摘要:

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 byinvivoneutron 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.

ISSN:0094-2405    

DOI:10.1118/1.596769

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0094-2405    

DOI:10.1118/1.596925

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

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 predictedaprioribecause 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.

ISSN:0094-2405    

DOI:10.1118/1.596926

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

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 including55Fe,67Ga,99mTc,111In,113mIn,115mIn,123I,125I,193mPt,195mPt,201Tl, and203Pb. 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.

ISSN:0094-2405    

DOI:10.1118/1.596927

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

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.

ISSN:0094-2405    

DOI:10.1118/1.596770

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

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.

ISSN:0094-2405    

DOI:10.1118/1.596771

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

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.

ISSN:0094-2405    

DOI:10.1118/1.596772

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A large plane‐parallel ionization chamber has been constructed to investigate interface effects in60Co 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 relevant60Co 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 atdmaxand 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 governing60Co 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 mediumZ(main interest in dosimetry and radiotherapy), but may underestimate up to 10% the backscatter from high‐Zmaterials even when transport parameters are optimized.

ISSN:0094-2405    

DOI:10.1118/1.596795

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

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 Klevenhagenetal. [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 Klevenhagenetal. 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.

ISSN:0094-2405    

DOI:10.1118/1.596796

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

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.

ISSN:0094-2405    

DOI:10.1118/1.596797

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY