An accessory device and an immobilization system including the device are provided for positioning at least a portion of a patient's body relative to an immobilization device including a low temperature thermoplastic sheet. The accessory device may include a patient fixation portion and a lock portion. The patient fixation portion is attached to or inserted in the patient during use. The lock portion may extend distally from the patient fixation portion generally along a lock portion axis. The lock portion includes a shape, surface feature, or configuration, such that the low temperature thermoplastic sheet may be conformed to the outer surface of the lock portion of the accessory device, such that unintended separation of the accessory device from the low temperature thermoplastic sheet is prevented. Methods of forming the accessory device and immobilization system are also provided.
