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Evaluation of a Heterogeneous Coarse Mesh Photon Transport Method Using a Simplified Lung Model

In this paper we test a new coarse-mesh transport method (COMET) recently developed for the transport of photons in human tissue as would occur in the treatment of patients undergoing radiation therapy. Two small heterogeneous benchmark problems (32 x 20 cm 2 ) are considered here. Both represent two very simple lungs and spinal column embedded in tissue. One benchmark problem uses the lung tissue definition while the other uses an inflated lung tissue definition to fill in the lung region. For both of these, a 4.5 MeV beam was placed along the entire 32 cm side of the problem, impinging normal to the surface. These problems are made up of 160 2x2 cm 2 blocks with each being composed of one of the specified materials. Each global benchmark is then decomposed into 40 coarse-meshes. In this case, each coarse-mesh may be heterogeneous as it may contain more than one material. EGSnrc was then used to obtain response functions for each unique coarse-mesh and stored in a library. COMET was then used to obtain the energy deposited in each region of the problem. Comparing the results to a reference solution obtained from EGSnrc, an average relative error of 0.581% and 0.885% was found for the lung case and inflated lung case, respectively. The maximum relative error was slightly higher for both the lung and inflated lung benchmark problems at 2.137% and 4.373%, respectively. Both of the COMET solutions required around 42 seconds. The lung COMET solution was over 902 times faster than the reference solution, while the inflated lung solution ran 855 times faster than the pure Monte Carlo results.