Correction of manual monitor unit calculation for asymmetric fields using Varian enhanced dynamic wedge / 中华放射医学与防护杂志

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.