DOSIMETRIC VERIFICATION OF THE 95-PERCENT ISODOSE SURFACE FOR A CONFORMAL IRRADIATION TECHNIQUE

In the treatment planning of conformal radiotherapy, field shapes are often designed in such a way that a high-value isodose surface fully e ncompasses the target volume. Therefore, knowledge about the accuracy with which the treatment planning system calculates the position of th at isodose surface is essential to prevent field shapes which are eith er too large or too small. To determine this accuracy for a conformal multi-field technique, the dose in the high-dose region must be measur ed with a high spatial resolution. A method is presented to reconstruc t and evaluate the experimental high-dose region from a set of water p hantom scans. This method, which assesses combined dose profiles for m ulti-field irradiation techniques, can be used for the commissioning a nd/or quality assurance of a 3-D treatment planning system. For a spec ific conformal technique, the measured and calculated 95% isodose posi tions along lines in several directions have been compared. It is show n that different dose values of single beam profiles determine the res ulting 95% isodose position, which is important to recognize for quali ty assurance of treatment planning calculations. It is further found t hat the uncertainty in the calculated 95% isodose surface can be descr ibed by a standard deviation in dose value, which relates to a positio nal uncertainty through the local dose gradient. Thus the confidence r egion of the calculated 95% isodose can be indicated in the treatment plan by plotting isodoses at the 95% level plus and minus its standard deviation. Such a procedure is recommended instead of plotting the 95 % isodose with a constant width. In addition, restrictions for the cum ulative dose-volume histogram of acceptable treatment plans can be for mulated, based on the sensitivity of the actual target coverage on the uncertainty with which the prescribed isodose surface is calculated. (C) 1998 American Association of Physicists in Medicine. [S0094-2405(9 8)00504-5].