bluuebunny/crossref_metadata_2025 huggingface dataset:
The correction for the gamma‐ray component in neutron therapy
Authors: Kellerer A. M. , Rassow J.
Published: August 2003

Neutron beams for therapy always contain some γ ray contamination that varies with depth and with distance from the beam axis. The problem therefore arises how the varying γ ray contribution should be accounted for in dose specification. Not infrequently a “total effective dose” DE is quoted that is equal to the neutron dose plus the γ ray dose divided by a constant weight factor τ. On general biophysical considerations this appears to be not a valid approach since it must be assumed that τ decreases with increasing dose. The nature and the magnitude of this dose dependence is derived in the present article. Application of the results to actual doses per fraction and to factual γ ray to neutron ratios demonstrates that the dose dependence of τ has, in fact, very minor influence on the numerical values of DE. Utilization of a constant value τ is therefore satisfactory in practice.
Postal intercomparison of absorbed dose for high energy x rays with thermoluminescence dosimeters
Authors: Bjärngard B. E. , Kase K. R. , Rudén B.‐I. , Biggs P. J. , Boyer A. L. , Johansson K. A.
Published: August 2003

This study concerns the accuracy and precision of the IAEA/WHO LiF TLD system used in intercomparison by mail of absorbed doses from 60Co γ‐radiation and 4–25 MV x rays. The system employs 160 mg LiF powder in polystyrene capsules, which are placed at 5 or 7 cm depth in water and irradiated to doses close to 200 rad (2.00 Gy). The dosimeters are mailed to the IAEA Dosimetry Laboratory and read out under conditions to minimize variations in instrument sensitivity. The precision of the readout technique, using 3 capsules per irradiation and the readout of 5 aliquots per capsule, is characterized by 0.2% standard deviation of the resulting mean. Since random errors during the irradiation are added, the detectable systematic discrepancy in dose delivery, at the 95% confidence level, is ±2% for 60Co γ and ±3% for high‐energy x rays.
Video techniques for x‐ray imaging and data extraction from roentgenographic and fluoroscopic presentations
Authors: Baily Norman A.
Published: August 2003

This review is concerned with the use of video‐fluoroscopic images in diagnostic radiology. It particularly emphasizes the potential of such images for performing quantitative procedures such as used in pulmonary and cardiac assessment, evaluation of contrast differentials, and tomographic (including CT) applications. This article also attempts to determine limitations, capabilities, and required developments for both the present and future hardware systems. Most importantly this review attempts to assess the present state of the art and to analyze the reasons for the limited application of such techniques in current radiology operations.
The influence of target thickness and backstop material on proton‐produced neutron beams for radiotherapy
Authors: Awschalom Miguel , Rosenberg Ivan , Kuo T. Y. , Tom J. L.
Published: August 2003

Results are presented of measurements of skin sparing, penetration and total dose per unit of incident charge for various target thicknesses and filtrations for a neutron beam generated by 42 MeV protons on beryllium. These results are contrasted to predictions outlined in a previous paper. The differences from these predictions are attributed to the contribution of low‐energy neutrons produced by the residual proton beam in the thick copper target backstop.
Using liquid dielectrics to obtain spatial thermal distributions
Authors: Kopecky Walter J.
Published: August 2003

A liquid dielectric has been used to determine thermal distributions for several clinically useful waveguide applicators as used in microwave hyperthermia. The system uses a saline/ethanol filled compartment placed within a temperature controlled water bath set to 37.5°C. Thermal distributions are taken with a thermistor probe placed perpendicular to the electric field. In this way longitudinal and transverse cross sections of the heat deposition due to microwave interaction with the liquid dielectric can be elicited. A similar arrangement was carried out using phantom material and an infrared camera as described by Guy. Comparison of the central axis data and cross‐sectional thermal distributions of these two systems shows good agreement.
Concerning radiotherapy standard dose meter calibration
Authors: Karzmark C. J.
Published: August 2003

Constancy measurements for nine ionization chambers were made using a 2 mg Ra source. (AIP)
Definition of source for head shielding requirements in linear accelerators may affect room shielding design
Authors: Herman Martin W. , Dockter Arden E. , Scallon Joseph E.
Published: August 2003

Siemens Corporation defines the source of the Mevatron 20 linear accelerator as a line source approximately 2 m in length. Calculations for secondary barriers for leakage radiation are generally based on 0.1% of the useful beam 1 m from the target. The difference in source terms can lead to calculating thinner barriers than are required. The placement of a 30×10×10‐cm block of lead behind the head was required to achieve acceptable radiation levels.
Computer assisted MTF determination in CT
Authors: Schneiders Nicholas J. , Bushong Stewart C.
Published: August 2003

A series of computer programs have been written to analyze a scan of the AAPM CT performance phantom resolution insert with little operator assistance. A representative analysis is done on a scan from an AS&E scanner as an example. Its point spread function, edge response function and modulation transfer function are presented.
Estimate of the risk in radiation therapy due to unwanted neutrons
Authors: Swanson William P.
Published: August 2003

The integral dose of accelerator‐produced leakage neutrons to patients undergoing high‐energy photon therapy is estimated and compared to other sources of integral dose. The leakage neutron component contributes about 5 g rad (1 rad=10−2 Gy) for a typical treatment course of 5000 rad. When averaged over a 70‐kg tissue volume, the corresponding dose amounts to only 0.36 rad. From this, the risk of inducing fatal malignancies by leakage neutrons is estimated to be about 50×10−6 per year following treatment. This is compared to other risks to which the patient is unavoidably exposed, and it is argued that the unwanted neutrons pose such small additional risk that regulatory intervention is not warranted. This assessment is performed without reference to neutron RBE or quality factor.
Clarification of the concept of “emitted radiation”
Authors: Attix F. H.
Published: August 2003

The quantity “emitted radiation” (due to Johns and Cunningham) is defined mathematically and its relationship to certain other quantities employed in brachytherapy dosimetry is shown.
Evaluation of the spectral response of the Wisconsin Mammographic kVp cassettes
Authors: Cruty Michael R. , Ghilardi‐Netto Thomaz
Published: August 2003

The spectral response of several Wisconsin Mammographic kVp cassettes was measured under various diagnostic conditions. The response for both unfiltered and externally filtered x‐ray beams from tungsten and molybdenum target tubes with different tube voltage ripple was studied for single phase x‐ray units. The response for a three phase generator with a molybdenum target tube was also measured. The maximum difference between these experimental response curves and the manufacturer's cassette calibration curve was 3.5 kVp for cassettes with Cu+Al step wedges and 6.0 kVp for cassettes with Cu+Ni step wedges under these widely different spectral conditions. The average difference between the experimental results and the manufacturer's calibration was ±2 kVp for Cu+Al step wedge cassettes and ±3.5 kVp for Cu+Ni cassettes. kVp measurements were made with an NBS calibrated voltage divider.
Specifying and evaluating the performance of computed tomography (CT) scanners
Authors: McCullough Edwin C.
Published: August 2003

Specifying and evaluating the performance of a computed tomographic (CT) scanner is necessary in order to ensure that a “typical” level of performance is achieved as well as providing baseline values for a program of quality assurance. Performance to be specified and tested include parameters related to: (a) slice geometry, (b) patient dosage, (c) artifactual behavior and (d) contrast–detail performance. Measures of performance are defined and specified. Tests which provide for performance evaluation in these four areas are detailed and discussed.
Displacement corrections used in absorbed dose determination
Authors: Cunningham J. R. , Sontag Marc R.
Published: August 2003

When an (exposure) calibrated ionization chamber is used for the determination of absorbed dose from a photon beam, the reading of the instrument must be multiplied by a number of factors, one of which is an attenuation correction for phantom material displaced by the chamber. The magnitude of this correction must depend on the size and shape of the ionization chamber as well as the energy of the radiation beam. For cobalt‐60 radiation, a single number, 0.985, has generally been used. Recent measurements, however, and “first scatter” calculations, of kerma suggest that a more appropriate value for a Farmer‐type chamber used in a water phantom would be 0.975. Such a change is small but would be important when dose calculations based on “in phantom” measurements are compared to calculations that are based on in air measurements. Values for the attenuation factor for other beam energies have not been generally available. We have carried out “first scatter” calculations for a rather wide range of energies and spectra. Measurements in 60Co beams and in a high‐energy (25 MV) photon beam support the calculations. A set of proposed displacement correction factors is presented.
The degradation function in conventional tomography
Authors: Israel James W. , Amtey Sharad R.
Published: August 2003

A method is developed by which the imaging characteristics of a tomographic x‐ray unit may be determined directly from a study of the mechanical motion of the unit. In this method we first describe the mechanical motions in terms of a vector equation. From this vector equation, a “degradation function” is derived which acts as a multiplicative factor, directly relating the optical transfer function of the system in a tomographic mode to the optical transfer function of the system in a stationary mode. Therefore, if the imaging properties of the system in the stationary mode are known (or, as is often the case, can be neglected), it is a simple matter to determine the imaging properties in the tomographic mode. The star pattern is used to verify the validity of this method.
Characteristics of an 18 MV photon beam from a Therac 20 Medical Linear Accelerator
Authors: Patterson Michael S. , Shragge Peter C.
Published: August 2003

The 18 MV photon beam characteristics of a Therac 20 Medical Linear Accelerator manufactured by Atomic Energy of Canada Ltd, are presented. Tissue phantom ratios (TPR's) and percent depth dose data are given; for a 10×10 cm field, the percent depth dose at a depth of 10 cm is 78.5 (SSD 100 cm). The relative dose factors (RDF'S) are given and are analyzed to elucidate the relative contributions from phantom scatter, collimator scatter, and backscatter from the top of the collimators into the monitor chambers. The effect of field size and depth on the penumbra is described. Crossplots of the beam at a depth of 5 cm indicate that the flattening filter could be improved; there are hot spots of 108% near the corners of 40×40 fields.
16O(n, p)16N: A fast neutron detector for rapid tissue inhomogeneity correction
Authors: Beach Joseph L. , Watts Ronald J. , Milavickas Lou R.
Published: August 2003

We have devised a transit dose technique for fast neutron therapy treatment planning based on the 16O(n, p)16N reaction in recirculating water, and have determined the effect of simulated bone and lung inhomogeneities in phantom. An effective threshold of 10.2 MeV in the 16O(n, p) reaction is exploited to detect transmitted neutrons without the need for detector collimation. This system has been demonstrated with 14 MeV (d, T) neutrons and with cyclotron produced p(42)+Be neutrons. 16N decays to the 6.13 MeV excited states of 16O in 7.14 s, allowing for easy identification by NaI(Tl) and for rapid recirculation. The transmission of fast neutrons can thus be related to an effective thickness of soft tissue, providing a rapid and direct measure of the effects of inhomogeneities under actual treatment conditions, with the 10 MeV threshold providing a useful degree of insensitivity to multiply scattered neutrons. Equivalent thicknesses of compact bone and lung relative to water were found to be 1.4 and 0.34 respectively, closely resembling the effective thicknesses for Cobalt‐60 gamma rays.
Distortion introduced in radionuclide camera views by multiformat imagers
Authors: Dunn William L. , Brown Manuel L. , Tuscan Michael
Published: August 2003

The degree of spatial distortion of radionuclide camera images introduced by the multiformat imager is compared in six different cameras. The instruments tested were the Ohio Nuclear 110 and 410S, Searle Pho Gamma LFOV, and the General Electric 400T. We found image nonlinearity variations of 4 to 25%; this illustrates the need for industrial standards applied to imager spatial distortion performance.
Second scatter contribution to dose in a cobalt‐60 beam
Authors: Wong J. W. , Henkelman R. M. , Fenster A. , Johns H. E.
Published: August 2003

Semi‐analytic and direct numerical integration approaches have been used to study the contribution of doubly scattered photons to dose at a point P in a homogeneous water medium irradiated by a cobalt‐60 point source. The semi‐analytic analysis was used for an infinite field radius to calculate the variation of second scatter dose with depth, the angular dependence of the second scatter dose, and the spectra. Direct numerical integration was used to calculate the second scatter dose at a depth of 10 cm for various finite field radii ranging from 0 to 25 cm. Maps were made of the locations of scattering sites that were important to the second scatter dose. These calculations show that the second scatter contribution is generally less than the first scatter dose with the exception of a few sites remote from the point of interest. In many ways, the pattern of dose deposition by second scatter is similar to first scatter with less pronounced features. In other respects, the second scatter appears more isotropic. The implications of these calculations for approximate radiotherapy dose calculations are discussed. Key words: photon scatter, second scatter dose, differential scatter air ratio
Effect of small inhomogeneities on dose in a cobalt‐60 beam
Authors: Wong John W. , Henkelman R. M. , Andrew J. W. , Van Dyk J. , Johns H. E.
Published: August 2003

The effect of small ring‐shaped cavities on the dose delivered by a cobalt‐60 beam to a homogeneous medium was studied experimentally and theoretically. Changes as small as 1×10−4 of the total dose were measured. Experimental results show that, depending on the position of the cavity, replacing water with a small cavity can either increase or decrease scatter dose to a point in the medium. The increase in scatter dose was not anticipated and to our knowledge is not predicted by any of the presently available inhomogeneity dose correction algorithms used in treatment planning. Calculations were based on the perturbations of first and second scatter dose contributions and show that the presence of a cavity in the medium introduces three processes that decrease scatter dose and five that increase it. The calculated net effect is in good agreement with experiment. Additional calculations show that the effect of a single small inhomogeneity cannot be easily extrapolated to larger inhomogeneities and that multiple inhomogeneities do not act independently. Results from this study demonstrate two constraints that must be satisfied by future dose calculation algorithms: (i) they must correctly determine dose in a homogeneous non‐unit density material, and (ii) they must account for the change in dose due to small inhomogeneities in the medium. Key words: computed tomography, dose calculations, small inhomogeneity, first scatter dose, second scatter dose
Regional myocardial flow estimation using computed tomography
Authors: Nassí Menahem , Brody William R.
Published: August 2003

A novel method is presented for estimating regional myocardial blood flow (RMBF) using x‐ray computed tomography (CT). Two major setbacks are removed from the existing methods; namely, the requirement for intra‐arterial bolus injection of tracer, and the inability to determine regional volumes of tracer distribution when the corresponding partition coefficients are unknown. The mathematical model developed for RMBF estimation combines both the tracer dispersion process and the CT measurement process. Intravenously administered contrast media (tracer) is assumed to be perfectly mixed by the myocardium. Tracer dynamics, as measured by CT (CT♯s. vs time curves) in the myocardial regions and in the left ventricular chamber, can then be used to compute cardiac output, regional volumes of tracer distribution, and absolute RMBF. These computations use the zeroth and the first moments of the measured CT♯s. vs time curves. Errors due to partial volume effects, and methods for their correction, are analysed. This formulation is readily adapted to existing computed tomographic systems having the capability to produce multiple sequential CT scans.
An x‐ray fluorescence technique to measure the mercury burden of dentists in vivo
Authors: Bloch P. , Shapiro I. M.
Published: August 2003

Mercury exposure in dental offices may represent a health hazard. Previous studies evaluated mercury exposure through blood, urine or hair analysis. The mercury content of body fluids in hair depends on the time since mercury exposure, mode of intake and its excretion, and sequestration by different tissues. Utilizing an x‐ray fluorescence technique, the mercury burden in bone, liver, lung, spleen, and kidney was measured in vitro. The x‐ray fluorescence, XRF, and chemical assays of the mercury content of these samples correlated. The XRF technique was used to assay the mercury content of tissues irradiated in vivo. It was found that the mercury signal arising at a depth in tissue was reduced as the thickness of overlying material increased. Thus, the XRF assay for tissues in vivo is confined to organs near the surface. The mercury content of the superficial layers of the head and wrist were determined in situ for a dental population. Concentrations as low as 20 μg/g could be detected with an exposure of 60 mR. Of the 298 dentists evaluated, 28% had greater than 20 μg/g of mercury in the head.
A formalism for calculation of absorbed dose to a medium from photon and electron beams
Authors: Loevinger Robert
Published: August 2003

A formalism is derived that relates the absorbed dose to a medium from photon and electron beams to the photon calibration factor of an ionization chamber. The formalism is applicable to the photon and electron beam energies that are currently of interest in radiation therapy. It is developed in terms of a cavity‐gas calibration factor, a quantity characteristic of the chamber and independent of the energy of the calibration beam assuming the energy expended per ion pair is energy independent. The cavity−gas calibration factor can be obtained from a chamber calibration performed in terms of exposure,absorbed dose to water,or air kerma. The perturbation corrections due to replacement of the surrounding medium by the chamber wall and cavity are identified as ratios of the photon energy fluence,or the electron fluence, at the position of the chamber center. The unmanageable complexities of a theory that covers an ionization chamber made of several materials are avoided by limiting the development to a chamber made of a single material with the expectation that the inhomogeneities of real chambers can be treated as perturbations. Attention is called to certain theoretical aspects of this dosimetry development that do not appear to have been previously recognized.
Voltage, energy, and material dependence of secondary radiation
Authors: Burgess A. E. , Pate G.
Published: August 2003

Measurement results of the relative contribution of primary and secondary radiation are presented. Measurements were done with both monoenergetic and broad spectrum x‐ray beams. A variety of phantoms were used: graphite, water, lucite, a skull phantom, and aluminum. We found that the primary radiation fraction is nearly independent of photon energy and tube potential. Calculations using a simple single Compton scattering model agree with the experiment results.
Calorimetric and ionimetric dosimetry intercomparisons I: U.S. neutron radiotherapy centers
Authors: McDonald Joseph C. , Ma I‐Chang , Liang John , Eenmaa Juri , Awschalom Miguel , Smathers James B. , Graves Robert , August Leon S. , Shapiro Philip
Published: August 2003

In the U.S. neutron radiotherapy trial centers, absorbed dose is routinely measured using commercially available A‐150 tissue equivalent (TE) plastic ionization chambers. The collecting volumes of these chambers are filled with either methane‐based tissue equivalent gas or air. Absorbed dose in A‐150 plastic, determined with these ionization chambers, was compared to that measured by an A‐150 plastic calorimeter in an A‐150 plastic phantom. These comparisons have yielded the following information: (1) Agreement of the total absorbed dose measured using the ionization chambers was within 2.5% of the calorimeter at all the centers visited to date. (2) For all the neutron fields measured, the product of the stopping power ratio between the A‐150 plastic chamber wall and TE gas, and the average energy expended in the gas per ion pair formed, , was computed assuming Bragg–Gray theory and found to be 31.0±0.7 J/C. (3) The displacement correction factor employed to normalize measurements at a depth in a phantom using the type IC‐17 ionization chamber was verified to be approximately 0.97±0.01.
The radiation therapy dosimetry network in the United States
Authors: Lanzl Lawrence H. , Rozenfeld Martin , Wootton Peter
Published: August 2003

The United States government and the governments of sixteen other countries have established radiation standards laboratories which house the primary standards dosimeters of the respective countries. In the United States, three regional calibration laboratories were established to disseminate the national radiation standards through the use of secondary standards dosimeters. These, in turn, are used for calibration of radiation field instruments used to measure radiation applied in therapy. These laboratories are periodically accredited by the American Assocation of Physicists in Medicine (AAPM). In 1976, AAPM established a Committee on Radiation Calibration Needs in Therapy to conduct a study of the adequacy of the therapy dosimetry network in the United States. According to this study, 1322 centers were performing radiotherapy in 1978, and 1057 radiation instruments were used to calibrate the therapy machines in these facilities. In more than 99% of the institutions, medical physicists are responsible for carrying out the necessary calibrations. The study showed that there is a need, though not an urgent one, for an additional regional calibration laboratory in the Far Western United States. Establishment of such a laboratory would not be excessively disruptive to the existing laboratories.
Limitations to iodine isolation using a dual beam non‐K‐edge approach
Authors: Riederer S. J. , Kruger R. A. , Mistretta C. A.
Published: August 2003

In dual‐beam selective iodine imaging, images of an object are made with each of two spectrally different x‐ray beams. The mean beam energies may either straddle the 33 keV iodine K‐edge or both lie above the K‐edge. Both patient exposure considerations and the availability of sufficient x‐ray flux make the latter approach favorable for tissue thicknesses exceeding 5 cm. Consider such an approach in which image contrast from tissue is suppressed in the difference image. It is proven theoretically that the residual bone–to–iodine contrast is a constant independent of the two mean beam energies used. This invariance principle is demonstrated experimentally by comparing images made from different pairs of x‐ray spectra. Observed contrast ratios match the predicted value very well. In dual‐beam imaging, contrast from only one material may be suppressed. Other substances yield residual signals which compete with the iodine. Subtleties of this incomplete cancellation are demonstrated, discussed, and quantitated. A contrast enhancement factor (CEF) is defined as the factor by which iodine contrast is enhanced in a multiple beam subtraction technique relative to monoenergetic imaging at 40 keV. CEFs are determined for tissue and bone cancellation separately and their limits are discussed. Images of a simulated artery containing iodine superimposed over a Rando head and neck phantom show that the CEF limitation for dual beam imaging is quite severe compared to a time dependent mask mode imaging approach. Finally, optimum energies for dual beam images are discussed.
Computer programs for output and depth dose from hyperbolic equations
Authors: Horn Richard A.
Published: August 2003

A compact and accurate equation has been developed that describes the output field size over the range from 4×4 to 35×35 cm2. An accuracy of ±0.2% has been demonstrated for teletherapy machines with energies from 250 kVp to 6 MV. The same equation form is used to describe depth dose tables with the same accuracy down to a depth of at least 30 cm. The relationship is described by a hyperbola whose x and y axes are displaced by corresponding values X0 and Y0 producing the equation y=Y0−C/(X0+x), where y=output and x=field size. By substituting three separate values for y and x into three simultaneous equations, values for Y0, X0 and c can be derived.
Airborne concentrations of toxic metals resulting from the use of low melting point lead alloys to construct radiotherapy shielding
Authors: McCullough Edwin C. , Senjem David H.
Published: August 2003

Determinations of airborne concentrations of lead, cadmium, bismuth, and tin were made above vessels containing a “fusible” lead alloy (158 °F melting point) commonly used for construction of radiotherapy blocks. Fume concentrations were determined by collection on a membrane filter and analysis by atomic absorption spectrophotometry. Samples were obtained for alloy temperatures of 200 °, 400 °, and 600 °F. In all instances, concentrations were much lower than the applicable occupational limits for continuous exposure. The results of this study indicate that the use of a vented hood as a means of reducing air concentrations of toxic metals above and near vessels containing low temperature melting point lead alloys commonly used in construction of radiotherapy shields appears unjustifiable. However, proper handling procedures should be observed to avoid entry into the body via alternate pathways (e.g., ingestion or skin absorption). Transmission data of a non‐cadmium containing lead alloy with a melting point of 203 °F was ascertained and is reported on.
Experimental measurements of the scatter reduction obtained in mammography with a scanning multiple slit assembly
Authors: Yester Michael V. , Barnes Gary T. , King Michael A.
Published: August 2003

A nonconventional and sensitive method of measuring scatter is described. The method was employed to quantitate the scatter imaged in mammography with a conventional unit, a unit equipped with a grid, and a unit equipped with a scanning multiple slit assembly (SMSA). The results indicate that the grid technique significantly reduces the scatter imaged, while the SMSA virtually eliminates it. The resultant increase in large area contrast is readily apparent on radiographs with greater improvement obtained with the SMSA than with the grid. The effect of the increase in contrast on small detail visibility was assessed with a phantom having simulated fibrils and calcifications. Significantly more fibril and calcification detail was visible with the grid and SMSA technique than with the conventional technique. The detail visible with the grid and SMSA technique was comparable, and the lack of better performance by the SMSA unit is attributed to its poorer MTF.
Use of a neutron remmeter to measure leakage neutrons from medical electron accelerators
Authors: Rogers D. W. O. , Van Dyk G.
Published: August 2003

It is argued that the technique of choice for measuring neutrons from medical electron accelerators is neutron remmeters with gold activation foils as thermal neutron detectors. This technique is to be preferred because of its simplicity, its accuracy, and its lack of dependence on any calculated responses or neutron spectral shapes. Measurements of the response of this detector are presented along with the results of representative measurements made at seven medical linacs and one betatron.
Some aspects of optimization of an invasive microwave antenna for local hyperthermia treatment of cancer
Authors: de Sieyes David C. , Douple Evan B. , Strohbehn John W. , Trembly B. Stuart
Published: August 2003

Hyperthermia has emerged as a promising alternative or adjunct to other forms of cancer therapy. In order to utilize hyperthermia in very localized volumes immersed in regions of vital normal tissue, an invasive microwave coaxial monopole antenna has been developed. An experimental approach has been taken to characterize and optimize the electromagnetic properties and heating capabilities of bare and insulated antennas imbedded in tissue equivalent phantoms and dog brain. Four methods have been used to visualize the thermal profiles of the microwave probes: the liquid crystal technique, the gelatin technique, and the direct measurement of temperature with thermistor probes in phantom and dog brain. Among the parameters studied are: antenna impedance, insertion depth, antenna insulation (dielectric constant and thickness), shaft insulation, and frequency.
Analytic formulae for estimation of dose along the central ray of diagnostic x‐ray beams
Authors: Chaney Edward L. , Batchelor D. A.
Published: August 2003

Empirically derived analytic formulae for estimation of dose delivered along the central ray by conventional diagnostic x‐ray procedures are presented. The formulae are based on a computational model employing tissue–air ratios along the central axis of beams with 2.5 and 3.5 mm Al filtration, and for 3φ waveform (15% ripple) throughout the range from 70 to 120 kVp.
A method for selective tissue and bone visualization using dual energy scanned projection radiography
Authors: Brody William R. , Butt Glenn , Hall Anne , Macovski Albert
Published: August 2003

Information contained in the x‐ray energy spectrum can be used to produce selective radiographic images of bone or soft tissue. A method has been devised to separate bone and soft tissue based upon differences in photoelectric absorption and Compton scattering using an appropriate combination of images obtained with radiographic exposures at 70 KVP and 140 KVP. Since photoelectric absorption is highly dependent upon atomic number, high atomic number materials such as calcium can be easily separated from water density substances. Using a prototype system for line‐scanned radiography, selective subtraction of bone or soft‐tissue has been implemented. Because this method uses a conventional broad‐spectrum x‐ray source, it was necessary to develop a nonlinear polynomial approximation to estimate tissue and bone thickness. The model was verified with phantom studies using water and aluminum. The application of this dual‐energy bone and soft‐tissue separation to chest radiography is demonstrated. This method allows accurate estimation of tissue and bone thickness and should find application to chest radiography for improved lesion detection and for bone mineral assessment.
Principles in positioning cross‐projecting lasers
Authors: Thomadsen Bruce R.
Published: August 2003

With the increased sophistication of lasers used for patient positioning in radiotherapy comes increased possibilities for mistakes in installing the lasers, which can result in serious patient positioning errors. Two principles must be followed when mounting cross‐projecting lasers: (1) the horizontal line must be on the same level as the treatment unit isocenter, and the vertical line must be coplanar with the gantry rotation, and (2) If one of the lines of the cross rotates, it must be the line which is perpendicular to the line joining the two holes from which the laser beams exit the laser casing.
CT alignment probe for the dose profile insert
Authors: Schneiders Nicholas J. , Glaze Sharon A. , Bushong Stewart C.
Published: August 2003

One of the most accepted methods of characterizing the dose in computed tomograhy (CT) is by measuring the dose profile. Thermoluminescent dosimeters (TLD's) arranged in a stack are scanned in a plexiglass phantom. Unfortunately with this system there is no assurance that the scan properly intercepts the stack. Mispositioning will not be apparent until the TLD's are read, entailing at least a 24 h delay before rescanning. We have designed a simple alignment probe that insures that the scan will be centered on the stack to within 1 mm.
Neutron flux‐density and secondary‐particle energy spectra at the 184‐inch synchrocyclotron medical facility
Authors: Smith A. R. , Schimmerling W. , Kanstein L. L. , McCaslin J. G. , Thomas R. H.
Published: August 2003

We have identified the sources of neutron production in the beam transport system of the 720‐MeV helium beam used for radiation therapy at the 184‐in synchrocyclotron of the Lawrence Berkeley Laboratory, and determined their magnitude. Measurements with activation detectors of differing energy response were used to unfold secondary particle spectra at various locations on the patient table. The effect of charged particles was estimated using a calculation of neutron‐flux densities derived from published cross sections. The absorbed dose, as a function of distance from the beam axis, was calculated using the unfolded spectra and evaluated fluence‐to‐dose conversion factors. The values of absorbed dose obtained from the unfolding of experimental data agree with the values obtained from the calculated spectra within the estimated uncertainty of ±25%. These values are ∼5×10−3 rad on the beam axis and ∼1×10−3 rad at distances greater than 20 cm, perpendicular to the beam axis, per rad deposited by the incident alpha‐particle beam in the plateau. Estimates of upper limits of dose to two critical organs, the lens of the eye and red bone narrow, are ∼25 rad and ∼5 rad, respectively, for a typical treatment plan.
Comparison of ion chamber and TLD dosimetry in mammography
Authors: Stanton Leonard , Day J. L. , Brattelli S. D. , Lightfoot D. A. , Vince M. A. , Stanton R. E.
Published: August 2003

An ionization chamber method has been developed to measure exposure vs depth in a uniform BR 12 “average breast” phantom. It employs a Memorial mammography chamber for exit exposure measurements; resulting data is then corrected for backscatter as well as for the exceptionally thin window of this chamber. A careful comparison has then been made with relative exposure vs depth curves obtained using TLD at several mammography beam qualities, for identical exposure factors and SSD values. Use of a correction for residual and background TL signals significantly improved agreement between TLD and ion chamber curves in the 28 to 35 kVp/0.03 mm Mo range of beam quality. Agreement was within ±5% for the Mo target tube, but TLD readings were 4%–8% higher than ion chamber values for the W/Mo target tube. At Xeromammography energies (45 kVp/1.6 mm Al), corrected TLD curve readings were 6% higher at depth than ion chamber curve values. TLD measurements with 28 to 35 kVp/0.03 mm Mo beams tend to underestimate dosage to the midbreast parenchyma. For example, in a 5 cm “average breast”, the underestimation ranges from 2%–10% for corrected, 10%–16% for uncorrected TLD readings.   Key words: Mammography dosimetry, mammography, thermoluminescent dosimetry, ion chamber dosimetry
“Battered data”—Some clinical effects of the abuse of multiple regression methods: The NSD
Authors: Herbert Donald E.
Published: August 2003

The NSD equation, D = 1850×T0.11×N0.24, is a celebrated transmogrification of Cohen's two well‐known collations of data on response to clinical irradiations (3° erythema and 0.90 ablation of skin cancer) in which the relation of D and T is fixed by the data selected by Cohen and the additional constraint that N correspond to a schedule of five treatments per week is imposed subsequently by Ellis. The present paper shows that the equation, if correct, would have little clinical significance because the proportion, P, in which the dose, D, elicits the 3° erythema is unspecified: D = D(P) = D(?). Since the two Cohen collations each summarize the measurements on a different set of observational units, it is questionable whether the equation can be correct. This paper further shows that the appropriate (Least Squares) estimates of the three‐parameter equation derived for the Cohen data under the Ellis constraint (five treatments per week) is in fact: . The paper shows that the NSD equation is also incorrect because the ad hoc method by which Ellis estimates the exponents is inconsistent with the constraints imposed by Cohen and Ellis upon the parameters of the multivariate frequency distribution of the data set. The paper shows that the method by which the correct LS estimates of the exponents were obtained from the Cohen–Ellis data is consistent with these constraints and, therefore, this equation is a correct graduation of any other set of treatment regimens which is also consistent with the Cohen and Ellis constraints. The paper further shows that for such data sets there are, in fact, only two independent continous variables, either D and T or D and N, since the Ellis constraint requires that N and T be collinear. Thus, the best linear graduation has the typical form: D≃1900T0.32. This is “best” in the usual sense: both prediction and confidence intervals are provided for the estimates of the conditional “tolerance dose” D; these are not inflated by the presence of a collinear variable. This equation is biased, however, by the absence of the collinear variable. The TDF and the CRE concepts are derived from the NSD and, therefore, the deficiencies of the latter concept which we discuss may be expected to encumber these progeny as well. The two characteristic features of the Cohen (Ellis)‐type data which impede the construction of useful estimates of the putative separate effects of N and T upon the response of tissues to irradiation are that (1) these data do not include specifications of either a tissue defect or its incidence, and (2) the variables N and T are collinear. Appendices I and II describe methods by which the effects of these features may be eliminated (I) or reduced (II).   Key words: NSD, response surface, dose response equations, isoeffect equations, management equations, least squares, constrained least squares, collinearity, factoral experiments, ridge regression, principal components
Rotating laminar emission camera with GE‐detector: Experimental results
Authors: Urie Marcia M. , Mauderli Walter , Fitzgerald Lawrence T. , Williams Clyde M. , Tosswill Christopher H.
Published: August 2003

Experimental results of a prototype rotating laminar emission camera (ROLEC) for nuclear medicine imaging are reported. A 11.5 mm thick, 45×45 mm high‐purity germanium detector is segmented into 30 1.47 mm wide parallel channels and collimated with 39 mm high parallel plates. Projection data acquired at multiple angular orientations as the detector‐collimator assembly is rotated about its center are mathematically reconstructed to image the activity distribution. The spatial resolution of the ROLEC is at least twice as good, at all distances, as that of gamma cameras with high resolution collimators. The better energy resolution of the germanium enhances the detection and resolution of the ROLEC in comparison to gamma cameras with NaI(T1) crystals, the relative superiority increasing with greater volumes and with greater depths. Adequate sensitivity is maintained while achieving these improvements in spatial resolution and in practice; ROLEC images are acquired in less time than pinhole collimator images with gamma cameras. Key words: emission camera, laminar camera, germanium camera
Improved method for the design of tissue compensators
Authors: Shragge Peter C. , Patterson Michael S.
Published: August 2003

A program, WBEAM, is described which calculates dose distributions in planes perpendicular to the beam axis, taking into account both field shape and patient contour. WBEAM can be used to design compensators which when placed in the beam will produce uniform dose distributions in the plane of calculation. The program was tested in three different situations: a 30×30 cm field with a flat contour, a “mantle” field with a flat contour, a “mantle” field with a human‐like contour. The program performs as designed: the dose distributions are accurate, and the compensators flatten the radiation beams to the specified limits. Key words: compensator, beam flattener, dose distribution, isodose distribution, mantle field
The tracks of the Compton effect
Authors: del Regato J. A.
Published: August 2003

The observation of scattered radiations of larger wavelength than the primary had been repeatedly rejected or explained away by many researchers, including Compton. After years of vacillations, he recognized the effect named after him and was the first to develop a quantal equation predicting the wavelength of scattered radiation. It became one of the most significant contributions to modern radiation physics, opening the doors of quantum mechanics. Key words: scattered radiations, Compton effect, William Duane, Karl Stenström
RC filter versus integrator data collection circuits for computed tomography
Authors: Olson E. A. , Smith C. R. , Pisano D. J.
Published: August 2003

This paper presents an RC filter network as an alternative to the standard integrator circuit used for data collection in CT scanners. It compares the two circuits for noise and resolution performance under the assumption of constant total x‐ray dose. The RC circuit exhibits great versatility in its ability to trade off resolution for a lower noise‐to‐signal ratio as a function of the time interval of data collection. The integrator circuit has a noise and resolution relationship which is fixed for a constant x‐ray dose.
Approximation formulas for least‐squares fitting of functions of the form f [ μ(x−x0)]
Authors: Jette David
Published: August 2003

Approximation formulas have been developed for least‐squares fitting of functions of the form f [ μ(x−x0)]. In particular, formulas are given for determining the best‐fit parameter values for the Laughlin and Shabason–Hendee representations of central‐axis depth dose of electron teletherapy beams. Comparison with exact values obtained through nonlinear least‐squares regression analysis, for the Laughlin and Shabason–Hendee representations applied to standard data sets, demonstrates that these approximation formulas are highly accurate.
Postal dose intercomparison of 60Co dosimetry using thermoluminescence dosimeters among secondary standard dosimetry laboratories
Authors: Boyd A. W. , Eisenlohr H. H. , Girzikowsky R.
Published: August 2003

A postal dose intercomparison of 60Co dosimetry at the radiotherapy level was carried out in the Secondary Standard Dosimetry Laboratory (SSDL) Network using LiF. The deviations between the dose quoted by the 22 Secondary Standard Dosimetry Laboratories taking part and that measured by the International Atomic Energy Agency's dosimetry laboratory were all less than 5% and in 16 cases were less than ±2%, the estimated uncertainty in the method.
Scattered radiation in fan beam imaging systems
Authors: Johns P. C. , Yaffe M.
Published: August 2003

Scatter‐to‐primary energy fluence ratios (S/P) have been studied for fan x‐ray beams as used in CT scanners and slit projection radiography systems. The dependence of S/P on phantom diameter, distance from phantom to image receptor, and kilovoltage is presented. An empirical equation is given that predicts S/P over a wide range of fan beam imaging configurations. For CT body scans on a 4th‐generation machine, S/P is approximately 5%. Scattered radiation can produce a significant cupping artefact in CT images which is similar to that due to beam hardening. When multiple slices are used in scanned slit radiography, they can be arranged such that the increase in S/P is negligible. Calculations of scatter‐to‐primary ratios for first order scattering showed that for fan beams the contribution of coherent scatter is comparable to or greater than that of incoherent first scatter.
The specific γ‐ray constant and exposure rate constant of 182Ta
Authors: Glasgow Glenn P. , Dillman L. T.
Published: August 2003

Reported values of the specific γ‐ray constant Γ for 182Ta range from the original value of 6.1 to 7.692 R cm2h−1mCi−1, recommended in NCRP Report No. 41. The original calculation of Γ was based on inadequate nuclear spectroscopy and decay scheme data. The higher value of Γ occurs because of a computational error in the relative intensity of the 1.2575‐MeV γ ray. Using nuclear spectroscopy data from the most recent Evaluated Nuclear Structure Data File (ENSDF), Γ is calculated to be 6.71±0.06 R cm2h−1mCi−1 and the exposure rate constant Γγ is 6.87±0.06 R cm2h−1mCi−1. These new calculations are presented and previously reported values of Γ and Γγ are reviewed.
The effects of hydrogenous and nonhydrogenous filters on the quality of a p(66)Be(49) neutron beam
Authors: Rosenberg Ivan , Awschalom Miguel , Ten Haken Randall K.
Published: August 2003

The hardening effects of hydrogenous and nonhydrogenous filters on a p(66)Be(49) neutron beam have been investigated. It was found that all materials studied, Teflon, aluminum, lead, steel, and polyethylene, harden the neutron beam, albeit polyethylene to a greater extent. Relationships were found to exist between the attenuation of a filter and its hardening effect, and also between the build‐up characteristics and the depth for half‐maximum dose of the hardened beams.
Alignment of x‐ray tube focal spots for spectral measurement
Authors: Nishizawa Kunihide , Maekoshi Hisashi , Kamiya Yoshihiro , Kobayashi Yoshio , Ohara Ken , Sakuma Sadayuki
Published: August 2003

A general method to align a diagnostic x‐ray machine for x‐ray spectrum measurement purpose was theoretically and experimentally investigated by means of the optical alignment of focal pinhole images. Focal pinhole images were obtained by using a multi‐pinholed lead plate. The vertical plane, including the central axis and tube axis, was decided upon by observing the symmetry of focal images. The central axis was designated as a line through the center of focus parallel to the target surface lying in the vertical plane. A method to determine the manipulation of the central axis in any direction is presented.
Optimal photon energies with respect to absorbed dose for visualization of soft tissue masses within adipose tissue
Authors: Ragozzino Mark W.
Published: August 2003

The photon energies which maximize the signal to noise ratio per unit absorbed dose (optimal photon energies) have been calculated for detection of soft tissue masses within adipose tissue 3–15 cm thick. The detection task simulates mammography in women with fatty breasts. Ideal, plain film and calcium tungstate screen–film systems were considered in the analysis. Net photographic density is not incorporated into this model; therefore, the optimal photon energy with respect to the signal to noise ratio per unit absorbed dose may not be the same as the photon energy which results in maximum film density per unit absorbed dose. Therefore, the model has limited applicability to film or screen–film systems. However, it is directly applicable to digital systems where brightness and contrast can be adjusted.