ABSTRACT
Objective To investigate the correction of manual monitor unit calculation for asymmetric fields using the Varian enhanced dynamic wedge.Methods Monitor unit (MU) was calculated when the field sizes ranged from 6 cm × 6 cm to 20 cm × 20 cm at a depth of 5 cm using Varian Eclipse and both 6 MV and 10 MV X-rays data from Varian Clinac 23EX for all seven available EDW angles,including 10°15°,20°,25°,30°,45°and 60° The field size was kept fixed,and the distance between geometry center of field and isocenter was increased in increments of 1 cm,ranging from -9 cm to 4 cm.When the field size was the same,the correction factor was defined as the ratio of MU calculated for asymmetric field to monitor unit calculated for symmetric field.To ensure the correction factors obtained above could be used in routine manual calculation for EDW fields,measurements were made at a depth of 5 cm for 30°and 45°EDW with field size of 10 cm × 10 cm using 6 MV X-rays.Results The correction factor was independent of field dimensions,so the average value was adopted to make practical calculation.Without correction,the maximum error was 18% for 30°,and 30% for 45.After the results of monitor unit calculation were corrected,the largest error was - 1.8% and - 1.7% for 30° and 45°EDW,respectively.The magnitude of errors was within the clinical tolerance limits.Conclusions For asymmetric EDW fields,there is very large difference between the prescribed dose by manual calculation using EDW factors measured for symmetric fields and that delivered during treatment in order to obtain correct dose to reference point.The errors are decreased to be acceptable after correction.The method of correction is simple and independent of machine specific beam parameters.
ABSTRACT
We developed a user-friendly program to independently verify monitor units (MUs) calculated by radiation treatment planning systems (RTPS), as well as to manage beam database in clinic. The off-axis factor, beam hardening effect, inhomogeneity correction, and the different depth correction were incorporated into the program algorithm to improve the accuracy in calculated MUs. A beam database in the program was supposed to use measured data from routine quality assurance (QA) processes for timely update. To enhance user's convenience, a graphic user interface (GUI) was developed by using Visual Basic for Application. In order to evaluate the accuracy of the program for various treatment conditions, the MU comparisons were made for 213 cases of phantom and for 108 cases of 17 patients treated by 3D conformal radiation therapy. The MUs calculated by the program and calculated by the RTPS showed a fair agreement within +/-3% for the phantom and +/-5% for the patient, except for the cases of extreme inhomogeneity. By using Visual Basic for Application and Microsoft Excel worksheet interface, the program can automatically generate beam data book for clinical reference and the comparison template for the beam data management. The program developed in this study can be used to verify the accuracy of RTPS for various treatment conditions and thus can be used as a tool of routine RTPS QA, as well as independent MU checks. In addition, its beam database management interface can update beam data periodically and thus can be used to monitor multiple beam databases efficiently.