Systematic set-up errors for IMRT in the head and neck region: effect on dose distribution.

BACKGROUND AND PURPOSE: There is a general concern about intensity modulated radiation therapy (IMRT) treatments being more sensitive to patient positioning than conventional treatments. The aim of this study was to evaluate the International Commission on radiation units and measurements (ICRU) method for taking systematic set-up errors into account for IMRT treatments and to compare the effects on the dose distribution with the effects of conventional treatments. MATERIAL AND METHODS: A planning margin to account for set-up errors was added to the clinical target volumes and to the spinal cords, for three head and neck patients, according to the ICRU. No margin was added to organs at risk with mainly parallel structure if they were situated adjacent to the target volume, for example, the parotid glands. The effects of set-up errors in six IMRT plans and three conventional plans were simulated in the planning system and analysed with physical dose parameters. RESULTS AND CONCLUSIONS: In general, the ICRU method of taking set-up errors into account works satisfactorily for IMRT treatments as well as for conventional treatments with no difference between the treatment techniques. The sensitivity to set-up errors regarding the target volume is dependent on the quality of the treatment plan, i.e. the part of the target covered with a dose >95 and <105% and the effect in the critical organs is dependent on the sharpness of the dose gradients outside the critical organ. However, the method makes it difficult to include organs at risk with mainly parallel structure if they are situated adjacent to the target volume.