Search PubMed⌕ Search

Biomedical subjects

A Brahme

Publications and source records attributed to A Brahme.

At least 91 records · Page 5Linked to original sources
PubMed1987

Design principles and clinical possibilities with a new generation of radiation therapy equipment. A review.

The main steps in the development of isocentric megavoltage external beam radiation therapy machines are briefly reviewed identifying three principal types or generations of equipment to date. The new fourth generation of equipment presented here is characterized by considerably increased flexibility in dose delivery through the use of scanned elementary electron and photon beams of very high quality. Furthermore the wide energy range and the possibility of using high resolution multileaf collimation with all beam modalities makes it possible to simplify irradiation techniques and increase the accuracy in dose delivery. The main design features are described including a dual dipole magnet scanning system, a photon beam purging magnet, a helium atmosphere in the treatment head, a beam's eye view video read-out system of the collimator setting and a radiotherapeutic computed tomography facility. Some of the clinical applications of this new type of radiation therapy machine are finally reviewed, such as the ease of performing beam flattening, beam filtering and compensation, and the simplification of many treatment techniques using the wide spectrum of high quality electron and photon beams. Finally the interesting possibility of doing conformation and more general isocentric treatments with non-uniform beams using the multileaf collimator and the scanning system are demonstrated.

Equipment Design↗
PubMed1986

Electron contamination from photon beam collimators.

Photon beam collimators are a source of secondary electrons that contaminate the beams and increase the surface dose. Calculations and measurements of this contamination have been made for 6 MV and 21 MV X-ray beams. Calculations are made using an electron transport model including the production of electrons by the photons and their transport and multiple scatter, both in the collimator and in the air. The variation with collimator material and geometry is investigated. The lowest contamination is obtained for high density and atomic numbers because of the small lateral electron range and large linear stopping power. The collimator geometry is also very important. If the collimator surfaces are hit by the photons, the collimator-produced electrons reaching the phantom may be increased by a factor of 2 or more. Calculations show that the surface dose from these electrons is less than 5% for 21 MV X-rays. This means that the dominating source of contaminating electrons at this energy is the beam flattening filter.

Electrons↗
PubMed1986

Chamber-dependent wall correction factors in dosimetry.

The calculation of the response of ionisation chambers to different photon beam qualities used in radiotherapy requires electron stopping-power data. These data have recently been revised and are used here to derive a consistent set of correction factors for ionisation chambers of different wall and build-up cap composition. Theoretically derived parameters km and katt which relate the exposure calibration of an ionisation chamber to the absorbed dose to the air of the cavity, are compared with the experimentally derived product kmkatt showing generally good agreement but also significant discrepancies for plastic-walled chambers with inner graphite coatings. A table of km values is given for a large number of commercial ionisation chambers. The new stopping-power data are also used to evaluate the wall-dependent correction factor (pwall) that enters into the determination of the absorbed dose to water in photon beams, results being given as a function of the quality of the beam. Our theoretical calculations of pwall are consistent with existing experimental data.

Cobalt Radioisotopes↗
PubMed1986

Stopping power data for high-energy photon beams.

Spencer-Attix stopping power ratios for the dosimetry of high-energy photon beams used in radiation therapy have been calculated using the Monte Carlo method. The stopping power ratios are calculated in a more consistent way than previously and are given as a function of the attenuation properties of the beam. The dependence of the stopping power ratio on the electron contamination of the beam as well as on depth and field size has also been investigated. Results are compared with stopping power ratios recommended in different dosimetry protocols and to experimental results. The agreement with most dosimetry protocols is within about one per cent and with recent experimental data is better than half a per cent.

Evaluation Studies as Topic↗
PubMed1985

Current algorithms for computed electron beam dose planning.

The field of electron beam dose planning has been in a state of very rapid development during the last decade. The essentially one-dimensional manual corrections used since the beginning of the sixties, has been replaced by at least two- and sometimes three-dimensional computer algorithms capable of taking all irregularities of the body cross-section and the properties of the various tissues into account. This is achieved by dividing the incoming broad beams in a number of narrow pencil beams, the penetration of which can be described by essentially one-dimensional formalisms. The constituent pencil beams are most often described by Gaussian, experimentally or theoretically derived distributions. The accuracy of different dose planning algorithms is discussed in some detail based on their ability to take the different physical interaction processes of high energy electrons into account. It is shown that those programs that take the deviations from the simple Gaussian model into account give the best agreement with experimental results. With such programs a dosimetric relative accuracy of about 5% is generally achieved except in the most complex inhomogeneity configurations. Finally, the present limitations and possible future developments of electron dose planning are discussed.

Computers↗
PubMed1985

Proton radiotherapy with the Uppsala cyclotron. Experience and plans.

From 1957 to 1968, the 230-cm synchrocyclotron at the Gustaf Werner Institute was used for clinical tests with a 185 MeV proton beam. The radiotherapeutic research was part of an extensive research programme in physics, chemistry, biology and medicine. Only a small series of patients were treated. A brief review of the early development and clinical experience of the cyclotron activities at Uppsala from 1957 to 1968 is given. The former accelerator is now being converted. Beams are expected to be available in the new radiotherapy treatment rooms in 1986. Plans for the new facilities with special reference to alternative methods of proton acceleration and beam transport, i.e. fixed beams or an gantry system are presented. The corresponding activities at the Institute of Theoretical and Experimental Physics (ITEP) in Moscow are also referred to thanks to a bilateral research programme which has existed in the past and from which the Uppsala group has benefited greatly.

Brain Neoplasms↗
PubMed1984

Computer assisted dosimetry of scanned electron and photon beams for radiation therapy.

A computer controlled beam forming system for energies up to 50 MeV has been developed in order to produce high quality electron and photon beams for radiation therapy. The desired radiation field shape and dose distribution are achieved by programming the scanning pattern of a narrow and unfiltered electron or photon beam. The computer that controls the scanning pattern also performs dosimetric analyses in the resultant radiation beams. The system allows real time display of the measured dose distributions at a rate of up to five discrete dose values per second for a 15 cm square field. Measurements in scanned as well as in stationary electron and photon beams at energies of 10, 20 and 50 MeV are presented. Finally, the consequences of photon generated electrons in the very broad high energy photon beams that can be produced by a scanning system are illustrated and discussed.

Computers↗
PubMed1984

Dosimetric precision requirements in radiation therapy.

Based on simple radiobiologic models the effect of the true distribution of absorbed dose in therapy beams on the response of uniform tumor volumes are investigated. Under assumption that the dose variation in the beam is small it is shown that the response of the tumor to radiation is determined by the mean dose to the tumor volume. Quantitative expressions are also given for the loss in tumor control probability as a function of the degree of dose variations around the mean dose level. When the dose variations are large the minimum tumor dose is best related to tumor control. It is finally shown that high tumor control rates can only be achieved with a very high accuracy in dose delivery. If the normalized dose response gradient is higher than 3, as is frequently the case, the relative standard deviation of mean dose in the target volume should be less than 3 per cent to achieve an absolute standard deviation in tumor control probability of less than 10 per cent.

Cell Survival↗
PubMed1983

Neutron beam characteristics from 50 MeV protons on beryllium using a continuously variable multi-leaf collimator.

The dose distributional properties of a p(50) Be neutron beam using a continuously variable multi-leaf collimator are presented and compared with a 6 MV photon beam. The differences in physical dose delivery between these two radiation modalities are generally insignificant for radiation therapy, and stringent comparisons of neutron and photon treatments should therefore be possible. The flexibility in field shaping with the multi-leaf collimator opens new possibilities in the treatment of complex irregular target volumes. The collimator consists of 40 wedge-shaped leaves that are independently moved under computer control with their collimating surfaces always aligned with the effective radiation source to minimize the penumbra. The leaf collimator eliminates the need for handling of heavy insert collimators and beam blocks at the same time that it allows dynamic conformation therapy with neutrons.

Beryllium↗
PubMed1983

Relation between kerma and absorbed dose in photon beams.

The relation between kerma and absorbed dose has been calculated using a diffusion equation approximation for the electron transport in photon beams and by equating the depth integral of the collision kerma with that of the absorbed dose. The results show that the absorbed dose in the transient equilibrium region practically coincides with the total kerma in water both for 60Co and 21 MV roentgen rays. The ratio between the absorbed dose at maximum build-up and the collision kerma at the surface (katt) was calculated. The results show that the dependence of katt on the atomic number is very small. For 60Co katt was determined for water and air to 0.992 and 0.991, respectively.

Electron Transport↗
PubMed1983

Geometric parameters of clinical electron beams.

The influence of different components in the treatment head of a therapy accelerator on the geometric properties of the delivered electron beams have been analysed based on the theory of small angle multiple scattering. The analyses show that it is possible to define and compute the properties of an effective electron source which, when placed in vacuum at some distance from the patient, produces exactly the same electron fluence at the patient as the real beam. The effective source is fully described by three parameters: its angular spread, its radial width and the distance from the patient. A numerical example indicates that the half width of the effective source could be considerable, often as large as 10 cm or more at low energies.

Computers↗
PubMed1983

Fluence and absorbed dose in high energy electron beams.

The depth dependence of fluence, vectorial and planar fluence, energy fluence, vectorial and planar energy fluence and absorbed dose has been calculated in 5 and 20 MeV electron beams in water. The shape of these distributions has been compared with analytic expressions for fluence and absorbed dose and the general mechanisms governing the shape of the depth dose curve have been explained. In particular it is demonstrated that the depth dependence of planar energy fluence and mean energy is very similar, and so is the case for fluence and absorbed dose and primary fluence and absorbed dose from primaries with inclusion of fast secondaries. The results are useful for dosimetry and dose planning in high energy electron beams.

Computers↗
PubMed1983

Electron beam dose planning using Gaussian beams. Improved radial dose profiles.

The Gaussian solution of the transport equation for electrons in a medium omits the large angle single scattering events. These events have been included by using Monte Carlo calculated radial dose profiles for point monodirectional beams. A sum of three Gaussian functions with different relative weights and widths have been fitted to the Monte Carlo calculated radial dose profiles. These profiles have been confirmed by measurements in an almost point monodirectional beam, and the importance of an adequate experimental set-up for determination of radial dose profiles is discussed. The analytic treatment when using three different functions in the Gaussian formalism is presented. Central axis depth dose curves for 10 MeV and 20 MeV have finally been calculated and compared with depth dose curves calculated using Monte Carlo technique and a single Gaussian function. Considerable errors (20-30%) result with the single Gaussian function at small field sizes whereas three components give good agreement with the Monte Carlo method.

Computers↗
PubMed1983

Absorbed dose distribution of electron beams in uniform and inhomogeneous media.

The absorbed dose at dose maximum in different materials or tissues irradiated by a given electron fluence can vary considerably depending on scattering and stopping properties of the media. A simple expression relating the dose levels in different materials have been derived and is found to be in good agreement with published experimental results for different phantom materials. This expression has been used to calculate the ratio of the absorbed dose to different tissues to that in water. The absorbed dose behind different inhomogeneities has been investigated experimentally. Based on this expression correction factors and scaling laws have been derived which allow the calculation of the absorbed dose distribution behind the inhomogeneity with good accuracy. The procedure only uses tabulated mass scattering and total stopping power data and the dose distribution in the uniform medium. The shift in penetration behind the inhomogeneity independent of its depth is shown to be accurately determined by the equivalent thickness of the inhomogeneity as obtained by multiplying the real thickness by the total stopping power ratio at the incident mean energy.

Adipose Tissue↗
PubMed1982

Solution of an integral equation encountered in rotation therapy.

An integral equation relating the lateral absorbed dose profile of a photon beam to the resultant absorbed dose distribution during single-turn rotating-beam therapy has been set up and solved for the case of a cylindrical phantom with the axis of rotation coinciding with the axis of symmetry of the cylinder. In the first approximation the results obtained are also valid when the axis of rotation is somewhat off-centred, even in a phantom that deviates from circular symmetry, provided the rotation is performed in both clockwise and counter clockwise directions. The calculated dose profiles indicate that improved dose uniformity can be achieved using a new type of non-linear wedge-shaped filter, which can easily be designed using the derived general analytic solution to the integral equation.

Humans↗
PubMed1982

Proliferation kinetics and specific behaviour of G2 cells of the Bp8 mouse ascites sarcoma in vivo.

Growth characteristics of the Bp8 ascites sarcoma were investigated by means of evaluation of total cell number, the cellular DNA content, the mitotic index and the percentage of dead cells. Following intraperitoneal inoculation of 18 x 10(6) cells into NMRI mice a plateau level of about 1400 x 10(6) cells was reached after 7 days followed by a daily decline of 5 per cent up to day 14, the terminal stage of life. Based on the increase of the cell number and the distribution of cells in the cell cycle the flow rate through and the duration of the various parts of the cell cycle were calculated. Generally, the durations of all phases of the cell cycle including mitosis increased with age, most markedly after day 8 when the durations tended to become infinite. At the plateau phase the total number of G1, S-phase and mitotic cells decreased significantly while G2 cells remained at a constant level. This finding may be of interest in view of the concept of ascites tumour cells behaving as a regulated cell system.

Animals↗
PubMed1982

Physical and biologic aspects on the optimum choice of radiation modality.

The most generally used radiation modalities, low energy photon and electron beams, have been gradually optimized with regard to their physical properties during the last decade. The advantages of increasing the energy range of these beams as high as 50 MeV are discussed based on the assumption that high quality beam flattening systems are used. Possible further developments of the conventional low LET electron and photon beams using grid irradiation and ultra short pulses are also indicated. The dose distributional properties of high quality electron and photon beams are compared with those of heavy particle beams indicating that in several cases only marginal advantages should be expected for the heavy charged particle beams.

Alpha Particles↗