Transfer errors of planning CT to simulator: a possible source of setup inaccuracies?

The purpose of this study was to analyse whether the intended patient setup, based on a CT scan, was different from the setup at the simulator. Furthermore, we investigated how these possible transfer errors between the planned patient setup and the actual simulator setup affected the resulting overall treatment setup accuracy. Two groups, of 15 prostate patients each, were studied. For one group (group II), the simulation time was about twice as large as for the other (group I), since digitally reconstructed radiographs (DRRs) were used to get a good visual agreement between the intended and the simulator setup. For the purpose of this study DRRs were also calculated for the patients in group I, and for both groups DRRs were matched with the simulator images to obtain quantitative data of the transfer errors. The resulting overall treatment setup accuracy was determined by comparing the DRRs with portal images. For group I, the standard deviations (SD) of the differences between the DRRs and the simulator images ('transfer errors') were 1.5 mm and 4.5 mm in the lateral (x) and cranio-caudal (y) direction, respectively. For group II the SDs were smaller: 1.4 mm and 1.5 mm in the x- and y-direction, respectively. For both groups, the magnitude of the overall mean was less than 1.3 mm. For group I, the SDs of the resulting overall setup deviations during treatment were 1.6 mm and 4.1 mm in the x- and y-direction, respectively. For group II, these figures were 2.4 mm and 2.6 mm, respectively. For both groups, the magnitude of the overall mean was less than 1.0 mm.(ABSTRACT TRUNCATED AT 250 WORDS)