Before the radiation therapy begins, you will have a "simulation visit" to plan for the radiation therapy. During this session, you will be fitted to a plastic mask to hold your head still during radiation. The mask is made of a material called "thermoplastic" that becomes soft when it is placed in warm water. While this plastic is soft, it will be pulled over your face to make a mold. When the plastic cools down, it will harden again and the mask will be completed. After the mask is made, a CT scan of your head will be done for treatment planning.