Most likely the idea is to get your skin away from the radiation beam. This is also why they insert cotton rolls to provide some separation (ie for lips etc). I had a tongue depressor to properly position and restrain the tongue with respect to the radiation field.... However, that needs to be done before/during the radiation planning.

Wax or plastic does NOT efficiently absorb xrays... If that really would work people would use plastic aprons etc and not use LEAD containers etc. Seen many bomb shelters with wax wands??

M