摘要:

It is well known that the dose calibrator response/unit exposure rate depends significantly upon source energy. However, investigation of137Cs,192Ir, and226Ra brachytherapy sources by empirical, analytical, and Monte Carlo techniques shows that source filtration significantly affects the calibrator reading to exposure rate conversion factor. The results demonstrate that for each clinically used filtration thickness an exposure calibrated standard source is required to establish the response of the well chamber. An interesting consequence of this analysis is that the Sievert point dose algorithm for clinical sources overestimates the dose on the order of 3% at distances of approximately 3.5 cm from the source.

ISSN:0094-2405    

DOI:10.1118/1.595234

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Fermi–Eyges multiple scattering theory for electrons is explained, and a general three‐dimensional formalism is developed for its application to problems of therapeutic electron dosimetry. The formalism is illustrated by a number of elementary examples: a rectangular beam, an isotropic point source, and a scanning line source.

ISSN:0094-2405    

DOI:10.1118/1.595235

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The calculation of activity yields from practical photonuclear target systems designed to produce short‐lived positron emitting radionuclides for nuclear medicine purposes requires certain basic information. These include a knowledge of the photon source (bremsstrahlung energy spectrum and intensity as a function of angle from the electron beam) and the γ,nactivation cross section of the secondary target element. A lack of adequate information concerning these parameters motivated the present study in which activity yields for the reactions12C(γ, n)11C,14N(γ, n)13N, and16O(γ, n)15O were measured as a function of energy of and angle from the electron beam between 16 and 30 MeV and 0° and 30.5°, respectively. The data indicate highly complex relationships between the activity yield and the experimental variables. Also indicated are possible applications of the data to indicate the energy of an electron beam producing a given bremsstrahlung field in which activation measurements are made.

ISSN:0094-2405    

DOI:10.1118/1.595295

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Iodine‐125 implants are being used to boost external beam treatments of unresectable pancreas and lung tumors. Calculations of the125I activity required to achieve a specific average peripheral dose are presented as a function of the average tumor dimension for spherical, ellipsoidal, and cylindrical implants. Both uniform and random seed spacings are investigated. The results indicate that the average peripheral dose is relatively insensitive to the seed distribution as well as to the seed activity. The average tumor dose is typically 20% greater than the average peripheral dose.

ISSN:0094-2405    

DOI:10.1118/1.595236

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A technique for rotational total skin electron irradiation is presented in which the patient stands on a slowly rotating platform (SSD=285 cm) in a large uniform linear accelerator electron field (E0=3.5 MeV). The beam is scattered by the transmission ionization chamber and by a special lead/aluminum scattering filter, and then degraded by a sheet of Lucite. A Farmer chamber is used as a patient dose monitor and a method for absolute dose calibration is presented. The field is uniform to within ±5% for dimensions of 180×40 cm2. The surface dose for rotational therapy is equal to 45% of the maximum dose in a stationary beam. The rotating beam exhibits a dose maximum on the surface, falls to 80% at 0.5 cm and has an x‐ray contamination of ∼4%. The surface dose rate is about 25 cGy/min for the rotating beam. The rotational beam percentage depth dose distributions, calculated using stationary beam information, agree well with measured data. The stationary beam exhibits a dose maximum at 4 mm in tissue, a surface dose of 93%, 80% dose at a depth of 1 cm, a practical range of 1.75 cm, and an x‐ray contamination of 2.5%. The rotational total skin electron irradiation significantly reduces the patient treatment and setup time and solves the problem of beam matching, when compared to standard multiple‐beam techniques.

ISSN:0094-2405    

DOI:10.1118/1.595296

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Misregistration of the projections in 360° computed tomographic (CT) scanners has been found to blur the image without generating artifacts. The effects of this error were investigated by analytical methods and by reconstruction of real and simulated data. The point‐spread function which results from shifting each projection by a constant distance ε consists of a two‐dimensional impulse function surrounding a region of negative density. The locus of the impulse function is a circle for parallel‐beam geometry and a sixth‐order curve for fanbeam geometry. The anisotropy and position dependence of the point‐spread function in fanbeam geometry have been characterized. The line‐spread function due to the error in parallel‐beam geometry consists of two delta functions located at ±ε. In fanbeam geometry, the line‐spread function consists of two delta functions separated by approximately 2ε, with the locations of the impulses dependent on the position and orientation of the line. This error, combined with other sources of blurring, results in a system edge‐response function which contains a flat region at one‐half the maximum density.

ISSN:0094-2405    

DOI:10.1118/1.595237

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Using a Monte Carlo computer technique, tables of parameters have been derived which are of use in the dosimetry of brachytherapy nuclides in a variety of tissues and organs. From the results, it has been possible to derive relationships linking the composition of a tissue with the dose received at any location around an implanted source. Other factors, such as the relative importance of scattered radiation, spectral degradation, and integral dose, are also discussed. In particular, attention is drawn to possible dosimetric problems arising from the clinical use of125I.

ISSN:0094-2405    

DOI:10.1118/1.595297

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The treatment of decompression sickness often involves both recompressing the victim and administering hyperbaric oxygen in the hope of more rapidly dissolving the bubbles which cause this malady. Although many hundreds of such treatments are conducted each year in the United States alone, the underlying physical principles governing the dissolution of such bubbles are not well understood and only empirically tested. In this paper, we present a mathematical theory of bubble dissolution that is verified by comparison with laboratory experiments. This theory suggests that the commonly employed treatment techniques would be only marginally effective, and that in many situations the bubbles that cause the disease cannot be adequately dissolved using existing techniques and facilities.

ISSN:0094-2405    

DOI:10.1118/1.595291

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A new analytical expression is presented to describe the full saturation curve of parallel‐plate ionization chambers. In contrast to the presently known expressions, which hold only for the near saturation region, this empirically determined expression is in excellent agreement with measurements in the whole collection efficiency range from 0 to 1 for x‐ray sources with effective energies from 20 to 150 keV and cobalt‐60 gamma rays. The dependence of the ion collection efficiency and the extrapolated electric field, which is a parameter in the new saturation curve expression, on electric field, dose, dose rate, beam quality, and chamber volume, is discussed. The effect of photoemission from the chamber polarizing electrode for low‐energy x‐ray beams on the saturation current is demonstrated. A universal ionization chamber constant is derived experimentally. It is shown that all parameters of the saturation curve equation and thus the saturation curve itself, can be calculated from one single measurement of ionization current at a given electric field and air gap thickness.

ISSN:0094-2405    

DOI:10.1118/1.595292

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Monte Carlo estimates of eight specific absorbed fractions associated with an I‐125 point source in water are presented for distances ranging from 1–10 cm.

ISSN:0094-2405    

DOI:10.1118/1.595293

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY