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Nov 1992

Volume 19, Issue 6, pp. 1349-1498

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Current status of physical measurements of the skeleton

Moses A. Greenfield

Med. Phys. 19, 1349 (1992); http://dx.doi.org/10.1118/1.596769 (9 pages) | Cited 3 times

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An overview is presented of some of the major methods of measuring the skeleton in the past 30 years. These include single photon absorptiometry (SPA), dual photon absorptiometry (DPA), quantitative computed tomography (QCT), and recently dual energy radiographic absorptiometry (DRA), also called DEXA (dual energy x‐ray absorptiometry). In addition to these methods, all attempting to measure bone mineral density, regional and total body calcium have been determined by in vivo neutron activation analysis (IVNAA). An attempt to determine bone quality as contrasted with bone quantity has been made using ultrasound, with measurements of speed of sound and of attenuation as useful parameters to characterize bone tissue. While the various methods for measuring bone density have been most useful, no one method includes all the features required to be entirely satisfactory: excellent precision and accuracy and the ability to measure volumetric density in gm/cm3. Least successful has been the ability to predict fracture risk, an essential goal in helping the patient.
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87.57.-s Medical imaging
87.85.Pq Biomedical imaging

Auger electron dosimetry: Report of AAPM Nuclear Medicine Committee Task Group No. 6

James G. Kereiakes and Dandamudi V. Rao

Med. Phys. 19, 1359 (1992); http://dx.doi.org/10.1118/1.596925 (1 page) | Cited 1 time

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Abstract Unavailable
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87.57.U- Nuclear medicine imaging
87.53.Bn Dosimetry/exposure assessment
32.80.Hd Auger effect (including Coster-Krönig transitions)
87.53.-j Effects of ionizing radiation on biological systems

Biological effects of the Auger emitter iodine‐125: A review. Report No. 1 of AAPM Nuclear Medicine Task Group No. 6

Kandula S. R. Sastry

Med. Phys. 19, 1361 (1992); http://dx.doi.org/10.1118/1.596926 (10 pages) | Cited 11 times

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The biological implications of Auger electron cascades following inner shell ionization of atoms have been of interest for over 25 years. By virtue of their decay via orbital electron capture and/or internal conversion, several biomedical radionuclides emit numerous low‐energy electrons spontaneously. The biological effects of such radionuclides incorporated into tissues cannot be predicted a priori because of the highly localized patterns of energy deposition by the electrons. Results of extensive research using Iodine‐125 as a model Auger electron emitter are now available. This article presents an up‐to‐date review of the physical and radiobiological data on this Auger emitter. Valuable concepts concerning the action of internal Auger emitters are identified phenomenologically, and questions that need to be answered are indicated. The present understanding provides a scientific basis toward estimation of risk associated with Auger emitters used in diagnosis, and suggests potential applications to therapy.
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87.53.-j Effects of ionizing radiation on biological systems
87.57.U- Nuclear medicine imaging
32.80.Hd Auger effect (including Coster-Krönig transitions)

Radiation spectra for Auger‐electron emitting radionuclides: Report No. 2 of AAPM Nuclear Medicine Task Group No. 6

Roger W. Howell

Med. Phys. 19, 1371 (1992); http://dx.doi.org/10.1118/1.596927 (13 pages) | Cited 15 times

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Radiation spectra for radionuclides currently provided by the MIRD Committee and ICRP do not include the very low‐energy N‐ and O‐shell Auger electrons. These electrons, emitted in large numbers by radionuclides decaying by electron capture and/or internal conversion, are important for determining the absorbed dose in microscopic volumes. Accordingly, the present AAPM Report employs Monte Carlo computational methods to obtain a self‐consistent set of complete radiation spectra for a variety of radionuclides including 55Fe, 67Ga, 99mTc, 111In, 113mIn, 115mIn, 123I, 125I, 193mPt, 195mPt, 201Tl, and 203Pb. Although the conventional spectra provided by MIRD and ICRP are adequate for most dosimetry calculations, the Auger electron spectra provided in this report are recommended for calculating the dose to target volumes <1 μm in diameter.  
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23.20.Nx Internal conversion and extranuclear effects (including Auger electrons and internal bremsstrahlung)
32.80.Hd Auger effect (including Coster-Krönig transitions)
87.53.Bn Dosimetry/exposure assessment
87.57.U- Nuclear medicine imaging

Analytic microdosimetry for radioimmunotherapeutic alpha emitters

T. G. Stinchcomb and J. C. Roeske

Med. Phys. 19, 1385 (1992); http://dx.doi.org/10.1118/1.596770 (9 pages) | Cited 14 times

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Analytic microdosimetry using Fourier transform techniques has been applied to internal alpha emitters. These techniques need revision and simplification for use with short‐lived radionuclides such as those which may be useful for radioimmunotherapy. Analytic methods may have advantages over Monte Carlo methods in some cases (e.g., where time is important). Applications to eight different source geometries show the usefulness of these techniques. Comparisons of some of the results to Monte Carlo calculations prove its accuracy. For a uniform source of 5.867‐MeV alphas spread throughout the volume outside a cell surface, the two methods agree well. Results are within 1% both for the average specific energy and for the number of hits. Analytic microdosimetry provides an alternate method to use for the critical evaluation of models that seek to predict the relation between alpha energy deposition and cell survival data. Similarly, it may be helpful to point the way toward the rational interpretation of general biological results for antibodies labeled with alpha emitters.
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87.53.Jw Therapeutic applications, including brachytherapy
87.53.Bn Dosimetry/exposure assessment

Microbeam radiation therapy

D. N. Slatkin, P. Spanne, F. A. Dilmanian, and M. Sandborg

Med. Phys. 19, 1395 (1992); http://dx.doi.org/10.1118/1.596771 (6 pages) | Cited 30 times

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It is proposed to carry out radiotherapy and radiosurgery for brain lesions by crossfiring an array of parallel, closely spaced microbeams of synchrotron‐generated x rays several times through an isocentric target, each microbeam in the array having an ≊25‐μm‐wide adjustable‐height rectangular cross section. The following inferences from the known tissue sparing of 22‐MeV deuteron microbeams in the mouse brain and the following exemplary Monte Carlo computations indicate that endothelial cells in the brain that are lethally irradiated by any microbeam in an array of adequately spaced microbeams outside an isocentric target will be replaced by endothelial cells regenerated from microscopically contiguous, minimally irradiated endothelium in intermicrobeam segments of brain vasculature. Endothelial regeneration will prevent necrosis of the nontargeted parenchymal tissue. However, neoplastic and/or nonneoplastic targeted tissues at the isocenter will be so severely depleted of potentially mitotic endothelial and parenchymal cells by multiple overlapping microbeams that necrosis will ensue. The Monte Carlo computations simulate microbeam irradiations of a 16‐cm diameter, 16‐cm‐long cylindrical human head phantom using 50‐, 100‐, and 150‐keV monochromatic x rays.
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87.53.Jw Therapeutic applications, including brachytherapy
87.85.Ox Biomedical instrumentation and transducers, including micro-electro-mechanical systems (MEMS)

A finite‐size pencil beam model for photon dose calculations in three dimensions

J. D. Bourland and E. L. Chaney

Med. Phys. 19, 1401 (1992); http://dx.doi.org/10.1118/1.596772 (12 pages) | Cited 34 times

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A three‐dimensional dose computation model employing a finite‐size, diverging, pencil beam has been developed and is demonstrated for Cobalt‐60 γ rays. The square cross‐section pencil beam is simulated in a semi‐infinite water phantom by convolving the pencil beam photon fluence with the Monte Carlo point dose kernel for Cobalt‐60. This finite‐size pencil beam is calculated one time and becomes a new data base with which to build larger beams by two‐dimensional superposition. The pencil beam fluence profile, angle correction for beam divergence, the Mayneord inverse square correction, radial and angular sampling rates, error propagation, and computation time have been investigated and are reported. Radial and angular sampling rates have a great effect on accuracy and their appropriate selection is important. Percent depth doses calculated by finite‐size pencil beam superposition are within 1% of values calculated by full convolution and the agreement with values from the literature is within 6%. The latter disagreement is shown to be due to a low‐energy photon component which is not modeled in other calculations. Computation time measurements show the pencil beam method to be faster than full convolution and one implementation of the differential‐scatter‐air‐ratio (dSAR) method.
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87.53.Bn Dosimetry/exposure assessment

A study of interface effects in 60Co beams using a thin‐walled parallel plate ionization chamber

B. Nilsson, A. Montelius, and P. Andreo

Med. Phys. 19, 1413 (1992); http://dx.doi.org/10.1118/1.596795 (9 pages) | Cited 12 times

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A large plane‐parallel ionization chamber has been constructed to investigate interface effects in 60Co beam. The designed geometry yields negligible perturbation from the side walls, as opposed to the large effects existing in commercially available plane‐parallel chambers. The chamber has been used to investigate interface phenomena in transition zones using a wide range of elements (Z=4‐82) as front‐ and back‐scattering media and a clinically relevant 60Co γ‐ray field size. The effects of varying the chamber height discretely (0.5–11 mm) and increasing the wall thickness (1–9 mg/cm2) have been investigated. The variation of the measured ionization with the experimental setup (air gap between backscatter material and chamber wall, measurements at dmax and at 5‐cm depth, varying the material both in front of and behind the chamber, etc.) has also been investigated. The simple geometry of the ion chamber has been found optimum for benchmark studies of Monte Carlo calculations. The ion chamber is suited for investigating experimentally the effects of varying transport parameters used in Monte Carlo simulations. The results presented show that the complex physical mechanisms governing 60Co interface dosimetry still make Monte Carlo condensed‐history (macroscopic) techniques uncertain. It has been found that the EGS4 Monte Carlo system, together with the user code DOSRZ V4.0 and the PRESTA algorithm, yields good agreement with experiments for low and medium Z (main interest in dosimetry and radiotherapy), but may underestimate up to 10% the backscatter from high‐Z materials even when transport parameters are optimized.
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87.53.Bn Dosimetry/exposure assessment

Simple calculation of the electron‐backscatter factor

Tatsuo Tabata and Rinsuke Ito

Med. Phys. 19, 1423 (1992); http://dx.doi.org/10.1118/1.596796 (4 pages) | Cited 6 times

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The authors have studied the dependence of the electron‐backscatter factor (EBF) on mean electron energy and on backscatterer atomic number by using the semiempirical depth‐dose code EDMULT. A plane‐parallel electron beam is assumed to be normally incident on a polystyrene slab, which is backed with a layer (backscatterer) of different materials of effectively semi‐infinite thickness. A small air cavity to measure ionization is embedded in the polystyrene slab at the boundary facing the backscatterer. The EBF is defined as the ratio of the ionization with the backscatterer to the ionization with a full polystyrene medium, and is approximated by the ratio of the doses computed at the depth of the cavity. Values of EBF obtained show trends similar to the experimental data of Klevenhagen et al. [Phys. Med. Biol. 27, 363–373 (1982)], although the former are generally lower than the latter. When the typical energy spread of clinical electron beams is taken into account, the difference between the experimental and the calculated values is reduced. The present results also show the same trend of increase of the backscatter factor with increasing energy as observed by Klevenhagen et al. in some series of measurements for the lead backscatterer at the lowest energies. This is explained by the rapid buildup of the dose with depth for electrons of low initial energies incident on the full polystyrene medium.
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87.53.Bn Dosimetry/exposure assessment

Uncertainty analysis of absorbed dose calculations from thermoluminescence dosimeters

T. H. Kirby, W. F. Hanson, and D. A. Johnston

Med. Phys. 19, 1427 (1992); http://dx.doi.org/10.1118/1.596797 (7 pages) | Cited 25 times

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Thermoluminescence dosimeters (TLD) are widely used to verify absorbed doses delivered from radiation therapy beams. Specifically, they are used by the Radiological Physics Center for mailed dosimetry for verification of therapy machine output. The effects of the random experimental uncertainties of various factors on dose calculations from TLD signals are examined, including: fading, dose response nonlinearity, and energy response corrections; reproducibility of TL signal measurements and TLD reader calibration. Individual uncertainties are combined to estimate the total uncertainty due to random fluctuations. The Radiological Physics Center’s (RPC) mail out TLD system, utilizing throwaway LiF powder to monitor high‐energy photon and electron beam outputs, is analyzed in detail. The technique may also be applicable to other TLD systems. It is shown that statements of ±2% dose uncertainty and ±5% action criterion for TLD dosimetry are reasonable when related to uncertainties in the dose calculations, provided the standard deviation (s.d.) of TL readings is 1.5% or better.
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87.53.Bn Dosimetry/exposure assessment
87.80.-y Biophysical techniques (research methods)
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Medical Physics is an official science journal of the AAPM and of the COMP/CCPM/IOMP
William R. Hendee, Editor
Departments of Radiology, Radiation Oncology, Biophysics, Community and Public Health, Medical College of Wisconsin
One Physics Ellipse
College Park, Maryland 20740
301-209-3352

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