Methods of immobilizing the head of a patient include heating a thermoplastic mask preform to a formable temperature, forming the thermoplastic mask over at least a mouth of the patient, and pushing a mouth receiving end of a bite piece into a portion of the thermoplastic mask preform that is positioned over the mouth of the patient, which moves the portion of the thermoplastic mask preform into the mouth of the patient. The methods also include allowing the thermoplastic mask preform to cool from the formable temperature, causing the portion of the thermoplastic mask moved into the mouth of the patient to harden.