Clinical implementation of an empirical method for electron output factor determination

Abstract

The objective of this work has been to develop and implement an empirical calculation method for the determination of clinical electron output factors. Electron beams with various energies, field sizes, and source to surface distances using cutouts of varying radii were used to measure dose output at the depth of maximum dose in water. A 30 cm x 30 cm x 17.8 cm water equivalent phantom with a 0.125 cc cylindrical ion-chamber (PTW Model 31010) was used. The calculation model predicted the output factor as a product of the cone factor, radius dependent cutout factor, the effective source to surface distance factor and the area dependent aspect ratio factor. A comparative analysis of clinical cutout output factors, determined through both empirical calculation and direct measurement was performed to evaluate the clinical viability of the calculation method before its implementation in our clinic. A total of 643 output factors for 294 different cutout shapes were determined through both traditional measurement and predictive calculation. Predictive calculation differed from definitive measurement by at most 3.5% for all cases, a majority of cases falling within 1%. The method developed successfully predicts electron output factors on the basis of cutout geometry with accuracy better than 96% for all cases and better then 98% for most cases. This ability holds true for all practical SSD, electron energy, cone, and irregular shape combinations. The method has been clinically implemented and in use at our center since 2007.