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

Abstract

To evaluate set-up reproducibility of a cabulite shell and determine CTV-PTV margins for head and neck intensity-modulated-radiotherapy. 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. 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. 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.