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G A Sandison

Publications and source records attributed to G A Sandison.

16 recordsLinked to original sources
PubMed1994

A solution to the Yang equation with electron energy loss following Harder's formula.

The Yang diffusion transport equation for charged particles was modified to allow the linear angular scattering power to vary with penetration depth in the scattering medium. Assuming charged particle energy loss to be a linear function of depth, conditional solutions to this transport equation have been found for the two cases of interest specified by Yang. The normalized excess path length distributions predicted for a 10-MeV electron beam show a shift toward larger excess path lengths compared to Yang's solutions.

Diffusion↗
PubMed1994

Inclusion of energy straggling in a numerical method for electron dose calculation.

Energy straggling along electron trajectories has been incorporated into a numerical algorithm for electron beam dose calculations. Landau's theory is used to predict, at any point in the absorber, the broadening of the primary electron energy spectrum due to energy loss straggling. Numerical calculations have been performed for electron beams with energies of 10-30 MeV incident upon water in order to determine the variation of dose with depth and variation of energy spectra with pathlength. These calculations are compared with the results of Monte Carlo simulations performed with the EGS4 code. The inclusion of energy loss straggling in the numerical calculations leads to predictions of energy spectra and dose deposition that are in good agreement with modified Monte Carlo simulations in which bremsstrahlung is ignored and the energy given to knock-on electrons is deposited at the site of their creation. Less satisfactory agreement was achieved when these calculations were compared to full Monte Carlo simulations that included the bremsstrahlung events and transported the knock-on electrons. It is concluded that bremsstrahlung energy loss must also be included into this algorithm, if an acceptable dose computation accuracy is to be achieved for clinical applications.

Computer Simulation↗
PubMed1994

A method for the calculation of electron energy-straggling spectra.

To calculate electron beam dose distributions accurately, numerical methods of electron transport calculations must account for the statistical variation (or "straggling") in electron energy loss. This paper shows that the various energy straggling theories that are applicable to short path lengths all derive from a single statistical model, known as the compound Poisson process. This model in turn relies on three assumptions: (1) the number of energy-loss events in a given path length is Poisson distributed; (2) events are mutually independent; and (3) each event has the same probability distribution for energy loss (i.e., the same energy-loss cross section). Applying the principles of the compound Poisson process and using fast Fourier transforms, a new method for calculating energy-loss spectra is developed. The spectra calculated using this method for 10, 20, and 30 MeV electrons incident on graphite and aluminum absorbers agreed with Monte Carlo simulations (EGS4) within 1% in the spectral peak. Also, stopping powers derived from the calculated spectra agreed within 1.2%, with stopping powers tabulated by the International Commission on Radiation Units and Measurements. Several numerical transport methods "propagate" the electron distribution (in position, direction, and energy) over small discrete increments of path length. Thus the propagation of our calculated spectra over multiple path length increments is investigated. For a low atomic number absorber (graphite in this case), calculated spectra agreed with EGS4 Monte Carlo simulations over the full electron range, provided the path length increments were sufficiently small (less than 0.5 g/cm2). It is concluded from these results that numerical methods of electron transport should restrict the size of path length increments to less than 0.5 g/cm2 if energy straggling is to be modeled accurately.

Electron Transport↗
PubMed1992

A reliable method for quantitating chromatin fragments by flow cytometry to predict the effect of total body irradiation and hyperthermia on mice.

The frequencies of chromatin fragments, including micronuclei, in murine thymus cells, spleen cells and bone marrow cells have been used as a quantitative indicator of gamma-ray induced chromosome damage and could be used to screen potential radioprotective agents as well. The yield of chromatin fragments induced in mice receiving different dosage levels of total body irradiation alone and in mice also given whole body hyperthermia as a potent radioprotector were assessed by flow cytometric analysis. Our results demonstrated that chromatin fragments induced by irradiation in vivo was clearly dose-dependent and that chromatin fragments could potentially serve as a biological indicator of radiation damage. One hour of whole body hyperthermia at 40 degrees C (+/- 0.2 degree C) given 20 hours before a lethal dosage (900 cGy) of total body irradiation protects 100% of DBA/2 mice from an LD 100/16 irradiation dose (dose of irradiation that killed 100% of the mice in 16 days). This is in good agreement with the percent of chromatin fragments formed in the cells of the protected animals, which showed no significant difference from those observed in the normal mice. The results indicate that whole body hyperthermia protected the thymus and bone marrow from irradiation damage. This study provides further evidence which supports that whole body hyperthermia can act as a potent radioprotector in vivo. Measurement of the frequencies of chromatin fragments by flow cytometry is simple and reliable. The method can be applied to screen radioprotective agents.

Animals↗
PubMed1992

A numerical method for electron transport calculations.

A numerical algorithm for calculating the penetration of electrons in dense media is presented. The numerical algorithm is intended for future application to radiotherapy dose calculations. The method is generic in the sense that it may be used with different theoretical models describing the angular scattering of electrons with depth. It is also general enough that it may be applied to electron dose calculations in heterogeneous as well as homogeneous media. The assumptions used in the algorithm are examined and equations describing the evolution of the distribution of electrons with depth are presented. Calculations have been performed for 10 MeV broad beams and pencil beams incident on water. It is shown that the Fermi-Eyges analytical solutions are recovered if the angular scattering process is assumed to be a Gaussian Markov process and the cumulative angle of electron travel remains small. In the case where the small angle approximation is not imposed, the numerical method qualitatively reproduces, at large depths, the wide angle scattering 'tails' seen in Monte Carlo generated profiles.

Algorithms↗
PubMed1990

Estimates of the effective dose equivalent, HE, in positron emission tomography studies.

The effective dose equivalent, HE, can be used as a standard radiation dose parameter in all imaging modalities that use ionizing radiation, including positron emission tomography (PET). A simplified method for evaluating approximate HE values for the positron emitters carbon 11, nitrogen 13, oxygen 15 and fluorine 18 is presented. HE values for a range of PET studies have been computed based on biodistribution data available in the scientific literature. Low-dose PET studies include a bolus administration of 1030 MBq CO15O (HE = 1 mSv) and 74 MBq [18F]-L dopa (HE = 1.3 mSv). High-dose PET studies include a 1-h (steady-state) inhalation of a total of 9250 MBq C15OO (HE = 9.4 mSv). The mean HE value of 13 diverse PET studies was computed to be 4.5 mSv.

Female↗
PubMed1989

The use of the effective dose equivalent, HE, for 99mTc labelled radiopharmaceuticals.

Using the concept of effective dose equivalent, HE, it is shown that a knowledge of the detailed biodistribution data in most organs and tissues taking up 99mTc labelled radiopharmaceuticals is unnecessary for the calculation of radiation risk. Reasonably precise dosimetry (+/- 25%) can be obtained from urine excretion data alone providing there is no significant uptake within the gonads or the thyroid. Special attention should only be paid to absorbed dose measurements in red bone marrow, skin, lungs, gonads and thyroid, with the greatest attention directed toward the retention and dosimetric aspects of radioactivity in the latter two organs. An example HE calculation for 99mTc labelled d,1-HMPAO is presented to illustrate the importance of these specific five organs to radiation risk.

Adult↗
PubMed1989

Radiation doses and detriment from chest x-ray examinations.

Radiation dose distributions for chest x-ray examinations have been measured in a Rando phantom for three views (AP, PA and lateral) as a function of kVp. On the basis of these data, the relationship between the surface dose, energy imparted and the effective dose equivalent have been determined. The mean energy imparted in a typical chest examination (PA + lateral views at 100 kVp) is 1.7 mJ and the corresponding value of the effective dose equivalent, HE, is 42 muSv. The measured radiation doses associated with chest x-rays were compared with the predictions of Monte Carlo calculations. The average difference between Monte Carlo and measured data for the HE was only about 16%. Demographic features (age/sex) of patients undergoing chest x-rays were investigated, and a population irradiation factor (PIF) introduced to estimate the radiation detriment to this population. The probability of expressed radiation-induced detriment to the patient population from chest x-ray examinations was computed to be about one half of that expected for a normal adult (working) population receiving the same dose. The radiation risk associated with chest x-ray examinations for this population was estimated to be less than 0.3 fatal cancers plus serious genetic disorders in the first two generations per million patient examinations.

Adult↗
PubMed1989

Patient doses from computed tomography in Manitoba from 1977 to 1987.

The number of patients undergoing computed tomographic (CT) examinations in the province of Manitoba is reported for the period 1977-1987. The annual patient throughput has increased from 4.2 per 10(3) population in 1978 to 18.2 per 10(3) population in 1987. Over the same period, the per capita population dose from CT has increased from 4.2 to 81.0 microSv. This substantial rise has occurred because of an increase in patient throughput, higher radiation doses associated with modern CT scanners and an increasing proportion of (higher dose) body CT studies. The mean patient dose on a second generation (EMI 5005) scanner was about 1.4 mSv, whereas the corresponding doses on third generation scanners operating in Manitoba were 3.9 mSv (GE 9800) and 5.6 mSv (Siemens DRH).

Humans↗
PubMedDate not supplied

Application of Fermi scattering theory to a magnetically scanned electron linear accelerator.

This paper uses a solution to the Fermi electron transport equation for an isotropic point source to characterize the magnetically scanned broad electron beams from the Sagittaire Therac 40 accelerator in the air space above patients. Thick lead collimation is shown to be adequately modeled by an infinitely thin absorbing plate when used to predict penumbra shape. A relationship between broad beam penumbra width and the value of the root-mean-square spatial Gaussian spread sigma (z) of an elementary pencil beam is derived. This relationship is applicable for any rectangular field size. Measurement of the variation in broad beam penumbra width with source-surface distance (SSD) for a 7-MeV beam locates the isotropic source to be coincident with the exit window of the accelerator and indicates that the scattering effect of the monitor chamber may be considered negligibly small. Using this source location accurate predictions of beam profile shape for any clinically used beam energy, SSD, or field size are made in the presence of lead trimmer collimation. Field penumbra beyond the photon collimation system is formed in each lateral direction by two lead blocks whose faces are aligned along a diverging ray emanating from the source. The photon collimator closest to the source restricts the field size causing a variation of both fluence and the mean square angle spread of the electrons across the plane at the level of the lower collimator. This variation is accounted for by introducing an empirical perturbation factor into the mathematical formalism. An interesting feature of this perturbation factor is that it is field size dependent and its effect on penumbra width may be scaled for both beam energy and SSD to accurately predict beam profile shape.

Humans↗
PubMedDate not supplied

Comparison of methods to determine electron pencil beam spread in tissue-equivalent media.

This study has intercompared the predictions of Fermi-Eyges theory for the rms spatial spread (sigma) of an electron pencil beam scattering in muscle-, lung- and bone-equivalent media with those of; two range straggling modifications to the theory, Monte Carlo simulations, and an empirical method based on broad beam penumbra. Systematic differences among the results obtained by these methods for the values of sigma have been identified. Monte Carlo simulations are lower than the predictions of Fermi-Eyges theory for sigma at all depths whereas the broad beam penumbra method results are in reasonable agreement with Fermi-Eyges theory at depths less than approximately 0.7 times the range of the incident electrons. All of the methods investigated have an increasing discrepancy from the predictions of Fermi-Eyges theory with depth, especially close to the end of the electron range. The two range-straggling modifications to Fermi-Eyges theory developed for soft tissue do not agree with either measured or Monte Carlo results for sigma in homogeneous scattering media of lung and bone.

Bone and Bones↗
PubMedDate not supplied

A restricted angular scattering model for electron penetration in dense media.

A restricted angular scattering model for electron penetration in dense media is presented. In the model, the Fermi-Eyges transport equation is modified through the addition of an extra term which may be interpreted as representing an apparent force opposing the scattering of electrons into wider angles. The introduction of this extra term allows the modeling of the measured saturation in the mean square angular spread of electrons with depth. The restricted scattering model retains the Gaussian features of the Fermi-Eyges model and, therefore, may be readily incorporated into existing dose computation algorithms. Good agreement is obtained with measured angular electron distribution data for a point monodirectional beam over a wide range of incident electron energies (5-20 MeV) and scattering media (atomic numbers of 6 to 82). Also, a comparison of the restricted scattering model predictions with measurements of the lateral pencil beam spread shows an improvement over the predictions of Fermi-Eyges model close to the end of the electron range. Broad beam profiles were generated using both the Fermi-Eyges and restricted scattering models. A comparison of predicted and measured beam profiles shows that the restricted scattering model is a significant improvement over the Fermi-Eyges model for the prediction of beam penumbra shape in homogeneous media.

Aluminum↗