The reason they want your skin clean and free of any cream or chemical of any sort prior to radiation is that many compounds serve as a bolus to the radiation, concentrating the beam where they don't want it and missing targetted areas.

I think a good rule of thumb is to do EXACTLY what your RO tells you to do; nothing less, and certainly nothing more unless the Doc approves it. You will develop such sensitive skin that you really don't want to do anything that will make it worse, and it's easy to develop a really nasty mess, doing nothing other than applying OTC creams.
Wayne