Impact of Multiple Beams on Plan Quality, Linear Energy Transfer Distribution, and Plan Robustness of Intensity Modulated Proton Therapy for Lung Cancer

Abstract

The increase of proton beam number might provide higher degrees of freedom in the optimization of intensity-modulated proton therapy planning. In this study, we aimed to quantitatively explore the potential benefits of the increased beam number, including dose volume histogram (DVH), linear energy transfer volume histogram, and DVH bandwidth metrics. Twelve patients with lung cancer are retrospectively selected. Four plans were created based on internal target volume (ITV) robust optimization for each patient using the RayStation treatment planning system. Four plans were generated using different numbers (three, five, seven, and nine) of evenly separated coplanar beams. The three-beam plan was considered as the reference plan. Biologically equivalent doses were calculated using both constant relative biological effectiveness (RBE) and variable RBE models, respectively. To evaluate plan quality, DVH metrics in the target [ITV: D 2%, CI, HI] and organs-at-risk [Lung: V 5Gy[RBE], V 20Gy[RBE], V 30Gy[RBE]; Heart D 2%; Spinal cord D 2%] were calculated using both RBE models. To evaluate LET distributions, LET volume histogram metrics [ITV LETmean and LET2%; Lung LETmean and LET2%; Heart LET2%; Spinal cord LET2%] were quantified. To evaluate plan robustness, the metrics using DVH bandwidth [ITV: D 2%, D 99%; Lung: V 5Gy[RBE], V 20Gy[RBE], V 30Gy[RBE]; Heart D 2%; Spinal cord D 2%] were also reported. For plan quality, the increase of proton beam number resulted in fewer target hot spots, improved target dose conformity, improved target dose homogeneity, lower median-dose lung volume, and fewer hot spots in spinal cord. As to LET distributions, target mean LET increased significantly as the beam number increased to seven or more. Lung LET hot spots were significantly reduced with the increase of proton beams. With respect to plan robustness, the robustness of target dose coverage, target hot spots, and low-dose lung volume were improved, while the robustness of heart hot spots became worse as the beam number increased to nine. The robustness of cord hot spots became worse using five and seven beams compared to that using three beams. As the proton beam number increased, plan quality and LET distributions were comparable or significantly improved. The robustness of target dose coverage, target dose hot spots, and low-dose lung volume were significantly improved.

Quantitative analysis of dose‐averaged linear energy transfer (LETd) robustness in pencil beam scanning proton lung plans

“…17,22,23 and from 3.1 to 3.2 keV/µm for the lung. 24 For the CTV in our study, the +3.5% and -3.5% range uncertainty scenarios without the setup uncertainties showed a systematic increase and decrease in the LET d , respectively. For the CTV, the variations in the mean LET d were found to be ≤ 0.5 keV/µm, with an average value of 0.4 keV/µm.…”

“…In the current study, we noticed that the mean LET d of the total lung in nominal plans ranged from 2.4 to 4.0 keV/µm, which was slightly higher than the mean LET d (0.8─1.8 keV/µm.) of lung reported by Shang et al 24 for a three-beam arrangement technique. Hahn et al 15 found the OARs of brain cancer patients being the most sensitive to range uncertainties when compared to prostate and head and neck cancer.…”

“…Previous studies 16–24 have explored LET d distributions in proton plans of various disease sites. LET d is very pronounced at the end of the range 1,35 .…”

“…However, the impact of treatment uncertainties on the LET d distributions in lung plans was not considered. 24 Additionally, the impact of different breathing phases on the LET d distributions was not quantified. 24 To date, it is not very clear how the range and setup uncertainties may impact the LET d distributions in robustly optimized (in terms of the range and setup uncertainties) PBS proton lung cancer plans.…”

Validation of automated complex head and neck treatment planning with pencil beam scanning proton therapy

“…Increasing the number of beams can reduce the impact of each of these uncertainties, and studies have shown that increasing the number of beams in a plan can improve robustness. 11,12 Combining these planning techniques; treating with multiple beams, using robust optimization, and optimizing and calculating with MC, can clearly be beneficial for the patient. However, it can be challenging and time consuming for the planning team.…”