SU-E-T-737: Accurately Estimating Electron Cutout Factors Using Only the Aperture Length and Width

Abstract

Purpose: To describe a calculation technique which can accurately approximate the cutout factor for an electron cutout using only the length and width of the cutout.Methods: Cerrobend electron cutouts for 10×10cm and 15×15cm applicators were fabricated with centered circular apertures having integer diameters from 2cm to 15cm. A Markus‐type ionization chamber in a solid water phantom was used to measure the cutout factors for these cutouts at five beam energies. The data was used model cutout factor as a function of cutout diameter. The proposed estimation method approximates an irregularly‐shaped custom cutout as an ellipse or rectangle, and requires measuring only the length and width of the aperture. A spreadsheet program was designed to calculate 360 radii of a rectangle or ellipse with the given dimensions. For each radius, the program calculates the cutout factor for an equivalent circle; the average is an estimate of the overall cutout factor. To test the accuracy of the spreadsheet, cutout factors for twenty‐seven 10×10 cm and fifteen 15x15 cm patient cutouts were measured at five energies. The length and width of each cutout were entered into the spreadsheet program to estimate the cutout factor, which was compared with the measured factor. Results: The mean absolute percent difference for the elliptical estimation method was 0.43% and 0.41% for the 10×10 cm and 15×15 cm cutouts, respectively. The mean absolute percent difference for the rectangular estimation method was 0.38% and 0.48% for the 10×10 cm and 15×15 cm cutouts, respectively. Conclusions: This work indicates that the length and width of an irregularly‐shaped aperture can be used to accurately estimate its cutout factor by approximating it as an ellipse or rectangle. The use of this technique improves the efficiency and accuracy of calculating electron cutout factors.