A new pencil beam model for photon dose calculations

The pencil beam method has been frequently used for dose calculations in intensity modulated radiation therapy (IMRT). In this work, a new pencil model is proposed for photon dose calculations in the heterogeneous media. To avoid the bias caused by oblique kernels and reduce the computation time, the dose distributions are calculated in a spherical coordinate system with pencil kernels of the different source to surface distance (SSD). To improve the accuracy of the dose calculations, the scaling of depth-directed components is modeled by moving the position of the entry point and modifying the value of SSD for a given beamlet. Thus, the lateral components are directly corrected by the density scaling method along the spherical shell without considering the densities in the previous layers. The proposed method was tested on several phantoms including lung-and bone-type heterogeneities. Comparisons with Monte Carlo (MC) simulations were made for several field sizes for 6MV energies beams. Results showed that the proposed method had a good agreement with MC simulations in these phantoms.