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E el-Khatib

Publications and source records attributed to E el-Khatib.

At least 19 recordsLinked to original sources
PubMed1999

Lung compensator design using an electronic portal imaging device.

Using a liquid filled electronic portal imaging device (EPID) installed on a linear accelerator and a composite chest phantom, exit dose measurements were carried out to establish an empirical relationship between the pixel values of the imaging detector and the corresponding equivalent thickness of the overlying phantom material. Results for 6 and 10 MV photons show that the relationship depends on the so-called input/output characteristics of the imaging device for a particular photon energy. For a chest irradiation, an EPID image obtained under treatment geometry provides the pixel value information that is used to calculate the tissue deficit over the lung region. The compensators are made of lead whose thickness is calculated from the established empirical relationship to replace the tissue deficit over lungs. The effectiveness of the method is demonstrated with thermoluminescent dosimetry (TLD) for 6 and 10 MV beams. With compensators in place, the dose uniformity was found to be within +/- 5%.

Humans↗
PubMed1998

The use of deconvolution and total least squares in recovering a radiation detector line spread function.

We present a method for obtaining the line spread function (LSF) of any radiation detector from measured data. The problem of finding a LSF is essentially a discrete deconvolution from known values of the input (Monte Carlo generated data) and the output (measured data) which can be put into matrix form. We applied the total least squares (TLS) method which is particularly useful when there are errors in both the input and output data. Results from computer simulation as well as from actual data are shown. In a practical application, however, our technique is currently limited by the ability of the Monte Carlo data to simulate correctly the inherent data from the head of the linear accelerator (linac). To overcome this difficulty we have solved by deconvolution and TLS for a more realistic inherent beam profile of our linac using the information from both profile data as measured with film and the film densitometer response function. The LSF of the densitometer was estimated with a simple method of direct measurement of a slit image and a full width at half maximum (FWHM) of 0.997 mm was recorded. Additionally, using the knowledge of this realistic inherent profile of the linac, a blurring function representing the finite source size effect missing in our current Monte Carlo profile simulation was determined. Finally, with the realistic inherent beam profile we have applied the deconvolution and TLS method to find a LSF for the Markus chamber and found a resulting FWHM of 5.39 mm. The TLS approach for deconvolving can find a useful application for both finding the LSF and correcting for the detector size effect once its LSF is known. This type of correction is required when a high spatial resolution is needed (e.g., in small field off-axis measurements). Convolved and measured profiles are also presented to illustrate the effect of the blurring due to different LSFs.

Biophysical Phenomena↗
PubMed1998

The use of an electronic portal imaging system to measure portal dose and portal dose profiles.

The dosimetric characteristics of a scanning liquid-filled ionization chamber (SLIC) electronic portal imaging device have been investigated. To assess the system's response in relation to incident radiation beam intensity, a series of characteristic curves are obtained for various field sizes and nominal energies of 6 and 10 MV photons. The response of the imaging system is dependent on incident radiation intensity and can be described to within 1% accuracy on central axis using a square root function. Portal dose measurements with the SLIC at the plane of the detector, on central axis of the beam using homogeneous attenuating phantom materials show that the imaging system is capable of measuring the portal (transmission) dose to within 3% of the ionization chamber results for homogeneous material. For two-dimensional dosimetry applications, the system is calibrated with a 10 cm Perspex block used as beam flattening material on the detector cassette to correct for variations in individual ion chamber sensitivity and the effect of nonuniform beam profiles produced by the flattening filter. Open and wedged dose profiles measured with the SLIC agreed with ion chamber measured profiles to within 3.5% accuracy.

Biophysical Phenomena↗
PubMed1995

Variation of electron beam uniformity with beam angulation and scatterer position for total skin irradiation with the Stanford technique.

PURPOSE: The influence of different scatterer-degraders and beam angulations on beam uniformity for total skin electron irradiation using the six dual beam Stanford technique is investigated. METHODS AND MATERIALS: The 6 MeV high dose rate total skin electron irradiation mode on a linear accelerator was used. Beam profiles and percentage depth doses in the patient plane for single, dual, and six dual beams were measured for different dual beam angulations and acrylic scatterer-degraders of different thicknesses mounted on the treatment head or in front of the patient in the treatment plane. RESULTS: It is demonstrated that, with the same electron nominal energy, total skin irradiation techniques with different beam penetrations can be obtained by inserting various beam scatterer-degraders into the beam, either mounted on the accelerator head or close to the patient. For our patient treatment, a beam penetration was selected so that the 80% dose lay at 8-9 mm and the 50% dose at 15-16 mm depth. This was achieved by mounting a 0.32-cm thick acrylic beam scatterer-degrader on the accelerator head. A uniform vertical profile was obtained for gantry angulations of +/- 21 degrees. CONCLUSIONS: To implement a total skin electron irradiation technique using the Stanford method, the required depth of penetration needs to be selected. Based on this, the appropriate combination of scatterer-degraders and dual beam angulations to produce a uniform beam in the treatment plane needs to be determined. Different techniques with different beam penetrations can be developed using the same high dose rate mode on the linear accelerator by a proper choice of scatterer-degraders and beam angulations.

Humans↗
PubMed1995

Total body irradiation with a sweeping 60Cobalt beam.

PURPOSE: This article describes the physical, technical, and dosimetric aspects of total body irradiation (TBI). METHODS AND MATERIALS: The continuous head swivel motion of a standard 60Cobalt unit has been used to obtain a sweeping beam that encompasses the entire length of the patient in TBI. A perspex beam flattener designed to remove the inverse square fall-off in beam intensity along the sweep axis provides a 90% field length of 200 cm in air at a treatment source-to-skin distance of 160 cm. The anterior-posterior parallel pair setup permits accurate placement of customized lead compensators to limit the dose to lungs. RESULTS: Measured beam profiles, dose buildup curves, and percentage depth dose for the technique are presented. With compensators in place, the variation in lung dose is shown to be within +/- 5% of the prescribed tumor dose. CONCLUSIONS: A sweeping beam TBI technique has been devised using a standard 60Cobalt unit. The technique allows the patient to be treated in supine and prone positions, which facilitates the accurate placement of lung compensators to limit the dose to lung tissue.

Cobalt Radioisotopes↗
PubMed1994

Dosimetry for asymmetric x-ray fields.

Conventional linear accelerators have four field-defining jaws or collimators. Usually, one set of the two opposing jaws moves concurrently to define the field width and the other set defines the field length. The resultant square or rectangular field will have the field centerline coincide with the collimator axis. However, some modern linacs have independent collimators or jaws that can be set asymmetrically. In this case, one of the two opposing jaws can be closed down independently of the other one to define an asymmetric field of smaller dimension. The field center now does not coincide with the collimator axis. Asymmetric collimators have found many clinical applications, but have complicated the dosimetry for physicists. Data acquisition and treatment planning implementations are tedious and complicated. An algorithm has been developed to correct for the reduced dose in the smaller asymmetric field. The approach used is similar in principle to the Day's equivalent field calculation. The difference in dose between an asymmetric and a symmetric radiation field is accounted for by a correction factor that is a function of the asymmetric and symmetric field sizes, off axis distance, and depth of measurement. The correction method presented here applies only to the closing down of one independent jaw. Beam profiles for asymmetric fields are measured for both the 6 and 10 MV photon beams.(ABSTRACT TRUNCATED AT 250 WORDS)

Humans↗
PubMed1993

Factors influencing lung density in experimental models: results of studies using CT densitometry.

Lung density measurements using computed tomography have been made in mice, rats, and pure-bred beagles and mongrel dogs. Density was found to be dependent on a number of factors including the location of the area scanned, and the age and species of the animal. Lung density was found to decrease by approximately 40% from initial values in a group of rats scanned at intervals between 30 and 800 days old. Age-related density changes were not seen for the lungs of mice or dogs; however measurements were not made over a prolonged period. A gradient of increasing density from the base to apex of the lung was seen for 30 and 175-day-old rats (scanned in the prone position) while an apex to base density gradient was seen for both types of dog studied (scanned in the supine position). In none of the species studied was there a systematic difference in density of the right and left lungs. Differences in average lung density between the mice, rats and dogs reported here are consistent with the relationship between lung morphology, metabolic rate and body size which obtains for a range of mammalian species.

Absorptiometry, Photon↗
PubMed1992

The influence of lung and bone dosimetry on the choice of radiation energy for total body irradiation.

The radiation absorbed dose to lung and bone is of importance for total body irradiation performed prior to bone marrow transplantation for hematologic malignancies. The measurement and calculation of radiation absorbed dose to low density materials such as lung has been discussed in several publications and most total body irradiation procedures account for the increased radiation dose to the lung. However, radiation absorbed dose to bone and soft tissues within bone is not calculated and is assumed to be the same as the dose to soft tissues. Because the bone is different in both density and atomic number from soft tissues, radiation dose calculations are more complex for bone than for lung. As the energy of the radiation beam changes, the dose to heterogeneous tissue varies. This variation of the radiation dose is investigated for radiation beams ranging in energy from 60Cobalt to x-ray beams produced at 18 MV. The radiation absorbed dose to soft tissues within bone is found to increase relative to the dose to soft tissue for higher megavoltage radiations, while at the same time there is a decrease in lung dose. Therefore, since for total body irradiation procedures the target tissues are the soft tissues within bone, a higher dose to those tissues would be an advantage.

Bone and Bones↗
PubMed1992

Conversion of ionization measurements to radiation absorbed dose in non-water density material.

The radiation absorbed dose to non-water equivalent materials of interest in radiotherapy is the dose to lung and the dose to bone. The measurement and calculation of dose to the lung has been of great interest and much effort has gone into the development of accurate lung dose calculation methods. The radiation absorbed dose to the bone is usually not calculated and most absorbed dose calculations have been done without correcting for the presence of bone. For the lower megavoltage photon beams this may be appropriate, however, as the energy of the photon beam increases, the region of electronic disequilibrium becomes larger and pair production which depends on the atomic number of the material becomes significant. Therefore the bone will produce greater perturbations of the dose distribution. The dose to lung-equivalent material is uniquely obtained from ionization measurements. However, in bone-equivalent materials two different calculations of absorbed dose are possible: the absorbed dose to soft tissue plastic (polystyrene) within bone-equivalent material and the dose to the bone-equivalent material itself. Both can be calculated from ionization measurements in phantoms. These two calculations result in significantly different doses in a heterogeneous phantom composed of polystyrene and aluminium (a bone substitute). The dose to a thin slab of polystyrene in aluminium is much higher than the dose to the aluminium itself at the same depth in the aluminium. Monte Carlo calculations confirm that the calculation of dose to polystyrene in aluminium can be accurately carried out using existing dosimetry protocols. However, the conversion of ionization measurements to absorbed dose to high atomic number materials cannot be accurately carried out with existing protocols and appropriate conversion factors need to be determined.

Bone and Bones↗
PubMed1990

Computerized tomography versus perfusion lung scanning in canine radiation lung injury.

Computerized tomographic (CT) measurements of lung density were obtained before and serially after thoracic irradiation in dogs to detect the alterations caused by radiation therapy. Fourteen mongrel dogs were given either 2000 cGy (Group A, 10 dogs, right lower zone irradiation), 1000 cGy (Group B, 2 dogs, right lower zone irradiation), or 500 cGy (Group C, 2 dogs, right lung irradiation) in one fraction. Once before and bi-weekly after irradiation, the anesthetized dogs had thoracic CT scans. CT numbers for the irradiated area were compared to their preirradiation control values. Macro-aggregated albumin (MAA) perfusion lung scans were also obtained before and at weekly intervals after irradiation and were evaluated visually and quantitatively for abnormalities. When both these tests were abnormal, or at the end of the scheduled study, the dogs were sacrificed to confirm radiation lung injury histologically. Our results showed that CT numbers (as a measure of tissue density) were higher with higher doses of radiation. Among all the techniques used, only the quantitative assessment of macro-aggregated albumin perfusion scan detected abnormalities in all the dogs given 2000 cGy. Their abnormalities correlated well with the presence of radiation lung damage histologically, however, the applicability of these methods in the detection of early injury has to be further evaluated.

Animals↗
PubMed1989

The use of CT densitometry in the assessment of radiation-induced damage to the rat lung: a comparison with other endpoints.

Changes in rat lung following irradiation have been assessed on the basis of survival, histopathology, function (breathing rate assay), and density changes measured by computed tomography (CT) scanning. CT densitometry is a non-invasive procedure which may be used without modification to assess lung damage in experimental animals and in man. An increase in the breathing rate is seen following irradiation of the thorax, the time of onset and severity of which are dose dependent and correlate with histopathological changes occurring at the same time. Lung density changes occurring after irradiation are more complex. For the lowest dose used (10.75 Gy) no density increase was observed and in fact density decreased with time after irradiation to a slightly greater extent than in non-irradiated controls. A post-irradiation increase in lung density was seen for rats given 13.0 Gy, but values fluctuated with maxima at 50 and 225 days after irradiation. Higher radiation doses (14.5, 16.0 Gy) were followed by a transient decrease in density before a dose-related density increase was observed. Density averaged over the whole lung proved to be a sub-optimal index of radiation-induced lung damage because of the focal nature of radiation-induced lung lesions and because of the apparently anomalous changes in post-irradiation damage which are observed. Further studies are being made to determine if regional density values will provide a more sensitive index.

Animals↗
PubMed1989

Lung density changes observed in vivo in rat lungs after irradiation: variations among and within individual lungs.

Lung density measurements using Computed Tomography have been used before at various intervals after irradiation to monitor radiation-induced changes in the lung. The average lung density, its standard deviation which was used as a measure of the density homogeneity throughout the lung, and the densities of smaller lung regions were measured before and up to 76 weeks after irradiation in rat lungs. Large differences in individual response to irradiation were observed. Both increases and decreases in lung density were measured. Regions of very low density were often found adjacent to dense foci of radiation damage. These compensatory changes made the measurement of changes in average lung density an insensitive index of radiation damage. However, the measurement of regional densities in smaller lung volumes, a method not previously applied to rodents, was a much more sensitive index of radiation damage. Changes from non-irradiated control lung densities were observed at earlier times and for lower radiation doses.

Absorptiometry, Photon↗
PubMed1989

Measured inhomogeneity correction factors for lung in electron beam treatments of the chest wall.

A set of clinically relevant measurements of percentage depth dose and inhomogeneity correction factors for electron beam irradiation of the chest wall and underlying lung is given. Electron beam nominal energies of 9, 12, 15, and 18 MeV and lung densities of 0.22 g/cm3 and 0.404 g/cm3 are considered. This data can serve in treatment planning to indicate the penetration of the beam into the lung and serve as a comparison for calculation algorithms which are used to calculate electron dose absorption in heterogeneous phantoms.

Algorithms↗
PubMed1986

Altered pulmonary epithelial permeability in canine radiation lung injury.

A radioaerosol scanning technique measuring regional clearance of sodium pertechnetate (99mTcO-4) and 99mTc-labeled diethylenetriaminepentaacetate (99mTc-DTPA) was used to assess changes in canine pulmonary epithelial permeability following lung irradiation. Doses of 2,000 cGy (11 dogs), 1,000 cGy (2 dogs), and 500 cGy (2 dogs) were given in one fraction to either the entire right hemithorax (500 cGy) or the right lower lung (1,000 and 2,000 cGy). Radioaerosol scans, chest roentgenograms, and computerized tomograms (CT) were obtained before and serially after irradiation. A dose of 2,000 cGy resulted in a decrease in regional pulmonary epithelial permeability to both 99mTcO4- and 99mTc-DTPA; both showed significant decreases from the 2nd wk postirradiation onward. In comparison, CT and chest roentgenogram did not become abnormal until 7.1 +/- 2.8 (SD) and 8.2 +/- 2.6 wk, respectively. Doses of 1,000 and 500 cGy produced reversible decreases in 99mTcO4- clearance. Lung morphology showed definite changes of radiation pneumonitis after 2,000 and 1,000 cGy but not after 500 cGy at approximately 9, 17, and 12 wk postirradiation, respectively. These results suggest that dose-dependent changes in pulmonary physiology may precede obvious structural alterations in radiation lung injury.

Aerosols↗
PubMedDate not supplied

Lung dose corrections for 6- and 15-MV x rays.

We have measured the radiation dose in simple heterogeneous phantoms and compared our results with those obtained by various methods of computation. Dose data were obtained both within and distal to simulated regions of lung in order to test the ratio of tissue-air ratios (TAR), Batho, and equivalent TAR methods. These procedures are used routinely in manual and computer-aided planning of radiation therapy, but have been validated primarily for cobalt-60 radiation. Tests performed with 6- and 15-MV x rays reveal that incorrect doses can be computed within or near to a low-density medium, particularly when the field size is small. In these cases, electronic equilibrium is not achieved in the lateral direction, thereby violating an implicit assumption of all the above calculation methods. We quantify the errors in dose calculation for simple slab phantoms, and support our interpretation with a Monte Carlo simulation in which the energy transported by charged particles away from sites of x-ray interactions is considered directly.

Animals↗
PubMedDate not supplied

Accuracy of lung dose calculations for large-field irradiation with 6-MV x rays.

In large-field irradiations of the upper half-body, there is a potential for severe respiratory complications. The incidence of severe radiation damage to lung can be reduced by limiting the lung dose and the volume of lung irradiated. It is necessary to have an accurate method of calculating the dose actually delivered to lung, and this work deals with such calculations and measurements in large (half-body) fields of 6-MV x rays. The results of various lung dose calculations by the Batho, the modified Batho (Lulu and Bjärngard), the simple scaled tissue-maximum ratio and the detailed equivalent tissue-air ratio methods are compared with doses measured in phantoms representing the mediastinum and adjacent lungs.

Humans↗
PubMedDate not supplied

Calculation of dose in homogeneous phantoms for partially attenuated photon beams.

Measured and calculated dose distributions under attenuators, which are of smaller cross-sectional dimensions than the radiation field, are presented. The study was performed on a 4-MV linac at a source-surface distance of 120 cm on the beam central axis in a water phantom for several thickness and cross sections of lead attenuators. Dose correction factors, which are used to multiply the open beam data to get dose distributions under partial attenuators, depend strongly on attenuator parameters and on depths in phantom. A method to calculate dose correction factors for any combination of attenuator parameters and any phantom depth is presented. The calculated dose distributions under partial attenuators agree well with measured data, which indicates that the method can be applied in clinical situations.

Humans↗