Quality assurance for non-invasive patient fixation during stereotactic convergent beam irradiation

Using a non-invasive mask this patient fixation system for stereotactic radiotherapy allows one to perform fractionated irradiation. Measuring the statistical quantities of patient motion and positioning uncertainties and calculating its effect on a spherically symmetric dose distribution showed that there is no severe disadvantage in most cases. Only if small field-size, excentric target volume localisation and extreme proximity to an organ at risk coincides have the statistical effects to be taken into account. The greatest measured standard deviation for positioning uncertainty was 2.3 mm. Its effect on the dose distribution with a field size of 9.1 mm increases the diameter of the 80% isodose to about 115%.