Cazz, I think they usually go by the amount of RAD's given to a particular area. Sometimes they will add another week of concentrated radiation to reach the maximum dose to a certain area.

Had the same thing, basically, neck and mouth irradiated for 6 weeks, then evaluated as to how well I took the radiation, and a "booster" week on 7th week just to the neck.

I wouldn't get real concerned, seems nothing is written in stone with these treatments, the need for the booster week probably is a result of mathematics rather than anything going on physically with your brother except possibly he is tolerating the radiation well.
Bob

SCC Tongue, stage IV diagnosed Sept, 2002, 1st radical neck dissection left side in Sept, followed by RAD/Chemo. Discovered spread to right side nodes March 2003, second radical neck dissection April, followed by more RAD/Chemo.