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Dec 2010

Volume 37, Issue 12, pp. 6113-6501

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Med. Phys. 37, 6357 (2010); http://dx.doi.org/10.1118/1.3515457 (11 pages)

Jeffrey Schlosser, Kenneth Salisbury, and Dimitre Hristov
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POINT/COUNTERPOINT: Co-60 tomotherapy is the treatment modality of choice for developing countries in transition toward IMRT

Patrick F. Cadman, M.Sc., Bhudatt R. Paliwal, Ph.D., and Colin G. Orton, Ph.D., Moderator

Med. Phys. 37, 6113 (2010); http://dx.doi.org/10.1118/1.3481358 (3 pages) | Cited 1 time

Online Publication Date: 8 November 2010

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87.55.-x Treatment strategy

RADIATION THERAPY PHYSICS: Target tracking using DMLC for volumetric modulated arc therapy: A simulation study

Baozhou Sun, Dharanipathy Rangaraj, Lech Papiez, Swetha Oddiraju, Deshan Yang, and H. Harold Li

Med. Phys. 37, 6116 (2010); http://dx.doi.org/10.1118/1.3511516 (9 pages) | Cited 1 time

Online Publication Date: 8 November 2010

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Purpose: Target tracking using dynamic multileaf collimator (DMLC) is a promising approach for intrafraction motion management in radiation therapy. The purpose of this work is to develop a DMLC tracking algorithm capable of delivering volumetric-modulated arc therapy (VMAT) to the targets that experience two-dimensional (2D) rigid motion in the beam’s eye view.
Methods: The problem of VMAT delivery to moving targets is formulated as a control problem with constraints. The relationships between gantry speed, gantry acceleration, MLC leaf-velocity, dose rate, and target motion are derived. An iterative search algorithm is developed to find numerical solutions for efficient delivery of a specific VMAT plan to the moving target using 2D DMLC tracking. The delivery of five VMAT lung plans is simulated. The planned and delivered fluence maps in the target-reference frame are calculated and compared.
Results: The simulation demonstrates that the 2D tracking algorithm is capable of delivering the VMAT plan to a moving target fast and accurately without violating the machine constraints and the integrity of the treatment plan. The average delivery time is only 29 s longer than that of no-tracking delivery, 101 versus 72 s, respectively. The fluence maps are normalized to 200 MU and the average root-mean-square error between the desired and the delivered fluence is 2.1 MU, compared to 14.8 MU for no-tracking and 3.6 MU for one-dimensional tracking.
Conclusions: A locally optimal MLC tracking algorithm for VMAT delivery is proposed, aiming at shortest delivery time while maintaining treatment plan invariant. The inconsequential increase of treatment time due to DMLC tracking is clinically desirable, which makes VMAT with DMLC tracking attractive in treating moving tumors.
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87.56.nk Collimators
87.55.dk Dose-volume analysis

RADIATION IMAGING PHYSICS: An iterative method for tomographic x-ray perfusion estimation in a decomposition model-based approach

Christoph Neukirchen, Marco Giordano, and Steffen Wiesner

Med. Phys. 37, 6125 (2010); http://dx.doi.org/10.1118/1.3495818 (17 pages) | Cited 5 times

Online Publication Date: 8 November 2010

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Purpose: X-ray based tomographic blood perfusion imaging requires recovery of contrast time-attenuation-curves from dynamic projection data. When using slowly rotating imaging systems, this task is challenging due to nonsimultaneous projection acquisition. A dynamic reconstruction method is proposed that aims at compensating the lack of simultaneously acquired information by incorporating prior knowledge about the expected temporal contrast dynamics.
Methods: A decomposition model using temporal basis functions to approximate time-attenuation-curves is integrated into an iterative tomographic reconstruction method. The computationally efficient implementation of the proposed approach makes use of standard forward-projections and backprojections, as well as scalar products in image space. The critical issue of projection noise propagation is tackled by the application of regularization which is realized by the early stopping of iteration cycles and by the proper selection of smooth temporal basis functions. The performance of the proposed dynamic reconstruction approach is evaluated in a simulation study concerning various aspects: Noise propagation and regularization, specification of the temporal model, and type of acquisition mode.
Results: The evaluation based on dynamic phantom data indicates that tomographic recovery of contrast time-attenuation-curves in tissue can be achieved with an average range of accuracy of ∼ 2% (with respect to dynamic peak attenuation) under ideal noise-free conditions. The relative estimation error for arterial time-attenuation-curves is in the range of 8%, which is due to faster contrast dynamics in the artery. In general, performance depends on the level of acquired information contained in the projection data, which is mainly influenced by the type of rotational acquisition mode; restrictions in angular range and speed can lead to limited accuracy. The analysis of propagated projection noise in a statistical bias-variance framework reveals relative noise levels in estimated time-attenuation-curves of 3%–4% in tissue regions and below 1% in vessels when using optimized settings for regularization. Here, the effect of noise suppression depends on the interrelation between the number of iteration cycles and the constraints imposed by the temporal decomposition model.
Conclusions: For usage with slowly rotating imaging systems, the presented model-based iterative dynamic reconstruction method is capable of recovering contrast time-attenuation-curves related to tissue perfusion. The proposed regularization framework is an effective means to limit the impact of projection noise, which is a factor dominating estimation accuracy in tissue regions.
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87.57.Q- Computed tomography
87.19.rh Fluid transport and rheology
87.59.bd Computed radiography
42.30.Wb Image reconstruction; tomography
87.57.nf Reconstruction
02.50.-r Probability theory, stochastic processes, and statistics

MEDICAL PHYSICS LETTERS: Multiscale entropy of laser Doppler flowmetry signals in healthy human subjects

Anne Humeau, Benjamin Buard, Guillaume Mahé, David Rousseau, François Chapeau-Blondeau, and Pierre Abraham

Med. Phys. 37, 6142 (2010); http://dx.doi.org/10.1118/1.3512796 (5 pages) | Cited 1 time

Online Publication Date: 8 November 2010

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Purpose: The cardiovascular system (CVS) regulation can be studied from a central viewpoint, through heart rate variability (HRV) data, and from a peripheral viewpoint, through laser Doppler flowmetry (LDF) signals. Both the central and peripheral CVSs are regulated by several interacting mechanisms, each having its own temporal scale. The central CVS has been the subject of many multiscale studies. By contrast, these studies at the level of the peripheral CVS are very recent. Among the multiscale studies performed on the central CVS data, multiscale entropy has been proven to give interesting physiological information for diagnostic purposes. However, no multiscale entropy analysis has been performed on LDF signals. The authors’ goal is therefore to propose a first multiscale entropy study of LDF data recorded in healthy subjects.
Methods: The LDF signals recorded in the forearm of seven healthy subjects are processed. Their period sampling is T = 50 ms, and coarse-graining scales from T to 23T are studied. Also, for validation, the algorithm is first tested on synthetic signals of known theoretical multiscale entropy.
Results: The results reveal nonmonotonic evolution of the multiscale entropy of LDF signals, with a maximum at small scales around 7T and a minimum at longer scales around 18T, singling out in this way two distinctive scales where the LDF signals undergo specific changes from high to low complexity. This also marks a strong contrast with the HRV signals that usually display a monotonic increase in the evolution of the multiscale entropy.
Conclusions: Multiscale entropy of LDF signals in healthy subjects shows variation with scales. Moreover, as the variation pattern observed appears similar for all the tested signals, multiscale entropy could potentially be a useful stationary signature for LDF signals, which otherwise are probe-position and subject dependent. Further work could now be conducted to evaluate possible diagnostic purposes of the multiscale entropy of LDF signals.
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87.63.lt Laser imaging
42.62.Be Biological and medical applications

RADIATION MEASUREMENT PHYSICS: High-rate x-ray spectroscopy in mammography with a CdTe detector: A digital pulse processing approach

L. Abbene, G. Gerardi, F. Principato, S. Del Sordo, R. Ienzi, and G. Raso

Med. Phys. 37, 6147 (2010); http://dx.doi.org/10.1118/1.3512804 (10 pages) | Cited 2 times

Online Publication Date: 8 November 2010

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Purpose:Direct measurement of mammographic x-ray spectra under clinical conditions is a difficult task due to the high fluence rate of the x-ray beams as well as the limits in the development of high resolution detection systems in a high counting rate environment. In this work we present a detection system, based on a CdTe detector and an innovative digital pulse processing (DPP) system, for high-rate x-ray spectroscopy in mammography.
Methods: The DPP system performs a digital pile-up inspection and a digital pulse height analysis of the detector signals, digitized through a 14-bit, 100 MHz digitizer, for x-ray spectroscopy even at high photon counting rates. We investigated on the response of the digital detection system both at low (150 cps) and at high photon counting rates (up to 500 kcps) by using monoenergetic x-ray sources and a nonclinical molybdenum anode x-ray tube. Clinical molybdenum x-ray spectrum measurements were also performed by using a pinhole collimator and a custom alignment device.
Results: The detection system shows excellent performance up to 512 kcps with an energy resolution of 4.08% FWHM at 22.1 keV. Despite the high photon counting rate (up to 453 kcps), the molybdenum x-ray spectra, measured under clinical conditions, are characterized by a low number of pile-up events. The agreement between the attenuation curves and the half value layer values, obtained from the measured spectra, simulated spectra, and from the exposure values directly measured with an ionization chamber, also shows the accuracy of the measurements.
Conclusions: These results make the proposed detection system a very attractive tool for both laboratory research and advanced quality controls in mammography.
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87.59.B- Radiography
87.64.kd X-ray and EXAFS

RADIATION IMAGING PHYSICS: Quantitative breast tomosynthesis: From detectability to estimability

Samuel Richard and Ehsan Samei

Med. Phys. 37, 6157 (2010); http://dx.doi.org/10.1118/1.3501883 (9 pages) | Cited 3 times

Online Publication Date: 8 November 2010

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Purpose: This work aimed to extend Fourier-based imaging metrics for modeling and predicting quantitative imaging performance. The new methodology was applied to the platform of breast tomosynthesis for investigating the influence of acquisition parameters (e.g., acquisition angle and dose) on quantitative imaging performance.
Methods: Two quantitative imaging tasks were considered: Area estimation and volume estimation of a 4 mm diameter spherical target. The maximum likelihood estimator yielded training data to generate a size estimation task function, which was combined with the MTF and NPS to predict estimation performance by computing an “estimability index” analogous to the detectability index. Estimation performance for the two tasks was computed as a function of acquisition angle and dose. The results were used for system optimization in terms of quantitation performance and further compared to the detectability index for the detection of the same spherical target.
Results: The estimability index computed with the size estimation tasks correlated well with precision measurements for area and volume estimation over a broad range of imaging conditions and provided a meaningful figure of merit for quantitative imaging performance and optimization. The results highlighted that optimal breast tomosynthesis acquisition parameters depend significantly on imaging task and dose. At nominal dose (1.5 mGy), mass detection was optimal at an acquisition angle of 85°, while area and volume estimation for the same mass were optimal at ∼ 125° and 164° acquisition angles, respectively.
Conclusions: These findings provide an initial validation that the Fourier-based metrics extended to estimation tasks can represent a meaningful metric and predictor of quantitative imaging performance. The optimization framework also revealed trade-off between anatomical noise and system noise in volumetric imaging systems potentially identifying different optimal acquisition parameters than currently used in breast tomosynthesis and CT.
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87.57.Q- Computed tomography
02.30.Nw Fourier analysis

NUCLEAR MEDICINE PHYSICS: Reduction of dental filling metallic artifacts in CT-based attenuation correction of PET data using weighted virtual sinograms optimized by a genetic algorithm

Mehrsima Abdoli, Mohammad Reza Ay, Alireza Ahmadian, Rudi A. J. O. Dierckx, and Habib Zaidi

Med. Phys. 37, 6166 (2010); http://dx.doi.org/10.1118/1.3511507 (12 pages) | Cited 6 times

Online Publication Date: 8 November 2010

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Purpose: The presence of metallic dental fillings is prevalent in head and neck PET/CT imaging and generates bright and dark streaking artifacts in reconstructed CT images. The resulting artifacts would propagate to the corresponding PET images following CT-based attenuation correction (CTAC). This would cause over- and/or underestimation of tracer uptake in corresponding regions thus leading to inaccurate quantification of tracer uptake. The purpose of this study is to improve our recently proposed metal artifact reduction (MAR) approach and to assess its performance in a clinical setting.
Methods: The proposed MAR algorithm is performed in the virtual sinogram space to overcome the challenges associated with manipulating raw CT data. The corresponding bins of the virtual sinogram affected by metallic objects are obtained by forward projection of segmented metallic objects in the original CT image. These bins are then substituted by weighted values of three estimates: the affected bins in the original sinogram, the bins in the corrected sinogram using spline interpolation, and the sinogram bins in the neighboring column of the sinogram matrix. The optimized weighting factors (α, β, and γ) were estimated using a genetic algorithm (GA). The optimized combination of weighting coefficients was obtained using the GA applied to 24 clinical CT data sets. The proposed MAR method was then applied to 12 clinical head and neck PET/CT data sets containing dental artifacts. Analysis of the results was performed using Bland and Altman plots and a method allowing analysis in the absence of gold standard called regression without truth (RWT). The proposed method was also compared to an image-based MAR method.
Results: Optimization of the weighting coefficients using the GA resulted in an optimum combination of parameters of α = 0.26, β = 0.67, and γ = 0.07. According to Bland and Altman plots generated for both CT and PET images of the clinical data, the proposed MAR algorithm is efficient for reduction of streak artifacts in CT images and such reduce the over- and/or underestimation of tracer uptake. The RWT method also confirmed the effectiveness of the proposed MAR method. The obtained figures of merit revealed that attenuation corrected PET data corrected using CTAC after applying the MAR algorithm are more similar to the assumed gold standard. Comparison with the knowledge-based method revealed that the proposed method mainly corrects the artifactualregions without modifying the unaffected regions. The knowledge-based method globally modifies the images including those that do not include metallic artifacts.
Conclusions: The proposed MAR algorithm improves the quality and quantitative accuracy of clinical head and neck PET/CT images and could be easily integrated in clinical setting.
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87.57.Q- Computed tomography
87.57.uk Positron emission tomography (PET)
87.57.nf Reconstruction
87.85.J- Biomaterials

RADIATION THERAPY PHYSICS: A comparison of the respiratory signals acquired by different respiratory monitoring systems used in respiratory gated radiotherapy

Yuki Otani, Ichirou Fukuda, Nobuhiro Tsukamoto, Yu Kumazaki, Hiroshi Sekine, Etsuko Imabayashi, Osamu Kawaguchi, Takayuki Nose, Teruki Teshima, and Takushi Dokiya

Med. Phys. 37, 6178 (2010); http://dx.doi.org/10.1118/1.3512798 (9 pages) | Cited 1 time

Online Publication Date: 8 November 2010

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Purpose: Respiratory monitoring systems are used to detect the respiratory phase of patients during the planning and administration of respiratory gated radiotherapy by using four-dimensional computed tomography (4DCT) or 4D positron-emission tomography/CT (4DPET/CT) and the linear accelerator (linac), respectively. Generally, identical respiratory monitoring systems are used for 4DCT, 4DPET/CT, and linac. However, different systems are sometimes used in combination because the accessibility of the respiratory monitoring systems may differ by manufacturer. The combined use of different respiratory monitoring systems in phase-based gating is of concern because the differences in the timing of tags (end-respiration signals algorithmically determined by the respiratory monitoring system), defined by the two systems, may result in phase differences. The purpose of this study is to estimate this difference and evaluate its effect on 4DCT data.
Methods: Ten patients (seven men and three women) with a median age of 75 yr (range: 57–84 yr) were treated by gated stereotactic body radiation therapy between April and December 2009. Two types of respiratory monitoring systems—RPM (Varian Medical Systems) and AZ-733V (Anzai MEDICAL)—were placed on the abdominal surface of the patients, and the respiratory signals were acquired by both systems. The relationship between the amplitude peak and the tag obtained by each respiratory system was analyzed for each patient. Further, the 4DCT images were reconstructed by using the signals obtained from both the RPM and the AZ-733V systems, and the tumor volumes and the tumor centroid positions in the craniocaudal plane were analyzed for each patient.
Results: The correlation factor between the respiratory signals from the RPM system and AZ-733V system was 0.990 (range: 0.940–0.994). The amplitude peak of the RPM system corresponded well with that of the AZ-733V system. The median±standard deviation of the phase difference for all the patients ranged from −4.3±7.1% to 3.5±2.2%. In the case of some patients, differences were noted between the two systems in the estimation of the tumor centroid position and tumor shape.
Conclusions: The estimation of the position of the tumor centroid and tumor shape may vary with the use of different respiratory monitoring systems. This implies that it is preferable to use the same respiratory monitoring system with 4DCT, 4DPET-CT, and linac.
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87.19.Wx Pneumodyamics, respiration
87.57.uk Positron emission tomography (PET)
87.57.Q- Computed tomography
87.57.nf Reconstruction
87.55.-x Treatment strategy

RADIATION IMAGING PHYSICS: Comparison of radiation exposure and associated radiation-induced cancer risks from mammography and molecular imaging of the breast

Michael K. O’Connor, Hua Li, Deborah J. Rhodes, Carrie B. Hruska, Conor B. Clancy, and Richard J. Vetter

Med. Phys. 37, 6187 (2010); http://dx.doi.org/10.1118/1.3512759 (12 pages) | Cited 4 times

Online Publication Date: 10 November 2010

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Purpose: Recent studies have raised concerns about exposure to low-dose ionizing radiation from medical imaging procedures. Little has been published regarding the relative exposure and risks associated with breast imaging techniques such as breast specific gamma imaging (BSGI), molecular breast imaging (MBI), or positron emission mammography (PEM). The purpose of this article was to estimate and compare the risks of radiation-induced cancer from mammography and techniques such as PEM, BSGI, and MBI in a screening environment.
Methods: The authors used a common scheme for all estimates of cancer incidence and mortality based on the excess absolute risk model from the BEIR VII report. The lifetime attributable risk model was used to estimate the lifetime risk of radiation-induced breast cancer incidence and mortality. All estimates of cancer incidence and mortality were based on a population of 100 000 females followed from birth to age 80 and adjusted for the fraction that survives to various ages between 0 and 80. Assuming annual screening from ages 40 to 80 and from ages 50 to 80, the cumulative cancer incidence and mortality attributed to digital mammography, screen-film mammography, MBI, BSGI, and PEM was calculated. The corresponding cancer incidence and mortality from natural background radiation was calculated as a useful reference. Assuming a 15%–32% reduction in mortality from screening, the benefit/risk ratio for the different imaging modalities was evaluated.
Results: Using conventional doses of 925 MBq Tc-99m sestamibi for MBI and BSGI and 370 MBq F-18 FDG for PEM, the cumulative cancer incidence and mortality were found to be 15–30 times higher than digital mammography. The benefit/risk ratio for annual digital mammography was >50:1 for both the 40–80 and 50–80 screening groups, but dropped to 3:1 for the 40–49 age group. If the primary use of MBI, BSGI, and PEM is in women with dense breast tissue, then the administered doses need to be in the range 75–150 MBq for Tc-99m sestamibi and 35 MBq–70 MBq for F-18 FDG in order to obtain benefit/risk ratios comparable to those of mammography in these age groups. These dose ranges should be achievable with enhancements to current technology while maintaining a reasonable examination time.
Conclusions: The results of the dose estimates in this study clearly indicate that if molecular imaging techniques are to be of value in screening for breast cancer, then the administered doses need to be substantially reduced to better match the effective doses of mammography.
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87.59.E- Mammography
87.53.Bn Dosimetry/exposure assessment
87.55.dk Dose-volume analysis
87.53.Jw Therapeutic applications, including brachytherapy
87.19.xj Cancer
87.57.uk Positron emission tomography (PET)

RADIATION IMAGING PHYSICS: Monte Carlo calculation of imaging doses from diagnostic multidetector CT and kilovoltage cone-beam CT as part of prostate cancer treatment plans

Aiping Ding, Jianwei Gu, Alexei V. Trofimov, and X. George Xu

Med. Phys. 37, 6199 (2010); http://dx.doi.org/10.1118/1.3512791 (6 pages) | Cited 1 time

Online Publication Date: 10 November 2010

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Purpose: To calculate imaging doses to the rectum, bladder, and femoral heads as part of a prostate cancer treatment plans, assuming an image guided radiation therapy (IGRT) procedure involving either the multidetector CT (MDCT) or kilovoltage cone-beam CT (kV CBCT).
Methods: This study considered an IGRT treatment plan for a prostate carcinoma patient involving 50.4 Gy from 28 initial fractions and a boost of 28.8 Gy from 16 fractions. A total of 45 CT imaging procedures, each involving a MDCT or a kV CBCT scan procedure, were carefully modeled using the MCNPX code version 2.5.0. The MDCT scanner model is based on the GE LightSpeed 16-MDCT scanner and the kV CBCT scanner model is based on the Varian On-Board Imager using parameters reported by the CT manufacturers and literatures. A patient-specific treatment planning CT data set was used to construct the phantom for the dose calculation. The target, organs-at-risk (OARs), and background voxels in the CT data set were categorized into six tissue types according to CT numbers for Monte Carlo calculations.
Results: For a total of 45 imaging procedures, it was found that the rectum received 78.4 and 76.7 cGy from MDCT and kV CBCT, respectively. The bladder received slightly greater doses of 82.4 and 77.9 cGy, while the femoral heads received much higher doses of 182.3 and 141.3 cGy from MDCT and kV CBCT, respectively. To investigate the impact of these imaging doses on treatment planning, OAR doses from MDCT or kV CBCT imaging procedures were added to the corresponding dose matrix reported by the original treatment plans to construct dose volume histograms. It was found that after the imaging dose is added, the rectum volumes irradiated to 75 and 70 Gy increased from 13.9% and 21.2%, respectively, in the original plan to 14.8% and 21.8%. The bladder volumes receiving 80 Gy increased to 4.6% from 4.1% in the original plan and the volume receiving 75 Gy increased to 7.9% from 7.5%. All values remained within the tolerance levels: V70<25%, V75<15% for rectum and V75<25%, V80<15% for bladder. The irradiation of femoral heads was also acceptable with no volume receiving >45 Gy.
Conclusions: IGRT procedures can irradiate the OARs to an imaging dose level that is great enough to require careful evaluation and perhaps even adjustment of original treatment planning in order to still satisfy the dose constraints. This study only considered one patient CT because the CT x rays cover a relatively larger volume of the body and the dose distribution is considerably more uniform than those associated with the therapeutic beams. As a result, the dose to an organ from CT imaging doses does not vary much from one patient to the other for the same CT settings. One factor that would potentially affect such CT dose level is the size of the patient body. More studies are needed to develop accurate and convenient methods of accounting for the imaging doses as part of treatment planning.
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87.55.dk Dose-volume analysis
87.57.Q- Computed tomography
87.55.-x Treatment strategy
87.55.K- Monte Carlo methods
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