Search PubMed⌕ Search

Biomedical subjects

A Brahme

Publications and source records attributed to A Brahme.

At least 109 records · Page 6Linked to original sources
PubMed1981

Electron beam dose planning using discrete Gaussian beams. Mathematical background.

A general method for computerized electron beam dose planning is proposed based on the use of a square or hexagonal grid or matrix of discrete Gaussian elementary beams. Using this method the distribution of absorbed dose in an electron beam of arbitrary cross-section incident on an irregular patient surface can be calculated taking the influence of tissue inhomogeneities into account. The derived expressions for the absorbed dose in narrow and broad uniform beams are compared with experimental data and Monte Carlo calculations. Good agreement is obtained over the clinically significant portion of the absorbed dose distribution at a fairly wide grid spacing of about 0.5 cm.

Computers↗
PubMed1981

Contamination of high-energy photon beams by scattered photons.

Absorbed dose and energy distribution of scattered photons that contaminate high-energy photon beams have been calculated for 6-MV and 21-MV bremsstrahlung sources using the Monte Carlo method. The percentage of the absorbed dose at the central axis is less than 3%. The results show that the main sources of scattered photons are the primary collimator for 6-MV and the flattening filter for 21-MV bremsstrahlung sources respectively. The absorbed dose from the phantom scattered photons has been calculated by a semi-empirical method. These calculations show that the dominating contribution to the absorbed dose outside the primary beam is due to these phantom scattered photons. In the surface region of the phantom, however, electrons from the air and collimators may give rise to a high absorbed dose.

Electrons↗
PubMed1981

Correction of a measured distribution for the finite extension of the detector.

Simple analytic expressions have been derived for the correction of a measured distribution for the finite extension of the detector in use. The expressions are simple to apply as they are only based on the actually measured data and on the size of the detector. The expressions are applied to two situations encountered in radiation dosimetry.

Mathematics↗
PubMed1980

Investigation of the possibility of using photoneutron beams for radiation therapy.

The possibility has been investigated of using electrons accelerated by a 50 MeV racetrack microtron for generation of photoneutron beams for radiation therapy. Central axis depth-dose curves have been measured in an A-150 tissue-equivalent phantom. Neutron half-value depths between 4.4 and 5.2 g cm-2 were obtained at an SSD of 100 cm for different converter materials and target geometries. At an absorbed dose ratio of 1:1 for neutrons and photons at the dose maximum, the total absorbed dose rates are estimated to be 0.1 Gy min-1 at 100 micronA electron current and a SSD of 100 cm. At a depth of 5 cm the neutron to photon absorbed dose ratio is typically 1:2 and the OER is expected to be about 1.8. Some dose distributional and radiobiological advantages of a physically mixed beam of neutrons and photons for external beam radiation therapy are discussed.

Electrons↗
PubMed1980

Collimation of high energy electron beams.

The collimation of high energy electron beams for radiation therapy is treated with special attention on the contamination of the beam by electrons which ideally should be stopped, but instead are scattered back into the beam through the edge of the collimator. It is demonstrated that the mean energy of these electrons is only 40 per cent of the mean energy in the beam and that the smallest electron contamination is obtained if the material closest to the beam is of high density.

Mathematics↗
PubMed1980

Electron and photon beams from a 50 MeV racetrack microtron.

The quality of broad high energy photon and electron beams can be improved considerably by the scanning of elementary narrow beams. Dose distributions produced in this way have advantages with respect to improved dose gradients and therapeutic ranges for electrons and lower surface doses, increased depth doses and half value depths for photons. The possibility of giving the whole therapeutic dose in a single local pulse of short duration is discussed as a new method to protect superficial tissues.

Electrons↗
PubMed1979

Absorbed dose from secondary electrons in high energy photon beams.

The absorbed dose in high energy photon beams due to scattered electrons from the irradiated air volume and from beam-shaping platforms has been calculated using the Fermi-Eyges theory of multiple scattering. The results are presented as lateral surface absorbed dose distributions across the field for three different radiation qualities, namely 60Co, 6 MV and 21 MV X-rays. For 60Co the relative absorbed dose due to electrons expelled in air reaches a value as high as 30% of the absorbed dose at dose maximum at a field size 40 X 40 cm2 and an SSD of 100 cm. The absorbed dose from electrons emanating from beam-shaping platforms contribute significantly to the absorbed dose at the surface when the platform is placed closer than 20--40 cm from the surface for field sizes greater than 10 X 10 cm2 to 40 X 40 cm2 respectively.

Electrons↗
PubMed1979

Radiation beam characteristics of a 22 MeV microtron.

The properties of the electron and photon beams from a 22 MeV clinical microton are presented. Favourable isodose distributions for radiation therapy are obtained for both modalities. For the electron beam this is accomplished using a dual scattering foil system in which the secondary foil is shaped to optimize uniformity and minimize energy loss and energy straggling. The photon beam is flattened by a composite filter to produce dose distributions of high uniformity over a wide depth interval.

Electrons↗
PubMed1979

Ferrous sulphate dosimetry for electrons. A re-evaluation.

A theory was developed for the perturbation correction to be applied in the calorimetric determination of the ferrous sulphate dosimeter G-value. Recent determinations of the molar extinction coefficient, epsilonm, have shown that systematic errors in epsilon may have influenced experimental G-values. With precision spectrophotometers this error is cancelled in the product epsilonm.G. It was shown that after applying the perturbation correction a very small spread was obtained in calorimetric epsilonm.G-values; the standard error of the mean being 0.2 per cent. The mean G-value is 2.3 per cent lower than that recommended in ICRU Report No. 21.

Absorption↗
PubMed1977

A 22 MeV microtron for radiation therapy.

The first 22 MeV micron installation for radiation therapy is described with regard to the general design, the beam transport and the gantry. The first measurements of the electron depth dose curves indicate that the distributions obtained are superior to those of most betatrons and linear accelerators. For the same electron beam energy the depth of dose maximum and the 90 or 80 per cent dose level is considerably increased. The 20.9 MV photon beam has similar depth dose characteristics as a 20 MV betatron.

Electrons↗
PubMed1975

Electron depth absorbed dose distribution for a 10 MeV clinical microtron.

Central axis depth absorbed dose distributions of the electron beam from a medical microtron accelerator have been measured. The measured distributions differ from those of existing betatrons and linear accelerators. A larger absorbed dose build-up and a sharper dose fall-off are obtained in close agreement with theoretical calcuations for monoenergetic electron beams. The differences from other accelerators are explained by the narrow energy spectrum, the clean geometry and the small amount of scattering material in the electron beam of the microtron.

Electrons↗
PubMedDate not supplied

Specification of electron beam quality from the central-axis depth absorbed-dose distribution.

The distribution of absorbed dose in a medium irradiated by broad electron beam has been analyzed from physical and therapeutic points of view. A number of parameters which describe the beam quality and the shape of the absorbed-dose distribution along the central axis are defined. Several comparisons are made of the values of these parameters obtained from measurements made on different therapy accelerators and from theoretically calculated dose distributions for monoenergetic and monodirectional beams. It is found that the major part of the observed differences between experimental and theoretical results can be attributed to the energy distribution of the accelerator beam and to processes in the scattering material, such as energy straggling and the production of secondary electrons and photons. A simple expression is derived relating the slower falloff in dose found for most accelerator beams to the large energy spread found in these beams. Further, a semiempirical relationship is found which makes it possible to obtain the mean electron energy at the phantom surface from the depth where the absorbed dose has decreased to 50% of its maximum value along the central axis.

Electrons↗
PubMedDate not supplied

Mean energy in electron beams.

The mean energy of the energy spectrum is an essential parameter for the dosimetry of therapeutic electron beams. Frequently it is assumed that the mean energy of such beams remains constant across the beam and only its degradation with depth is considered. The present work analyzes the variation of the mean energy of primary electrons with depth and lateral position in an electron beam using the Monte Carlo method. Results are compared with relations commonly employed for determination of mean energy at a depth. For the variation of the mean electron energy with depth in broad beams, good agreement was found between Monte Carlo results and an analytic continuous slowing down expression, which takes the variation of radiation stopping power with depth into account. Due to the calculated lateral variation of the mean energy, the relative absorbed dose profile determined with an air ionization chamber in a clinical beam should differ by less than 1% from the measured ionization profile.

Computers↗
PubMedDate not supplied

Optimization of the dose delivery in a few field techniques using radiobiological objective functions.

A method for finding optimal primary fluence profiles for multiple field external beam radiation therapy techniques has been developed using a radiobiologically based objective function that quantifies the probability of achieving complication-free tumor control, P+. The objective function P+ has the valuable property of giving the highest possible dose to the tumor without causing severe damage to normal tissues at risk. This radiobiologically based objective function selects suitable dose levels but also takes into account the dose homogeneity in the target volume to the extent that it is not causing an excessive risk of local recurrence or damage to surrounding normal tissues. The biological parameters used can either be patient specific, as determined by a predictive assay on biopsy specimens, or taken from a library of radiobiological parameter values characteristic for different tissue types of a reference patient. In its present form the method can be used to determine the optimum incident photon fluence profiles for each beam. The method has been used to investigate for a given target volume a large number of combinations of beam entry directions to find the best beam orientations with respect to the probability of achieving complication-free tumor control. It is demonstrated that when nonuniform dose delivery is available it is unsuitable to combine parallel opposed beams in two-beam techniques and to a lesser extent also to use perpendicular beams. In two-beam techniques the best angle between the beams is generally in the 100 degrees-120 degrees range. The major symmetry characteristics of the P+ phase space for two-beam techniques are also identified. The method can easily be extended from two to three dimensions and noncoplanar geometry, but it is presented here in its two-dimensional form for clarity and speed of calculation.

Algorithms↗
PubMedDate not supplied

Electron beam dose planning using Gaussian beams. Energy and spatial scaling with inhomogeneities.

The penetration of an electron beam in matter is characterized by only two material dependent parameters within the range of validity of the small angle diffusion equation. These parameters characterize the relative importance of multiple scattering and energy loss for a given material and energy. By comparison with more accurate Monte Carlo calculations it is shown that the same two parameters can be used to scale the dose distribution from one material and energy to an equivalent energy in an arbitrary material with high precision also outside the limits of the small angle approximation. This scaling procedure has been generalized to the non-uniform case with plane parallel slab inhomogeneities by assuming the angular distribution on each side of the interfaces to be equal. The generalized recursive scaling procedure is finally tested by measurements on almost point monodirectional beams in water with inserted aluminum plates of varying thicknesses. The agreement with the scaling procedure is of the order of one per cent with regard to the radial dose profiles.

Electrons↗
PubMedDate not supplied

Correction for the angular dependence of a detector in electron and photon beams.

Based on experimentally obtained angular response functions a simple analytic function has been found which can be accurately fitted to such data. Using this function formulas are derived for the variation of the detector response as the angular distribution of the electrons are changing with depth in a phantom both in electron and photon beams. The formulas show that for a flat liquid ionization chamber, a response variation of 2 to 4 per cent is normally obtained in the build-up region of electron and photon beams whereas the response is practically constant beyond dose maximum.

Electrons↗