摘要:

A historical review of conventional fractionation offers little confidence that such treatment is optimal for all tumors. Thus manipulation of time–dose schedules may provide a relatively inexpensive yet potentially useful technique for improving therapeutic results in radiation therapy. Consideration of basic radiobiological principles and animal model data illustrates the complex and heterogeneous nature of normal tissue and tumor response to time–dose effects and supports the hypothesis that better time–dose prescriptions can be found in clinical practice. The number of possible time–dose prescriptions is very large, and a review of the clinical trials using nonconventional fractionation demonstrates that the sampled portion of the total three‐dimensional space of time, fraction number, and dose has been very small. Only carefully designed clinical trials can establish the therapeutic advantage of a new treatment schedule, and methods for selecting the most promising schedules are discussed. The use of simple data reduction formulas for time–dose effects should be discarded since they ignore the very complexity and heterogeneity of tissues and tumors which may form the basis of improved clinical results.

ISSN:0094-2405    

DOI:10.1118/1.594728

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A brief review of flow cytometry is presented. Emphasis is on the construction of relatively simple instruments and on their application to problems in medical physics. Among these are the problems of measuring the biological parameters that determine the response of tumors to therapy and of measuring the biological damage in people exposed to ionizing radiations.

ISSN:0094-2405    

DOI:10.1118/1.594729

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A technique for determining the absolute activity of I‐123 in early thyroidal uptake tests is presented. The method is independent of geometry, tissue attenuation, and extrathyroidal neck activity (ENA). It utilizes the efficiency factors(εxTand εγT) derivable for the thyroid gland using coincidence counting techniques, and a quantity, α, characteristic of the vascularity of the individual patient. Although ENA is not determined in absolute terms (e.g., percent of injected dose), count rates due to ENA are determined. After subtracting the ENA from the total count rates thyroidal uptake can be determined as absolute activity by means of the efficiency factors. The validity and usefulness of the technique have been demonstrated in phantom and patient studies. Thyroid uptakes of less than 0.1% of injected dose can be measured accurately in our patient studies.

ISSN:0094-2405    

DOI:10.1118/1.594705

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Preliminary results on a system for delivering dynamic pion radiotherapy treatments are reported. The desired treatment volume is scanned across a small, focused pion beam using a computer‐controlled treatment couch. A computer‐controlled rangeshifting device modulates the stopping pion depth distribution in coordination with the couch motion to conform the dose to the shape of the treatment volume. For certain shaped tumors, the system can result in substantial normal tissue dose sparing and better field flatness as compared to irradiation by static treatments with broad beams. The characteristics of this new system, plus preliminary results for typical dose distributions as measured with thermoluminescent dosimeters (TLDs), are presented.

ISSN:0094-2405    

DOI:10.1118/1.594730

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Accurate determination of diagnostic x‐ray spectra requires, among many other technical considerations, correction of the experimental data for the energy‐dependent response of the x‐ray detector. In this study, we investigated the energy response of three types of detectors (intrinsic germanium, lithium‐drifted silicon, and sodium iodide) commonly used in the diagnostic energy range. We used a Monte Carlo simulation technique to compute photopeak efficiencies, escape peak losses, and Compton fractions as a function of incident x‐ray energy for a variety of detector thicknesses and diameters. Calculated responses were compared with experimental results obtained for the Ge and Si(Li) detectors. The measurements, in which we used fluorescent x ray to simulate the monoenergetic beams assumed for the calculations, indicate good agreement between theoretical and experimental approaches. Techniques for correction of measured spectra for the known response of the detector were also evaluated.

ISSN:0094-2405    

DOI:10.1118/1.594706

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The Poisson noise inherent in multispectral x‐ray imaging systems is formulated to give the SNR under a limited exposure constraint. The SNR value is maximized with respect to the exposure partition among different x‐ray energies. The study shows how to choose suitable spectra and the dose weights. The robustness of these dose weights is also demonstrated for systems of varying bone and tissue contents.

ISSN:0094-2405    

DOI:10.1118/1.594731

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The theory of a new B‐mode registration test procedure is presented along with experimental verification of the detailed theoretical predictions. The new procedure separately tests the B‐scan articulated arm calibration and the equipment acoustic velocity calibration. Readily verifiable secondary standards are incorporated into the procedure for self‐calibration purposes. The procedure provides a specific and reproducible analysis of angular misregistration errors while maintaining an acceptable level of sensitivity. No coupling gels or oils or operator skill is required. A special test has been found which permits the service person to verify that the equipment is capable of attaining proper B‐mode registrationbeforebeginning the recalibration procedure.

ISSN:0094-2405    

DOI:10.1118/1.594732

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The applicability of 25 keV neutrons to thein vivoactivation 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.

ISSN:0094-2405    

DOI:10.1118/1.594733

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Limitations of the most commonly used dose calculation methodologies as apply to medium and high‐energy x rays are discussed. A method is presented whereby the dose to a point in medium is divided into several components with expressions given for various components. The depth of transient‐charged particle equilibrium is chosen as the reference point. Equations are derived for calculating and relating the dose parameters including the percent depth dose, the tissue‐reference ratio, and the scatter‐reference ratio. Particular emphasis is placed on the accuracy and adequacy of the technique in the region of buildup. The applicability of the present method to megavoltage photons is established for a number of x‐ray beams in the range 4–45 MeV maximum energy.

ISSN:0094-2405    

DOI:10.1118/1.594721

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

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 in60Co beams and in a high‐energy (25 MV) photon beam support the calculations. A set of proposed displacement correction factors is presented.

ISSN:0094-2405    

DOI:10.1118/1.594817

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY