摘要:

The effective atomic numbers for total photon interaction in bone, muscle, liver, spleen, fat, and water are determined and found to decrease up to 50% as the energy increases from 10 to 200 keV. Muscle, spleen, liver, and water are found to behave in an approximately similar manner, in this energy region, as far as photon interaction is concerned.

ISSN:0269-3127    

DOI:10.1118/1.595658

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Uneven air gaps, beam obliquity, or sloping contours create complex dosimetry problems in electron beam treatment planning. We find that the depth dose distribution in an obliquely incident beam can be calculated from the data for a normally incident beam by using inverse square law and obliquity factors. The obliquity factors have been determined for various energy beams in the range of 6–22 MeV and can be used for manual treatment planning.

ISSN:0269-3127    

DOI:10.1118/1.595659

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The perturbation of electron beam dose distributions in the vicinity of medium interfaces is calculated. Two different solutions to the Boltzmann equation, one an energy averaged solution and the other a diffusion approximation solution, are presented for the calculation of the electron distribution in the vicinity of interfaces. An extension of the energy averaged formalism is presented for media with high atomic number. A new effective depth approximation is introduced to calculate the beam conditions beneath the interface. The transformation from electron distribution to dose distribution is made and dose calculations are compared with published electron beam dose measurements.

ISSN:0269-3127    

DOI:10.1118/1.595660

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Contaminant radiation within a therapeutic beam has been studied for accelerator‐produced 24‐MV x rays by a direct measurement utilizing independent jaws. A carefully positioned diode was exposed to secondary radiation for various collimator settings that project field sizes between [10×(0+5)] and [26×(0+13)]cm at a source‐to‐axis distance of 1 m. [The notation [L×(W1+W2)] means the projected field area due to lower (L) and upper (W1,W2) jaws.] Measurements were taken by placing polystyrene sheets with density thicknesses ranging from 0.21 to 4.53 g cm−2in front of the detector. The data strongly demonstrate that with increasing field size, the ratio of the dose due to electrons to that due to photons increases rapidly. The characteristic feature of the electron depth dose curves may be linked to the observed shift in the depth of dose maximum (dmax) with field size. Data taken with a magnetic field of 0.15 T permit analysis of photons and electrons with regard to their intensity, energy, and behavior in a phantom. From an analysis of 12.5×12.5 cm data, various radiation components have been studied and are correlated with the observed contaminants.

ISSN:0269-3127    

DOI:10.1118/1.595661

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Two experimental methods are described for obtaining average energies of neutron spectra produced by an electron accelerator in a medical treatment room. In the first method, the dose equivalent is measured by means of indium activation foils in an Andersson–Braun moderator. Fluences are determined by the usual activation foil techniques. These data lead to the conversion factor, neutrons/(cm2 s) per mrem/h, which uniquely defines the effective energy of the unknown spectrum. In the second method, the fast neutron broad‐beam attenuation in polyethylene is obtained from measurement of the thermalized fluence versus depth curve, whose slope in the exponential region parallels that of the fast neutron spectrum. It was found for a 24‐MV linear accelerator that the effective energy varied from 0.28 MeV at the isocenter under closed collimators to 0.016–0.024 MeV in the entrance maze. These results compare favorably with other measurements and calculations. It is concluded that the dose equivalent can be measured to within ±50% with an activation foil in an Andersson–Braun moderator and that the characteristic energy obtained by attenuation at the isocenter closely approximates the average energy of the neutron spectrum escaping the treatment head.

ISSN:0269-3127    

DOI:10.1118/1.595662

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In single photon emission computed tomography (SPECT), Compton scattering produces a background that degrades the image quality and contributes erroneously to quantitative measurements. A clinically implementable compensation algorithm has previously been reported that subtracts a Compton scatter image, acquired in an energy window set below the energy of the photopeak, from the primary image acquired in the photopeak window. We present an evaluation and justification of the assumptions made in the previous empirical development of the subtraction algorithm. A Monte Carlo model of the SPECT system in which the Compton scattered events may be followed independently of the nonscattered events was used to evaluate this subtraction technique. Simulation shows that the assumptions made in the experimental application of this algorithm were valid. Specifically (1) the “scatter” energy window used in the experiment (91–125 keV for imaging Tc‐99m) contains only scattered events, (2) the shape of the line spread function (LSF) for the events in the scatter window is a reasonable approximation to the shape of the scatter in the photopeak window, and (3) the ratio of the number of scattered events in the photopeak window to the number of events in the scatter window is 0.57, close to the value of 0.5 derived heuristically. Thus, Monte Carlo simulation validates the basic assumptions underlying the empirical implementation of the scatter subtraction algorithm.

ISSN:0269-3127    

DOI:10.1118/1.595663

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Measurements of dose distribution for square fields with sizes ranging from 1×1 to 30×30 cm for a 9‐MV x‐ray beam from a Neptune 10 linear accelerator, manufactured by CGR, are reported. Special attention was paid to field sizes smaller than 4×4 cm, used in radiosurgery techniques. To express the dose‐monitor units relationship, total, collimator, and phantom scatter correction factors were obtained by experimental measurements. A strong dependence of these factors on the smallest field sizes (<4×4 cm) was shown. Measurements of the maximum depth dosedmax, plotted as a function of field size, showed a maximum at about 5×5 cm, in good agreement with previous results.dmaxwas also measured for the smallest fields, demonstrating that the contaminating electron component of the x‐ray beam was not responsible for thedmaxshift. Analysis of the penumbra width of cross dose distributions, as a function of field sizes, allowed us to postulate that thedmaxshift could be due to the phantom scattered photons, which in turn were generated by the collimator scattered photons. Newly derived tissue‐maximum ratio and scatter‐maximum ratio data were used for dose profile calculations of 2×2, 4×4, and 10×10 cm field sizes. The agreement between experimental and calculated data was found to be ±2% within the geometrical edges of the fields and ±6% outside of them. A dose profile from the isocenter of a 2×2 cm square field moving through a 360° rotation arc was obtained and compared with that from the center of a125I shielded source, as measured by Ling. Advantages and problems relating to the use of x‐ray beams from linear accelerators in radiosurgery are discussed.

ISSN:0269-3127    

DOI:10.1118/1.595664

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A “gradient correction factor” and an “electron fluence correction factor” are used in the AAPM Task Group 21 dosimetry protocol, the product of the two being the “replacement correction factor.” The separation of two physical processes, implied by the concepts, is theoretically unfounded. The proof of this statement rests on Fano's theorem, the conditions for which constitute a case without variations in fluence or its directional and energy distributions. Deviations from these conditions represent gradients in the radiation field, affect the response of a measurement cavity, and do so by perturbing the fluence of electrons that deposit energy in the cavity. Thus, in beams of both electrons and photons, corrections are required when fluence variations exist in the vicinity of the cavity. The rationale for an electron fluence correction based on “in‐scattering” and “obliquity” is weak, since the effects only occur when gradients are present. The choice of values for the two correction factors at the depth of maximum dose are particularly inconsistent with the actual characteristics of the radiation field.

ISSN:0269-3127    

DOI:10.1118/1.595665

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Polyacrylamide‐based tissue‐equivalent phantoms simulating cortical bone and muscle are described. The equivalency is based upon similar elemental composition and density, and partial similarity in the morphology of bone. Satisfactory results were obtained when the phantoms were tested at low (20 keV) and high (15 MeV) gamma radiation. Applicability of this phantom material to neutron transport is discussed. The material can be molded and shaped and its composition is easily modified by altering the proportions of the constituents. Trace elements or radionuclides are easily added. Details of the physical and radiation characteristics of the formulated systems are given together with the manufacturing procedures.

ISSN:0269-3127    

DOI:10.1118/1.595666

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Abutment of unmodified electron fields to irradiate large areas can lead to significant dose inhomogeneities in the region of junction of the fields. In this paper we describe the design and dosimetric characteristics of a device developed to broaden the electron beam penumbra and thereby to improve the dose uniformity in the overlap region. The device is a high‐density triangular‐toothed comb capable of reducing the beam intensity without seriously degrading the beam energy. The effect of the comb is such that a single device will generate a beam penumbra which is broad and very nearly linear at all depths for all clinically used beam energies. Results are shown for various field configurations and energies. With a gap of 5.0 cm between the treatment cone and phantom surface the dose “ripple” in the region beneath the teeth was found not to exceed ±5% at 0.5‐cm depth.

ISSN:0269-3127    

DOI:10.1118/1.595667

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY