Characterization of Generalized Elliptical Sources in a Medical Linear Accelerator by Using the BEAMnrc/EGSnrc Monte Carlo System

Two recently developed elliptical source models were generalized in the BEAMnrc/EGSnrc Monte Carlo (MC) system to investigate both source shape and off-axis effects. These models are extensions of a previous study in which we showed the applicability of a center-oriented elliptical Gaussian source model in linac head simulations. The two-dimensional (2D) transformation and bivariate Gaussian distribution methods were applied to both uniform and Gaussian elliptical source models to move the center and to rotate the shape of the sources. To validate the models, two 2D transformation sources and six bivariate uniform/Gaussian sources were used in the thin target model. The shapes and the distributions for all the sources were well produced, as expected. Then, we investigated the 2D transformation source model effect in linac head simulations; the 6-MV photon beam of a Varian 21EX linac head model was used with four different source locations/shapes in BEAMnrc. Two phase space files were obtained for each source at source-to-surface distances of 27 cm and 100 cm. Planar and energy fluence profiles were acquired and analyzed to see the fluence variations by using the first phase space file. The second phase space file was re-used to calculate the dose distribution for field sizes of 10 × 10, 20 × 20, and 30 × 30 cm2 in the DOSXYZnrc user code. In the results, a reverse planar fluence effect was observed while energy fluences corresponded to the location of each source. The dose distributions conformed well to the energy fluence distributions of the individual sources. The generalized elliptical source models could be useful in understanding the dosimetric variation of elliptical sources as functions of position and shape in linac head MC simulations. KCI Citation Count: 0