Accessory device, a patient immobilization system, and a method of forming a patient immobilization system

An accessory device configured to position a patient relative to an immobilization device including a low temperature thermoplastic sheet, the accessory device comprising:

• a patient fixation portion configured for fixation on or insertion into the patient during use;
• and
• a lock portion extending distally from the patient fixation portion generally along a lock portion axis;
• wherein the accessory device is configured to be engaged by and to contact a proximal surface of the low temperature thermoplastic sheet by forming the proximal surface of the low temperature thermoplastic sheet around the lock portion of the accessory device, such that the proximal surface of the low temperature thermoplastic sheet conforms to the lock portion and when engaged prevents unintended separation of the accessory device from the low temperature thermoplastic sheet during use;
• wherein when engaged, the lock portion does not penetrate the low temperature thermoplastic sheet from the proximal surface of the low temperature thermoplastic sheet to a distal surface of the low temperature thermoplastic sheet;
• wherein the proximal surface of the low temperature thermoplastic sheet is positionable proximal the patient and the distal surface of the low temperature thermoplastic sheet is opposite from the proximal surface;
• and
• wherein the accessory device has an air passage such that air may pass through the accessory device.