Clinical Electron Beams Research Articles

The mass angular scattering power method for determining the kinetic energies of clinical electron beams

The method is based on the measurement in air of the spatial spread of a pencil electron beam which is produced from the broad clinical electron beam. As predicted by the Fermi-Eyges theory, the dose distribution measured in air on a plane, perpendicular to the incident direction of the initial pencil electron beam, is Gaussian. The square of its spatial spread is related to the mass angular scattering power which in turn is related to the kinetic energy of the electron beam. The measured spatial spread may thus be used to determine the mass angular scattering power, which is then used to determine the kinetic energy of the electron beam from the known relationship between mass angular scattering power and kinetic energy. Energies obtained with the mass angular scattering power method agree with those obtained with the electron range method.

Effect of backscatter on cell survival for a clinical electron beam

Relative to homogeneous scattering conditions in tissue-like materials, a large increase in dose for clinical electron beams can occur upstream from high atomic number heterogeneities due to backscattered electrons. The degree of this dose increase is uncertain due to the unknown energy distribution of the backscattered electron fluence. Cell survival after irradiation was studied for Chinese hamster cells at the depth of maximum dose in a clinical 6 MeV electron beam under normal scattering conditions and with the addition of a lead backscatterer. No significant difference in cell survival was found between the two geometries when the dose increase due to the backscattered electron fluence was approximated by the product of the measured ionization and the normal scattering stopping power ratio.

Reduction of the Bremsstrahlung component of clinical electron beams: implications for electron arc therapy and total skin electron irradiation

The dose due to Bremsstrahlung in stationary electron beams of nominal energies in the range 6-20 MeV is typically between 1-7% of the maximum dose and is usually not clinically significant. However, in treatments using rotational or multiple electron beams where the X-ray dose from several beams is added the X-ray dose will reach much higher proportions and will be of clinical significance. Moreover, this does often is located in normal tissue beyond the target volume. Reduction of this X-ray dose is therefore desirable. In the present study a reduction of the X-ray component of electron beams produced by a Clinac 2100C accelerator by a change of the transmission ion chamber and scattering foils is reported. A reduction in Bremsstrahlung of up to 50% can be achieved.

Film dosimetry of clinical electron beams

Film dosimetry is used widely to obtain relative dose distributions of clinical electron beams in phantoms. Nevertheless, measurement results obtained with film dosimetry may lack precision and reliability. In this paper well defined and reproducible methods in film dosimetry are discussed. By application of these methods, film dosimetry appears to be adequate in measuring relative dose distributions of clinically applied electron beams, with an accuracy of 1% to 2% of the dose maximum, in water and plastics as well as in heterogeneously composed material.

Film dosimetry of clinical electron beams : International Journal of Radiation Oncology, Biology, Physics, Vol. 18, No. 1, January 1990, pp. 69–76

Film dosimetry is used widely to obtain relative dose distributions of clinical electron beams in phantoms. Nevertheless, measurement results obtained with film dosimetry may lack precision and reliability. In this paper well defined and reproducible methods in film dosimetry are discussed. By application of these methods, film dosimetry appears to be adequate in measuring relative dose distributions of clinically applied electron beams, with an accuracy of 1% to 2% of the dose maximum, in water and plastics as well as in heterogeneously composed material.

Stopping-power ratios and their uncertainties for clinical electron beam dosimetry

The influence of energy and angular spread and electron and photon contamination on the water/air stopping-power ratios for 'realistic' electron beams of 10 MeV has been investigated using the Monte Carlo method. Differences smaller than 0.5% have been found in the sW,air value at the depth of maximum absorbed dose compared with sW,air(E0,z) determined according to most dosimetry protocols. The use of broad independent energy and angular distributions to sample the initial state of electrons for Monte Carlo simulations has been analysed. Uncertainties in sSAW,air values, evaluated with a Monte Carlo iterative technique, are approximately 0.5% at dmax. The combination of uncertainties in sSAW,air due to the calculation procedure and to the 'sW,air(E0,z) method' yields an estimated total uncertainty of approximately 1% for the sW,air at dmax in clinical electron beams with energies around 10 MeV, which is smaller than values quoted recently.

A study on the secondary electrons in a clinical electron beam

The central axis dose of a 12 MeV clinical electron beam is investigated in terms of an axial component due to primary electrons in the central ray and a lateral component due to secondary electrons originating from multiple scattering of electrons in the off-axis rays. To this effect secondary electron fluence measurements in a polystyrene medium irradiated with a collimated beam are made with a sensitive diode detector. This leads to a construction of secondary electron depth-dose profiles for beam sizes of diameters ranging from 1.7 to 17.4 cm. The results indicate that the lateral electrons account for 25% of the dose in the therapeutic region. For these electrons, the depth of dose maximum is correlated with diffusion depth and maximum lateral excursion in the medium. Dose component due to backscatter electrons at depths is also investigated using a thin-window parallel-plate ion chamber. The role of lateral and backscatter electrons in characterising central axis per cent depth-dose is discussed.

Methods of Calculating the Output Factors of Rectangular Electron Fields

The dose output of a clinical electron beam exhibits a complex dependence upon field size, beam energy and collimation system design. A variety of methods have been developed in the past to calculate the output of an electron beam of arbitrary field dimensions. This paper describes three of these methods and indicates the advantages and disadvantages of each. Comparisons with measured data are also presented.

Calculation of electron beam depth-dose curves and output factors for arbitrary field shapes

A previously presented method to calculate depth-dose curves and output factors for arbitrarily shaped electron beams [6] is evaluated. The method employs a Gaussian pencil model for direct incident and applicator scattered electrons; the parameter values are derived from measured central axis depth-dose distributions. In addition, an empirical model is used to compute the dose due to electrons scattered by field-defining frames. In this way, the properties of the clinical electron beams are taken into account. In this paper, calculations and measurements for electron beams with energies between 6 and 20 MeV, treatment field dimensions between 3 and 14 cm, and various applicator sizes are compared. The results demonstrate the importance of irregular field dose calculations and the scope of the present method. Agreement better than 3% in dose and 0.2 cm in depth is achieved. For electron beams without applicators, the calculations show the same accuracy. Another method in electron treatment planning that derives values for the radial width parameter of the pencil beam from measured broad beam profiles is also investigated. This method gives good results for dose calculations in beams without applicator scatter. It should be used with care, however, for beams that contain such a scatter component. When electrons scattered by the applicator walls and field-defining frames are neglected, differences between measured and calculated dose up to 8% are found.

Accuracy in clinical electron beam dose planning using pencilbeam algorithms

The accuracy of two different pencil beam models for electron beam dose planning is shown by comparisons with measured dose distributions. The investigation is restricted to two-dimensional geometries. In one model the multiple scattering of the electrons is considered by the small angle Gaussian approximation of Fermi and Eyges. In the other, the generalized Gaussian model, the large angle single scattering events are also considered. The parallel slab approximation is used in both models. The comparisons have been made for two different anatomical phantoms. One phantom was constructed to simulate a transversal cross-section through the chest wall and lung in radiotherapy of breast cancer. The other phantom was made to simulate the head at the level of the nose. The measurements in these phantoms were made with thermoluminescence dosimetry, LiF rods. The accuracy of the oblique incidence case was investigated in a homogenous water phantom. The results show that for oblique incidence and geometries where the semi-infinite slab approximation is reasonably good, the generalized Gaussian model is more accurate. Within and behind low density cavities, in the phantom, the Gaussian model often gives a better agreement to experimental data. This is shown to be due to mutually balancing errors from the semi-infinite slab approximation and the Gaussian approximation.

The in-air scattering of clinical electron beams as produced by accelerators with scanning beams and diaphragm collimators

The electron distribution F(x, y, z, theta x, theta y) in air has been evaluated for a clinical electron beam emanating from a scanning beam accelerator in which the collimation of the beam is performed by means of diaphragm collimators. The multiple scattering theory of Fermi turns out to be adequate in describing this electron distribution. In this theory, the only parameter to be determined experimentally is the angular variance at the level of the collimator blocks. Generally, this angular variance features the same energy dependence as the angular scattering power and its value at an arbitrary energy can be derived from measuring the penumbra widths of off-axis profiles in air, at various distances beyond the collimator blocks. Then, the angular variance at the level of a secondary diaphragm collimator can be calculated, as well as off-axis profiles in air at arbitrary distances. In this way, the relative electron distribution at the surface of patients can be calculated easily. This in turn serves adequately as input to the calculation of patient dose distributions in radiation therapy planning.

A new model for computerized clinical electron beam dosimetry.

Clinical electron beams consist of primary electrons, primary bremsstrahlung generated in the regular photon and electron collimator system determining the composite beam, and some short-range contaminant photon and electron scatter arising from the lower parts of the standard or regular electron applicator. Any beam-shaping insert placed inside the applicator causes some extra ("contaminant") bremsstrahlung and electron scatter. The new dose calculation model is based on separate treatment of these components. For the calculation of the primary electron dose we use experimentally determined electron scatter functions and differential electron scatter functions. The primary bremsstrahlung is treated as an unflattened but otherwise regular x-ray beam. The contaminant components arising from the rim area of the regular electron collimator and from beam-shaping inserts are considered separately. The behavior of the in-air ionization profiles is described using the concepts of effective electron source position and effective electron source diameter. The model has been tested for several electron energies.

Determination of three parameters describing the uncollimated electron beam in air

The parameters that describe the electron dose distribution phi (r, theta , z) produced in air by an uncollimated clinical electron beam are accurately determined. For the determination of these parameters the multiple scattering theory of Fermi (1940) is assumed. A new method which determines the angular variance at the phantom surface is introduced and the results appear to be in good agreement with the multiple scattering theory. Knowledge of the values of these parameters is essential for a numerical determination of the dose distribution in air and in the patient.

The use of computed tomography numbers in dose calculations for radiation therapy.

Although corrections for 'beam hardening' and 'scattering' have been implemented in currently available CT scanners, systematic differences exist between a real CT image and an ideal, artefact-free and monochromatic image. The appearance and magnitude of these differences are discussed. Conversion to the ideal image, i.e. conversion from CT number to X-ray attenuation coefficient at diagnostic photon energies, turns out to be possible with an accuracy of 5 per cent. In order to use the CT 'density' information from patients, in clinical photon and electron beam dose calculations, conversions must be made from the X-ray attenuation coefficient at diagnostic energies to relevant high energy radiation interaction properties. These conversions turn out to be possible within an accuracy also of 5 per cent. These limited accuracies cause errors in the photon beam dose calculation of less than 1 per cent of the dose maximum and errors in electron beam dose calculations of less than 2 per cent of the dose maximum.

3. Characteristics of Clinical Electron Beams

3. Characteristics of Clinical Electron Beams

3. Characteristics of Clinical Electron Beams

3. Characteristics of Clinical Electron Beams

On the steady-state drift conditions of a water calorimeter in clinical megavoltage photon and electron beams.

We have investigated the characteristic behavior of a water calorimeter in a clinical environment where the temperature is not strictly controlled. Since the ambient temperature plays a crucial role in the successful operation of a calorimeter, its effects were closely monitored. The results were compared with theoretical predictions based on a simple model of heat transfer by conduction. They were found useful in analyzing the radiation dose measurements. In particular, a relationship between the absorbed dose and the pre- and postirradiation thermal drifts was examined. Finally, some suggestions are made concerning the desirable conditions under which a water calorimeter may be operated.

Evaluation of diamond radiation dosemeters

The effect of beam energy, dose rate and temperature on seven commercial diamond electrical conductivity detectors was studied. The diamond dimensions were approximately 1 mm*1 mm*3 mm. Clinical photon and electron beams were used for irradiation. The photon energy response normalised to 60Co was observed to have a maximum of 1.37+or-0.06 at 122 keV and the dose rate response was slightly non-linear, in agreement with the theory for insulators. Temperature effects were small and different for each detector. Two of the seven detectors gave repeatable results (+or-5%) over five years, two others were much less satisfactory and three were rejected after preliminary tests. It is concluded that natural diamond can be used for the manufacture of reliable, small and near soft-tissue-equivalent dosemeters but that until the crystal selection and final product testing have been adequately standardised, the user needs to select the reliable dosemeters from a batch.