SU‐E‐T‐554: Dosimetric Impact of Implementing Kernel Hardening and Material‐Specific Kernels in the Convolution/Superposition Method

Purpose: To investigate the dosimetric impact of implementing two improvements to the convolution/superposition (C/S) method that are generally ignored in traditional implementations of the algorithm. First, the impact of taking into account spectral changes of the incident photon beam in the energy deposition kernel (EDK) calculation (kernel hardening method), rather than using a single polyenergetic kernel that reflects the spectrum at a single location, was investigated. Second, the impact of implementing material‐specific kernels, rather than performing density scaling of water kernels, was investigated. Methods: To investigate the impact of kernel hardening, depth dose curves were calculated for a clinical 6MV photon beam incident on a water phantom using two different implementations of the collapsed cone C/S method (Mobius Medical Systems, Houston, TX), one using a single polyenergetic kernel and one that fully takes into account spectral changes in the kernel calculation. To investigate the impact of material‐specific kernels, depth dose curves were calculated for a simplified titanium implant geometry (4×4×4 cm³ titanium cavity inside a water phantom) using both a traditional C/S implementation that performs density scaling of water kernels and a novel implementation using titanium kernels (generated using the EGSnrc user code EDKnrc). Results: Implementation of kernel hardening increased the PDD value at 25 cm depth by 2.1% to 5.8% depending on the field size. Implementation of titanium kernels gave a 1.5% higher dose upstream of the metal cavity (i.e., higher backscatter dose) and a 5.9% lower dose downstream of the cavity. Conclusion: Implementation of kernel hardening affected the shape of the C/S‐calculated depth dose curves and thus has the potential to affect beam modeling parameters obtained in the commissioning process. For metal implants, the C/S algorithms generally underestimate the dose upstream and overestimate the dose downstream of the implant; implementation of material‐specific kernels mitigated both of these errors. This work was supported by Public Health Service grants CA010953, CA081647, and CA21661 awarded by the National Cancer Institute, United States Department of Health and Human Services.