Abstract: A disposable mask system for positioning a patient relative to a patient restraint board during radiation therapy includes a piece of low-melt thermoplastic material affixed to a rigid, non-thermoplastic frame. A lock-down mechanism is integrally incorporated into the rigid, non-thermoplastic frame, such that a manipulable portion of the lock-down mechanism extends upward from the surface of the rigid frame so that it is accessible by the user. An engagement member extends downward from the underside of the rigid frame such that it can engage corresponding holes in a patient restraint board, such that when the rigid frame is pushed down so that its engagement members fully engage the corresponding holes of the patient restraint board, then the thermoplastic mask system may by tightly secured to the patient restraint board by activating the lock-down mechanism.