Purpose: To evaluate the effectiveness of an immobilization treatment system used for claustrophobic patients in head-and-neck intensity-modulated radiation therapy (IMRT).
Methods and materials: Instead of the thermoplastic facemask, the Vac Fix (S & S Par Scientific, Odense, Denmark) mold is used for immobilization of claustrophobic patients at the University of Florida in head-and-neck IMRT. The immobilization procedure combines the use of commercial stereotactic infrared (IR) ExacTrac camera system (BrainLAB, Inc., Westchester, IL) for patient setup and monitoring. The Vac Fix mold is placed on the headrest and folded up as needed to provide support before the mold is hardened. For the camera system, a frame referred to as a "tattoo-free immobilization accessory" is fabricated, on which the IR markers can be placed. A patient-specific dental impression is made with the bite tray. The movement of the markers, connected through the dental impression of the patient, accurately represents the overall patient motion. Patient movement is continuously monitored and repositioning is performed whenever patient movement exceeds the predefined tolerance limit. Monitored patient movements are recorded at a certain frequency. Recorded data are analyzed and compared with those of patients immobilized with the thermoplastic facemask plus the camera system that is the standard immobilization system in our clinic.
Results: For three patients treated with the Vac Fix mold plus the camera system, on average, the histogram-based uncertainties, U(95)(5), U(95)(20), and mean displacement, R(mean) (mm) were 1.03, 1.08, and 0.60, respectively. These values are close to those obtained with the mask plus the camera system. The Vac Fix mold plus the camera system often requires more beam interruptions because of repositioning than the mask plus the camera system (on average, the Vac Fix mold plus the camera system required repositioning 7.7 times and the mask plus the camera system required repositioning 1.8 times during 20 treatments).
Conclusion: The Vac Fix mold immobilization procedure plus the camera monitoring system has been set up for patients who are claustrophobic or cannot tolerate a mask during head-and-neck IMRT. Although this system causes more frequent beam delivery interruptions, it is as effective as the mask plus the camera system in immobilizing patients within the tolerance limit.