摘要:

I have compared the four sets of tissue–air ratio (TAR) data in the literature as a basis for recommending data to include in the revision of the Medical Physics Data Book, NBS Handbook 138 [Edited by T. N. Padikal and S. P. Fivozinshy (U.S. Department of Commerce, Washington, DC, 1982)].

ISSN:0269-3127    

DOI:10.1118/1.596625

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In this study the feasibility of utilizing nonlinear ultrasound propagation to control the power deposition patterns during ultrasound hyperthermia treatments of malignant tumors was investigated. From theinvivoresults it appears that the power deposition pattern and the temperature distributions can be significantly altered by increasing the temporal peak power of the source while reducing the duty cycle to maintain the same acoustic time average power. The maximum temperature gains obtained during hyperthermia sonication (between 1.5 and 2) were significantly less than one could obtain by varying the frequency and the other parameters of transducers and thus, the linear characteristics of the ultrasound fields should be used when hyperthermia systems are designed and treatments are planned. Then during the treatment nonlinear propagation can be used to provide on‐line control of the power deposition patterns. The transducer characteristics determined the magnitude of the temperature gain and the nonlinear propagation region could not be reached when sharply focused (Fnumber=1) transducers were used at the frequencies between 0.25 to 1.68 MHz due to the high pressure amplitudes that exceeded the transient cavitation threshold.

ISSN:0269-3127    

DOI:10.1118/1.596626

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Heating rate (HR) patterns in cylindrical structures were studied with inhomogeneous limb phantoms. These phantoms, arm and thigh models consisting of fat, bone, and muscle material, were heated with Clini‐Therm L, M, and MS applicators at 915 MHz. The thigh model is 18 cm in diameter with 2.65‐cm‐thick fat on the outside and a 4‐cm‐diam bone in the center. The arm model is 9 cm in diameter with 1.35‐cm‐thick fat and 2‐cm‐diam bone. All models are 29 cm long with phantom muscle in the space between fat and bone and were heated with their long axes parallel or perpendicular to theEfield. HR patterns in the transverse and longitudinal planes were obtained thermographically. A large water bolus, with the water channels parallel to theEfield, was used in every case. In the thigh model, maximum heating was in the muscle for both L and M applicators when theEfield was parallel to the long axis. When it was perpendicular, the maximum heating occurred in the fat layer. However, the peak HR in the fat remained about the same for bothEfield orientations. For the small applicator, heating was mostly in the fat, and the two field orientations did not cause much difference in the heating pattern. In the arm model, the maximum heating of all three applicators occurred mostly in the muscle for bothE‐field orientations. However, the maximum HR was reduced by a factor of 2 to 3 when theEfield was perpendicular rather than parallel to the long axis. These results indicate that placing the applicators with theEfield parallel to the long axis of cylindrical structures can optimize muscle heating, minimize required power, and reduce stray radiation.

ISSN:0269-3127    

DOI:10.1118/1.596627

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Two phantoms for assessing the performance of ultrasound scanners regarding detectability of small focal lesions are described. The spherical simulated lesions in the phantoms have diameters of 2.4, 3.0, and 4.0 mm and backscatter coefficients which are 16, 9, and 6 dB below that of the surrounding tissue‐mimicking material, the latter simulating normal tissue such as liver. Random positioning and relatively large numbers of lesions deal with the statistical problem related to mistaking a normal textural fluctuation, characteristic of ultrasound images of parenchymal tissue, to be a small lesion. All lesions have ultrasonic properties that are the same as those of the surrounding material except for the backscatter coefficient; therefore, artifacts resulting from attenuation, reverberation, and/or refraction are absent, and detectability of deeper lesions is not influenced by the presence of more proximal ones. A method is outlined for using the phantoms to determine bounding proximal and distal depths of resolution, or detectability, of focal lesions of a given object contrast and size. Tests for observer dependence involving five subjects resulted in standard deviations in the distal and proximal depths of less than 0.5 cm. A repeat of the observer dependence tests 18 months later showed little change in the results. In addition, results obtained by two experienced observers are also given for a variety of specific scanners, scanning heads, and scanning parameters.

ISSN:0269-3127    

DOI:10.1118/1.596589

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The energy‐averaged transport model for the calculation of dose rate distributions is applied to uniform, spherical source distributions in homogeneous media for radii smaller than the electron range. The model agrees well with Monte Carlo based calculations for source distributions with radii greater than half the continuous slowing down approximation range. The dose rate distributions can be written in the medical internal radiation dose (MIRD) formalism.

ISSN:0269-3127    

DOI:10.1118/1.596590

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Methods are introduced by which dose rate distributions due to nonuniform, irregularly shaped distributions of beta emitters can be calculated using dose rate distributions for uniform, spherical source distributions. The dose rate distributions can be written in the MIRD formalism.

ISSN:0269-3127    

DOI:10.1118/1.596591

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The sensitivity and precision of teflon‐imbedded CaSO4:Dy microthermoluminescent dosimeters (micro‐TLDs) were determined. The micro‐TLDs were sectioned from miniature TLDs (200 μm×400 μm×5 mm) that were fabricated using standard techniques. In order to measure absorbed dose, the miniature TLDs can be implanted directly into tissues (e.g., tumor xenografts) that have received injections of radiolabeled monoclonal antibodies. Micro‐TLDs recovered from tissue sections cut with a microtome can be read out to determine local absorbed dose. The precision of dose estimation was quantified for uniformly irradiated 32‐, 96‐, and 192‐μm TLD chips; coefficients of variation ranged from 22% to 41%, depending on chip size. The coefficients of variation were reduced to less than 12% using individual relative sensitivity factors for each micro‐TLD. The spatial resolution of the micro‐TLDs was studied by placing miniature TLDs across the sharp penumbral region of a linear accelerator x‐ray field. TLDs were sectioned into 32‐μm chips which were read out to determine the relative absorbed dose. The sharpness of the penumbra was readily quantified by the micro‐TLDs.

ISSN:0269-3127    

DOI:10.1118/1.596750

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A method of incorporating dose‐volume considerations within the framework of conventional linear programming is presented. This method is suitable for the optimization of beam weights and angles using a conformal treatment philosophy (i.e., tailoring the high‐dose region to the target volume only). Dose‐volume constraints are introduced using the concept that volumes of normal tissue nearer the target volume will be allowed higher dose constraints than volumes of normal tissue distal to the target volume. Each involved normal structure is divided into high‐dose and low‐dose volumes. These two volume partitions are represented by constraint points with either high‐dose or low‐dose constraints, respectively. Optimized treatment plans for three clinical sites demonstrate that this technique meets or surpasses the original dose‐volume constraints for a conformal‐type treatment plan using straightforward linear programming in a time frame that is comparable to other linear programming problems.

ISSN:0269-3127    

DOI:10.1118/1.596592

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Treatment planning for multiarc radiosurgery is an inherently complex three‐dimensional dosimetry problem. Characteristics of small‐field x‐ray beams suggest that major simplification of the dose computation algorithm is possible without significant loss of accuracy compared to calculations based on large‐field algorithms. The simplification makes it practical to efficiently implement accurate multiplanar dosimetry calculations on a desktop computer. An algorithm is described that is based on data from fixed‐beam tissue‐maximum‐ratio (TMR) and profile measurements at isocenter. The profile for each fixed beam is scaled geometrically according to distance from the x‐ray source. Beam broadening due to scatter is taken into account by a simple formula that interpolates the full width at half maximum (FWHM) between profiles at isocenter at different depths in phantom. TMR and profile data for two representative small‐field collimators (10‐ and 25‐mm projected diameter) were obtained by TLD and film measurements in a phantom. The accuracy of the calculational method and the associated computer program were verified by TLD and film measurements of noncoplanar multiarc irradiations from these collimators on a 4‐MV linear accelerator. Comparison of film measurements in two orthogonal planes showed close agreement with calculations in the shape of the dose distribution. Maximal separation of measured and calculated 90%, 80%, and 50% isodose curves was ≤0.5 mm for all planes and collimators. All TLD and film measurements of dose to isocenter agreed with calculations to within 2%.

ISSN:0269-3127    

DOI:10.1118/1.596593

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Using the EGS4 Monte Carlo simulation program, a general purpose code has been written to calculate Bragg–Gray and Spencer–Attix stopping‐power ratios for use in radiation dosimetry. The stopping‐power ratios can be calculated in any material in any region in a general cylindrical geometry with a large number of source geometries possible. The calculations take into account for the first time the differences between the stopping powers and the inelastic scattering of positrons and electrons. The results show that previous calculations ignoring these effects were accurate. The present results agree, typically within 0.1%, with the Spencer–Attix water‐to‐air stopping‐power ratios for broad parallel beams of electrons given in the AAPM and IAEA protocols except at the surface where the present calculations follow the buildup of secondary electrons in more detail and see a 2% reduction in the stopping‐power ratios.

ISSN:0269-3127    

DOI:10.1118/1.596594

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY