摘要:

The effect of detector size to the broadening of the measured beam penumbra has been a subject of numerous studies. Based on measured data, linear and quadratic curves have been proposed to describe the relationship between the measured penumbra width, between 10%–90% and 20%–80% intensities, and the detector size. Extrapolations of these curves to zero detector size also suggest that the inherent penumbras can be deduced. However, due to experimental noise, especially when a small ionization chamber is used, and the inherent penumbra is not known, it is difficult to discern the superiority of either model. In this study, one‐dimensional convolution using a thin circular disk shape detector was employed to analyze the effect of detector size to the broadening of the penumbra. A set of beams with different inherent penumbra widths, ranging from 0 to 350 in arbitrary unit, was first generated. Each beam was then convoluted with the response function of the detectors with different sizes from 10 to 280, in arbitrary unit. The result is an output signal with penumbra that is wider than the inherent penumbra. The plots of penumbra widths to detector radii are a family of concave parabolic curves with different inherent penumbra widths. The concave portions of the curve represent results from scanning with detectors equal to or smaller than the penumbra width, and the linear portions represent results from scanning with detectors much larger than the penumbra. The curves are nothing but different scales of one curve. The extrapolations of the curves to the penumbra axis when the detector radius approaches zero give the deduced inherent penumbra widths. The deduced inherent penumbra widths approximate the inherent penumbra widths satisfactorily. From the graphs provided, the inherent penumbra can be deduced using the detector radius and the measured penumbra width.

ISSN:0269-3127    

DOI:10.1118/1.597724

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Clinical applications of electrons may involve oblique incidence of beams, and although dose variations for angles up to 60° from normal incidence are well documented, no results are available for highly oblique beams. Surface dose measurements in highly oblique beams were made using parallel‐plate ion chambers and both standard LiF:Mg, Ti, and carbon‐loaded LiF Thermoluminescent Dosimeters (TLD). Obliquity factors (OBF) or surface dose at an oblique angle divided by the surface dose at perpendicular incidence, were obtained for electron energies between 4 and 20 MeV. Measurements were performed on a flat solid water phantom without a collimator at 100 cm SSD. Comparisons were also made to collimated beams. The OBFs of surface doses plotted against the angle of incidence increased to a maximum dose followed by a rapid dropoff in dose. The increase in OBF was more rapid for higher energies. The maximum OBF occurred at larger angles for higher‐energy beams and ranged from 73° for 4 MeV to 84° for 20 MeV. At the dose maximum, OBFs were between 130% and 160% of direct beam doses, yielding surface doses of up to 150% ofDmaxfor the 20 MeV beam. At 2 mm depth the dose ratio was found to increase initially with angle and then decrease asDmaxmoved closer to the surface. A higher maximum dose was measured at 2 mm depth than at the surface. A comparison of ion chamber types showed that a chamber with a small electrode spacing and large guard ring is required for oblique dose measurement. A semiempirical equation was used to model the dose increase at the surface with different energy electron beams.

ISSN:0269-3127    

DOI:10.1118/1.597873

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A new system of radiation dose mapping based on laser heating of thermoluminescent dosimetric plates (TLDP) has been developed. Application of this technique to intraoperative radiotherapy (IORT) has been investigated. Preliminary results show that TLDP are a possible alternative to radiological films and ionization chambers since they provide some advantages such as 2D dose measurements with digital storage, large dynamic dose range, and easy processing.

ISSN:0269-3127    

DOI:10.1118/1.597725

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Considerable effort is generally made in the quality control of radiation therapy units and in patient chart checking to ensure accurate delivery of treatment to the patients. Record and verify (RV) systems have been implemented which will inhibit the beam from being turnedONif the parameters set on the medical linear accelerators do not match a preprogrammed file. It should be emphasized, however, that RV systems are designed only to ensure proper setup of parameters on the therapy units and bear no direct link with proper patient position. The only real link between treatment planning and delivery to the patient isin‐vivodosimetry. Diodes provide a convenient way of measuring the dose received by the patient in real time and, after the initial tedious calibrations, require very little additional effort. In order to facilitate diode use by the therapists in our institution, the radiation management system (RMS) database in use with our linear accelerators was interfaced with the output obtained from a commercially available diode dosimetry system. The chart printout format was altered to read this value from the RMS database allowing a near real‐time recording of the actual dose received by the patient for up to three sites concurrently. This provides for an immediate additional check to the planned and received doses already recorded by the RMS RV software. This procedure, if carried out during the first treatment and subsequently on a weekly basis, would ensure that differences of the order of 5% or larger between planned and delivered dose would not go undetected.

ISSN:0269-3127    

DOI:10.1118/1.597744

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

To determine the shape of a radiation beam aperture a margin is typically applied to the clinical target volume (CTV) to yield the planning target volume (PTV), and the aperture is then determined from the projection of the PTV onto the aperture plane. This margin accounts for setup variability and organ motion originating from respiration or other physiologic processes. The use of either a uniform margin, or alternatively one which takes into account only the expected magnitude and direction of target motion, fails to account for the three‐dimensional nature of the target; such a method neglects the volumetric effect of target shape on the fractional target volume irradiated when the target shifts partially out of the aperture. A mathematical framework is developed to analyze and illustrate the consequences of irradiating an irregular target shape in the presence of target motion. The effect of target shape on volume coverage is demonstrated for selected cases involving conventional BEV aperture design techniques. The volumetric implications of target shape are considered from two complementary points of view. The first involves transformation into a “displacement space,” which isolates the volumetric effect of the shape of the target allowing it to be studied independently of the probability distribution of target motion. The second point of view combines the effects of the 3D target shape and the probability distribution of motion in a manner independent of beam direction to yield a 3D “target distribution.” The two points of view represent distinct starting points for computation of the expected value of fractional target volume coverage in the presence of target motion. In certain cases it may be beneficial to (1) employ “target distributions” for the target and normal tissues in place of the conventional static PTV and, (2) include the aperture shape, on equal footing with parameters such as beam weights and energies, into a quantitative optimization process explicitly accounting for uncertainties in the position of the target volume and critical structures.

ISSN:0269-3127    

DOI:10.1118/1.597874

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Construction has begun of prototype europium‐152 sources, with the principal goal of providing cost‐effective teletherapy for emergent nations where prices for cobalt‐60 are excessive, owing to replacement needs, and the use of linacs is economically prohibitive and technologically unfeasible.152Eu sources (T1/2=13.4yvsT1/2=5.26yfor60Co) are designed for absorbed dose rates ≳1.5 Gy/min at 1 m, with specific activities ≳5 TBq/gm, allowing substitution into existing irradiators. Costs of152Eu compare favorably with60Co, taking account of extended useful lifetime.

ISSN:0269-3127    

DOI:10.1118/1.597726

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The purpose of this study was to calculate photon absorbed fractions in tissue surrounding a radioactive source where both the source and the surrounding tissue are assumed to have cylindrical geometry. Specifically, we treated two cases: the case of a cylindrical source of homogeneous activity placed in air, and second, the case of a cylindrical source of homogeneous activity placed in a cylindrical nonradioactive absorbing material. In this study we offer an analytical solution to these problems followed by numerical integration. The computer program allowed for very general calculations, e.g., different tissues, different geometrical setups. Tables of absorbed fractions have been developed for commonly used radionuclide energies and tissue‐equivalent material. A comparison between our results and the results of other related studies showed the advantages and limitations of this approach.

ISSN:0269-3127    

DOI:10.1118/1.597727

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Computer algorithms for electron binding correction to Compton scattering and for detailed simulation of K‐edge characteristic x‐ray production were incorporated into EGS4 unix version 2.0. Based on detailed modelling of the internal structures of sources, the modified version was used to calculate dose rate constants, radial dose functions, and anisotropy functions on the long axis for an125I model 6711 source,169Yb Type 5 and Type 8 sources, and a stainless steel clad (SS)192Ir source. The geometry of these sources is cylindrically symmetric. Calculated results are generally in good agreement with corresponding values recommended by TG‐43 and Monte Carlo results published by other authors. The influence of electron binding in Compton scattering on the calculated dose distribution for an125I model 6711 source in water, and of different characteristic x‐ray production models for125I model 6711 and192Ir SS sources, were also studied.

ISSN:0269-3127    

DOI:10.1118/1.597728

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The objective of this study was to develop a computer‐aided diagnosis (CAD) scheme for automated detection of lung nodules in digital chest images in order to help radiologists to improve their performance in detecting lung nodules. Lung nodules in chest radiographs are the important sign of lung cancer which is the leading cause of cancer death in men and women in the United States, with the 5‐year survival rate only about 13%. Early detection and treatment are crucial for the improvement of this survival rate for patients with lung cancer. This dissertation included the following four topics: (1) accurate determination of ribcage boundary in digital chest radiographs; (2) detection of right and left hemidiaphragm edges and delineation of lung fields in digital chest radiographs; (3) development of a CAD scheme for automated detection of lung nodules in digital chest radiographs; and (4) ROC studies of effects of CAD output on radiologists' performance for detection of lung nodules on chest radiographs. Our CAD scheme for automated detection of lung nodules in digital chest images was based on the segmentation of lung fields, the difference image technique, image feature analysis, and an artificial neural network. The performance of the CAD scheme was evaluated by the FROC methodology. The CAD scheme achieved an improved performance of 70% sensitivity with 1.7 false positives per image, on average. The ROC analysis results indicated that radiologists increased their performance significantly in detecting lung nodules by use of the CAD output as a “second opinion.” Radiologists did not increase their reading time when the CAD output was used. [Thesis copies are available upon request from the author.]

ISSN:0269-3127    

DOI:10.1118/1.597875

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Efficient RIT depends onaprioriknowledge of the radiation absorbed dose distribution. Straightforward application of published table ofSvalues, which are for standard‐size homogeneous activity sources in a standard‐size body geometry, is not sufficient for accurate RIT dosimetry calculations. A new, fast, and functional treatment planning approach is developed that considers heterogeneous activity distributions. Calculation of the three‐dimensional absorbed dose distribution requires convolution of a cumulated activity distribution with a point‐source kernel; both represented by large matrices. To reduce the computation time required, a novel implementation of convolution using FHT is realized. Absorbed dose calculation time was reduced from 80 hours to 5 minutes. Volume measurement error has a direct impact on the absorbed dose calculation when mean activity information for an organ is used. Since identification of the tumor and other organs of interest is time‐consuming, a semi‐automated segmentation toolkit is developed. It is now possible to determine the volume of a structure in half the time with the same accuracy. The developed system was used to calculate and display as isodose contours the absorbed dose distribution in a patient's tumor and for a bone marrow sample. [For copies contact the author or University Microfilms, Inc., Ann Arbor, MI 48106 (Phone: 1‐800‐521‐0600).]

ISSN:0269-3127    

DOI:10.1118/1.597781

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY