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. 2020 Feb;21(2):26-37.
doi: 10.1002/acm2.12803. Epub 2020 Jan 3.

Sensitivity analysis of Monte Carlo model of a gantry-mounted passively scattered proton system

Milad Baradaran-Ghahfarokhi  1 , Francisco Reynoso  1 , Michael T Prusator  1 , Baozhou Sun  1 , Tianyu Zhao  1
Affiliations

Sensitivity analysis of Monte Carlo model of a gantry-mounted passively scattered proton system

Milad Baradaran-Ghahfarokhi et al. J Appl Clin Med Phys. 2020 Feb.

Abstract

Purpose: This study aimed to present guidance on the correlation between treatment nozzle and proton source parameters, and dose distribution of a passive double scattering compact proton therapy unit, known as Mevion S250.

Methods: All 24 beam options were modeled using the MCNPX MC code. The calculated physical dose for pristine peak, profiles, and spread out Bragg peak (SOBP) were benchmarked with the measured data. Track-averaged LET (LETt ) and dose-averaged LET (LETd ) distributions were also calculated. For the sensitivity investigations, proton beam line parameters including Average Energy (AE), Energy Spread (ES), Spot Size (SS), Beam Angle (BA), Beam Offset (OA), and Second scatter Offset (SO) from central Axis, and also First Scatter (FS) thickness were simulated in different stages to obtain the uncertainty of the derived results on the physical dose and LET distribution in a water phantom.

Results: For the physical dose distribution, the MCNPX MC model matched measurements data for all the options to within 2 mm and 2% criterion. The Mevion S250 was found to have a LETt between 0.46 and 8.76 keV.μm-1 and a corresponding LETd between 0.84 and 15.91 keV.μm-1 . For all the options, the AE and ES had the greatest effect on the resulting depth of pristine peak and peak-to-plateau ratio respectively. BA, OA, and SO significantly decreased the flatness and symmetry of the profiles. The LETs were found to be sensitive to the AE, ES, and SS, especially in the peak region.

Conclusions: This study revealed the importance of considering detailed beam parameters, and identifying those that resulted in large effects on the physical dose distribution and LETs for a compact proton therapy machine.

Keywords: MCNP; Monte Carlo simulation; passively scattered proton; sensitivity analysis.

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Figures

Figure 1
Figure 1
A cross‐section view of the simulated Mevion S250 treatment nuzzle for the deep group (distance from the proton source to the isocenter was 205 cm).
Figure 2
Figure 2
Comparison between the MCNPX derived (red) and measured (black) lateral beam profiles for options 6 (large group), 13 (deep group), and 24 (small group). The lateral beam dose profiles for options 13 and 24 are multiplied by 0.8 and 0.6, respectively, to show all three comparisons in one graph.
Figure 3
Figure 3
Distributions of LETt (green) and LETd (blue) as a function of depth for each main Bragg peak of the options 3 (large group), 14 (deep group), and 23 (small group).
Figure 4
Figure 4
The absolute difference (ΔR) between the baseline and MC derived depth of D90% as a function of changes in the AE for large, deep, and small groups.
Figure 5
Figure 5
Variations of the flatness of lateral profile for small groups due to 9% increase in the AE.
Figure 6
Figure 6
Absolute difference between the baseline and sensitivity derived width of the pristine peaks (a) and peak‐to‐plateau ratios (b) due to change in the ES for the studied groups (large, deep, and small). Peak‐to‐plateau ratio was derived based on the ratio of the peak‐to‐plateaus of sensitivity results to the baseline values (peak‐to‐plateauSensitivity/peak‐to‐plateauBaseline).
Figure 7
Figure 7
The increase in distal width and decrease in peak‐to‐plateau ratio of the pristine peaks due to uncertainty associated with SS in large, deep, and small groups. Increase in distal width was calculated based on the absolute difference of the baseline and sensitivity derived results. Peak‐to‐plateau ratio was derived based on the ratio of the peak‐to‐plateaus of sensitivity results to the baseline values (peak‐to‐plateauSensitivity/peak‐to‐plateauBaseline).
Figure 8
Figure 8
Variations of the peak‐to‐plateau ratio due to changes of the incident angle of the proton beam. Peak‐to‐plateau ratio was derived based on the ratio of the peak‐to‐plateaus of sensitivity results to the baseline values (peak‐to‐plateauSensitivity/peak‐to‐plateauBaseline).
Figure 9
Figure 9
Variations of the flatness and symmetry of the large group profiles due to uncertainty in the OA.
Figure 10
Figure 10
Changes in the flatness (a) and symmetry (b) of the profiles due to uncertainty associated with second scatter offset from the central axis.
Figure 11
Figure 11
Changes of the depth of D90% as a function of FS thickness for large and deep options.
Figure 12
Figure 12
Changes of the LETd (dash line) and LETt (solid line) distributions due to increase in the AE for the large group.
Figure 13
Figure 13
Changes of the LETd (dash line) and LETt (solid line) distributions due to increase in the ES for the large group.
Figure 14
Figure 14
Variations of the LETd (dash line) and LETt (solid line) distributions due to increase in the SS for the small group.
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