MO-D-BRB-01: Non-Voxel-Based-Broad-Beam (NVBB) Framework for IMRT Treatment Planning-II . Ultra-Fast Intermediate Dose Calculation

Abstract

Purpose:IMRT treatment plan optimization requires a fast yet accurate algorithm to calculate intermediate dose in each iteration. Conventional finite size pencil beam (FSPB) approaches have limitations such as: limited resolution, significant pre‐calculation time, huge memory demand, and limited accuracy in the presence of heterogeneity. In this work, we present a fast dose calculation algorithm that overcomes the limitations of FSPB approaches. Material and Methods: By decomposing the infinitesimal beam dose kernel into the central axis (CAX) and lateral (LSF) components and taking the beam eye view (BEV), we established a Fluence‐map‐Convolution‐Broad‐Beam (FCBB) dose calculation formula. The Collapsed‐cone convolution/superposition (CCCS) doses on water phantom with standard setups and various field sizes are used to derive LSF and CAX, the commissioning data for FCBB. The proposed dose calculation involves a 2D fluence map convolution with LSF followed by table‐lookup with inverse square correction of CAX based on radiological distance calculated via ray‐tracing the density volume. The complexity of FCBB is O(N3) both spatially and temporally, which is orders of magnitude smaller than FSPB in spatial complexity and orders of magnitude faster than CCCS. We implemented FCBB algorithm in C++ language and compared it with CCCS using both simulated and clinical cases. The clinical cases include prostate, H&N and lung patients that were optimized with TomoTherapy TPS. Results: For all tested cases, simulated or clinical, the dose calculation time for CCCS varied from hundreds to thousands of seconds when run on a single PC. It was reduced to sub‐seconds to seconds for FCBB on the same PC. As for the dose differences between FCBB and CCCS, about 90–95% of voxels have Gamma indexes less than 1 for the 3mm/3% criteria. Conclusions: The FCBB algorithm has low memory demand, is ultra‐fast and accurate enough for intermediate dose calculation during IMRT optimization.