@article{5abe5e6b955e4004866be8060bda93bc,
title = "PTV-based IMPT optimization incorporating planning risk volumes vs robust optimization",
abstract = "Purpose: Robust optimization leads to intensity-modulated proton therapy (IMPT) plans that are less sensitive to uncertainties and superior in terms of organs-at-risk (OARs) sparing, target dose coverage, and homogeneity compared to planning target volume (PTV)-based optimized plans. Robust optimization incorporates setup and range uncertainties, which implicitly adds margins to both targets and OARs and is also able to compensate for perturbations in dose distributions within targets and OARs caused by uncertainties. In contrast, the traditional PTV-based optimization considers only setup uncertainties and adds a margin only to targets but no margins to the OARs. It also ignores range uncertainty. The purpose of this work is to determine if robustly optimized plans are superior to PTV-based plans simply because the latter do not assign margins to OARs during optimization. Methods: The authors retrospectively selected from their institutional database five patients with head and neck (HN) cancer and one with prostate cancer for this analysis. Using their original images and prescriptions, the authors created new IMPT plans using three methods: PTV-based optimization, optimization based on the PTV and planning risk volumes (PRVs) (i.e., {"}PTV+PRV-based optimization{"}), and robust optimization using the {"}worst-case{"} dose distribution. The PRVs were generated by uniformly expanding OARs by 3 mm for the HN cases and 5 mm for the prostate case. The dose-volume histograms (DVHs) from the worst-case dose distributions were used to assess and compare plan quality. Families of DVHs for each uncertainty for all structures of interest were plotted along with the nominal DVHs. The width of the {"}bands{"} of DVHs was used to quantify the plan sensitivity to uncertainty. Results: Compared with conventional PTV-based and PTV+PRV-based planning, robust optimization led to a smaller bandwidth for the targets in the face of uncertainties {clinical target volume [CTV] bandwidth: 0.59 [robust], 3.53 [PTV+PRV], and 3.53 [PTV] Gy (RBE)}. It also resulted in higher doses to 95% of the CTV {D95%: 60.8 [robust] vs 59.3 [PTV+PRV] vs 59.6 [PTV] Gy (RBE)}, smaller D5% (doses to 5% of the CTV) minus D95% {D5% - D95%: 13.2 [robust] vs 17.5 [PTV+PRV] vs 17.2 [PTV] Gy (RBE)}. At the same time, the robust optimization method irradiated OARs less {maximum dose to 1 cm3 of the brainstem: 48.3 [robust] vs 48.8 [PTV+PRV] vs 51.2 [PTV] Gy (RBE); mean dose to the oral cavity: 22.3 [robust] vs 22.9 [PTV+PRV] vs 26.1 [PTV] Gy (RBE); maximum dose to 1% of the normal brain: 66.0 [robust] vs 68.0 [PTV+PRV] vs 69.3 [PTV] Gy (RBE)}. Conclusions: For HN cases studied, OAR sparing in PTV+PRV-based optimization was inferior compared to robust optimization but was superior compared to PTV-based optimization; however, target dose robustness and homogeneity were comparable in the PTV+PRV-based and PTV-based optimizations. The same pattern held for the prostate case. The authors' data suggest that the superiority of robust optimization is not due simply to its inclusion of margins for OARs, but that this is due mainly to the ability of robust optimization to compensate for perturbations in dose distributions within target volumes and normal tissues caused by uncertainties.",
keywords = "IMPT, planning risk volume, robust optimization, robustness evaluation",
author = "Wei Liu and Frank, {Steven J.} and Xiaoqiang Li and Yupeng Li and Zhu, {Ron X.} and Radhe Mohan",
note = "Funding Information: The authors would also like to thank Ms. Kathryn Carnes from the Department of Scientific Publications at their institution for editorial review of this paper. This research was supported by the National Cancer Institute Grant No. P01CA021239, by the University Cancer Foundation via the Institutional Research Grant program at The University of Texas MD Anderson Cancer Center, by the National Cancer Institute of the National Institutes of Health under Award No. K25CA168984, and by the MD Anderson Cancer Center support Grant No. CA016672 from the NCI. The content is solely the responsibility of the authors and does not necessarily represent the official views of the National Institutes of Health. FIG. 1. Dose distributions in the transverse plane for a representative patient illustrate the insensitivity of the robustly optimized plan (left column) to range and setup uncertainties compared with the conventional PTV+PRV-based optimized plan (middle column) and the conventional PTV-based optimized plan (right column). Top panels (a), (d), and (g) show dose distributions in nominal position; whereas the middle panels (b), (e), and (h) show corresponding data with 3.5% larger range and the bottom panels (c), (f), and (i) are for 3.5% larger range and 3 mm superior shift. Color scheme is CTV- middle shadowed area, Brainstem- bottom shadowed area. Isodose lines (red 100%) in the PTV-based and PTV+PRV-based plans are perturbed to a significantly greater degree than in robustly optimized plan. For instance, CTV is not adequately covered with the prescribed dose in panels (e), (f), (h), and (i). FIG. 2. Color wash represents the DVH bands for dose distributions covering all setup and range uncertainties for the CTV and various organs in the robustly optimized plan (left column), the PTV-based plan (middle), and the PTV+PRV-based plan (right) for one H&N case. The solid lines are the DVHs for the nominal dose distribution (i.e., without consideration of uncertainties). The narrowness of the CTV bands for the robustly optimized plan relative to the others indicates its superior robustness. Sparing of the brainstem, oral cavity, and optic chiasm was also perceptibly better. FIG. 3. Left of dashed line: Sparing of OARs resulting from use of the three planning methods. D 1cc is shown for the spinal cord and brainstem, D mean for the oral cavity and right parotid, and D 1% for other organs. Right of dashed line: Target (CTV) doses resulting from use of the three methods. CTV D 95% is from the dose distribution obtained by choosing the minimum of the 21 doses in each voxel in the CTV and CTV D 5% − D 95% . CTV D 5% is from the dose distribution obtained by choosing the maximum of the 21 doses in each voxel in the CTV. FIG. 4. Color wash represents the DVH bands for dose distributions covering all setup and range uncertainties for the CTV and various organs in the robustly optimized plan (left column), the PTV-based plan (middle), and the PTV+PRV-based plan (right) for the prostate case. The solid lines are the DVHs for the nominal dose distribution (i.e., without consideration of uncertainties). The narrowness of the CTV bands for the robustly optimized plan relative to the other plans indicates its superior robustness. Sparing of the rectum, bladder, and pubic symphysis was also perceptibly better. TABLE I. Patient prescriptions, target volumes, and beam angles. Prescriptions (Gy) Target volumes (cm 3 ) Beam angles (gantry, couch) Patient Number of fractions GTV CTV GTV CTV Field 1 Field 2 Field 3 1 33 66 16.8 245 0 ,0 0 285 0 ,40 0 100 0 ,0 0 2 35 70 66 10.7 45.8 60 0 ,0 0 290 0 ,0 0 320 0 ,0 0 3 30 66 66 3.0 179.1 180 0 ,90 0 310 0 ,30 0 70 0 ,340 0 4 33 70 70 5.2 20.12 290 0 ,20 0 70 0 ,340 0 285 0 ,90 0 5 37 74 74 5.5 20.9 300 0 ,90 0 270 0 ,0 0 75 0 ,0 0 6 a 28 50.4 50.4 9.6 42.3 315 0 ,45 0 45 0 ,315 0 180 0 ,0 0 a Prostate case. TABLE II. Beam parameters used in the optimization of IMPT plans. Axial margins a (cm) Lateral margins a (cm) Nominal energy Number of Spot spacing Patient Fields (proximal, distal) (X1, Y1, X2, Y2) (MeV) layers (cm) 1 Field 1 0.0, 0.0 0.9, 0.9, 0.9, 0.8 136.4 43 1.08 Field 2 0.0, 0.0 0.9, 0.9, 0.9, 0.8 129.2 39 Field 3 0.0, 0.0 0.9, 0.9, 0.9, 0.8 159.5 53 2 Field 1 0.3, 0.3 1.0, 1.0, 1.0, 1.0 131.0 40 0.5 Field 2 0.3, 0.3 1.0, 1.0, 1.0, 1.0 144.9 42 Field 3 0.3, 0.3 0.8, 0.8, 0.4, 0.8 141.6 46 3 Field 1 0.0, 0.0 1.0, 1.0, 1.0, 1.0 206.3 53 0.7 Field 2 0.0, 0.0 1.0, 1.0, 1.0, 1.0 203.7 58 Field 3 0.0, 0.0 1.0, 1.0, 1.0, 1.0 201.0 54 4 Field 1 0.0, 0.0 0.9, 0.9, 0.9, 0.9 122.5 24 0.7 Field 2 0.0, 0.0 0.9, 0.9, 0.5, 0.5 143.2 33 Field 3 0.0, 0.0 0.5, 0.9, 0.9, 0.9 153.2 21 5 Field 1 0.4, 0.8 0.7, 0.7, 0.7, 0.7 206.3 20 0.7 Field 2 0.6, 0.9 0.6, 0.6, 0.6, 0.6 206.3 29 Field 3 0.8, 1.0 0.7, 0.7, 0.7, 0.7 203.7 25 6 b Field 1 0.2, 0.2 1.0, 1.0, 1.0, 1.0 125.6 51 0.5 Field 2 0.2, 0.2 1.0, 1.0, 1.0, 1.0 131.0 54 Field 3 0.2, 0.2 1.0, 1.0, 1.0, 1.0 131.0 37 a Margins are used for placement of spots to ensure target coverage. b Prostate case. ",
year = "2013",
month = feb,
doi = "10.1118/1.4774363",
language = "English (US)",
volume = "40",
journal = "Medical physics",
issn = "0094-2405",
publisher = "AAPM - American Association of Physicists in Medicine",
number = "2",
}