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W V Prestwich

Publications and source records attributed to W V Prestwich.

At least 37 records · Page 2Linked to original sources
PubMed1993

Spatial dose-rate distribution for an 194Ir point source in water.

The spatial dose-rate distribution arising from a unit activity point source of 194Ir embedded in an infinite water medium has been calculated. These results may prove to be both useful and timely; 194Ir is a beta-particle-emitting radionuclide that is a suitable candidate for radioimmunotherapy, and a means for producing it fairly easily has recently been made available. Calculation of the beta-ray dose-rate distribution is based on a new set of monoenergetic electron dose point kernels which are results from an improved Monte Carlo (ETRAN) calculation. The spatial dose-rate distribution for 32P was also calculated and compared with results from the EGS4-PRESTA and ACCEPT Monte Carlo codes. The ETRAN-based distribution agrees with that from EGS4-PRESTA to within 2% out to 3.6 mm from the source, a distance over which 90% of the source energy is deposited. The ETRAN-based distribution also agrees well with that from ACCEPT between 0.6 and 4 mm from the source; at distances < 0.6 mm, values from ACCEPT are about 6% larger than ones calculated here.

Beta Particles↗
PubMed1993

A reanalysis of microdosimetric data pertaining to the quality factor of tritium.

A recently developed procedure for estimating the quality factor from microdosimetric data is applied to a published experiment involving tritium. Arguments are presented indicating that the observed value is most likely an estimate of a lower limit for this quantity. It is concluded that the data support a value of 2 for the tritium quality factor.

Radiometry↗
PubMed1989

Beta dose point kernels for radionuclides of potential use in radioimmunotherapy.

Beta dose point kernels for 32P, 67Cu, 90Y, 131I, 186Re, and 188Re nuclides appropriate for radioimmunotherapy are calculated based upon Monte Carlo results. The calculations are shown to differ significantly from values based upon solutions to the electron transport equation. Agreement with experiment for 32P is found to be improved for the former as compared with the latter. Values of the scaled dose point kernels are tabulated at 4% intervals of the continuous slowing down approximation range for each of the six radionuclides. Beta dose distributions are also tabulated at corresponding distances from the source. This data may be used to calculate the spatial dose distribution expected following administration of radiolabeled monoclonal antibodies, aiding in optimum selection of the appropriate radionuclide. Parameters for functions providing analytic representation of the calculated scaled dose point kernels of individual beta groups are presented.

Beta Particles↗
PubMed1987

Beta dosimetry for radioimmunotherapy of cancer using labeled antibodies.

Starting with the tabulation of scaled monoenergetic electron dose point kernels F in water, according to the calculations of Berger, we have developed analytic approximations that facilitate the task for convolving F with beta spectra of specific nuclides to derive their dose point kernels J. J is well fitted by a single log-normal distribution, involving three independent parameters. For large atomic numbers, the addition of an exponential function near the origin is required. The radiation dose distribution D in a homogeneous medium of soft tissue with an activity distribution C is given by the convolution of J with C. Computer programs using fast Fourier transform techniques have been developed to evaluate the convolution efficiently. To study the perturbation of tissue inhomogeneity for D, we have used a point source of 32P and measured the increase in backscattered dose from a polystyrene/aluminum interface simulating soft tissue/bone. We have also measured the decrease in dose at a polystyrene/air interface.

Antibodies↗
PubMed1979

On the deconvolution of exponential response functions.

The deconvolution or unfolding of exponential response functions from experimental data has been examined through the use of a Bayesian based algorithm. The algorithm, which is founded upon the concepts of probability, ensures positivity of solution. This constraint leads to a significant reduction in the growth of statistical noise in deconvolved data when compared with the more common linear unfolding techniques. The algorithm is an iterative procedure which, in the absence of statistical noise, can ultimately result in complete signal recovery. When noise is present one must balance the degree with which the response function is removed against the growth in the noise and, at some point, terminate the iterative process. Criteria for determining the point at which this 'best estimate' is attained are examined and an operationally realisable test is given. Comparison of results is made with the inverse filter solution which, for an exponential response function, is shown to consist of the sum of the observed data and its first derivative.

Mathematics↗
PubMedDate not supplied

System design for in vivo neutron activation analysis measurements of manganese in the human brain: based on Monte Carlo modeling.

Manganese is an essential nutrient required by the human body, but conversely, over exposure to the element may cause central nervous system damage. The technique of in vivo neutron activation analysis, using the McMaster KN-accelerator, is being investigated as a possible method of noninvasively determining manganese concentrations within the human body. Since the brain is the primary target of damage from exposure it would be the ideal site for measurements. Thus, Monte Carlo simulations have been undertaken to define the optimum experimental parameters for such a measurement, examining the use of possible moderator, reflector and collimator materials.

Basal Ganglia↗
PubMedDate not supplied

In vivo activation analysis of spinal calcium using 25 keV neutrons.

The applicability of 25 keV neutrons to the in vivo activation analysis of spinal calcium is examined both theoretically and experimentally. It is shown that the use of this energy results in an increase in sensitivity over higher energy neutrons so that patient dose may be significantly reduced.

Activation Analysis↗
PubMedDate not supplied

Calculation of radiation doses for nonuniformity distributed beta and gamma radionuclides in soft tissue.

In cancer radioimmunotherapy nonuniform distributions of radionuclides in geometrically irregular tumor volumes will be encountered. If the tumor is surrounded by a homogeneous layer of soft tissue, the radiation dose distribution in the tumor due to a beta-emitting nuclide can be calculated by convolution of the appropriate Loevinger's point source dose function, J, with the nuclide concentration distribution. Computer programs using fast Fourier transform techniques have been developed to evaluate the three-dimensional spatial convolution accurately and efficiently. Equivalent techniques have also been applied to gamma-emitting nuclides by replacing J in the convolution by the photon point isotropic specific absorbed fraction. Illustrative results are presented for the radiation dose distribution in a 1-ml target volume with activity distribution which is (i) uniform, (ii) limited by diffusion into the target volume, or (iii) limited by diffusion and shedding of radionuclide in the peripheral layers of the target volume. Calculations for the three cases are presented for 32P, 131I, and 15-, 30-, and 100-keV photon emitters.

Antibodies, Monoclonal↗
PubMedDate not supplied

Effect of tissue inhomogeneity on beta dose distribution of 32P.

In a homogeneous medium of soft tissue the radiation dose distribution due to a nonuniformly distributed beta source can be calculated by convolution of the beta dose point kernel of the nuclide with the source distribution. A possible extension of the technique to the calculation of the dose distribution in heterogeneous media involving relatively simple geometric interfaces requires the knowledge of the resulting perturbation to the beta point kernels in individual media. We simulated a soft-tissue-bone planar interface by a polystyrene (PST)-aluminum junction and measured the change in beta dose from the dose value in homogeneous PST due to a point source of 32P using 7LiF thermoluminescent dosimeters. With the point source at the interface, the dose rates at 0-31, 125-156, and 283-314 mg/cm2 separations from the interface were increased by (12 +/- 3)%, (8 +/- 2)%, and (3 +/- 2)%, respectively, compared with homogeneous PST. With the point source at a PST-air planar interface to simulate a soft-tissue-air junction, the dose rates at 0-31, 139-170, and 283-314 mg/cm2 from the interface were decreased by (25 +/- 4)%, (11 +/- 7)%, and (5 +/- 2)%, respectively. The changes in dose rates for these two interfaces have also been measured with degraded spectra of 32P. Comparison of the experimental data with Monte Carlo calculation for a point source and the two-group method of calculation for a plane source is also presented.

Biophysical Phenomena↗
PubMedDate not supplied

Effect of tissue inhomogeneity on dose distribution of point sources of low-energy electrons.

Perturbation in dose distributions of point sources of low-energy electrons at planar interfaces of cortical bone (CB) and red marrow (RM) was investigated experimentally and by Monte Carlo codes EGS and the TIGER series. Ultrathin LiF thermoluminescent dosimeters were used to measure the dose distributions of point sources of 204Tl and 147Pm in RM. When the point sources were at 12 mg/cm2 from a planar interface of CB and RM equivalent plastics, dose enhancement ratios in RM averaged over the region 0-12 mg/cm2 from the interface were measured to be 1.08 +/- 0.03 (SE) and 1.03 +/- 0.03 (SE) for 204Tl and 147Pm, respectively. The Monte Carlo codes predicted 1.05 +/- 0.02 and 1.01 +/- 0.02 for the two nuclides, respectively. However, EGS gave consistently 3% higher dose in the dose scoring region than the TIGER series when point sources of monoenergetic electrons up to 0.75 MeV energy were considered in the homogeneous RM situation or in the CB and RM heterogeneous situation. By means of the TIGER series, it was demonstrated that aluminum, which is normally assumed to be equivalent to CB in radiation dosimetry, leads to an overestimation of backscattering of low-energy electrons in soft tissue at a CB-soft-tissue interface by as much as a factor of 2.

Bone Marrow↗
PubMedDate not supplied

An evaluation of the EGS4 and CYLTRAN Monte Carlo codes with regard to boundary beta-ray dosimetry by comparison with experimental beta-ray dose backscatter factors.

Beta-ray dose backscatter factors or dose ratios at planar soft-tissue boundaries were calculated using the EGS4/RESTA and CYLTRAN (version 2.1) Monte Carlo codes and these data were compared with experimental results. Since the beta-ray source was 32P, this work addressed the transport of, and energy deposition by, electrons less energetic than 2 MeV. In particular, the simulations targeted the codes' performances with regard to the transport of low energy electrons across material boundaries and the backscattering of low energy electrons. In general, backscatter factors calculated at 7.25 mg/cm2 from several soft-tissue interfaces agreed with experimental values to within about five percent. CYLTRAN was also used to calculate the variation of backscatter factor with distance from aluminum/soft-tissue and air/soft-tissue interfaces and was found to reproduce the shapes of experimental backscatter factor depth profiles.

Beta Particles↗