摘要:

Beta‐ray dose backscatter factors with respect to soft tissue were measured using an extrapolation chamber. The beta‐ray dose backscatter factor is a measure of the change effected in absorbed dose to a soft‐tissue medium when part of the medium is replaced by a material other than soft tissue (i.e., a scatterer); the source is located at the boundary between the two media. The dependencies of backscatter factor on scatterer atomic number and on source geometry are investigated, and the variation of backscatter factor with distance from the boundary is determined. For a32P point source, backscatter factors with respect to Mylar, a soft‐tissue substitute, at 0.55 mg/cm2from the boundary, are, 29.65[0.12]%, 31.07[0.24]%, 19.30[0.48]%, 16.27[0.35]%, 5.46[0.11]% and −26.44[0.02]% for bismuth, tungsten, cadmium, copper, aluminum, and air scatterers, respectively. Backscatter factors measured for a32P planar source are generally smaller than those for a point source. The variation of backscatter factor with distance from the boundary is well represented analytically by sums of exponentials. Therefore, the rate of decrease of backscatter factor with distance can be specified by a relaxation length, defined as the depth through which the backscatter factor is reduced by 1/e, whereeis the base of the natural logarithm. For example, with a32P planar source, relaxation lengths in Mylar are 588[7] mg/cm2and 238[2]mg/cm2for bismuth and aluminum scatterers, respectively. Qualitative interpretation of backscatter factor depth profiles is presented. In addition, the variations of backscatter factor with scatterer atomic number and with source geometry are discussed with reference to existing experimental findings on beta particle reflection.

ISSN:0269-3127    

DOI:10.1118/1.597089

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Neglect of tissue and applicator heterogeneities in brachytherapy treatment planning is due in part to lack of accurate, general, and fast three‐dimensional (3D) dose‐computation algorithms. A novel dose‐calculation algorithm that accounts for the lateral dimensions and location of the heterogeneity as well as its thickness has been developed. For simple 2D, water‐equivalent density heterogeneities, the algorithm is shown to be applicable to a wide range of photon energies and is 500–1000 times more efficient than Monte Carlo photon‐transport calculations. The model is based upon reducing the bounded 2D heterogeneity problem to two 1D problems by dividing the scattering volume into two regions: A cone‐shaped region that subtends the heterogeneity with its apex at the source and the complementary cone that contributes scatter dose by diffusion around the heterogeneity. The input data consist of precalculated scatter‐to‐primary ratios (SPRs) for collimated isotropic point sources. The central‐axis “mini‐beam” problem for a slab heterogeneity is solved by a simple 1D scatter integration model that accounts for both the thickness of the heterogeneity and its location relative to the point of interest. The scatter contribution arising outside the mini‐beam is modeled as the difference in SPRs corrected for transmission through the heterogeneity. The algorithm agrees, on average, with sample Monte Carlo calculations within 1% to 7% for125I,192Ir, and 100 keV point sources along the axes of water‐equivalent cylindrical heterogeneities (ρ=0–12.6 g/cm3, 3.6, and 24 mm diameters, and dose‐perturbation factors of 0.44–1.33 relative to the homogeneous case). The potential of generalizing the scatter‐subtraction approach to encompass 3D heterogeneities, those consisting of high‐atomic number media, and those of irregular shape, is discussed.

ISSN:0269-3127    

DOI:10.1118/1.597090

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An inexpensive and easy method of calibration of brachytherapy137Cs tubes is described. Cesium brachytherapy sources are calibrated relative to an NIST‐calibrated source. The detection system is a NaI detector and single channel analyzer with the window set on the photo peak.

ISSN:0269-3127    

DOI:10.1118/1.597091

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The examination of brachytherapy sources in a quality assurance program can result in significant exposure to staff. Even though exposure may be within prescribed limits attempts should be made to reduce such exposure to as low as reasonably achievable (ALARA concept). In this report, a system that can be used to examine small spherical radiation sources while minimizing exposure to staff is described.

ISSN:0269-3127    

DOI:10.1118/1.597092

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.597097

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.597099

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.597132

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.597137

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1002/j.2473-4209.1993.tb00066.x

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY