摘要:

The problem of neutrons produced by many of the high‐energy x‐ray therapy machines (10 MV and above) is reviewed, and the possible risk their presence poses to radiotherapy patients is estimated. A review of the regulatory background containing a summary of the recommendations of the U. S. Council of State Governments (USCSG), and of the International Electro‐Technical Commission (IEC), as well as an indication that recommendations will be forthcoming from the National Council on Radiation Protection (NCRP) and the International Commission of Radiological Protection (ICRP) is presented. The neutrons in question are produced by high‐energy photons (x rays) incident on the various materials of the target, flattening filter, collimators, and other essential components of the equipment. The neutron yield (per treatment dose) increases rapidly as the megavoltage is increased from 10 to 20 MV, but remains approximately constant above this. Measurements and calculations of the quantity, quality, and spatial distribution of these neutrons and their concomitant dose are summarized. Values of the neutron dose are presented as entrance dose, midline dose (10‐cm depth), and integral dose, both within and outside of the treatment volume. These values are much less than the unavoidable photon doses which are largely responsible for treatment side effects. For typical equipment, the average neutron integral dose from accelerator‐produced neutrons is about 4–7 g cGy (per treatment cGy), depending on the treatment plan. This translates into an average dose of neutrons [averaged over the body of a typical 70‐kg (154 1b) patient] of 0.06–0.10 cGy for a treatment of 1000 cGy. Using these neutron doses and the best available neutron risk coefficients, it is estimated that 50×10−6fatal malignancies per year due to the neutrons may follow a typical treatment course of 5000 rads of 25‐MV x rays. This is only about 1/60th of the average incidence of malignancies for the general population. Thus, the cancer risk to the radiotherapy patient from accelerator‐produced neutrons poses an additional risk to the patient that is negligible in comparison.

ISSN:0269-3127    

DOI:10.1118/1.595497

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Analytical techniques have been developed to generate electron beam isodose distributions from a modest set of experimental data. These techniques include (1) development of a compact yet powerful linear least‐squares optimization program with a graphic display of the results, (2) coordinate transformations to allow straightforward fitting of the isodose distributions with simple polynomial functions, and (3) accurate interpolation methods to allow production of treatment planning dose matrices. The program, written infortran, has been implemented on a minicomputer and uses the standard peripherals of a commercially available treatment planning system (AECL TP‐11). The computer‐generated isodose and central‐axis distributions correspond very closely with the data for electron energies from 5 to 20 MeV and for all field sizes.

ISSN:0269-3127    

DOI:10.1118/1.595498

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Experimental techniques have been developed for obtaining microdosimetric spectra on a hospital‐based linear accelerator. Teletherapy beams of 10 and 15 MeV bremsstrahlung x rays from a Varian Clinac‐18 and Clinac‐20, respectively, have been produced at ultralow dose rates (50–200 μGy/h) which enables direct measurements of lineal energy distributions with a conventional Rossi‐type gas proportional counter. Extensive measurements have been made to insure that the dosimetric properties of these low dose rate beams are nearly identical to those produced under high dose rate clinical conditions. Analytical procedures have been developed to correct measured lineal energy spectra for pileup caused by the low duty factor of the linear accelerator. The lineal energy spectra of these megavoltage beams differ significantly from Co‐60, with dose averaged lineal energies (ȳD) being 20%–30% lower than for Co‐60. Although such differences may not be important at clinical doses, the theory of dual radiation action does predict a lower biological effectiveness for these beams at very low dose levels.

ISSN:0269-3127    

DOI:10.1118/1.595499

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The commonly used Blunck and Leisegang electron energy‐loss distribution falls off too rapidly with increasing energy loss. Also, for large thicknesses and/or low‐Zmedia, where their distribution should approach Landau's, it normalizes to 0.92 rather than 1.0, it overestimates the number of very small energy‐loss events, and its peak is shifted from λ=−0.225 to 0.1. Because of these shortcomings, calculations made using this distribution yield a mean straggled energy loss which is lower than the value predicted by the continuous slowing down approximation (CSDA). An improved version of the Blunck–Leisegang distribution, which exhibits better normalization and falloff, has been developed. Further, an algorithm was created which (depending on the CSDA energy loss,Z,A, electron energy, and transport step size) samples the electron's straggled energy loss from the more accurate of the available distribution functions.

ISSN:0269-3127    

DOI:10.1118/1.595500

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In digital subtraction angiography, hybrid subtraction provides selective vessel images free of soft‐tissue motion artifacts but with a lower signal‐to‐noise ratio (SNR) than temporal subtraction images. An image processing method called measurement‐dependent filtering has been developed to enhance the SNR of hybrid images without losing resolution or selectivity. Linear combinations of four images consisting of a pre‐ and postcontrast dual‐energy measurement pair form both the hybrid image and a lower noise but less selective vessel image. The noise‐reduced image is derived by combining the low‐frequency components of the hybrid image with the high‐frequency components of the lower noise image in a variety of ways. The results of the filtering method, when tested on both phantom and clinical data, display images with about the same degree of conspicuity as the hybrid image and a SNR approaching that of the temporal image.

ISSN:0269-3127    

DOI:10.1118/1.595501

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Heavy‐ion beams used in biomedical studies suffer a substantial amount of nuclear reactions (fragmentation) as they traverse matter. Since it has been demonstrated that dose and linear energy transfer (LET) are not a sufficient description of a beam for the purpose of understanding its biological effects, it is necessary to be able to separate the components of a complex beam so that their individual effects can be analyzed. A simple and small assembly consisting of a thin silicon LET detector, in time coincidence with a thick germanium residual energy detector has been used in measurements of the components of Ne‐20 and Si‐28 high‐energy ion beams. The detector system can be placed at any experimental area without difficulty and it can carry out a beam analysis in a few minutes, making it very appropriate for fast on‐line measurements and verification of beam characteristics. LET values measured by the silicon detector agree well with results of the Bethe stopping‐power calculations, and the dose measured for the beam components can be used to obtain Bragg curves that are in good agreement with those obtained by ionization chamber measurements on the same beams. The numbers and LET distribution of primaries and fragments at different positions of the Bragg curves, as well as fractional dose contributed by the different components are determined directly from the experimental data. Particle velocity distributions can be obtained for the higherZfragments. Limitations and advantages of the simple measurement technique are discussed.

ISSN:0269-3127    

DOI:10.1118/1.595502

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

We have reexamined the Batho power law for computing the dose within and beyond lung irradiated with small and large fields of cobalt‐60 and 6‐MV x rays. Using slab phantoms consisting of two materials, agreement between calculated and measured doses was within 2% inside lung for 6‐MV x irradiation, but much poorer (9%) for cobalt‐60 irradiation. For cobalt‐60 irradiation, tissue–air ratios (TARs) were used initially in the Batho equation, while for 6‐MV x rays, tissue–maximum ratios (TMRs) were used. When we substituted TMR values instead of TAR values for cobalt‐60, we found marked improvement by nearly 5% in the accuracy of dose calculated within lung. This was confirmed by numerical comparison of the Batho expression with an analytic solution of the primary and first‐scattered radiation. We therefore encourage the use of TMRs for cobalt‐60 radiation, expecially for larger radiation fields, and provide measured data tables for field sizes up to 50×50 cm2, and depths up to 30 cm. In addition to unifying the dosimetry for all megavoltage irradiation, this approach improves the accuracy of doses calculated within lung.

ISSN:0269-3127    

DOI:10.1118/1.595495

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The effect of various digital parameters, such as the sampling aperture, sampling distance, and display aperture, on the modulation transfer function (MTF) of digital radiographic imaging systems was investigated by means of theoretical simulation studies. The MTFs were also determined experimentally to confirm the relationship used in the simulation studies. The results indicate that the overall MTF of a digital system cannot specify the resolution properties in the same way as can the MTFs of analog systems. The MTF of a digital system may include a “false” response due to aliasing, which could lead to an incorrect interpretation of the resolution properties. The magnitude of aliasing that will occur in a digitized signal depends on the sampling parameters chosen and on the frequency content of the radiologic object being imaged. Thus, the type of object to be detected as well as various digital parameters must be considered in the design and evaluation of digital imaging systems.

ISSN:0269-3127    

DOI:10.1118/1.595629

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Equations are derived and the results of numerical calculations shown that illustrate the effect of counting system deadtime on measured thyroid uptake of radioiodine. It is predicted that the observed uptake is higher than the true uptake due to system deadtime. This is shown for both paralyzing and nonparalyzing deadtime. The effect of increasing the administered activity is shown to increase the measured uptake, in a manner predicted by the paralyzable and nonparalyzable deadtime models.

ISSN:0269-3127    

DOI:10.1118/1.595496

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Measurements were made to calibrate a Philips RT‐50 contact therapy unit using an EG&G extrapolation chamber. The measured half‐value layer of 0.27 mm in aluminum agrees closely with published data. Exposure rates in air for 4‐cm source‐to‐surface distance (SSD) cones fall off faster than can be explained by the inverse square law alone. Lining the cone with lead results in exposure rates that follow the inverse square law at all distances from the cone end. Depth dose measurements made for 2‐ and 4‐cm SSD cones produced the anomalous result of greater depth dose for the shorter SSD. This is due to a low‐energy component arising from the end of the 4‐cm SSD cone, producing an artificially high surface dose. Lining the cone with lead reduces the component and significantly increases the depth dose. The low‐energy component is believed to be 6–8 keV characteristic x rays that arise from the stainless‐steel cone. Lining the cone with lead either absorbs these x rays or prevents their production.

ISSN:0269-3127    

DOI:10.1118/1.595503

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY