A VERIFICATION PROCEDURE TO IMPROVE PATIENT SET-UP ACCURACY USING PORTAL IMAGES

The purpose of this study was to establish which level of geometrical accuracy can be obtained during radiotherapy, using portal image analysis, with a minimum number of patient set-up measurements and corrections. A set-up verification and correction procedure using decision rules for improving the set-up of a patient during radiotherapy was investigated by means of a computer simulation. In this simulation study, set-up deviations were assumed to be the sum of random and systematic deviations and Varying ratios of random and systematic deviations were studied. The distribution of random deviations (SD equal to sigma) was assumed to be equal for all patients of a specific treatment site. Set-up deviations are measured during the first N consecutive fractions after the start of the treatment or after a patient set-up correction. A set-up is corrected when the deviation averaged over these measurements is larger than an N-dependent action level. This action level is specified by alpha/root N, in which alpha is a variable initial action level parameter. After the start of the treatment or after each correction, N-max measurements are made to decide on a possible (further) correction. By varying alpha and N-max, the relation between the overall accuracy and the workload has been analyzed. It was possible to obtain a resulting overall accuracy level which is almost independent of the initial distribution of systematic deviations. When the standard deviation Sigma of the distribution of systematic deviations ranges from sigma to 3 sigma, the parameters alpha = 2 sigma and N-max = 2 result in an accuracy with less than 5% of the set-ups larger than 3 sigma. Using these parameters, for Sigma = 2 sigma, 3.2 measurements and 0.9 corrections per patient on average are required. The probability that a patient has to be corrected more than once is 22.5%. A specific set-up with a initial large systematic deviation equal to 4.5 sigma results in an average deviation smaller than 2 sigma with a probability equal to 99%.