[Evaluation of a method for correcting systematic setup error in external-beam radiation therapy].

BACKGROUND AND PURPOSE We verified the propriety of our systematic error reduction strategy by means of a computer simulation based on our data of position error with a prone fixation device for prostate IMRT. MATERIALS AND METHODS Computer simulations of the off-line correction method for systematic setup errors based on the portal imaging taken on the first several days of the treatment session were performed. Using the computer simulations, an optimal number of portal images were evaluated for the SD value, from 0.5 mm to 1.5 mm at a 0.25 mm interval, and the respective required setup margins were calculated. RESULTS The value of systematic error was reduced as the frequency of data obtained increased. Moreover, the reduction rate was so remarkable that random error was large.