Po-Thur Eve General-32: Evaluation of calculated total scatter factor based on predicted collimator scatter factor and phantom scatter factor for kilovoltage x-ray radiation fields

Abstract

Kilovoltage x‐ray tube therapy beams are used for treatment of keloids and localized superficial malignancies. The purpose of this work is to calculate the total scatter factor for kilovoltage x‐ray radiation fields based on the predicted collimator scatter factor and the predicted phantom scatter factor. The radiation field is defined by a variable collimator with or without lead sheets on the patient's skin. Measurement and prediction have been done at tube potentials of 60, 100, 120 and 250 kVp. Our approach uses three different sets of field types: square fields defined by the collimator and square and circular fields defined by lead sheets. Two calculation methods are employed: the equivalent field method and Clarkson's method. Calculation and measurement are also done for rectangular and irregular shape fields. It is shown that while the equivalent field method may lead to errors up to about 6% for irregular fields, Clarkson's method is more accurate with errors up to 2.3%. The choice of the field type for the data base is not significant. Using the measured data for square fields defined by the collimator together with Clarkson's method is recommended. The implementation of this method requires a minimum number of measurements, which are acquired during the commissioning of the unit and can be applied in dose calculation for a variety of field shapes and sizes.