Abstract
Monte Carlo (MC)‐based dose calculations are generally superior to analytical dose calculations (ADC) in modeling the dose distribution for proton pencil beam scanning (PBS) treatments. The purpose of this paper is to present a methodology for commissioning and validating an accurate MC code for PBS utilizing a parameterized source model, including an implementation of a range shifter, that can independently check the ADC in commercial treatment planning system (TPS) and fast Monte Carlo dose calculation in opensource platform (MCsquare). The source model parameters (including beam size, angular divergence and energy spread) and protons per MU were extracted and tuned at the nozzle exit by comparing Tool for Particle Simulation (TOPAS) simulations with a series of commissioning measurements using scintillation screen/CCD camera detector and ionization chambers. The range shifter was simulated as an independent object with geometric and material information. The MC calculation platform was validated through comprehensive measurements of single spots, field size factors (FSF) and three‐dimensional dose distributions of spread‐out Bragg peaks (SOBPs), both without and with the range shifter. Differences in field size factors and absolute output at various depths of SOBPs between measurement and simulation were within 2.2%, with and without a range shifter, indicating an accurate source model. TOPAS was also validated against anthropomorphic lung phantom measurements. Comparison of dose distributions and DVHs for representative liver and lung cases between independent MC and analytical dose calculations from a commercial TPS further highlights the limitations of the ADC in situations of highly heterogeneous geometries. The fast MC platform has been implemented within our clinical practice to provide additional independent dose validation/QA of the commercial ADC for patient plans. Using the independent MC, we can more efficiently commission ADC by reducing the amount of measured data required for low dose “halo” modeling, especially when a range shifter is employed.
Highlights
The use of Pencil Beam Scanning (PBS) is expanding rapidly in proton therapy, in large part because the approach produces highly conformal dose distributions and facilitates optimized delivery, without the requirement of field‐specific hardware such as compensators or apertures, in contrast to conventional double scattering and uniform scanning delivery
Monte Carlo (MC)‐based dose calculation is generally superior to analytical algorithms commonly used in treatment planning system (TPS) in modeling the dose distribution for PBS treatments.[1,2,3]
This is true when protons propagate through bone–soft tissue, soft tissue–air, and bone–air interfaces in treatment sites such as head and neck and lung, as multiple Coulomb scattering (MCS) can lead to a distortion of the field and inadequate target coverage.[2,3]
Summary
The use of Pencil Beam Scanning (PBS) is expanding rapidly in proton therapy, in large part because the approach produces highly conformal dose distributions and facilitates optimized delivery, without the requirement of field‐specific hardware such as compensators or apertures, in contrast to conventional double scattering and uniform scanning delivery. Due to the generation of secondary products as well as the particle transport within the air gap, it is difficult to model the dose calculation with a range shifter analytically given a limited measured data set,[5] an approach such as MC is desirable; the broader MC generated dataset is valuable for analytically approximating the low‐dose halo[5] using multi‐Gaussian lookup tables in water or in air after a range shifter given the magnification of potential MCS and halo calculation inaccuracies by various range shifter thicknesses and air gaps.[7,8]
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