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.