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. 2012 Jul 7;57(13):4095-115.
doi: 10.1088/0031-9155/57/13/4095. Epub 2012 Jun 7.

Comprehensive analysis of proton range uncertainties related to patient stopping-power-ratio estimation using the stoichiometric calibration

Ming Yang  1 , X Ronald Zhu, Peter C Park, Uwe Titt, Radhe Mohan, Gary Virshup, James E Clayton, Lei Dong
Affiliations

Comprehensive analysis of proton range uncertainties related to patient stopping-power-ratio estimation using the stoichiometric calibration

Ming Yang et al. Phys Med Biol. .

Abstract

The purpose of this study was to analyze factors affecting proton stopping-power-ratio (SPR) estimations and range uncertainties in proton therapy planning using the standard stoichiometric calibration. The SPR uncertainties were grouped into five categories according to their origins and then estimated based on previously published reports or measurements. For the first time, the impact of tissue composition variations on SPR estimation was assessed and the uncertainty estimates of each category were determined for low-density (lung), soft, and high-density (bone) tissues. A composite, 95th percentile water-equivalent-thickness uncertainty was calculated from multiple beam directions in 15 patients with various types of cancer undergoing proton therapy. The SPR uncertainties (1σ) were quite different (ranging from 1.6% to 5.0%) in different tissue groups, although the final combined uncertainty (95th percentile) for different treatment sites was fairly consistent at 3.0-3.4%, primarily because soft tissue is the dominant tissue type in the human body. The dominant contributing factor for uncertainties in soft tissues was the degeneracy of Hounsfield numbers in the presence of tissue composition variations. To reduce the overall uncertainties in SPR estimation, the use of dual-energy computed tomography is suggested. The values recommended in this study based on typical treatment sites and a small group of patients roughly agree with the commonly referenced value (3.5%) used for margin design. By using tissue-specific range uncertainties, one could estimate the beam-specific range margin by accounting for different types and amounts of tissues along a beam, which may allow for customization of range uncertainty for each beam direction.

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Figures

Figure 1
Figure 1
Images of (a) the RMI 467 tissue characterization phantom and (b) the head- and body-size calibration phantoms.
Figure 2
Figure 2
(a) Measured versus theoretical CT numbers of the tissue substitutes; (b) the relative difference between the measured and theoretical CT numbers of the tissue substitutes.
Figure 3
Figure 3
Uncertainties associated with tissue variations. The stoichiometric calibration curve is shown with the reference (triangles) and the individualized (red cloud) human tissues. (a) and (b) are are magnifications of the soft and bone tissue regions, respectively.
Figure 4
Figure 4
Relative error in the calculated proton range using SPRs based on a single energy of 175 MeV (red) and 100 MeV (blue) versus the projected range in water for adipose and cortical bone tissue.
See this image and copyright information in PMC

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