SU‐FF‐T‐410: Simulation of Large X‐Ray Fields Using Independently Measured Source and Geometry Details

Abstract

Purpose: Measure and simulate dose distributions from a 6 and 18 MV x‐ray linac with open jaws, including asymmetry, with independent measurements of the source and geometry details. Materials and Methods: Energy of the incident electron beam was measured by removing the target and flattening filter and measuring the percent‐depth ionization curve of the raw beam. Spot size was measured with an alternating layer spot camera. Treatment head parameters were determined from disassembly of the treatment head, or deduced from the penumbra position. Flattening filter density was determined by weighing it. Distance between target and flattening filter determined from position of the penumbra in the diagonal profiles. Simulations were with EGSnrc. Measurements and simulations were done both with and without the flattening filters. Results: Measured and simulated dose distributions inside the treatment field agreed to 1%/1mm. The exception was the buildup region with no flattening filters, where measurements were greater than simulation by 3–5%, corresponding to a 1 mm shift. This suggests the Roos chamber over‐responded to low energy electrons. Conclusions: Close agreement betweem measurement and simulation was obtained, with simulation parameters determined independently. It was not necessary to remove the flattening filters to obtain the simulation parameters.Support from NIH R01 CA104777‐01A2 and Siemens OCS.