Regarding the field concept...
xrays do not just stop they go through and through tissue (consider an x-ray picture), and they are a lot more energetic. The way the radiation field is created is by changing the angle of the irradiation (and intensity). This will create a irradiation volume where the all the radiations add up. However, you still have the individual "shots" the path the radiation takes. If you have very sensitive cells (i.e. hair follicles) in their path they may be killed (both exit and entry damage). Generally, the stuff at the back of the head comes back.

BTW: This is why proton therapy is very interesting, here the particles stop after a certain penetration depth. So you would not have any exit radiation collateral damage.!

Markus