measurement of the immobilisation efficacy of a head fixation system

background: in order to assign appropriate planning target volume (ptv) margins, each centre should measure the patient positioning deviations for their set-up techniques. at the royal marsden hospital, uk, a conformal shell (cast) system is used when a stereotactic frame is not suitable. in this paper, we report on a series of measurements with the aim of obtaining the systematic and random components of positioning error when using the above-mentioned shell system. materials and methods: the verification protocol was based on orthogonal pairs of anterior-posterior and lateral electronic portal images (epis) used to check the isocentre position. the isocentre verification results of paediatric patients were analysed. a practical ‘off-line’ patient set-up correction strategy had been used with the aim of reducing systematic errors. the verification protocol involved epi acquisition on the first three fractions and then on a weekly basis. additional images were taken if an isocentre movement was applied based on a 3 mm tolerance for a consistent 1d discrepancy. results: four patients required isocentre corrections ranging between 2 mm and 4 mm. following the off-line corrections, the residual systematic errors in each direction were within 0.5 mm while the 1d random variation was about 1.0 mm. conclusions: the head fixation system in conjunction with the correction strategy successfully kept the random and systematic positioning errors within an acceptable level well within the 3 mm tolerance. the measured components of positioning error can be used to define appropriate ptv margins.