摘要:

Quantitative standards for protection against exposure to ionizing radiation were first formulated in the 1930s. Since that time, standards have been restated periodically in different radiation units and conceptual frameworks that reflect improved understanding of the biological effects of radiation interactions and their consequences for human health. In the 1970s the expression of protection standards shifted from a dose‐ to a risk‐based approach, with dose limits established to yield risks to radiation workers comparable with those for workers in other “safe” industries. Over the years, radiation protection standards have exhibited a downward trend to more rigorous limits that require increased commitments of personnel and resources for their enforcement. There are several reasons for this trend, including increased recognition of the long‐term health effects of radiation, improved protective measures that permit radiation use at lower levels of exposure, growing numbers of persons exposed occupationally to radiation, and probably a greater intolerance to involuntary risks in society, with radiation targeted as a highly visible source of involuntary risks in the form of nuclear power plants and radioactive waste sites. In the past few years, reports of the Radiation Effects Research Foundation, United Nations Scientific Committee on the Effects of Atomic Radiation, and the National Research Council of the U.S. National Academy of Sciences have presented increased risk estimates for radiation exposure as a consequence of ongoing epidemiological analyses of human populations exposed to ionizing radiation. These risk estimates have enhanced public concern about radiation exposure and set the stage for discussions about the desirability of further reductions in exposure standards for radiation workers and members of the public. Such reductions would directly affect the professional activities, educational responsibilities, and administrative burdens of most medical and health physicists. These persons should understand the process of deriving risk estimates from epidemiological data, the factors that influence the risk estimates, how risk estimates are integrated into radiation protection standards, and the possible impact of more rigorous standards on the use of radiation in medicine.

ISSN:0269-3127    

DOI:10.1118/1.597153

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The breakdown of Molière's multiple‐scattering theory for short pathlengths occurring during Monte Carlo simulations with charged particles is demonstrated. It has been found that in certain conditions where the theory is assumed to be valid, significant distortions of the angular distribution occur that make the sampling of the polar angle questionable in numerous steps of Monte Carlo simulations. The limits of the theory have been investigated, both using a large number of terms in the Molière's series and using steps of Molière's theory where 1/Bexpansions are not involved. AtB=4.5 the commonly accepted 3‐term series expansion yields differences up to ±6% compared with the evaluation of the complete Molière angular distribution, and up to 7 terms in the series are needed in order to achieve agreement within ±2%. One percent agreement requiresB=10. Numerical values of the full distribution are given in terms of Molière's parametersBand reduced angle ϑ. By using the general dependence of the distribution results are valid for both electron and proton Monte Carlo simulations in any material.

ISSN:0269-3127    

DOI:10.1118/1.596982

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The distribution of absolute dose per unit fluence from polyenergetic photon beams impinging upon a water phantom was calculated using two convolution approaches that properly account for beam hardening effects. Dose deposition kernels calculated previously using theegs4Monte Carlo code are convolved with the primary terma to give the dose for monoenergetic photon beams of energies ranging from 100 kev to 50 MeV. A polyenergetic dose distribution is composed of separately calculated monoenergetic components, which are appropriately weighted with the fluence spectrum to yield the polyenergetic dose distribution. Alternatively, a single convolution for the polyenergetic beam is considered, where a composite polyenergetic kernel is convolved with the respective polyenergetic terma. The effects of the polyenergetic kernel variance due to beam hardening as well as the effect of tilting the kernels for a diverging beam geometry were also examined. The depth dose data produced using the two proposed methods were compared with measured data and Monte Carlo simulations and showed good agreement.

ISSN:0269-3127    

DOI:10.1118/1.597154

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In order to specify arbitrarily shaped beam apertures for three‐dimensional radiation treatment planning, aperture contours (or outlines) are often manually drawn using a beam's eye view display of the target volume and nearby normal structures. This can be a very time consuming process, and can be impractical for multileaf collimation and computer‐aided optimization of a large number of fields. A method has been developed that allows automatic generation of aperture shapes that outline the target volume and may spare neighboring structures whenever desired. Margins of user‐specified sizes (positive or negative) around the target and normal structures are also incorporated. For a chosen beam orientation, a 3D surface of each anatomic structure of interest is formed and projected onto a plane at the beam's isocenter. The outlines of each projected object are detected by an edge following algorithm, and margins are added. The outlines of normal structures are combined with that of the target volume to obtain the final aperture shape. This is done by overlaying filled versions of the outlines in such a way that regions of the target overlapped by normal structures are cut away, leaving only the target volume region to be irradiated. The remaining target volume outline is again detected to produce an aperture contour. Normal structures may split the aperture into several pieces, so this method detects any number of disjoint aperture contours. The results of the algorithm are illustrated with apertures generated for nasopharynx and prostate tumors, including sparing of normal tissues.

ISSN:0269-3127    

DOI:10.1118/1.596983

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This paper describes the errors in rebinning photon dose point spread functions and pencil beam kernels (PBKs) from cylindrical to Cartesian coordinates. An area overlap method, which assumes that the fractional energy deposited per unit volume remains constant within cylindrical voxels, provides large deviations (up to 20%) in rebinned Cartesian voxels while conserving the total energy. A modified area overlap method is presented that allows the fractional energy deposited per unit volume within cylindrical voxels to vary according to an interpolating function. This method rebins the kernels accurately in each Cartesian voxel while conserving the total energy. The dose distributions were computed for a partially blocked beam of uniform fluence using the Cartesian coordinate kernel and the kernels rebinned by both methods. The kernel rebinned by the modified area overlap method provided errors less than 1.7%, while the kernel rebinned by the area overlap method gave errors up to 4.4%.

ISSN:0269-3127    

DOI:10.1118/1.596984

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A procedure of separating the primary‐ and scatter‐dose components in therapeutic photon beams is examined. It is based on the observation that the scatter‐dose component is proportional to the variablez=rd/(r+d), whereranddare beam radius and depth in phantom, respectively. It is, therefore, possible to express an absorbed dose in the form of a linear equationD(r)=P+Nz, where at a fixed depthd, both primary dosePand coefficientNare constant. A method of linear extrapolation of an absorbed doseD(r) to “zero‐field” size, i.e.,r=0, is utilized. Since Monte Carlo technique is capable of scoring separately the primary‐ and scatter‐dose components, it is used to evaluate the accuracy of the linear extrapolation method within the range of60Co ‐ 15‐MV nominal photon energies. The results demonstrate that this method is sufficiently accurate to obtain the primary dose component in photon beams. For60Co γ radiation in water, tabulated sets of measured depth‐dose data are analyzed by the linear extrapolation method to review “zero‐field” dose values [percentage depth dose (PDD) and tissue–air ratio (TAR) tables of theBritishJournalofRadiology, Suppl. 17]. The “zero‐field” PDD data are found to be accurate within limits of experimental uncertainties. Inconsistencies in the TAR table are illustrated and discussed.60Co tables of relative doses,D(r,d)/P(dmax), including “zero‐field” values for both fixed SSD and isocentric geometries, are generated. Dose calculation in irregular fields is considered. The linear extrapolation method is recommended as a standard procedure for separating primary dose from depth‐dose data in high‐energy photon beams.

ISSN:0269-3127    

DOI:10.1118/1.597155

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A one‐dimensional model is described for illustrating the perturbing effects of mixed photon/electron fields on planar dose‐versus‐depth profiles in lowZmaterials (Z≤20), and at moderate photon energies (0.3–10 MeV). An electron field may arise from photon interactions in filters upstream of a dose‐versus‐depth experiment. The effects of upstream mass, scattering of electrons out of a gap between a filter and the experiment, and differences between the incident electron energy distribution and locally produced KERMA are modeled and compared to Discrete Ordinates calculations. At transitional depths in a dose‐versus‐depth experiment, the distortions can be significant and obscure the profile of the photon component alone.

ISSN:0269-3127    

DOI:10.1118/1.597100

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.597101

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.597102

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.597103

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY