutes more each day because they stop for ten minutes or so to get the OK from the RO on staff that session (the RO may make an adjustment on the spot).

Anyone out there who knows better on this please pipe in.