You shouldn't be going anywhere near the sun during the treatment John; that advice about suncream is for the future.
If you need to use moisturiser during the RT, be very careful what you use - nothing perfumed. Baby moisturiser or Ph-neutral products only. Check with the RT nurses before using anything. Also use only very plain shower gel- again, baby wash is best.
The area goes from your hip, across your belly and round to the other hip, and from your pubic hair up towards your belly button. If you have dark skin you may hardly be affected but men with fair skin can get sore skin, just like if you have been in the sun for too long.