Dose intercomparison at the radiotherapy centers in The Netherlands. 2. Accuracy of locally applied computer planning systems for external photon beams.

As part of a quality assurance program in The Netherlands, the performance of computer planning systems was tested. Relative dose values, determined with an ionization chamber, were compared with dose values obtained from locally applied computer planning systems. Several clinically relevant situations were investigated: perpendicular incident beams, wedged beams, oblique incident beams, variable source-surface distances (SSD) and off-axis planes. The mean value of the ratios of calculated to measured dose values is 0.994, with an uncertainty of 2.4% (1 S.D.) and a maximum deviation of 9%, for all combinations of energies, planning systems and geometries investigated. The uncertainty for each situation separately was less than 2% (1 S.D.), except for the wedged beams and off-axis plane, which showed uncertainties of 2.6% (1 S.D.). Part of the additional uncertainty for the wedged beams originates from the value chosen for the wedge factor. Systematic deviations between calculated and measured dose values were investigated for three commercially available planning systems, separately. The mean deviation was smaller than 1% (1 S.D.), for most situations. Only for the wedged beams, larger deviations, up to a mean deviation of 2.6%, were observed.