Biomedical subjects

M A Ebert

Publications and source records attributed to M A Ebert.

12 recordsLinked to original sources

PubMed2001

System validation and work practice efficiency gains of a new localization method for stereotactic radiotherapy.

The increased procedural demands of stereotactic localization techniques when compared with conventional treatment practices reduces machine efficiency, an outcome likely to be greatly magnified by the introduction of fractionation to stereotactic techniques. Currently in Australia and New Zealand there are no guidelines for the definition of efficiency. We sought to devise a system to simultaneously validate the accuracy and efficiency of the technique. The frameless relocation methods employed in the Medtronic Sofamor Danek (MSD) stereotactic radiotherapy (SRT) system were studied in the clinical setting. Accuracy has been determined according to the accumulation of errors throughout the planning and treatment process. The clinical demands of the system (staffing and resources) were analysed relative to conventional treatment approaches. Timing studies indicate a mean time of 19.7 min for treatment of a daily SRT fraction (4-5 arcs, single isocentre). Cost and staffing requirements are similar to those for conventional radiotherapy. It is concluded that with the system used, SRT is efficient for routine clinical implementation, with the level of efficiency increasing with increasing patient numbers. It is recommended that a common acceptance standard be developed to allow cross-institutional comparison of the clinical efficiency of new treatment techniques.

Brain Neoplasms↗

PubMed2001

Dose perturbation by air cavities in megavoltage photon beams: implications for cavity surface doses.

Gas-filled cavities in the body are known to perturb megavoltage photon beams in radiation therapy. The bowel represents one such cavity where cavity dimensions can vary throughout a treatment course. This has implications for doses to the surface of the bowel. Doses to the first 0.1 mm and 1.0 mm of cavity surfaces were calculated for 6 MV and 18 MV photon beams for a range of cavity sizes. It is found that relatively minor variations in cavity surface doses result over the typical range of rectal cavity size (2-3 cm). However, the variations in surface dose should be considered whenever bowel reactions must be considered, or when the target cells of interest line the bowel itself.

Air↗

PubMed2000

Viability of the EUD and TCP concepts as reliable dose indicators.

The concept of equivalent uniform dose (EUD) was introduced to provide a method of reporting radiotherapy dose distributions which takes account of the nonlinearity of tissue dose-response, whilst not attempting to make predictions of absolute outcome. The purpose of this investigation was to determine the level of sensitivity of EUD to model parameters for significant variations in dose distribution and consequently the reliability of the factor as a dose-indicator, and to compare EUD with the more familiar index, tumour control probability (TCP). EUD and TCP, derived from the linear-quadratic formalism, were investigated for a test tissue being irradiated non-uniformly. Variations in the parameters of the model (tissue cell characteristics, dose heterogeneity, fractionation parameters) indicated the sensitivity of EUD and TCP to them. For time independent factors--cell density, cell radiosensitivity, radiosensitivity heterogeneity (population averaged) and ratio alpha/beta--EUD was found to vary insignificantly in comparison with TCP, though this is a function of the actual form of the dose distribution under consideration. For fractionated treatments where the mean dose per fraction is varying (due to dosimetric/positioning errors for example), both EUD and TCP showed little variation with the degree of dose non-uniformity. For other time dependent factors, fractionation rate and cell repopulation times, TCP again showed significant variation relative to EUD. The relative insensitivity of EUD implies that this index will be useful for dose evaluation when parameters are not known with accuracy, for the intercomparison of dose control studies and as a radiobiologically based optimization objective. However, given confidence in model parameters, the sensitivity of TCP would make it a more reliable tool for indicating potentially successful and unsuccessful irradiation strategies. It is suggested that both parameters be used in conjunction, with EUD and TCP results viewed with an appreciation of the characteristics of each model.

Dose Fractionation, Radiation↗

PubMed2000

Modeling dose response in the presence of spatial variations in dose rate.

Nonuniform dose rates are an inevitability in treatments involving internal sources, arising from electronic disequilibrium effects as well as nonuniformity in activity distribution. These dose-rate nonuniformities are of consequence for protracted treatments (when dose-delivery times are of the order of cell-repair times). The influence of nonuniform dose rates on tumor control probability (TCP) has thus been considered. A model for TCP has been developed by merging established (linear-quadratic based) TCP models for dose nonuniformity, with dose-rate effects as influenced by cell repair and proliferation capacities. This model has been examined by considering treatment of spherical tumors of varying sizes filled with uniform distributions of several beta-emitting isotopes. Dose (or dose-rate) volume histograms (DVHs) were calculated for the combinations of tumor size and isotope, and applied to the developed TCP model. Comparison of the results identified several characteristics of the effect of nonuniform dose rate, including the balance between minimum dose and cell number as they vary with tumor size, the dominance of minimum dose (dose rate) on TCP, and the influence of cell-proliferation effects on effective delivered dose (and the effective DVH). The model was also used to determine TCPs for simulated 90Y-labeled microsphere treatments of liver metastases using both uniform and clustered-microsphere models for activity distributions, and for varying tumor size. Despite significantly higher doses being achieved via clustered (nonuniform) activity distributions, the minimum dose for clustered distributions is consistently lower than that of the corresponding uniform distributions, and TCP is always higher for the uniform distributions.

Cell Division↗

PubMed1999

Implementation of a stereotactic radiotherapy system.

The tests carried out on a recently-installed stereotactic radiotherapy system capable of fully frameless stereotaxy are outlined. The two principle investigations carried out on the planning system were a dosimetry test, and a test to verify the accuracy of localisation from CT examination through to patient treatment. The method employed in frameless relocation for fractionated stereotactic radiotherapy--position detection using stereoscopic infra-red cameras and a set of fiducials attached to a bite-block--was tested for reproducibility. The results of these tests suggest a mean accuracy in treatment localisation of 1.23 mm (maximum 2.72 mm) for single fraction techniques, and 1.77 mm (maximum 4.53 mm) for treatments involving repeated (frameless) localisation. The implications of these accuracies for treatment with single, multiple-fraction and multi-isocentre techniques are discussed.

Brain Diseases↗

PubMed1997

Possibilities for tailoring dose distributions through the manipulation of electron beam characteristics.

The influence of the properties of an electron beam on resulting dose distributions, and the potential benefits for dose conformity and optimizing dose distribution characteristics by electron beam manipulation, are theoretically examined. A simulated annealing routine is used to weight electron pencil beams of discrete energies incident at discrete locations and angles on one side of a phantom. The resulting optimal electron phase space provides a dose distribution which most closely approaches a desired distribution on the basis of a physical comparison. For simple desired distributions, intuitive results are obtained such as the benefits of energy modulation for distributing dose with depth, of angular and spatial modulation for overcoming disequilibrium effects and their combination in boosting surface doses. For a complex desired dose distribution, the optimization routine instigates a complex interplay of energy, angular and spatial modulation in attempting to achieve dose conformity. A significant result shows that, for a suitably selected beam energy, angular modulation can compensate for the variation in the depth of the distal edge of a superficial target. The effects of varying just energy for normally incident electrons are compared with those of varying the distribution of incidence angle (for monoenergetic electrons) and the combination of both, indicating the relative merits of the manipulation of available degrees of freedom.

Biophysical Phenomena↗

PubMed1997

Optimisation in radiotherapy I: defining the problem.

Optimisation in radiotherapy should incorporate a very wide set of variables, including the combinations of dose due to external beam radiotherapy, brachytherapy, internally administered radionuclides and the effects of chemotherapy, surgery, hyperthermia, other biological and chemical defences, alternative treatment techniques, lifestyle and mental state of the patient, the economics of cancer treatment, and consideration of tolerable levels of adverse effects and palliation. A full treatment optimisation would consider the influence and covariance of all these variables and any future techniques, as well as the complex constraints imposed by the biological systems being irradiated. This series of reviews concentrates on optimisation of radiotherapy through the treatment planning component of the treatment process--an area of radiotherapy research that has received a great deal of attention as the attached lists of references will testify. It hopes to provide the medical physics and engineering community (and hopefully the clinical community) with a background into the mathematical bases for the manipulation of radiation for clinical benefit. It also examines the potential benefits of research into these techniques in the light of recent approaches to optimisation in radiotherapy, and provides pointers to more concise accounts in the literature. In this first article, the incentive for radiotherapy optimisation research is established, and the actual radiotherapy optimisation problem (in terms of the manipulation of degrees of freedom in radiation delivery) is defined. The degrees of freedom associated with radiotherapy treatment are identified, and it is shown how these degrees of freedom translate into the mathematical parameters of the problem, including the dose distributions they produce. The constraints and objectives of the problem are also discussed from both physical and radiobiological perspectives. Subsequent articles will consider algorithms for performing the optimisation, their relative utility, and their application in clinical practice.

Humans↗

PubMed1996

Some characteristics of tumour control probability for heterogeneous tumours.

Some mathematical characteristics of tumour control probability (TCP) have been examined in the light of potential intra-tumour variations in clonogenic cell characteristics. In particular, an explicit expression for the relationship between the dose distribution and the probability of local clonogen eradication is obtained which maximizes the TCP using both general and specific TCP models, for the case of fixed energy deposition into the tumour volume. Characteristics of this expression are considered for the cases of varying clonogen cell density (uniform radiosensitivity) and uniform cell density (uniform radiosensitivity), yielding the 'uniform TCP' and 'uniform dose' results respectively for TCP maximization. These situations are examined graphically to highlight the possible consequences of neglecting intra-tumour heterogeneity in dose prescription.

Cell Survival↗

PubMed1996

Modelling clinical accelerator beams: a review.

Radiotherapy dose calculation algorithms currently under development (and some in use) require knowledge of the characteristics of particles in linear accelerator-generated radiation beams. A range of techniques have been developed which allow determination of those characteristics requiring minimal interaction with the physical beam. Such techniques may be analytical in nature, making use of analytical transport results and cross sections, or they may be numerical, employing the increasing utility of Monte Carlo techniques. These techniques provide us with an extensive description of clinical beams and the ability to refine beam production and collimation systems. This review details several published analytical and numerical approaches to megavoltage photon and electron beam modelling, the characteristics that they provide, and their relative accuracy and utility.

Algorithms↗

PubMed1996

The energy and angular characteristics of the applicator scattered component of an electron beam.

Using a previously developed Monte Carlo based model for determining the characteristics of particles scattered from electron beam applicators (cones), the properties of such particles are investigated for the 6 MeV and 12 MeV beams of a Siemens Mevatron KD-2 linear accelerator. Properties examined are energy and angular spectra averaged across the full beam, mean energy and angle and particle number with radial distance from isocentre, and mean energy with angle for particles scattered from each of the three trimmers of the applicator considered. The particles examined are electrons scattered from applicator trimmers, and photons generated in the applicator materials by primary beam electrons. It is found that the properties observed are characteristic of the effective source that each applicator trimmer represents.

Biophysical Phenomena↗

PubMed1995

A Monte Carlo investigation of electron-beam applicator scatter.

An EGS4 Monte Carlo investigation into applicator scatter in clinical electron beams has been undertaken in order to establish the characteristics of electrons incident on the patient surface which have interacted with collimation systems. The applicator scattered component of an electron beam (including that from irregularly shaped cutouts) should be considered when modeling the electron phase space since it represents a component of the beam incident on the patient with widely varying characteristics to those of the primary beam. Scattering off an edge of applicator material is considered in terms of the types and characteristics of incident primary beam, the resulting interactions in the edge, and the fluence and energy characteristics of the emerging particles. Results indicate that the principal component to consider is scattered electrons due to the electron component of the primary beam, and that the fluence and energy characteristics of this component are dependent upon primary beam energy and the configuration of the applicator apertures.

Computer Simulation↗

PubMed1995

A model for electron-beam applicator scatter.

Applicators (or cones), used in conjunction with patient specific cutouts in electron-beam radiotherapy, may interact with the primary electron beam to produce a secondary beam component (applicator scatter). This component affects machine output as well as the shape of resulting dose distributions. A model has been developed to simulate this scatter component for applicators consisting of trimming plates of arbitrary shape. This model involves sampling established kernels of scatter from edge elements of appropriate materials, obtained through Monte Carlo simulations. The result of the model is a phase space (position, direction, energy, charge, weighting) of applicator scattered particles which can be incorporated into a further Monte Carlo simulation, or as input into another advanced treatment planning algorithm. This model is evaluated by comparison of measured profiles and applicator scatter component depth dose curves with Monte Carlo simulations using simulated phase-space data as input. Results are very consistent and reveal information on the angular and spatial variation characteristics of this beam component. The results obtained verify the developed model as an accurate predictor of the characteristics of applicator scattered particles.

Computer Simulation↗