Brainlab RT/RS Hardware User's Manual page 66

Stereotactic Mask System
Treating through 3.2 mm material (with no less than 6 MeV) can be done without significant
irritation of skin. If the treatment is performed with static fields rather than convergent
arcs, we recommend avoiding the double-layer areas of the mask with the beam path if
skin irritation starts to occur.
66 Hardware User Guide Rev. 4.9 RT / RS Stereotactic Hardware