B Gino Fallone
University of Alberta, Medical Physics, Faculty Member
L'invention concerne un systeme de radiotherapie comprenant une source de rayonnement permettant de produire un faisceau de rayonnement et un dispositif d'imagerie par resonance magnetique. Une interface situee entre la source de... more
L'invention concerne un systeme de radiotherapie comprenant une source de rayonnement permettant de produire un faisceau de rayonnement et un dispositif d'imagerie par resonance magnetique. Une interface situee entre la source de rayonnement et le dispositif d'IRM permet d'appliquer un faisceau de rayonnement et d'obtenir des images par resonance magnetique simultanement. Le dispositif d'IRM et la source de rayonnement sont couples de facon que le systeme puisse etre utilise dans un mode rotatif dans lequel la source de rayonnement peut irradier un patient a partir de n'importe quel angle, sans que la qualite de l'image d'IRM soit reduite.
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Purpose: Electronic portal imaging devices (EPIDs) are potentially useful for dosimetric verification in integrated MRI-linac systems. This work presents the reproducibility, linearity, image lag, and radiation field profiles in a... more
Purpose: Electronic portal imaging devices (EPIDs) are potentially useful for dosimetric verification in integrated MRI-linac systems. This work presents the reproducibility, linearity, image lag, and radiation field profiles in a conventional EPID, with and without a 0.5 T parallel magnetic field present in a 6 MV photon beam. Methods: An aS500 EPID was modified to function in strong magnetic fields. All measurements were made using the linac-MR installed at the Cross Cancer Institute. The EPID remained stationary on the couch between the measurements made with and without magnetic field. We measured shortterm reproducibility of dark and flood fields, signal linearity from 1 to 500 MU irradiations, and image lag post 100 MU irradiation. An ion chamber was used to measure any linac output variations to correct the EPID signal due to these variations for the duration of experiment. X-axis and Y-axis radiation field profiles were obtained from the EPID image resulting from a 10 9 10 c...
PURPOSE Investigate 3D (spatial and temporal) convolutional neural networks (CNNs) for real-time on-the-fly magnetic resonance imaging (MRI) reconstruction. In particular, we investigated the applicability of training CNNs on a... more
PURPOSE Investigate 3D (spatial and temporal) convolutional neural networks (CNNs) for real-time on-the-fly magnetic resonance imaging (MRI) reconstruction. In particular, we investigated the applicability of training CNNs on a patient-by-patient basis for the purpose of lung tumor segmentation. Methods: Data were acquired with our 3T Philips Achieva system. A retrospective analysis was performed on six non-small cell lung cancer patients who received fully sampled dynamic acquisitions consisting of 650 free breathing images using a bSSFP sequence. We retrospectively undersampled the six patient's data by 5x and 10x acceleration. The retrospective data was used to quantitatively compare the CNN reconstruction to gold truth data via the Dice coefficient (DC) and centroid displacement to compare the tumor segmentations. Reconstruction noise was investigated using the normalized mean square error (NMSE). We further validated the technique using prospectively undersampled data from a volunteer and motion phantom. Results: The retrospectively undersampled data was reconstructed at 5x and 10x acceleration using patient specific trained CNNs. The patient average DCs for the tumor segmentation at 5x and 10x acceleration were 0.94 and 0.92, respectively. These DC values are greater than the inter- and intra-observer segmentations acquired by radiation oncologist experts as reported in a previous study of ours. Furthermore, the patient specific CNN can be trained in under 6 hours and the reconstruction time was 65 ms per image. The prospectively undersampled CNN reconstruction data yielded qualitatively acceptable images. Conclusions: We have shown that 3D CNNs can be used for real-time on-the-fly dynamic image reconstruction utilizing both spatial and temporal data in this proof of concept study. We evaluated the technique using six retrospectively undersampled lung cancer patient data sets, as well as prospectively undersampled data acquired from a volunteer and motion phantom. The reconstruction speed achieved for our current implementation was 65 ms per image. .
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Accurate and efficient patient dose calculations are essential for treatment planning in Magnetic Resonance Imaging guided Radiotherapy (MRIgRT). Achieving reasonable performance for a space-angle discontinuous finite element method... more
Accurate and efficient patient dose calculations are essential for treatment planning in Magnetic Resonance Imaging guided Radiotherapy (MRIgRT). Achieving reasonable performance for a space-angle discontinuous finite element method (DFEM) grid based Boltzmann solver (GBBS) with magnetic fields for clinical MRIgRT applications largely depends on how the transport sweep is orchestrated. Compared to classical Discrete Ordinates, DFEM in angle introduces increased angular degrees of freedom and eliminates ray-effect artifacts. However, the inclusion of magnetic fields introduces additional serial dependencies such that parallelization of the space-angle transport sweeps becomes more challenging. Novel techniques for the transport sweep and right-hand source assembly are developed, predicated on limiting the number of bulk material densities modeled in the transport sweep scatter calculations. Specifically, k-means clustering is used to assign sub-intervals of mass-density for each spatial element to execute the scatter-dose calculations using batched multiplication by pre-inverted transport sweep matrices. This is shown to be two orders of magnitude more efficient than solving each elemental system individually at runtime. Even with discrete material densities used in the transport sweep scatter calculations, accuracy is maintained by optimizing the material density assignments using k-means clustering, and by performing the primary photon fluence calculations (ray-tracing) using the underlying continuous density of the computed tomography (CT) image. In the presence of 0.5 T parallel and 1.5 T perpendicular magnetic fields, this approach demonstrates high levels of accuracy with gamma 1%/1 mm passing rates exceeding 94% across a range of anatomical sites compared to GEANT4 Monte Carlo dose calculations which used continuous densities. This deterministic GBBS approach maintains unconditional stability, produces no ray-effect artifacts, and has the benefit of no statistical uncertainty. Runtime on a non-parallelized Matlab implementation averaged 10 minutes per beam averaging 80,000 spatial elements, paving way for future development based on this algorithmically efficient paradigm.
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Real-time tracking of lung tumors using magnetic resonance imaging (MRI) has been proposed as a potential strategy to mitigate the ill-effects of breathing motion in radiation therapy. Several autocontouring methods have been evaluated... more
Real-time tracking of lung tumors using magnetic resonance imaging (MRI) has been proposed as a potential strategy to mitigate the ill-effects of breathing motion in radiation therapy. Several autocontouring methods have been evaluated against a "gold standard" of a single human expert user. However, contours drawn by experts have inherent intra- and interobserver variations. In this study, we aim to evaluate our user-trained autocontouring algorithm with manually drawn contours from multiple expert users, and to contextualize the accuracy of these autocontours within intra- and interobserver variations. Six nonsmall cell lung cancer patients were recruited, with institutional ethics approval. Patients were imaged with a clinical 3 T Philips MR scanner using a dynamic 2D balanced SSFP sequence under free breathing. Three radiation oncology experts, each in two separate sessions, contoured 130 dynamic images for each patient. For autocontouring, the first 30 images were use...
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Half-block techniques in radiotherapy may be used to junction two abutting radiation fields. Removable half blocks for linear accelerators not equipped with asymmetrical jaws can accomplish this technique providing the block edge is... more
Half-block techniques in radiotherapy may be used to junction two abutting radiation fields. Removable half blocks for linear accelerators not equipped with asymmetrical jaws can accomplish this technique providing the block edge is properly secured with respect to the beam central axis to achieve a smooth beam junction. Successive block edge cuts with a milling machine are, however, required to achieve an acceptable junction of the fields. The design of a half-block device that allows secure adjustment of the block edge on the linac thus alleviating the need for successive milling procedures is presented.
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ABSTRACT Research into the use of amorphous selenium (a-Se) based detectors in digital radiography has seen a dramatic increase in the past few years due to the development of thin film transistor technology. Direct detection active... more
ABSTRACT Research into the use of amorphous selenium (a-Se) based detectors in digital radiography has seen a dramatic increase in the past few years due to the development of thin film transistor technology. Direct detection active matrix flat-panel imagers (AMFPIs), which use a-Se to directly convert X-rays into charge, have been studied for use in mammography, chest radiography and fluoroscopy and have shown great promise due to their high intrinsic resolution. Before these detectors can be optimized to fulfill their maximum potential, however, the physics of the interaction of X-rays with a-Se must be understood in a quantitative manner, Although the interaction of visible light with a-Se is adequately understood, the interaction with X-rays has been under debate for some time, In this work, we develop a formalism to directly analyze the signal produced due to the interaction of X-rays with a-Se. We simulate the creation of electron-hole pairs, their transport within the detector, and their recombination along the secondary electron tracks. We show that our analysis agrees with recently published experimental results of the pair creation energy W± as a function of both the applied electric field and the incident X-ray energy
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Research Interests: Physics, Optics, Biomedical Engineering, Computer Aided Design, Medical Physics, and 12 moreMedicine, Computer Simulation, Rf Power Amplifier, Radio Frequency, Waveguide, Theoretical Models, Reproducibility of Results, Linear Accelerator Market, Sensitivity and Specificity, Equipment Design, Equipment Failure Analysis, and Particle Accelerators
Research Interests: Physics, Computed Tomography, TomoTherapy, Nuclear medicine, Medicine, and 15 moreImage-guided radiation therapy, Head and Neck, Humans, Standard Deviation, Movement, Glioblastoma, Male, Radiologic Technology, Clinical Sciences, Glioblastoma Multiforme, Brain Neoplasms, Helical tomotherapy, Prostatic neoplasms, Physical Phenomena, and Head and neck neoplasms
Research Interests: Chemistry, Molecular Biology, Mass Spectrometry, Magnetic Resonance Imaging, Biology, and 15 moreMedicine, Cell Culture, Lipids, In Vitro, Humans, Exploration, In Vivo, Ionization, Bioreactors, Cell Proliferation, Amino Acid Sequence, Cell Survival, Cell Growth, Magnetic resonance image, and Glycopeptides
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Modern effort in radiotherapy to address the challenges of tumor localization and motion has led to the development of MRI guided radiotherapy technologies. Accurate dose calculations must properly account for the effects of the MRI... more
Modern effort in radiotherapy to address the challenges of tumor localization and motion has led to the development of MRI guided radiotherapy technologies. Accurate dose calculations must properly account for the effects of the MRI magnetic fields. Previous work has investigated the accuracy of a deterministic linear Boltzmann transport equation (LBTE) solver that includes magnetic field, but not the stability of the iterative solution method. In this work, we perform a stability analysis of this deterministic algorithm including an investigation of the convergence rate dependencies on the magnetic field, material density, energy, and anisotropy expansion. The iterative convergence rate of the continuous and discretized LBTE including magnetic fields is determined by analyzing the spectral radius using Fourier analysis for the stationary source iteration (SI) scheme. The spectral radius is calculated when the magnetic field is included (1) as a part of the iteration source, and (2)...
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Electronic portal imaging devices (EPIDs) are potentially useful for dosimetric verification in integrated MRI-linac systems. This work presents the reproducibility, linearity, image lag, and radiation field profiles in a conventional... more
Electronic portal imaging devices (EPIDs) are potentially useful for dosimetric verification in integrated MRI-linac systems. This work presents the reproducibility, linearity, image lag, and radiation field profiles in a conventional EPID, with and without a 0.5 T parallel magnetic field present in a 6 MV photon beam. An aS500 EPID was modified to function in strong magnetic fields. All measurements were made using the linac-MR installed at the Cross Cancer Institute. The EPID remained stationary on the couch between the measurements made with and without magnetic field. We measured short-term reproducibility of dark and flood fields, signal linearity from 1 to 500 MU irradiations, and image lag post 100 MU irradiation. An ion chamber was used to measure any linac output variations to correct the EPID signal due to these variations for the duration of experiment. X-axis and Y-axis radiation field profiles were obtained from the EPID image resulting from a 10 × 10 cm radiation field delivery. The average pixel value (±standard deviation) of flood field with and without magnetic fields were 57,876 ± 379 and 57,703 ± 366, respectively, and the corresponding average dark field pixel values were -32.05 ± 0.85 and -32.19 ± 0.97. The maximum difference in image linearity data with and without magnetic field is 0.2% which is well within the measurement uncertainty of 0.65%. Similarly, the image lag curves, with and without the magnetic field, were nearly identical. The first measured point, with mean lag signal of 1.44% without and 1.41% with magnetic field, shows that the largest difference is well below the uncertainty in the EPID signal measurement. The radiation field profiles obtained with and without magnetic fields were nearly identical; 91.3% of the X-axis and 95.2% of the Y-axis profile points pass a gamma criterion of 1% and 1 mm. A conventional EPID imager with a 0.1 cm copper plate responds to 6 MV photons similarly irrespective of the strong magnetic field being off or on in the parallel orientation to the radiation beam. Thus, the EPID is a potentially useful tool for pretreatment dosimetric verification in linac-MR systems using parallel magnetic field.
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This work presents a real-time dynamic image reconstruction technique, which combines compressed sensing and principal component analysis (CS-PCA), to achieve real-time adaptive radiotherapy with the use of a linac-magnetic resonance... more
This work presents a real-time dynamic image reconstruction technique, which combines compressed sensing and principal component analysis (CS-PCA), to achieve real-time adaptive radiotherapy with the use of a linac-magnetic resonance imaging system. Six retrospective fully sampled dynamic data sets of patients diagnosed with non-small-cell lung cancer were used to investigate the CS-PCA algorithm. Using a database of fully sampled k-space, principal components (PC's) were calculated to aid in the reconstruction of undersampled images. Missing k-space data were calculated by projecting the current undersampled k-space data onto the PC's to generate the corresponding PC weights. The weighted PC's were summed together, and the missing k-space was iteratively updated. To gain insight into how the reconstruction might proceed at lower fields, 6× noise was added to the 3T data to investigate how the algorithm handles noisy data. Acceleration factors ranging from 2 to 10× were ...
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ABSTRACT Cascade analysis is a powerful tool which can be used to calculate the signal and noise properties of medical imaging detectors. It involves the conceptual separation of the imaging chain into stages which consist of either pure... more
ABSTRACT Cascade analysis is a powerful tool which can be used to calculate the signal and noise properties of medical imaging detectors. It involves the conceptual separation of the imaging chain into stages which consist of either pure amplification or pure dislocation stages. It is, however, not always possible to break the physical processes down to these elementary stages. In this work we derive a new cascade equation which is applicable to any stage which involves multiple amplifications and dislocations. The equation simplifies to the known equations for pure amplification and pure dislocation stages in the appropriate limits, and can be numerically calculated using Monte Carlo techniques for more complicated situations. We demonstrate the use of this equation with an example: we derive an expression for the DQE of a metal/phosphor detector for megavoltage imaging with our formalism, and evaluate the expression with Monte Carlo techniques. We have found that there is excellent agreement between theory and experimental results, and believe that the formalism could be useful for other applications where the amplification and dislocation processes cannot be divided into elementary stages.
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ABSTRACT There has been increased interest in developing real‐time MR‐guided radiotherapy systems because of the potential of 3D imaging in real time with the improved soft‐tissue contrast provided by MR. An overview of the issues related... more
ABSTRACT There has been increased interest in developing real‐time MR‐guided radiotherapy systems because of the potential of 3D imaging in real time with the improved soft‐tissue contrast provided by MR. An overview of the issues related to the integration of a radiotherapy source (e.g. linac) with a low field magnetic system are discussed. The effects of the MR magnetic fields on radiation treatment plans for the various generic designs, and on the performance of the linac are presented. The effects of radio frequency fields from the linac upon the MR image acquisition process is introduced. Advantages of low fields are discussed: minimization of the magnetic field‐induced distortions on plans; use conventional RTP instead of Monte Carlo for high fields; avoidance of hot/cold radiation spots at air‐tissue interfaces seen with high fields; reduced susceptibility artifacts; reduced geometric distortions; reduced magnetic shielding issues; exploitation of the increased T1 contrasts at low fields for fast imaging. The main disadvantage at low fields is decreased SNR. Some proposed designs are given. In particular, the first reported successful system providing any MR imaging during radiotherapy irradiation is discussed. This head prototype involves a 6 MV linac mounted onto an opening of a biplanar 0.2 T MR system. The linac‐MR gantry would rotate together to the prescribed angle of irradiation delivery. The only observable difference between the MR images obtained with linac‐radiation ON to linac OFF is the small changes in SNR. Other possible enhancements (replacing low field with high field MR if required, different types of magnets, etc) to the generic design are also given.The lecture will offer an overview of the advantages and disadvantages of low field MR for real‐time MR‐guided radiotherapy (with the introduction of a working prototype).Learning Objectives:1. Understand the technical issues in integrating an MR system with a radiotherapy source, especially with a linac2. Understand the issues related to distortions of the characteristics of dose deposition because of the MR magnetic field3. Understand the imaging advantages of low field MR, as well as, its disadvantage4. Understand the technical advantages in the integration of low field system with a linac5. Understand the advantages/disadvantages of the different generic designs proposed to date
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The purpose of this work was to investigate the effect of J-coupling interactions on the quantification and T2 determination of 1.3-ppm lipid methylene protons at 3 T. The response of the 1.3-ppm protons of hexanoic, heptanoic, octanoic,... more
The purpose of this work was to investigate the effect of J-coupling interactions on the quantification and T2 determination of 1.3-ppm lipid methylene protons at 3 T. The response of the 1.3-ppm protons of hexanoic, heptanoic, octanoic, linoleic and oleic acid was measured as a function of point-resolved spectroscopy (PRESS) and stimulated echo acquisition mode (STEAM) TE. In addition, a narrow-bandwidth refocusing PRESS sequence designed to rewind J-coupling evolution of the 1.3-ppm protons was applied to the five fatty acids, to corn oil and to tibial bone marrow of six healthy volunteers. Peak areas were plotted as a function of TE, and data were fitted to monoexponentially decaying functions to determine Mo (the extrapolated area for TE = 0 ms) and T2 values. In phantoms, rewinding J-coupling evolution resulted in 198%, 64%, 44%, 20% and 15% higher T2 values for heptanoic, octanoic, linoleic and oleic acid, and corn oil, respectively, compared with those obtained with standard ...
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Measurements of the neutron flux generated at depth in water by an 18 MV photon beam of a Varian 2100C linac are presented. The thermal neutron flux at depth exhibits a maximum at around 5 cm of 1.37×10 6 n/cm2/s for a 40×40 cm2 field... more
Measurements of the neutron flux generated at depth in water by an 18 MV photon beam of a Varian 2100C linac are presented. The thermal neutron flux at depth exhibits a maximum at around 5 cm of 1.37×10 6 n/cm2/s for a 40×40 cm2 field size, 100 cm SSD and a photon output of 400 MU/min. Because of the low
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Formation of γH2AX foci (a marker of DNA double-strand breaks), rates of foci clearance and apoptosis were investigated in cultured normal human fibroblasts and p53 wild-type malignant glioma cells after exposure to high-dose... more
Formation of γH2AX foci (a marker of DNA double-strand breaks), rates of foci clearance and apoptosis were investigated in cultured normal human fibroblasts and p53 wild-type malignant glioma cells after exposure to high-dose synchrotron-generated microbeams. Doses up to 283 Gy were delivered using beam geometries that included a microbeam array (50 µm wide, 400 µm spacing), single microbeams (60-570 µm wide) and a broad beam (32 mm wide). The two cell types exhibited similar trends with respect to the initial formation and time-dependent clearance of γH2AX foci after irradiation. High levels of γH2AX foci persisted as late as 72 h post-irradiation in the majority of cells within cultures of both cell types. Levels of persistent foci after irradiation via the 570 µm microbeam or broad beam were higher when compared with those observed after exposure to the 60 µm microbeam or microbeam array. Despite persistence of γH2AX foci, these irradiation conditions triggered apoptosis in only ...
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To assess late toxicity and outcomes in high-risk prostate cancer patients treated with hypofractionated radiation treatment with androgen suppression therapy. Sixty high-risk prostate cancer patients were enrolled. IMRT prescription was... more
To assess late toxicity and outcomes in high-risk prostate cancer patients treated with hypofractionated radiation treatment with androgen suppression therapy. Sixty high-risk prostate cancer patients were enrolled. IMRT prescription was 68 Gy/25 fractions (2.7 Gy/fraction) to the prostate and proximal seminal vesicles (SV). The pelvic lymph nodes (PLN) and distal SV concurrently received 45 Gy/25 fractions (1.8 Gy/fraction). The patients were treated with helical TomoTherapy-based IMRT and underwent daily megavoltage CT image-guided verification before each treatment. RTOG Toxicity scores were recorded for a 5-year period. Sixty patients completed RT with median follow-up of 63 months (range, 7 to 80 mo).At 5 years follow-up timepoint: Grade (G)2 and G3 late genitourinary toxicity was experienced in 7 (17.0%) and 1 (2.44%), respectively; gastrointestinal G2 as highest toxicity recorded in only 1 (2.44%) patient. There was no G3 gastrointestinal toxicity recorded at this timepoint.With 63-month median follow-up (mean of 65.41±1.72 mo), the 5-year overall survival was 86.67%; 5 years freedom from biochemical failure was 91.67% and freedom from clinical failure was 96.67%. Dose escalation and hypofractionated radiation treatment with IMRT treating the prostate and proximal SV concurrently with the pelvic lymph nodes and distal SV and long-term androgen suppression therapy is well tolerated with respect to acute and late toxicity with 5-year actuarial overall survival 86.67%, freedom from biochemical failure 91.38%, and freedom from clinical failure 96.67%. Longer follow-up will provide more information on 10-year survival outcomes.
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This paper presents a complete set of formulae for the position (D50) and the normalized slope (gamma50) of the dose-response relationship based on the most commonly used radiobiological models for tumours as well as for normal tissues.... more
This paper presents a complete set of formulae for the position (D50) and the normalized slope (gamma50) of the dose-response relationship based on the most commonly used radiobiological models for tumours as well as for normal tissues. The functional subunit response models (critical element and critical volume) are used in the derivation of the formulae for the normal tissue. Binomial
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To determine from the number of trials, n, and the number of observed successes, k the most probable value, the variance and the confidence limits of the probability of success, p, in animal experiments and clinical studies subject to... more
To determine from the number of trials, n, and the number of observed successes, k the most probable value, the variance and the confidence limits of the probability of success, p, in animal experiments and clinical studies subject to binomial statistics. In such experiments the probability of success is an unknown parameter. The Bayesian approach to the problem is advocated, based on constructed distribution of the probability of success. A simple Matlab code for the calculation of the confidence limits according to the proposed method is provided. The most probable, the mean, the variance and the confidence limits are calculated applying the usual definitions of these characteristics. The proposed method works for any number of trials--large and small and all possible values of the number of successes, including k=0 and k=n, providing exact formulae for the calculation of the confidence limits in all cases.
Research Interests: Algorithms, Research Design, Probability, Stochastic processes, Biological Sciences, and 12 moreSoftware, Humans, Animals, Physical sciences, Theoretical Models, Clinical Study, Reproducibility of Results, Bayes Theorem, Confidence Limit, Cohort Studies, Bayesian approach, and Medical and Health Sciences
ABSTRACT Purpose: To investigate the effects of radiation damage, tumor repopulation and cellular sublethal damage repair. Method and Materials: An expression of the Zaider‐Minerbo model obtained by Stavreva et al. is used to fit... more
ABSTRACT Purpose: To investigate the effects of radiation damage, tumor repopulation and cellular sublethal damage repair. Method and Materials: An expression of the Zaider‐Minerbo model obtained by Stavreva et al. is used to fit published dose‐response in vitro data from two cellular megacolony cultures. Results: The data analysis shows the importance of the linear‐quadratic mechanism of cell damage for the description of in vitro cell dynamics. In a previous work, where in vivo data were analyzed, the employment of the single hit model and cell repopulation produced the best fit, while ignoring the quadratic term in the current analysis leads to poor fits. Also, the best‐fit value of the probability of sublethal damage repair, τ, for both cell cultures tends to infinity, indicating that full recovery of the cells occurs between any two consecutive fractions. Conclusion: We conclude that the Zaider‐Minerbo model assuming full recovery of the cells between fractions accompanied by cell repopulation best fits the data from both cellular cultures investigated in the current work. However, a model assuming no repopulation returned a fit statistically indistinguishable from the fit produced by the Zaider‐Minerbo model, though at the expense of unusual best fit values of the cell radiosensitivity characteristics and a large value of τ. Therefore, we recommend the design of experiments using different fractionation regimes producing diverse data to help better analyze the TCP models and rank the models in accordance with statistical criteria.
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In this work we further develop the modeling of tumor dynamics by proposing a mechanism of tumor resensitization that is based on the process of reoxygenation. Reoxygenation is modeled using the concept of nonstationary diffusion of... more
In this work we further develop the modeling of tumor dynamics by proposing a mechanism of tumor resensitization that is based on the process of reoxygenation. Reoxygenation is modeled using the concept of nonstationary diffusion of oxygen. This leads to the derivation of an explicit expression for the radiosensitivity parameter that predicts a radiosensitivity that increases with time. To account for the resensitization mechanism, the time-dependent expression for the radiosensitivity is then incorporated within a tumor control probability (TCP) model that already includes tumor cell repopulation and repair. We fit a set of experimental animal TCP curves corresponding to several different fractionation regimes using both the modified (with resensitization) and unmodified (without resensitization) versions of the TCP model. In comparison to the unmodified model, the modified model produces statistically superior fits, and is able to describe an "inverse" dose-fractionation behavior present in the data.
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In the current state-of-the art of clinical inverse planning, the design of clinically acceptable IMRT plans is predominantly based on the optimization of physical rather than biological objective functions. A major impetus for this trend... more
In the current state-of-the art of clinical inverse planning, the design of clinically acceptable IMRT plans is predominantly based on the optimization of physical rather than biological objective functions. A major impetus for this trend is the unproven predictive power of radiobiological models, which is largely due to the scarcity of data sets for an accurate evaluation of the model parameters. On the other hand, these models do capture the currently known dose-volume effects in tissue dose-response, which should be accounted for in the process of optimization. In order to incorporate radiobiological information in clinical treatment planning optimization, we propose a hybrid physico-biological approach to inverse treatment planning based on the application of a continuous penalty function method to the constrained minimization of a biological objective. The objective is defined as the weighted sum of normal tissue complication probabilities evaluated with the Lyman normal-tissue complication probability model. Physical constraints specify the admissible minimum and maximum target dose. The continuous penalty function method is then used to find an approximate solution of the resulting large-scale constrained minimization problem. Plans generated by our approach are compared to ones produced by a commercial planning system incorporating physical optimization. The comparisons show clinically negligible differences, with the advantage that the hybrid technique does not require specifications of any dose-volume constraints to the normal tissues. This indicates that the proposed hybrid physico-biological method can be used for the generation of clinically acceptable plans.
Research Interests: Algorithms, Biomedical Engineering, Radiation Protection, Medical Physics, Risk assessment, and 12 moreHumans, Computer Simulation, Male, Radiometry, Reproducibility of Results, Risk Assessment, Treatment planning, Sensitivity and Specificity, Cell Survival, Objective function, Prostatic neoplasms, and lung neoplasms
The charging characteristics of ionographic latent images are discussed in terms of the saturation characteristics of ionographic chambers. It is shown that latent images are essentially foil electrets with surface charge densities... more
The charging characteristics of ionographic latent images are discussed in terms of the saturation characteristics of ionographic chambers. It is shown that latent images are essentially foil electrets with surface charge densities modulated by the x-ray transmission through the object. Empirical methods for the calculation of saturation current densities and extrapolated electric fields are given. Both the saturation current density and the extrapolated field consist of two components: one, which is linear with the air gap thickness, is produced by primary photon interactions in the chamber-sensitive volume; the other, exhibiting an exponential saturation, is attributed to photoelectrons backscattered from the polarizing electrode into the chamber-sensitive volume. The minimum applied electric field needed for an optimized charge collection in the ionographic chamber is presented in terms of both the characteristic polarization time and the electret relaxation time. The maximum possible surface charge density in the latent image is calculated, first exhibiting direct proportionality with the polarizing voltage V0, and then following a V0 (1/2) dependence for voltages larger than the breakpoint voltage. The optimum exposure for latent image production is calculated. Exposures below this optimum give insufficient charge densities for subsequent development, while exposures above it degrade the image and eventually result in a uniform foil electret charged to its maximum theoretical value.
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A new analytical expression is presented to describe the full saturation curve of parallel-plate ionization chambers. In contrast to the presently known expressions, which hold only for the near saturation region, this empirically... more
A new analytical expression is presented to describe the full saturation curve of parallel-plate ionization chambers. In contrast to the presently known expressions, which hold only for the near saturation region, this empirically determined expression is in excellent agreement with measurements in the whole collection efficiency range from 0 to 1 for x-ray sources with effective energies from 20 to 150 keV and cobalt-60 gamma rays. The dependence of the ion collection efficiency and the extrapolated electric field, which is a parameter in the new saturation curve expression, on electric field, dose, dose rate, beam quality, and chamber volume, is discussed. The effect of photoemission from the chamber polarizing electrode for low-energy x-ray beams on the saturation current is demonstrated. A universal ionization chamber constant is derived experimentally. It is shown that all parameters of the saturation curve equation and thus the saturation curve itself, can be calculated from one single measurement of ionization current at a given electric field and air gap thickness.