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L H Lanzl

Publications and source records attributed to L H Lanzl.

27 records · Page 2Linked to original sources
PubMedDate not supplied

Application of fission track detectors to californium-252 neutron dosimetry in tissue near the radiation source.

Fission track detectors were applied to a unique problem in neutron dosimetry. Measurements of neutron doses were required at locations within a tumor of 1 cm diameter implanted on the back of a mouse and surrounded by a square array of four 252Cf medical sources. Measurements made in a tissue-equivalent mouse phantom showed that the neutron dose rate to the center of the tumor was 2.18 rads micrograms-1 h-1 +/- 8.4%. The spatial variation of neutron dose to the tumor ranged from 1.88 to 2.55 rads micrograms-1 h-1. These measurements agree with calculated values of neutron dose to those locations in the phantom. Fission track detectors have been found to be a reliable tool for neutron dosimetry for geometries in which one wishes to know neutron dose values which may vary considerably over distances of 1 cm or less.

Animals↗
PubMedDate not supplied

Analytic representation of electron central-axis depth dose data.

We have examined a number of analytic representations of electron central-axis depth dose data current in the literature, testing them against sets of standard depth dose data. One of them, a two-parameter model of Shabason and Hendee, is recommended in situations in which good accuracy (approximately 2%) is desired, with the values of the parameters determined by an approximation formula which we have developed elsewhere. For higher accuracy, we have developed a polynomial model which gives, typically, a standard deviation of the fitting polynomial from the data points of 1%, and a maximum deviation of 2%. Fitting polynomials obtained with this method possess the property of having zero slope at the position of actual maximum dose, and generally a fifth-order polynomial (requiring four nonzero coefficients) provided the most acceptable fit. The four parameters involved are determined through inversion of a 4 x 4 matrix, and we have tabulated these four coefficients for the standard data sets. The polynomial model is designed for interpolation in the range between the 100% dose depth and the 10% dose depth, and another fitting curve of the same type can be adjoined to cover depths less than the 100% dose depth.

Computers↗
PubMedDate not supplied

Product representations of teletherapy dose distributions.

Product representations are frequently used for teletherapy dose distributions. For example, the dose in a central plane is often written as the product of two factors, one dependent on the depth and the other on the transverse variable. We have answered the following question: given a (two-dimensional) set of data, how closely is it possible to represent these data by a product of two such factors, and what factors would give a best-fit representation? We thus have developed a quantitative test with which to judge any proposed product representation, for a given set of data. As an example, we have applied our method in analyzing the accuracy of a model proposed by van de Geijn for representing central-plane Cobalt-60 dose data through a product representation on decrement lines intersecting the source.

Humans↗
PubMedDate not supplied

An overview of errors in line source dosimetry for gamma-ray brachytherapy.

The generally used approach for routine brachytherapy dosimetry depends on a value of source strength provided by the source supplier, the theoretical concept of exposure rate constants for different nuclides, and the applicability of inverse square law to point sources. Corrections for filtration of the radiation within the capsule of the source and attenuation in tissue medium are effected based on attenuation coefficients and data published in the literature. Therefore there is no unique or ideal way of selecting the required data and there could be a spread in the dose values derived by different users. The errors inherent in the current practice of dosimetry of linear sources in brachytherapy are discussed. If the brachytherapy sources are specified in terms of exposure rate at a distance from the source by the source suppliers, the overall uncertainty in the dosimetry at regions of clinical interest around the source could be limited to about +/- 6%.

Brachytherapy↗
PubMedDate not supplied

Can the AAPM Task Group 21 protocol lead to optimum ion chamber designs?

The recently published AAPM Task Group 21 protocol for high-energy dosimetry is complicated in that it requires the physicist to obtain the values of about a dozen different physical variables by looking them up in tables or graphs. This should be compared with the procedure of earlier protocols using the concept of a single multiplier C lambda. We have investigated how the physical principles outlined in the improved AAPM protocol could be utilized for the redesign of the therapy-level ion chambers in such a way that one can reduce the number of factors that need to be looked up in tables or graphs for the calibration of high-energy teletherapy photon beams. In our analysis presented in this paper we found that one such design could be for an ion chamber having a wall acrylic or Bakelite of a thickness not exceeding 0.1 g/cm2 and having an inner diameter of 6 mm, and used in conjunction with a cobalt-60 buildup cap of thickness 0.35 g/cm2 made of acrylic, Bakelite, or Tufnol. If a chamber of such a design is used in a water phantom, the dosimetry practically reduces to the simplicity of the former protocols of depending on a single value of energy-dependent multiplier to be obtained from a table. With the above design parameters, it becomes possible to eliminate the explicit need to incorporate the factors Pwall, Prepl, Awall, beta wall, and the variable alpha, representing the fraction of ionization due to electrons from the wall material of the chamber.

Animals↗
PubMedDate not supplied

Collimated electron beams and their associated penumbra widths.

The Fermi-Eyges multiple-scattering theory for electrons is applied to calculate profiles of collimated electron beams. The dose profile below the collimator is a convolution of the intensity distribution of the electrons at the level of the collimator and the distribution arising from the propagation of a Gaussian point source from the collimator to the level of the calculation. The electrons at the level of the collimator possess an angular distribution characteristic of the configuration of the electron beam at the vacuum window. Hence, the dose profile and its associated penumbra width can be expressed in terms of the angular moments of the distribution of the electrons at the collimator. The dependence of the penumbra width on the configuration-dependent angular spread of the electrons at the collimator accounts for differences in the size of the penumbra between two broad-beam configurations. These differences are also seen experimentally. We have also studied the dependence of the angular moments of the electrons upon scattering foils present above the collimator and the position of the beam-broadening device in the accelerator head.

Electrons↗
PubMedDate not supplied

Electron dose calculation using multiple-scattering theory: thin planar inhomogeneities.

In this article in our series on electron dose calculation using multiple-scattering theory, we apply the Fermi-Eyges theory to the problem of a thin planar inhomogeneity present in an otherwise-layered medium. We derive expressions for the distribution function P and the location distribution L (which multiplied by the restricted mass collision stopping power is the dose directly deposited by the primary electrons) for various types of incident beams: a completely arbitrary distribution, a Gaussian point source, a pencil beam, an isotropic point source, and a broad parallel beam. We show how divergent-beam dose distributions can be determined from parallel-beam calculations, through use of equivalent configurations dependent upon the depth of dose calculation. Also, we indicate how this work can be applied to the design of wedges (or "compensators") for beam shaping to provide desired dose distributions or to match juxtaposed radiation fields. Explicit formulas for thin plates are then worked out, and we examine the appearance of hot and cold spots distal to the edge of a localized inhomogeneity, for thin half-slabs and for narrow strips. Finally, considering the case of a thin straight wedge-shaped inhomogeneity, we theoretically discover the phenomenon of a "focused hot spot" without an accompanying cold spot, and suggest the design of a "multiple-scattering lens".

Electrons↗