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 <= 3 mm, and 99% <= 5 mm. The overall systematic error was 0.9 mm (+/- 1.0SD) in the Right-Left, 0.7 mm (+/- 0.90) 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. (C) 2005 Published by Elsevier Ireland Ltd.