Abstract
The highly conformal dose distributions produced by scanned proton pencil beams (PBs) are more sensitive to motion and anatomical changes than those produced by conventional radiotherapy. The ability to calculate the dose in real-time as it is being delivered would enable, for example, online dose monitoring, and is therefore highly desirable. We have previously described an implementation of a PB algorithm running on graphics processing units (GPUs) intended specifically for online dose calculation. Here, we present an extension to the dose calculation engine employing a double-Gaussian beam model to better account for the low-dose halo. To the best of our knowledge, it is the first such PB algorithm for proton therapy running on a GPU. We employ two different parameterizations for the halo dose, one describing the distribution of secondary particles from nuclear interactions found in the literature and one relying on directly fitting the model to Monte Carlo simulations of PBs in water. Despite the large width of the halo contribution, we show how in either case the second Gaussian can be included while prolonging the calculation of the investigated plans by no more than 16%, or the calculation of the most time-consuming energy layers by about 25%. Furthermore, the calculation time is relatively unaffected by the parameterization used, which suggests that these results should hold also for different systems. Finally, since the implementation is based on an algorithm employed by a commercial treatment planning system, it is expected that with adequate tuning, it should be able to reproduce the halo dose from a general beam line with sufficient accuracy.
Highlights
Fast dose calculation finds use in a variety of radiotherapy applications and is an active area of research [1]
It should be noted that, the computational PBs (CPBs) widths are calculated as described in the original publication, the values of the parameters ES and δ, which enter the width calculation as free parameters in the implementation, have to be adjusted in the double-Gaussian beam model
For the Soukup model, the halo fraction is close to 0 at the surface, which means that the width of the primary contribution at the surface should be given by the total pencil beams (PBs) width in air, as was the case for the single-Gaussian beam model
Summary
Fast dose calculation finds use in a variety of radiotherapy applications and is an active area of research [1]. Due to the high level of dose conformity, the small number of treatment fields, and the sensitivity to material changes in the beam path, adaptive treatment techniques relying on fast, repeated dose calculation are of particular interest in proton therapy. Frontiers in Oncology | www.frontiersin.org da Silva et al. Fast Double-Gaussian Dose Calculation would require the calculation time of individual energy layers to be short in comparison with the time between energy layers or the length of a typical motion phase. Fast Double-Gaussian Dose Calculation would require the calculation time of individual energy layers to be short in comparison with the time between energy layers or the length of a typical motion phase For such applications, GPU MC dose calculation on a single workstation remains too slow by at best one, and generally two or more, orders of magnitude. The halos are responsible for the low-dose region further away from the target, which might be of interest when trying to predict the risk of developing side effects or secondary tumors
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