Thickness of shielding required for dose reduction with 4-22 MeV electrons

In electron therapy, it is common practice to shape the treatment area by the use of lead or cerroband alloy on the skin, or as insert at the electron collimator. We have studied the effect of the position of these shields on the skin dose and the thickness of the shield to reduce the dose to less than 5%. With energies above 12 MeV, it is not practicable to reduce the dose to 5% or less with reasonable thickness of the shield at either position. For lower energies the shield is more effective at the collimator rather than on the skin.