SU-E-T-738: Verification of an Automated Weighted Sector-Integration Algorithm for Determining Output Factors for Electron Cutouts

Abstract

Purpose: To verify the accuracy of an automated weighted‐sector‐integration electron output factor calculation algorithm for a variety of cone and cutout sizes, beam energies and clinical cutout shapes in a multi‐site radiation medicine department. Methods: A new weighted sector‐integration algorithm for electron output factor estimation has been implemented on a custom database structure. To automate the cutout shape determination, the cutout contour points are imported electronically from the treatment planning system, or digitized on a computer monitor using a digitally reconstructedradiographimage from an Electronic Medical Records system. The contour points are converted to polar coordinates and a sector‐integration summation performed with a weighting scheme corresponding to the angular size of each contour sector. Sector output factors used in the summation process are taken from a database of circular cutout measurements for a range of cone sizes, radii and energies. The algorithm was validated over a range of cutout sizes (2–20 cm), electron cones (10×10 cm2 to 20×20 cm2), cutout shapes (rectangular, circular and irregular including narrow fields and fields with concavities or convexities) and nominal beam energies (6–18 MeV) used in clinical electron beam therapy.Results: Differences between calculated and measured output factors for the patient cutouts studied were less than 2%, except in the case of long and narrow cutout shapes measured at low energies, which remained within 2.5% Conclusions: In a busy multi‐hospital environment, measurement of output factors for electron cutouts places a considerable time burden on physics staff. An estimation method that provides sufficiently accurate output factors for the wide variety of cutouts is a valuable clinical tool. Further work with handling unusually concave and long, narrow shapes at lower energies will be of value to extend the extreme limits of the algorithm's accuracy.