摘要:

The use of a single portal image for the three‐dimensional verification of patient placement during radiotherapy has been investigated. In this study, a deviation in the patient positioning is quantified by a three‐dimensional translation and rotation of the patient. The parameter values are obtained by fitting the projections of anatomical match points in the simulator and the portal image, using the three‐dimensional gantry coordinates of these points in the intended treatment setup. Two methods for finding an analytical solution for the fitting problem are presented. One method yields a solution from the affine transformation of the portal image (shift, rotation, magnification, and stretching in a specified direction) that fits the projections of the match points on the simulator image. The second and more accurate method yields a solution that expresses the estimated parameter values and their covariances as functions of image and three‐dimensional gantry coordinates of the selected matchpoints. The robustness and sensitivity of the solution is implicitly given by these expressions. The applications of these methods are illustrated by experiments with a human pelvic bone.

ISSN:0269-3127    

DOI:10.1118/1.597075

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The scatter factors, i.e., the ratios between the primary and scatter components of absorbed dose, have been determined for a 25‐MV x‐ray beam. The primary dose is described by attenuation and head‐scatter factors. The attenuation coefficient decreased from 0.028 cm−1at the surface to 0.025 cm−1at 30 cm depth. The head‐scatter factor varied about 8% for collimator settings from 5×5 to 40×40 cm2. To determine the scatter factors, measured tissue‐output ratios were divided by the attenuation and head‐scatter factors for the same depth and field size. The results were fitted, with agreement within ±1%, to a model which assumes a linear relation between the scatter factor and the field size if the ratio between field size and depth is constant. The model requires transient electron equilibrium, which is present when the depth exceeds 6 cm and the field size 6×6 cm2. The scatter factor increases to 1.33 at 30‐cm depth and 40×40 cm2field size. The peak scatter factor (PSF) at 3.5‐cm depth reaches 1.06 for the largest field. Both PSF and NPSF, the normalized peak scatter factor, are affected by electron disequilibrium, which causes some conceptual ambiguities and numerical uncertainties.

ISSN:0269-3127    

DOI:10.1118/1.597076

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Backscatter from the asymmetric collimators of a linac into the beam monitor chamber (BMC) has been investigated for two accelerators having different BMC configurations. The effect has been quantified as a function of field size and collimator jaw position for 6 and 18 MV beams. The results indicate a maximum 2.5% (6 MV) and 4% (18 MV) decrease in output in one case and a negligible effect in the other case. The experiments indicate that the difference can be attributed to the different construction of the BMC's for the two accelerators.

ISSN:0269-3127    

DOI:10.1118/1.597077

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The dependence of the wedge factor and central axis depth dose on field size was evaluated for 6‐, 10‐, and 24‐MV wedged photon beams for field sizes up to 40×40 cm2. The wedge factor for 60°, 45°, 30°, and 15° wedges in a 24‐MV beam was found to vary by as much as 25%, 12%, 9%, and 5%, respectively, over a field size range of 5×5 to 40×40 cm2. For 10 and 6 MV wedged beams, the wedge factors varied by up to 17% and 15%, respectively, over the same field size range. The depth dose curves for the wedged beams differed significantly from the open beam profiles. At 6 MV, the wedges caused beam hardening while at 24 MV, with the exception of the 15° wedge, all wedged beams were softer than the open beams, for all field sizes. At 10 MV, wedged fields of size less than 20×20 cm2were hardened relative to the open beam, whereas larger wedged fields had depth dose values within ±1% of the 10‐MV open‐beam depth dose data. Accurate treatment planning for large wedged fields and high‐energy photon beams thus requires the use of wedged beam depth dose curves and field size specific wedge factors. It was established that an equivalent square field for a rectangular wedged field can be determined using the standard open beam formulation. The largest difference between the wedge factor for a rectangular beam and its equivalent square beam was 2.5% and occurred for 24‐MV elongated fields. The depth dose values for a rectangular wedged field and its equivalent square beam were within ±1% at all energies studied.

ISSN:0269-3127    

DOI:10.1118/1.597078

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The anatomy at the edge of a treatment machine portal film may differ from that shown by the delineator lines on a simulator film by up to 10 mm, even if both the simulator and the treatment machine meet accepted criteria for mechanical tolerances. To assure that this possibility is minimized requires some form of overall alignment check between the simulator and the treatment machine. A new test device, the edge tolerance test tool (ET3), has been designed to permit a quick and accurate check on whether portal film disagreement with simulator films is due to an accumulation of tolerances. Its use should eliminate one source of this common problem.

ISSN:0269-3127    

DOI:10.1118/1.597129

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The effect of air gaps under bolus material on the surface dose for electron beams is investigated. Dose measurements were performed for air gaps from 0.0 to 3.0 cm and bolus thicknesses of 0.5 and 1.0 cm using the various electron energies and cone sizes available on an electron linear accelerator. Our results show that the surface dose decreases for lower electron energies, smaller field sizes, thicker boluses, and larger air gaps.

ISSN:0269-3127    

DOI:10.1118/1.597079

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A CT‐based simulator has been assembled based on a commercial CT scanner, virtual simulation software developed at the University of North Carolina and a laser drawing device to transfer the radiation portals from the virtual simulator to the patient. The simulation process can be completed in approximately 1 h; under most cases, the treatment portals can be designed and the patient marked in one session. The device has an inherent accuracy of ±1 mm. The portal projection accuracy in clinical cases is observed to be better than 2 mm.

ISSN:0269-3127    

DOI:10.1118/1.597080

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.597165

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The effect of eye plaque materials (gold backing and silastic seed‐carrier insert) on the dose distribution around a single125I seed has been measured, using cubic lithium fluoride thermoluminescent dosimeters (TLDs) 1 mm on an edge, in a solid water eye phantom embedded in a solid water head phantom. With an125I seed (model 6711) positioned in the center slot of the silastic insert for a 20‐mm plaque of the design used in the collaborative ocular melanoma study (COMS), dose was measured at 2‐mm intervals along the plaque central axis (the seed's transverse axis) and at various off‐axis points, both with and without the COMS gold backing placed over the insert. Monte Carlo calculations (morsecode) were performed, as well, for these configurations and closely the same geometry but assuming a large natural water phantom. Additional Monte Carlo calculations treated the case, both for 20‐ and 12‐mm gold plaques, where the silastic insert is replaced by natural water. Relative to previous measurements taken in homogeneous medium of the same material (without the eye plaque), the dose reduction found by both Monte Carlo and TLD methods was greater at points farther from the seed along the central axis and, for a given central‐axis depth, at larger off‐axis distances. Removal of the gold backing from the plaque did not make measurable difference in the dose reduction results (10% at 1 cm).

ISSN:0269-3127    

DOI:10.1118/1.597164

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

An easy‐to‐use phantom has been constructed for checking the accuracy of a stereotactic computed tomography localization system. This phantom has been used on a commercially available stereotactic radiosurgery system. With this system, the phantom reference point, whose location is established by means of a precision measuring implement, can be localized by a computed tomography (CT) scanner with a standard deviation of measurement that is less than 0.3 mm in three orthogonal axes.

ISSN:0269-3127    

DOI:10.1118/1.597034

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY