Assessment of a customised immobilisation system for head and neck IMRT using electronic portal imaging.

PURPOSE: To evaluate set-up reproducibility of a cabulite shell and determine CTV-PTV margins for head and neck intensity-modulated-radiotherapy. MATERIALS AND METHODS: Twenty patients were entered into the study. A total of 354 anterior and lateral isocentric electronic portal images (EPIs) were compared to simulator reference images. RESULTS: About 94% of all translational displacements were < or =3 mm, and 99% < or =5 mm. The overall systematic error was 0.9 mm (+/-1.0SD) in the Right-Left, 0.7 mm (+/-0.9SD) in the Superior-Inferior and -0.02 mm (+/-1.1SD) in the Anterior-Posterior directions. The corresponding SDs of the random errors were +/-0.4, +/-0.6 and +/-0.7 mm. The estimated margins required from CTV-PTV were calculated according to the Van Herk formula was 2.9, 2.6 and 3.3 mm, respectively. CONCLUSIONS: This head and neck immobilisation system is of sufficient accuracy for its use with IMRT treatments and a 3 mm CTV-PTV margin has been adopted.