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Purpose: To present a proton computed tomography (pCT) reconstruction approach that models the integral depth dose (IDD) of the clinical scanning proton beam into beamlets. Using a multilayer ionization chamber (MLIC) as the imager, the proposed pCT system and the reconstruction approach can minimize extra ambient neutron dose and simplify the beamline design by eliminating an additional collimator to confine the proton beam. Methods: Monte Carlo simulation was applied to digitally simulate the IDDs of the exiting proton beams detected by the MLIC. A forward model was developed to model each IDD into a weighted sum of percentage depth doses of the constituent beamlets separated laterally by 1 mm. The water equivalent path lengths (WEPLs) of the beamlets were determined by iteratively minimizing the squared L2‐norm between the forward projected and simulated IDDs. The final WEPL values were reconstructed to pCT images, that is, proton stopping power ratio (SPR) maps, through simultaneous algebraic reconstruction technique with total variation regularization. The reconstruction process was tested with a digital cylindrical water‐based phantom and an ICRP adult reference computational phantom. The mean of SPR within regions of interest (ROIs) and the WEPL along a 4 mm‐wide beam (WEPL4mm${\rm{WEP}}{{\rm{L}}_{4{\rm{mm}}}}$) were compared with the reference values. The spatial resolution was analyzed at the edge of a cortical insert of the cylindrical phantom. Results: The percentage deviations from reference SPR were within ±1% in all selected ROIs. The mean absolute error of the reconstructed SPR was 0.33%, 0.19%, and 0.27% for the cylindrical phantom, the adult phantom at the head and lung region, respectively. The corresponding percentage deviations from reference WEPL4mm${\rm{WEP}}{{\rm{L}}_{4{\rm{mm}}}}$ were 0.48 ± 0.64%, 0.28 ± 0.48%, and 0.22 ± 0.49%. The full width at half maximum of the line spread function (LSF) derived from the radial edge spread function (ESF) of a cortical insert was 0.13 cm. The frequency at 10% of the modulation transfer function (MTF) was 6.38 cm–1. The mean signal‐to‐noise ratio (SNR) of all the inserts was 2.45. The mean imaging dose was 0.29 and 0.25 cGy at the head and lung region of the adult phantom, respectively. Conclusion: A new pCT reconstruction approach was developed by modeling the IDDs of the uncollimated scanning proton beams in the pencil beam geometry. SPR accuracy within ±1%, spatial resolution of better than 2 mm at 10% MTF, and imaging dose at the magnitude of mGy were achieved. Potential side effects caused by neutron dose were eliminated by removing the extra beam collimator. [ABSTRACT FROM AUTHOR]

Purpose: This paper presents a novel method for the calculation of three‐dimensional (3D) Bragg–Gray water‐to‐detector stopping power ratio (sw,det) distributions for proton and carbon ion beams. Methods: Contrary to previously published fluence‐based calculations of the stopping power ratio, the sw,det calculation method used in this work is based on the specific way GATE/Geant4 scores the energy deposition. It only requires the use of the so‐called DoseActor, as available in GATE, for the calculation of the sw,det at any point of a 3D dose distribution. The simulations are performed using GATE‐RTion v1.0, a dedicated GATE release that was validated for the clinical use in light ion beam therapy. Results: The Bragg–Gray water‐to‐air stopping power ratio (sw,air) was calculated for monoenergetic proton and carbon ion beams with the default stopping power data in GATE‐RTion v1.0 and the new ICRU90 recommendation. The sw,air differences between the use of the default and the ICRU90 configuration were 0.6% and 5.4% at the physical range (R80 — 80% dose level in the distal dose fall‐off) for a 70 MeV proton beam and a 120 MeV/u carbon ion beam, respectively. For protons, the sw,det results for lithium fluoride, silicon, gadolinium oxysulfide, and the active layer material of EBT2 (radiochromic film) were compared with the literature and a reasonable agreement was found. For a real patient treatment plan, the 3D distributions of sw,det in proton beams were calculated. Conclusions: Our method was validated by comparison with available literature data. Its equivalence with Bragg–Gray cavity theory was demonstrated mathematically. The capability of GATE‐RTion v1.0 for the sw,det calculation at any point of a 3D dose distribution for simple and complex proton and carbon ion plans was presented. [ABSTRACT FROM AUTHOR]

Purpose: To study the accuracy with which proton stopping power ratio (SPR) can be determined with dual‐energy computed tomography (DECT) for small structures and bone–tissue–air interfaces like those found in the head or in the neck. Methods: Hollow cylindrical polylactic acid (PLA) plugs (3 cm diameter, 5 cm height) were 3D printed containing either one or three septa with thicknesses tsepta = 0.8, 1.6, 3.2, and 6.4 mm running along the length of the plug. The cylinders were inserted individually into a tissue‐equivalent head phantom (16 cm diameter, 5 cm height). First, DECT scans were obtained using a Siemens SOMATOM Definition Edge CT scanner. Effective atomic number (Zeff) and electron density (ρe) images were reconstructed from the DECT to produce SPR‐CT images of each plug. Second, independent elemental composition analysis of the PLA plastic was used to determine the Zeff and ρe for calculating the theoretical SPR (SPR‐TH) using the Bethe–Bloch equation. Finally, for each plug, a direct measurement of SPR (SPR‐DM) was obtained in a clinical proton beam. The values of SPR‐CT, SPR‐TH, and SPR‐DM were compared. Results: The SPR‐CT for PLA agreed with SPR‐DM for tsepta ≥ 3 mm (for CT slice thicknesses of 0.5, 1.0, and 3.0 mm). The density of PLA was found to decrease with thickness when tsepta < 3 mm. As tsepta (and density) decreased, the SPR‐CT values also decreased, in good agreement with SPR‐DM and SPR‐TH. Conclusion: Overall, the DECT‐based SPR‐CT was within 3% of SPR‐TH and SPR‐DM in the high‐density gradient regions of the 3D‐printed plugs for septa greater than ~ 3mm in thickness. [ABSTRACT FROM AUTHOR]

Purpose: In proton therapy planning, the accuracy of the Stopping Power Ratios (SPR) calculated in the stoichiometric CT calibration is affected by, among others, uncertainties on the mean excitation energies (I‐values) of human tissues and water. Traditionally, the contribution of these uncertainties on the SPR has been conservatively estimated of the order of 1% or more for a reference tissue of known composition. This study provides a methodology that enables a finer estimation of this uncertainty, eventually showing that the traditional estimates of the uncertainty are too conservative. Methods: Since human tissues contain water, a correlation exists between the I‐values of tissues and water. As the SPR is the ratio of the tissue stopping power to that of water, this correlation decreases the uncertainty of the SPR. Our formalism considers this by expressing the I‐value of the tissue as a function of the water weight fraction and the I‐value of water, while applying Bragg's additivity rule only to the nonaqueous mixture of the tissue. For 22 reference tissue compositions, the SPR uncertainty was estimated by randomly sampling Gaussian distributions, based on ICRU data, for the I‐values of water and the nonaqueous mixture, as well as for the water weight fraction. Results: The relative standard deviation of the SPR, estimated at 150 MeV, is in the range of 0.1%–0.3% for soft tissues with an average water weight percentage of at least 60%. For tissues with a low water content (e.g., adipose and bones), this uncertainty is in the range of 0.5%–0.7%. Conclusion: Uncertainties on the I‐values of human tissues and water appear to have a significantly lower impact on the SPR uncertainty than traditionally expected. In the future, this may provide a rationale for using smaller distal and proximal margins on the target volume, provided that all other range uncertainty components are correctly estimated too. [ABSTRACT FROM AUTHOR]

Purpose: Accurate calculation of proton stopping power ratio (SPR) relative to water is crucial to proton therapy treatment planning, since SPR affects prediction of beam range. Current standard practice derives SPR using a single CT scan. Recent studies showed that dual‐energy CT (DECT) offers advantages to accurately determine SPR. One method to further improve accuracy is to incorporate prior knowledge on human tissue composition through a dictionary approach. In addition, it is also suggested that using CT images with multiple (more than two) energy channels, i.e., multi‐energy CT (MECT), can further improve accuracy. In this paper, we proposed a sparse dictionary‐based method to convert CT numbers of DECT or MECT to elemental composition (EC) and relative electron density (rED) for SPR computation. Method: A dictionary was constructed to include materials generated based on human tissues of known compositions. For a voxel with CT numbers of different energy channels, its EC and rED are determined subject to a constraint that the resulting EC is a linear non‐negative combination of only a few tissues in the dictionary. We formulated this as a non‐convex optimization problem. A novel algorithm was designed to solve the problem. The proposed method has a unified structure to handle both DECT and MECT with different number of channels. We tested our method in both simulation and experimental studies. Results: Average errors of SPR in experimental studies were 0.70% in DECT, 0.53% in MECT with three energy channels, and 0.45% in MECT with four channels. We also studied the impact of parameter values and established appropriate parameter values for our method. Conclusion: The proposed method can accurately calculate SPR using DECT and MECT. The results suggest that using more energy channels may improve the SPR estimation accuracy. [ABSTRACT FROM AUTHOR]

Purpose The main objective of this study is to propose an alternative parameterization for the empirical relation between mean excitation energies ( I-value) and effective atomic numbers ( Zeff) of human tissues, and to present a simplified formulation (which we called DEEDZ- SPR) for deriving the stopping power ratio ( SPR) from dual-energy ( DE) CT data via electron density ( ρe) and Zeff calibration. Methods We performed a numerical analysis of this DEEDZ- SPR method for the human-body-equivalent tissues of ICRU Report 46, as objects of interest with unknown SPR and ρe. The attenuation coefficients of these materials were calculated using the XCOM photon cross-sections database. We also applied the DEEDZ- SPR conversion to experimental DECT data available in the literature, which was measured for the tissue-characterization phantom using a dual-source CT scanner at 80 kV and 140 kV/Sn. Results It was found that the DEEDZ- SPR conversion enables the calculation of SPR simply by means of the weighted subtraction of an electron-density image and a low- or high- kV CT image. The simulated SPRs were in excellent agreement with the reference values over the SPR range from 0.258 (lung) to 3.638 (bone mineral-hydroxyapatite). The relative deviations from the reference SPR were within ±0.6% for all ICRU-46 human tissues, except for the thyroid that presented a −1.1% deviation. The overall root-mean-square error was 0.21%. Application to experimental DECT data confirmed this agreement within the experimental accuracy, which demonstrates the practical feasibility of the method. Conclusions The DEEDZ- SPR conversion method could facilitate the construction of SPR images as accurately as a recent DECT-based calibration procedure of SPR parameterization based directly on the CT numbers in a DECT data set. [ABSTRACT FROM AUTHOR]

Purpose: The accuracy of proton dose calculation is dependent on the ability to correctly characterize patient tissues with medical imaging. The most common method is to correlate computed tomography (CT) numbers obtained via single-energy CT (SECT) with proton stopping power ratio (SPR). CT numbers, however, cannot discriminate between a change in mass density and change in chemical composition of patient tissues. This limitation can have consequences on SPR calibration accuracy. Dual-energy CT (DECT) is receiving increasing interest as an alternative imaging modality for proton therapy treatment planning due to its ability to discriminate between changes in patient density and chemical composition. In the current work we use a phantom of known composition to demonstrate the dosimetric advantages of proton therapy treatment planning with DECT over SECT. Methods: A phantom of known composition was scanned with a clinical SECT radiotherapy CT-simulator. The phantom was rescanned at a lower X-ray tube potential to generate a complimentary DECT image set. A set of reference materials similar in composition to the phantom was used to perform a stoichiometric calibration of SECT CT number to proton SPRs. The same set of reference materials was used to perform a DECT stoichiometric calibration based on effective atomic number. The known composition of the phantom was used to assess the accuracy of SPR calibration with SECT and DECT. Intensity modulated proton therapy (IMPT) treatment plans were generated with the SECT and DECT image sets to assess the dosimetric effect of the imaging modality. Isodose difference maps and root mean square (RMS) error calculations were used to assess dose calculation accuracy. Results: SPR calculation accuracy was found to be superior, on average, with DECT relative to SECT. Maximum errors of 12.8% and 2.2% were found for SECT and DECT, respectively. Qualitative examination of dose difference maps clearly showed the dosimetric advantages of DECT imaging, compared to SECT imaging for IMPT dose calculation for the case investigated. Quantitatively, the maximum dose calculation error in the SECT plan was 7.8%, compared to a value of 1.4% in the DECT plan. When considering the high dose target region, the root mean square (RMS) error in dose calculation was 2.1% and 0.4% for SECT and DECT, respectively. Conclusions: DECT-based proton treatment planning in a commercial treatment planning system was successfully demonstrated for the first time. DECT is an attractive imaging modality for proton therapy treatment planning owing to its ability to characterize density and chemical composition of patient tissues. SECT and DECT scans of a phantom of known composition have been used to demonstrate the dosimetric advantages obtainable in proton therapy treatment planning with DECT over the current approach based on SECT. [ABSTRACT FROM AUTHOR]

Purpose: The aim of this study was to investigate whether the stopping power ratio (SPR) of a deformable, silicone-based 3D dosimeter could be determined more accurately using dual energy (DE) CT compared to using conventional methods based on single energy (SE) CT. The use of SECT combined with the stoichiometric calibration method was therefore compared to DECT-based determination. Methods: The SPR of the dosimeter was estimated based on its Hounsfield units (HUs) in both a SECT image and a DECT image set. The stoichiometric calibration method was used for converting the HU in the SECT image to a SPR value for the dosimeter while two published SPR calibration methods for dual energy were applied on the DECT images. Finally, the SPR of the dosimeter was measured in a 60 MeV proton by quantifying the range difference with and without the dosimeter in the beam path. Results: The SPR determined from SECT and the stoichiometric method was 1.10, compared to 1.01 with both DECT calibration methods. The measured SPR for the dosimeter material was 0.97. Conclusions: The SPR of the dosimeter was overestimated by 13% using the stoichiometric method and by 3% when using DECT. If the stoichiometric method should be applied for the dosimeter, the HU of the dosimeter must be manually changed in the treatment planning system in order to give a correct SPR estimate. Using a wrong SPR value will cause differences between the calculated and the delivered treatment plans. [ABSTRACT FROM AUTHOR]

Many papers discussed the I value for water given by the ICRU, concluding that a value of about 80±2 eV instead of 67.2 eV would reproduce measured ion depth-dose curves. A change in the I value for water would have an effect on the stopping power and, hence, on the water-to-air stopping power ratio, which is important in clinical dosimetry of proton and ion beams. For energies ranging from 50 to 330 MeV/u and for one spread out Bragg peak, the authors compare the impact of the I value on the water-to-air stopping power ratio. The authors calculate ratios from different ICRU stopping power tables and ICRU reports. The stopping power ratio is calculated via track-length dose calculation with SHIELD-HIT07. In the calculations, the stopping power ratio is reduced to a value of 1.119 in the plateau region as compared to the cited value of 1.13 in IAEA TRS-398. At low energies the stopping power ratio increases by up to 6% in the last few tenths of a mm toward the Bragg peak. For a spread out Bragg peak of 13.5 mm width at 130 mm depth, the stopping power ratio increases by about 1% toward the distal end. [ABSTRACT FROM AUTHOR]