Ellen El-Khatib

Evaluation of lung dose correction methods for photon irradiations of thorax phantoms

Radiation absorbed dose in lung is measured and calculated using several algorithms available on commercial treatment planning systems. Phantoms resembling the human thorax are used and irradiated with small and large photon beams of 60Co, 4, 6, and 10 MV X ray energies. The applicability and usefulness of the different calculation methods in clinical situations is discussed.

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.

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.

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 Days 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)

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.

Evaluation of film and thermoluminescent dosimetry of high‐energy electron beams in heterogeneous phantoms

Film and thermoluminescent dosimetry (TLD) are investigated in heterogeneous phantoms irradiated by high-energy electron beams. Both film and TLD are practical dosimeters for multiple and moving beam radiotherapy. The accuracy and precision of these dosimeters for radiation dose measurements in homogeneous water-equivalent phantoms has been discussed in the literature. However, film and TLD are often used for dose measurements in heterogeneous phantoms. In those situations perturbations are produced which are related to the density and atomic number of the phantom material and the physical size and orientation of the dosimeter. In our experiments the relative dose measurements in homogeneous phantoms were the same regardless of dosimeter or dosimeter orientation. However, significant differences were observed between the dose measurements within the inhomogeneity. These differences were influenced by the type and orientation of the dosimeter in addition to the properties of the heterogeneity. These differences could be reproduced with Monte Carlo calculations and modeling of the experimental conditions.

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.

The use of ct densitometry to predict lung toxicity in bone marrow transplant patients

Total body irradiation (TBI) is considered an integral part of the preparation of patients with hematological malignancies for marrow transplantation. One of the major causes of death following bone marrow transplantation is interstitial pneumonia. Its pathogenesis is complex but radiation may play a major role in its development. Computed tomography (CT) has been used in animal and human studies as a sensitive non-invasive method for detecting changes in the lung following radiotherapy. In the present study CT scans are studied before and up to 1 year after TBI. Average lung densities measured before TBI showed large variations among the individual patients. On follow-up scans, lung density decreases were measured for patients who did not develop lung complications. Significant lung density increases were measured in patients who subsequently had lung complications. These lung density increases were observed prior to the onset of respiratory complications and could be correlated with the clinical course of the patients, suggesting the possibility for the usage of CT lung densitometry to predict lung complications before the onset of clinical symptoms.

Broad beam and narrow beam attenuation in Lipowitz's metal

Attenuation properties of Lipowitzs metal have been studied for narrow and broad beams of cobalt-60 gamma rays and 4-10 MV x-rays. The measured transmitted fraction for geometries used in radiotherapy depends on the field size and depth of measurement. Therefore a calculation of dose for partially attenuated beams based on narrow beam attenuation coefficients can cause large errors in dosimetry. Our simple calculation of transmitted fractions based on primary attenuation and scattered radiation agrees quite well with the measured data for therapeutic geometries. Also given is a table for linear, mass attenuation, and mass energy absorption coefficients of Lipowitzs metal in the photon energy range from 10 keV to 10 MeV.

The effect of lead attenuators on dose in homogeneous phantoms

In radiotherapy, the radiation beam is sometimes shaped so as to deliver different doses to different organs or give a homogeneous dose to structures of different densities. This objective is achieved by the use of attenuating materials introduced into the beam. These attenuators alter the primary as well as the scattered radiation components of the beam. There is at present no accurate method of dose calculation for these situations. Most calculations are performed considering only the effect of the attenuators on the primary radiation beam and can produce large errors in dosimetry. In the present study, the broad beam attenuation is investigated in homogeneous phantoms for various radiation field sizes, photon beam energies, and depths in phantom. A calculational method taking account of primary as well as first scatter radiation is developed. This method predicts reasonably well the transmission through lead attenuators for the various experimental conditions investigated.