High- and low-density interface modeling in photon radiotherapy: An inter-comparison of EGSnrc Monte Carlo code and prowess panther treatment planning system

In this study, the accuracy of 3D ray tracing algorithm based Prowess Panther treatment planning system (TPS) was benchmarked against that of Monte Carlo (MC) simulations. MC Simulations were carried out in homogeneous and heterogeneous water phantoms with lung equivalent heterogeneities both in presence and absence of bone equivalent heterogeneities. Percentage depth doses (PDDs) were calculated for a) 5×5, b) 10×10, c) 15×15 and d) 20×20 cm2 field sizes with 6 MV photon beam by using IAEA phase space files. For comparison, doses were calculated with the similar phantom geometries using Prowess Panther TPS. The results were found in good agreement with MC results for field sizes of 10×10 cm2 and greater, but deviate in smaller field sizes, especially in presence of heterogeneities. An analysis of average deviation of TPS calculated values from MC values showed that TPS dose calculation is somehow dependent on field sizes, with a severe under-dosage at lower field size and a slight over-dosage as field size increases. The confidence limits (CL) were also calculated for comparison of the two results. For homogeneous phantom, CL decreases with increasing field sizes. For water-lung phantom, initially CL decreases with increasing field sizes but a slight increase is observed afterward. For water-lung-bone phantom, a trend similar to homogeneous water phantom is observed i.e. with an increase in field size, CL reduces.