Affiliations: Department of Radiation Oncology, University of
Oklahoma Health Sciences Center, Oklahoma City, OK, USA
Note: [] Corresponding author: Salahuddin Ahmad, Ph.D., University of
Oklahoma Health Sciences Center, Department of Radiation Oncology, 825 N.E.
10th, Suite 1430, Oklahoma City, Oklahoma, OK 73104, USA. Tel.: +1 405 271
5641; Fax: +1 405 271 8297; E-mail: salahuddin-ahmad@ouhsc.edu
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 ×
30 cm × 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.
Keywords: Electron output factor, radiation therapy, cancer treatment
