Biomedical subjects

Robert Jeraj

Publications and source records attributed to Robert Jeraj.

At least 19 recordsLinked to original sources

PubMed2006

Automatic registration of megavoltage to kilovoltage CT images in helical tomotherapy: an evaluation of the setup verification process for the special case of a rigid head phantom.

Precise daily target localization is necessary to achieve highly conformal radiation delivery. In helical tomotherapy, setup verification may be accomplished just prior to delivering each fraction by acquiring a megavoltage CT scan of the patient in the treatment position. This daily image set may be manually or automatically registered to the image set on which the treatment plan was calculated, in order to determine any needed adjustments. The system was tested by acquiring 104 MVCT scans of an anthropomorphic head phantom to which translational displacements had been introduced with respect to the planning image set. Registration results were compared against an independent, optically guided positioning system. The total experimental uncertainty was within approximately 1 mm. Although the registration of phantom images is not fully analogous to the registration of patient images, this study confirms that the system is capable of phantom localization with sub-voxel accuracy. In seven registration problems considered, expert human observers were able to perform manual registrations with comparable or inferior accuracy to automatic registration by mutual information. The time to compute an automatic registration is considerably shorter than the time required for manual registration. However, human evaluation of automatic results is necessary in order to identify occasional outliers, and to ensure that the registration is clinically acceptable, especially in the case of deformable patient anatomy.

Algorithms↗

PubMed2005

Imaging dose management using multi-resolution in CT-guided radiation therapy.

In image-guided radiation therapy, megavoltage computerized tomography (MVCT) delivers higher dose to the patient for lower image quality than diagnostic kilovoltage CT (kVCT). One way to reduce the mean imaging dose is to reduce the imaging volume, which is often sufficient for registration and dosimetry purposes. The filtered back projection using truncated data causes artefacts that degrade the image quality. Those artefacts can be effectively reduced by wavelet-based multi-resolution analysis (WMRA), in which the detail and approximate information are reconstructed separately to bypass the non-locality of filtered back projection. In this study, WMRA was used to reconstruct local images from both very low-dose kVCT scans from a bench-top tomotherapy unit and MVCT scans from helical tomotherapy. Results show that mean imaging dose can be significantly reduced by imaging a small region of interest. In simulation, the root-mean-square error brought by the truncation is smaller than 1-2% and depends on the level of dose reduction. On the other hand, the same mean dose that would have been delivered by a low-quality global CT can be conformed to a smaller volume to improve the visibility of low-contrast organs and fine structures using WMRA. Organs at risk can be avoided during repeated daily CT imaging when irregular-shaped reconstruction areas are used. WMRA does not involve computationally expensive iterations and is suitable for image-guided radiation therapy where imaging speed is essential. Compared with extrapolation methods, errors are further reduced to improve the detection of low contrast and fine structures.

Algorithms↗

PubMed2005

Dose calibration of nonconventional treatment systems applied to helical tomotherapy.

Current dosimetric protocols based on the absorbed dose (AAPM TG-51 and IAEA TRS-398 protocols) require calibration measurements under reference conditions. For some radiotherapy systems, this requirement cannot be met, and calibration has to be performed under nonreference experimental conditions. In order to solve this problem, both protocols can be extended by inclusion of the measured-to-reference conversion factor, k(mr). In order to determine this factor, basic dosimetric quantities, like stopping power ratios, mass attenuation coefficients and chamber correction factors have to be calculated. If measurements are not feasible, accurate Monte Carlo modeling is required. The extension of the protocols is illustrated using the case of the helical tomotherapy radiation unit, where the typical calibration measurement conditions are the 10 x 5 cm2 field size and the 85 cm surface source distance, limited by the system design. It was calculated that the k(mr) factor for this conditions is close to unity (0.997+/-0.001). In addition, the deviation of the measurement conditions from the reference conditions results in the change of the quality conversion factor (approximately 0.995-0.998, depending on the ionization chamber used). This change is the same regardless of the used calibration protocol. For smaller field sizes the corrections become more significant, resulting in the total correction factor compared to the reference conditions of up to 1.5% for the smallest considered field size of 2 x 2 cm2.

Algorithms↗

PubMed2005

A novel method to correct for pitch and yaw patient setup errors in helical tomotherapy.

An accurate means of determining and correcting for daily patient setup errors is important to the cancer outcome in radiotherapy. While many tools have been developed to detect setup errors, difficulty may arise in accurately adjusting the patient to account for the rotational error components. A novel, automated method to correct for rotational patient setup errors in helical tomotherapy is proposed for a treatment couch that is restricted to motion along translational axes. In tomotherapy, only a narrow superior/inferior section of the target receives a dose at any instant, thus rotations in the sagittal and coronal planes may be approximately corrected for by very slow continuous couch motion in a direction perpendicular to the scanning direction. Results from proof-of-principle tests indicate that the method improves the accuracy of treatment delivery, especially for long and narrow targets. Rotational corrections about an axis perpendicular to the transverse plane continue to be implemented easily in tomotherapy by adjustment of the initial gantry angle.

Equipment Design↗

PubMed2005

Confirmation, refinement, and extension of a study in intrafraction motion interplay with sliding jaw motion.

The interplay between a constant scan speed and intrafraction oscillatory motion produces interesting fluence intensity modulations along the axis of motion that are sensitive to the motion function, as originally shown in a classic paper by Yu et al. [Phys. Med. Biol. 43, 91-104 (1998)]. The fluence intensity profiles are explored in this note for an intuitive understanding, then compared with Yu et al., and finally further explored for the effects of low scan speed and random components of both intrafraction and interfraction motion. At slow scan speeds typical of helical tomotherapy, these fluence intensity modulations are only a few percent. With the addition of only a small amount of cycle-to-cycle randomness in frequency and amplitude, the fluence intensity profiles change dramatically. It is further shown that after a typical 30-fraction treatment, the sensitivities displayed in the single fraction fluence intensity profiles greatly diminish.

Body Burden↗

PubMed2004

Fast treatment plan modification with an over-relaxed Cimmino algorithm.

A method to quickly modify a treatment plan in adaptive radiotherapy was proposed and studied. The method is based on a Cimmino-type algorithm in linear programming. The fast convergence speed is achieved by over-relaxing the algorithm relaxation parameter from its sufficient convergence range of (0, 2) to (0, infinity). The algorithm parameters are selected so that the over-relaxed Cimmino (ORC) algorithm can effectively approximate an unconstrained re-optimization process in adaptive radiotherapy. To demonstrate the effectiveness and flexibility of the proposed method in adaptive radiotherapy, two scenarios with different organ motion/deformation of one nasopharyngeal case were presented with comparisons made between this method and the re-optimization method. In both scenarios, the ORC algorithm modified treatment plans have dose distributions that are similar to those given by the re-optimized treatment plans. It takes us using the ORC algorithm to finish a treatment plan modification at least three times faster than the re-optimization procedure compared.

Algorithms↗

PubMed2004

Radiation characteristics of helical tomotherapy.

Helical tomotherapy is a dedicated intensity modulated radiation therapy (IMRT) system with on-board imaging capability (MVCT) and therefore differs from conventional treatment units. Different design goals resulted in some distinctive radiation field characteristics. The most significant differences in the design are the lack of flattening filter, increased shielding of the collimators, treatment and imaging operation modes and narrow fan beam delivery. Radiation characteristics of the helical tomotherapy system, sensitivity studies of various incident electron beam parameters and radiation safety analyses are presented here. It was determined that the photon beam energy spectrum of helical tomotherapy is similar to that of more conventional radiation treatment units. The two operational modes of the system result in different nominal energies of the incident electron beam with approximately 6 MeV and 3.5 MeV in the treatment and imaging modes, respectively. The off-axis mean energy dependence is much lower than in conventional radiotherapy units with less than 5% variation across the field, which is the consequence of the absent flattening filter. For the same reason the transverse profile exhibits the characteristic conical shape resulting in a 2-fold increase of the beam intensity in the center. The radiation leakage outside the field was found to be negligible at less than 0.05% because of the increased shielding of the collimators. At this level the in-field scattering is a dominant source of the radiation outside the field and thus a narrow field treatment does not result in the increased leakage. The sensitivity studies showed increased sensitivity on the incident electron position because of the narrow fan beam delivery and high sensitivity on the incident electron energy, as common to other treatment systems. All in all, it was determined that helical tomotherapy is a system with some unique radiation characteristics, which have been to a large extent optimized for intensity modulated delivery.

Electrons↗

PubMed2004

Treatment plan optimization incorporating respiratory motion.

Similar to conventional conformal radiotherapy, during lung tomotherapy, a motion margin has to be set for respiratory motion. Consequently, large volume of normal tissue is irradiated by intensive radiation. To solve this problem, we have developed a new motion mitigation method by incorporating target motion into treatment optimization. In this method, the delivery-breathing correlation is determined prior to treatment plan optimization. Beamlets are calculated by using the CT images at the corresponding breathing phases from a dynamic (four-dimensional) image sequence. With the displacement vector fields at different breathing phases, a set of deformed beamlets is obtained by mapping the dose to the primary phase. Optimization incorporating motion is then performed by using the deformed beamlets obtained by dose mapping. During treatment delivery, the same breathing-delivery correlation can be reproduced by instructing the patient to breathe following a visually displayed guiding cycle. This method was tested using a computer-simulated deformable phantom and a real lung case. Results show that treatment optimization incorporating motion achieved similar high dose conformality on a mobile target compared with static delivery. The residual motion effects due to imperfect breathing tracking were also analyzed.

Biophysical Phenomena↗

PubMed2003

Adaptive anisotropic diffusion filtering of Monte Carlo dose distributions.

The Monte Carlo method is the most accurate method for radiotherapy dose calculations, if used correctly. However, any Monte Carlo dose calculation is burdened with statistical noise. In this paper, denoising of Monte Carlo dose distributions with a three-dimensional adaptive anisotropic diffusion method was investigated. The standard anisotropic diffusion method was extended by changing the filtering parameters adaptively according to the local statistical noise. Smoothing of dose distributions with different noise levels in an inhomogeneous phantom, a conventional and an IMRT treatment case is shown. The resultant dose distributions were analysed using several evaluating criteria. It is shown that the adaptive anisotropic diffusion method can reduce statistical noise significantly (two to five times, corresponding to the reduction of simulation time by a factor of up to 20), while preserving important gradients of the dose distribution well. The choice of free parameters of the method was found to be fairly robust.

Anisotropy↗

PubMed2003

Optimizer convergence and local minima errors and their clinical importance.

Two of the errors common in the inverse treatment planning optimization have been investigated. The first error is the optimizer convergence error, which appears because of non-perfect convergence to the global or local solution, usually caused by a non-zero stopping criterion. The second error is the local minima error, which occurs when the objective function is not convex and/or the feasible solution space is not convex. The magnitude of the errors, their relative importance in comparison to other errors as well as their clinical significance in terms of tumour control probability (TCP) and normal tissue complication probability (NTCP) were investigated. Two inherently different optimizers, a stochastic simulated annealing and deterministic gradient method were compared on a clinical example. It was found that for typical optimization the optimizer convergence errors are rather small, especially compared to other convergence errors, e.g., convergence errors due to inaccuracy of the current dose calculation algorithms. This indicates that stopping criteria could often be relaxed leading into optimization speed-ups. The local minima errors were also found to be relatively small and typically in the range of the dose calculation convergence errors. Even for the cases where significantly higher objective function scores were obtained the local minima errors were not significantly higher. Clinical evaluation of the optimizer convergence error showed good correlation between the convergence of the clinical TCP or NTCP measures and convergence of the physical dose distribution. On the other hand, the local minima errors resulted in significantly different TCP or NTCP values (up to a factor of 2) indicating clinical importance of the local minima produced by physical optimization.

Algorithms↗

PubMed2003

Treatment plan modification using voxel-based weighting factors/dose prescription.

Under various clinical situations, it is desirable to modify the original treatment plan to better suit the clinical goals. In this work, a method to help physicians modify treatment plans based on their clinical preferences is proposed. The method uses a weighted quadratic dose objective function. The commonly used organ-/ROI-based weighting factors are expanded to a set of voxel-based weighting factors in order to obtain greater flexibility in treatment plan modification. Two different but equivalent modification schemes based on Rustem's quadratic programming algorithms--modification of a weighting matrix and modification of prescribed doses--are presented. Case studies demonstrated the effectiveness of the two methods with regard to their capability to fine-tune treatment plans.

Algorithms↗

PubMed2003

The method of intercepts in parameter space for the analysis of local minima caused by dose-volume constraints.

The local minima problem in radiotherapy optimization has been a concern for both researchers and physicians. In this work, local minima induced by dose-volume histogram (DVH) constraints are discussed. The non-convex property of the feasible set formed by DVH constraints is discussed in beam weight space. An intuitive explanation of the origin of this type of local minima is given by a two-beam model setup. Some interesting properties and insights about the DVH-induced local minima are found. Based on this, a heuristic non-random initial guess sampling method is proposed and applied to a clinical nasopharyngeal case, where some significantly different local minima are located.

Algorithms↗

PubMed2003

Image guidance for precise conformal radiotherapy.

PURPOSE: To review the state of the art in image-guided precision conformal radiotherapy and to describe how helical tomotherapy compares with the image-guided practices being developed for conventional radiotherapy. MATERIALS AND METHODS: Image guidance is beginning to be the fundamental basis for radiotherapy planning, delivery, and verification. Radiotherapy planning requires more precision in the extension and localization of disease. When greater precision is not possible, conformal avoidance methodology may be indicated whereby the margin of disease extension is generous, except where sensitive normal tissues exist. Radiotherapy delivery requires better precision in the definition of treatment volume, on a daily basis if necessary. Helical tomotherapy has been designed to use CT imaging technology to plan, deliver, and verify that the delivery has been carried out as planned. The image-guided processes of helical tomotherapy that enable this goal are described. RESULTS: Examples of the results of helical tomotherapy processes for image-guided intensity-modulated radiotherapy are presented. These processes include megavoltage CT acquisition, automated segmentation of CT images, dose reconstruction using the CT image set, deformable registration of CT images, and reoptimization. CONCLUSIONS: Image-guided precision conformal radiotherapy can be used as a tool to treat the tumor yet spare critical structures. Helical tomotherapy has been designed from the ground up as an integrated image-guided intensity-modulated radiotherapy system and allows new verification processes based on megavoltage CT images to be implemented.

Animals↗

PubMed2003

Radiotherapy dose calculations in the presence of hip prostheses.

The high density and atomic number of hip prostheses for patients undergoing pelvic radiotherapy challenge our ability to accurately calculate dose. A new clinical dose calculation algorithm, Monte Carlo, will allow accurate calculation of the radiation transport both within and beyond hip prostheses. The aim of this research was to investigate, for both phantom and patient geometries, the capability of various dose calculation algorithms to yield accurate treatment plans. Dose distributions in phantom and patient geometries with high atomic number prostheses were calculated using Monte Carlo, superposition, pencil beam, and no-heterogeneity correction algorithms. The phantom dose distributions were analyzed by depth dose and dose profile curves. The patient dose distributions were analyzed by isodose curves, dose-volume histograms (DVHs) and tumor control probability/normal tissue complication probability (TCP/NTCP) calculations. Monte Carlo calculations predicted the dose enhancement and reduction at the proximal and distal prosthesis interfaces respectively, whereas superposition and pencil beam calculations did not. However, further from the prosthesis, the differences between the dose calculation algorithms diminished. Treatment plans calculated with superposition showed similar isodose curves, DVHs, and TCP/NTCP as the Monte Carlo plans, except in the bladder, where Monte Carlo predicted a slightly lower dose. Treatment plans calculated with either the pencil beam method or with no heterogeneity correction differed significantly from the Monte Carlo plans.

Algorithms↗

PubMed2003

Image reconstruction and the effect on dose calculation for hip prostheses.

High atomic number inserts, such as hip prostheses and dental fillings, cause streak artifacts on computed tomography (CT) images when filtered back-projection (FBP) methods are used. These streak artifacts severely degrade our ability to differentiate the tumor volume. Also, incorrect Hounsfield numbers yield incorrect electron density information that may lead to erroneous dose calculations, and, as a result, compromise clinical outcomes. The aim of this research was to evaluate the dosimetric consequences of artifacts during radiotherapy planning of a prostate patient containing a hip prosthesis. The CT numbers corresponding to an iron prosthesis were inserted into the right femoral head of an existing CT image set. This artifact-free image was used as the standard image set. CT projections through the image set formed the sinogram, from which filtered back projection and iterative deblurring methods were used to create reconstructed image sets. These reconstructed image sets contained artifacts. Prostate treatment plans were then calculated using a Monte Carlo system for the standard and reconstructed CT image sets. Close to the prosthesis, the CT numbers between the reconstructed and standard image sets differed substantially. However, because the CT number differences covered only a small area, the dose distributions on the reconstructed and standard image sets were not significantly different. The dose-volume histograms for the prostate, rectum, and bladder were virtually identical. Our results indicate that even though CT image artifacts restrict our ability to differentiate tumors and critical structures, the dose distributions for a prostate plan containing a hip prosthesis, calculated on both artifact-free image sets and image sets containing artifacts, are not significantly different.

Algorithms↗

PubMed2003

Benchmarking beam alignment for a clinical helical tomotherapy device.

A clinical helical tomotherapy treatment machine has been installed at the University of Wisconsin Comprehensive Cancer Center. Beam alignment has been finalized and accepted by UW staff. Helical tomotherapy will soon be clinically available to other sites. Clinical physicists who expect to work with this machine will need to be familiar with its unique dosimetric characteristics, and those related to the geometrical beam configuration and its verification are described here. A series of alignment tests and the results are presented. Helical tomotherapy utilizes an array of post-patient xenon-filled megavoltage radiation detectors. These detectors have proved capable of performing some alignment verification tests. That is particularly advantageous because those tests can then be automated and easily performed on an ongoing basis.

Benchmarking↗

PubMed2002

Re-optimization in adaptive radiotherapy.

In routine clinical practice, radiotherapy treatment planning is performed based on the patient CT images obtained during the patient setup procedure. However, the actual delivered dose to the patient might be different from the planned dose because of various reasons such as patient motion. Under such situations, it is desirable to modify the original treatment plan in order to partially remedy the dose delivery errors in the subsequent dose delivery process. Such modification can be implemented by modifying the original treatment plan using re-optimization. In this work, issues such as the re-optimization dose prescription, optimization constraints in re-optimization, re-optimization in multiple fractionation schemes and re-optimization procedure with generalized dose-based objective functions were investigated and corresponding mathematical schemes proposed. The derived results were applied to a clinical case study in which it was shown that the proposed re-optimization method is able to remedy the cold spots in tumour while delivering low dose to normal structures. Thus the potential effectiveness of the method was demonstrated.

Algorithms↗

PubMed2002

A flattening filter for brachytherapy skin irradiation.

Radioactive sources in close contact offer an alternative to superficial radiation in the treatment of skin lesions. A flattening filter was designed for a lead surface applicator to improve the skin dose distribution of a high dose rate (HDR) brachytherapy unit (Nucletron). At three heights from the opening (10, 15 and 25 mm) of the cylindrical applicator, the 192Ir source can be driven into the centre of the applicator. Thin sheets of lead foil (0.2 mm) were cut into circular shapes and placed in the opening to build a cylindrical cone that acts as a flattening filter. The shape of the cone was optimized in an iterative process using a spreadsheet and the resulting dose distribution under the applicator was determined using radiosensitive film. The use of the filter improved the dose distribution in a plane perpendicular to the beam axis to be within +/- 5% of the central axis dose. The present applicator and flattening filter together with an HDR brachytherapy unit offer an alternative for skin irradiation where a superficial unit is not available or will be replaced with a more flexible device. As the depth dose characteristics can be modified using different source-to-surface distances, the dose throughout the patient's skin can be shaped as desired by the radiation oncologist using a compensator design type approach.

Brachytherapy↗