摘要:

Photon dose fractions (PDFs) have been measured in and around a neutron radiotherapy beam with a tissue‐equivalent proportional counter (TEPC) and with paired ion chambers. The PDFs were found to increase linearly with increasing field size and width depth in phantom. PDFs were shown to decrease with decreasing phantom size and to be larger in the shielded region of the phantom than in the direct beam. Uncertainties in the PDF values were estimated to be 10%–15% for the TEPC measurements but about 50% for the measurement made with ion chambers.

ISSN:0094-2405    

DOI:10.1118/1.594323

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Since published results for the fast‐neutron dose per x‐ray rad from high‐energy therapy installations have differed by as much as a factor of 300, we have measured the neutron production from our 25‐MeV betatron. Fast‐neutron activation of aluminum foils was the method used. The effect of photoneutron production in the detectors, which has affected some past work, has been eliminated. A major source of neutrons in the treatment field was the platinum target. The neutron spectra used in the calculations of fluence were obtained by interpolation between published energies and between neighboring elements. Fluences per rad of x rays without a phantom were largely independent of field size and energy in the range 18–23 MeV. At 100 cm SSD and 23 MeV a large beam flattener contributed 15% of the neutrons, the remainder coming equally from the target and background. A phantom increased the neutron fluence/rad of x rays by 0%–10% depending on the field size. At 23 MeV we estimated the neutron dose to a patient to be 2.2×10−4rad per rad of x rays inside the treatment field and 3×10−5at 20 cm outside the field. The uncertainty in these figures is believed to be ±50%. In the electron beam the neutron dose per rad was about 50 times smaller than in the x‐ray beam. Estimates were made of neutron fluences at other energies and target thicknesses. We discuss our results in comparison with those of others.

ISSN:0094-2405    

DOI:10.1118/1.594324

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Backscatter factors for kVps and half‐value layers typical of mammographic beams have been determined as a function of beam diameter and half‐value layer. The method utilized thin TLD‐100 chips with the scattering medium alternately present and absent. Results indicate that the field‐size dependence of the backscatter factor for lower half‐value layers is less than that shown in theBritish Journal of Radiology Supplement 11.

ISSN:0094-2405    

DOI:10.1118/1.594312

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The computer‐simulation technique for imaging of vessels in angiography is being recognized as a useful tool for the study of the effect of physical parameters on the vessel image. We have examined the validity of this technique by comparing the computed image distribution directly with experimental results. The input x‐ray pattern of a blood vessel phantom, the LSF of the focal spot, and the LSF of a screen–film system were measured. From the data obtained, the final image distributions of the phantom were computed and compared with the experimentally determined distributions. A high‐precision device for the alignment and positioning of the vessel phantom was employed in the experiments. Results indicate that computer simulation can correctly predict the image distribution of the blood vessel phantom.

ISSN:0094-2405    

DOI:10.1118/1.594325

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In order to use negative pions for the treatment of large deep‐seated tumors in radiotherapy, it is necessary to produce depth‐dose distributions tailored to specific shapes. We present here a method of beam shaping which utilizes a fluid‐filled piston having a programmable, computer‐controlled, time‐dependent thickness. The fluid alters the residual range of the pions such that predetermined depth‐dose distributions can be obtained. Changing from one distribution to another can be accomplished simply and rapidly without access to the treatment room. Depth‐dose distributions which are flat over a range in depth up to 10 cm have been produced. Distributions tailored to produce flat “effective dose” versus depth have also been obtained.

ISSN:0094-2405    

DOI:10.1118/1.594313

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Cellular, animal, and human radiobiology studies are in progress at the Los Alamos Meson Physics Facility as part of a joint University of New Mexico and Los Alamos Scientific Laboratory pion therapy project. To support these activities, dosimetry has been performed on many different pion beam configurations. The effect of both static and dynamic momentum spreaders and of collimators on beam profiles, depth‐dose distributions, and peak‐to‐plateau ratios have been studied. The absorbed dose is obtained by the application of Bragg–Gray cavity theory to ionization chamber measurements. Calculations have been made for the effectiveWvalues and average mass‐stopping‐power ratios needed for the Bragg–Gray equation. Kerma corrections are applied to transform the dose from the chamber wall to dose in muscle.

ISSN:0094-2405    

DOI:10.1118/1.594326

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The current method for determining absorbed dose from high‐energy photons is to use in water a Cobalt‐60 exposure‐calibrated ionization chamber andCλ. Recently questions have arisen about the correctness of currently recommendedCλvalues and whether or not the Cobalt‐60 buildup cap should be used for in‐water dose measurements. The effect of the buildup cap onCλwas obtained by measuring Farmer chamber response in water with and without the buildup cap. Results show no measurable change at Cobalt‐60 or 4 MV but an increase of 1%–2% above 20 MV. The effects of the wall materials and the displacement factors onCλare also considered. It is shown that at high energies errors of 3%–5% can be made in determining dose in water using commercially available Farmer or Farmer‐like chambers and currently recommendedCλ. The exact size of the error depends upon the materials and thicknesses of the wall and cap, the chamber cavity volume, and whether or not the buildup cap is used.

ISSN:0094-2405    

DOI:10.1118/1.594327

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The characteristics of wedged fields which affect their clinical use have been examined for the 10‐MV x‐ray beam from the Clinac‐18. The methods used for obtaining and analyzing the wedged‐field data are discussed. These characteristics have been examined in terms of (a) the wedge angle, (b) the variation of the wedge angle with field size, (c) the variation in the angle through which isodose curves between the approximate depths of 5 and 15 cm are turned relative to the central axis as a function of depth, and (d) the variation of wedge central‐axis transmission factor with field size. Analysis of the data pertinent to these categories is presented for the 15‐, 30‐, 45‐, and 60‐deg wedges. In addition, the effect of the 60‐deg wedge upon the position of maximum dose on the central axis was determined.

ISSN:0094-2405    

DOI:10.1118/1.594322

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The tissue‐maximum ratios for high‐energy betatron x rays, measured at target‐axis distances of 90, 110, and 130 cm, exhibit a strong dependence on the target‐axis distance in the buildup region as well as a significant dependence in the exponential region. This dependence and the field‐size dependence of the tissue‐maximum ratios at a fixed target‐axis distance are studied with the use of phantom and collimator scatter factors. The depth and field‐size dependence of these scatter factors are determined experimentally. The measurements indicate that the collimator scatter factor is dependent on both the collimator opening and the target‐axis distance. These components are quantitated separately and are associated primarily with high‐energy electron scatter from the collimating system. An equation involving these components of the collimator scatter factor is derived for the conversion of tissue‐maximum ratios from one target‐axis distance to another. With the use of this equation, the tissue‐maximum ratios measured for one target‐axis distance can be used to calculate within 1% those at other target‐axis distances.

ISSN:0094-2405    

DOI:10.1118/1.594328

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Traditional methods for correcting for the presence of tissue inhomogeneities may produce errors as great as 10% at points within or close to the inhomogeneity. A more accurate method is presented which employs tissue–air ratios raised to some power dependent on the relative electron densities of the inhomogeneities involved. Corrections may be made for points that lie within or below an inhomogeneity as well as for multiple inhomogeneities. Measurements were made in phantoms containing aluminum or cork inhomogeneities. Agreement between measured and predicted results was usually within 2%–3%.

ISSN:0094-2405    

DOI:10.1118/1.594329

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY