摘要:

When x rays were discovered by Wilhelm Roentgen in November, 1895, the news spread rapidly through Europe, Great Britain, and the United States and many individuals became involved in their development. Some of the more prominent participants shared the name of Thompson or Thomson, which causes confusion when the history of x rays is discussed because of their similar pronunciation. In Britain they were William Thompson (Lord Kelvin), J. J. Thomson, and Silvanus P. Thompson and in the United States it was Elihu Thomson. In addition, one of the first books on x rays published in the United States was written by Edward Thompson.

ISSN:0269-3127    

DOI:10.1118/1.597068

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

For megavoltage radiotherapy photon beams, EGS4 Monte Carlo calculations show, and experimental measurements confirm with an accuracy of 0.2%, that glass or quartz‐walled vials used in Fricke dosimetry increase the dose in the Fricke solution. This is mainly caused by increased electron scattering from the glass which increases the dose to the Fricke solution. The dose perturbation is shown to vary from nothing in a60Co beam up to 2% in a 24‐MV beam. For plastic vials of similar shapes, calculations demonstrate that the effect is in the opposite direction and even at high energies it is much less (0.2% to 0.5%).

ISSN:0269-3127    

DOI:10.1118/1.597128

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

To follow up on the theoretical comparison of the IAEA 1987 and AAPM 1983 protocols for dosimetry calibration of high‐energy photons and electrons [Med Phys.18, 26–35 (1991)], results of a set of dosimetric measurements made with a Farmer type PTW and Capintec ionization chambers in solid water, PMMA, and polystyrene phantoms and exposed to a 4 MV photon beam from a Varian Clinac 4S at Yale, a 10 MV photon beam and 6 and 15 MeV electron beams from a Varian Clinac 1800 at Phelps Radiation Center, University of Connecticut, and a 25 MV photon beam from a Sagittaire at Yale, are presented. Because different methods are used for the determination of electron beam energies, the values of mean electron energy determined by the two protocols are different by up to 8%. However, for dose intercomparison, the overall agreement between the two protocols is within 1% in most cases, with a maximum discrepancy of 3.3% in one case. For photons, the IAEA results are smaller than the AAPM results by 0.7% on the average, while maximum discrepancies are in the range of −0.4%–−1%. In the case of 15 MeV electrons, the discrepancies between the two protocols are found to be in the range of −0.1%–1% and have an average value of 0.5%. In contrast to the above, a large discrepancy is observed between the two protocols for 6 MeV electrons. Depending upon the choice of phantom and ion chamber, this discrepancy is found to be in the range of −0.1%–−3.3%. A largest discrepancy of −3.3% is observed when a Capintec chamber is used in a polystyrene phantom. A most likely cause for this discrepancy is in the values of fluence correction factors used in the two protocols. Until better values for these parameters are available, a Farmer type chamber should not be used in a polystyrene phantom for the calibration of low‐energy electrons (6 MeV). In contrast to the theoretical intercomparison reported earlier, the present results of dosimetry measurements show improved agreement (by up to 1.4%) between the two protocols for electron beams, and about the same level of overall agreement (within 0.5%) for photon beams. The standard deviation of dose calibration for electron beams determined by the two protocols using different chambers and phantoms are about the same, approximately equal to ±1.5% for both protocols. It is recommended that either the AAPM should revise and update its 1983 protocol or adopt the international protocol produced by the IAEA. The IAEA protocol [Technical Report Series No. 277 (IAEA, Vienna, 1987), pp. 1–98]has fewer erroneous equations, uses more recent interaction data, and has an international appeal.

ISSN:0269-3127    

DOI:10.1118/1.597138

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The use of optical radiation in medical physics is important in several fields for both treatment and diagnosis. In all cases an analytic and computable model of the propagation of radiation in tissue is essential for a meaningful interpretation of the procedures. A finite element method (FEM) for deriving photon density inside an object, and photon flux at its boundary, assuming that the photon transport model is the diffusion approximation to the radiative transfer equation, is introduced herein. Results from the model for a particular case are given: the calculation of the boundary flux as a function of time resulting from a δ‐function input to a two‐dimensional circle (equivalent to a line source in an infinite cylinder) with homogeneous scattering and absorption properties. This models the temporal point spread function of interest in near infrared spectroscopy and imaging. The convergence of the FEM results are demonstrated, as the resolution of the mesh is increased, to the analytical expression for the Green's function for this system. The diffusion approximation is very commonly adopted as appropriate for cases which are scattering dominated, i.e., where μs≫μa, and results from other workers have compared it to alternative models. In this article a high degree of agreement with a Monte Carlo method is demonstrated. The principle advantage of the FE method is its speed. It is in all ways as flexible as Monte Carlo methods and in addition can produce photon density everywhere, as well as flux on the boundary. One disadvantage is that there is no means of deriving individual photon histories.

ISSN:0269-3127    

DOI:10.1118/1.597069

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A method for the calculation of three‐dimensional dose distributions for high‐energy photon beams is presented. The main features are (i) the calculation is fast enough to allow interactive three‐dimensional treatment planning, and (ii) irregularly shaped or compensated fields, which are required to fit three‐dimensional dose distributions to target volumes, are adequately taken into consideration. The method is based on the pencil beam convolution technique and shares its features concerning accuracy. A considerable gain in speed is achieved by decomposing the pencil beam kernel into three separated terms, thus reducing the required number of two‐dimensional convolutions. The convolutions are performed in the frequency domain via the fast Hartley transform. Using these techniques, the calculation time for the convolutions is only about 8 s on a DEC VAX station 3100. This is one‐fourth to one‐third of the calculation time for the ray tracing through the three‐dimensional CT data set, which has to be performed in any case. Results of the calculation are compared with measurements in a homogeneous phantom for 15 MV photons. Two irregular fields shaped with a multileaf collimator are considered. The deviations between measured and calculated absolute dose values are smaller than ±2%.

ISSN:0269-3127    

DOI:10.1118/1.597070

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Ion‐chamber responses are calculated for graphite, PMMA, and aluminum‐walled ion chambers free‐in‐air in60Co beams and 200‐keV beams for a graphite chamber. The EGS4 Monte Carlo system is used with various electron step‐size algorithms, in particular the PRESTA algorithm, the much simpler ESTEPE constraint on the energy loss per step, and in combination. Contrary to previous reports, it is found that there are variations in the calculated ion chamber response of up to 3% in60Co beams and up to 8% in the 200‐keV beam. It is recommended that all ion chamber calculations be done with the PRESTA algorithm plus an additional constraint on the energy losses per step of 1% (or less for higher‐Zmaterials). The systematic uncertainty in the calculations for60Co beams is found to be 1% (1σ) and somewhat higher in 200‐keV beams because of sensitivity to transport parameters such as AE and ECUT.

ISSN:0269-3127    

DOI:10.1118/1.597071

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A solid heterogeneous phantom made up of 25‐ and 50‐mm cubes of materials with different electron densities was used to verify the accuracy of a three‐dimensional (3‐D) dose calculation algorithm. This algorithm uses 3‐D information obtained from contiguous CT (computed tomography) slices, spaced 5 mm apart. Primary and scatter doses at a point are calculated by using information from ray‐tracing CT voxels. The algorithm was developed on a Stardent model 1500 Supergraphic workstation. Cubes of materials with different electron densities were stacked up to simulate finite heterogeneities in three dimensions. This design allows verification of the algorithm for surface contour corrections and finite heterogeneities in the treatment field. Thermoluminescent lithium fluoride chips were placed in grooves milled on the cubes for dose measurement at various points. Different experiments were performed to investigate both the accuracy of the dose calculation algorithm and the utility of the versatile test phantom.

ISSN:0269-3127    

DOI:10.1118/1.597072

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The calibration of parallel‐plate chambers for absolute dosimetry is an unsettled matter. The medical physics community has not yet agreed on a practical method of obtainingNgas, although several researchers are working on this problem. If the photon and electron fluence perturbation factors,KwallKcomp, were known for chambers of standard construction with full buildup provision, then an in‐air Co‐60 calibration could be applied to these, as is done with cylindrical chambers. We have obtained such correction factors for five commercially available chambers based on measurements in air and in homogeneous phantoms relative to matched cylindrical chambers of known dosimetric parameters. For three of the chambers (Markus, Holt and Exradin) we find thatKwallKcomp=1.000±0.008, in excellent agreement with available results from Monte Carlo calculations. The values for the other two chambers (NACP and Capintec) are different than 1. Our results are compared to recently published values, both calculated and measured.

ISSN:0269-3127    

DOI:10.1118/1.597073

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An equation for a recombination correction factor for a pulsed swept beam of electrons was derived by Boag. This equation is based on an integration technique, which assumes that a large number of spot beams cover the radiation field, that the field size is much larger than the spot beam size, and that the spot beam size is much larger than the chamber size. However, for computer‐controlled pulsed swept beams of electrons, the spot beam pattern can be altered, may not cover all of the field area, and the locations are reproducible. In this report, a summation method is proposed for this type of beam. Calculations for two such beams are demonstrated with the chamber located in the center of the field, and with the chamber half‐way to and at the edge of the field for a linear accelerator. The results lie between the integration pulsed swept and pulsed beam curves. Moving the ionization chamber from the center toward the edge of the field produces curves closer to the pulsed swept beam curve. Increasing spot beam size produces curves closer to the pulsed beam curves. It is therefore concluded that the pulsed swept beam cannot be characterized by a single recombination correction factor curve. The actual curve will be bounded by the integration pulsed swept and pulsed beam curves.

ISSN:0269-3127    

DOI:10.1118/1.597139

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Using high‐energy photon and electron beams, the response of two solid water parallel‐plate chambers is evaluated against a PTW (Physicallsch‐Technische Werkstatten) Farmer‐type chamber. These two chambers are (1) a Memorial “Holt” chamber which was further modified by this author to take advantage of electrically conductive solid water, and (2) a thin window parallel‐plate chamber designed by Attix. The evaluations are made for (1) stability and reproducibility, (2) polarity effect, (3) ion recombination correction, (4) field size dependence on output, (5) dose rate dependence on output, and (6) absorbed dose comparison in high‐energy photon and electron beams using the American Association of Physicists in Medicine Task Group 21 (TG21) protocol.

ISSN:0269-3127    

DOI:10.1118/1.597074

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY