Abstract
Purpose: To develop a hybrid stochastic‐deterministic method, COMET‐PE, for dose calculation in external beam photon radiotherapy. Methods: To calculate dose, COMET‐PE solves the coupled Boltzmann Transport Equations for photons and electrons. The method uses a deterministic iteration to compose response functions that are pre‐computed using Monte Carlo. Thus, COMET‐PE takes advantage of Monte Carlo physics without incurring the computational costs typically required for statistical convergence. Dose distributions are calculated for a heterogeneous benchmark problem using both COMET‐PE and DOSXYZnrc (Monte Carlo) methods. The benchmark consists of a CT‐based lung phantom, composed of air, lung, soft tissue, and bone, irradiated by a 2 cm × 2 cm photon field. The 6 MV source spectrum comes from Monte Carlo simulation of a Varian Clinac 2100. The COMET‐PE solution is computed at resolution of 1 mm (27,648,000 voxels) before being reduced to a resolution of 5 mm (221,184 voxels) for compatibility with the DOSXYZnrc reference solution. Results: The agreement between dose distributions calculated with COMET‐PE and Monte Carlo is excellent. Of voxels receiving greater than 10% of the maximum dose, 98.73% pass the 2% (point‐wise relative difference) or 2 mm (distance‐to‐agreement) criterion and 99.38% pass the 3% / 3 mm criterion. Localized discrepancies are observed at the beam corners; these are caused by the difficulty of using a continuous representation for the discontinuous primary fluence. Most of the failures, however, occur where the beam exits the phantom and where Monte Carlo uncertainties are the highest. The COMET‐PE calculation is over 10 times faster than the Monte Carlo reference solution. Conclusion: The COMET‐PE method calculates dose with accuracy comparable to Monte Carlo while using only a fraction of the time and providing a solution with orders of magnitude more detail.
Published Version
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