摘要:

Electron linac fields are usually characterized by the central‐axis practical range in water,Rp, and the depth of half maximum dose,R50, for dosimetry, quality assurance, and treatment planning. The quantitative relations between the range parameters and the intrinsic linac beam's energy structure are critically reviewed. The spectral quantity 〈E0〉* is introduced which is defined as the mean energy of the incident spectral peak, termed the “peak mean energy.” An analytical model is constructed to demonstrate the predicted relation between polyenergetic spectral shapes and the resulting depth‐dose curves. The model shows that, in the absence of electrons at the patient plane with energies outside about 〈E0〉*±0.1〈E0〉*,RpandR50are both determined by 〈E0〉*. This analytical approximation is confirmed by a Monte Carlo calculation comparing two different idealized incident spectra. The effect of contaminant lower energy or wide‐angle scattered electrons is also discussed. The effect of the width of the intrinsic energy spread on the shape of the depth‐dose curve is investigated using Monte Carlo depth‐dose simulations based on measured linac energy spectra having energy spreads (full width at half maximum) as large as 20%. These simulations show that the energy spread has only a small effect on the shape of the central‐axis depth‐dose curve.

ISSN:0094-2405    

DOI:10.1118/1.597192

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The Yang diffusion transport equation for charged particles was modified to allow the linear angular scattering power to vary with penetration depth in the scattering medium. Assuming charged particle energy loss to be a linear function of depth, conditional solutions to this transport equation have been found for the two cases of interest specified by Yang. The normalized excess path length distributions predicted for a 10‐MeV electron beam show a shift toward larger excess path lengths compared to Yang's solutions.

ISSN:0094-2405    

DOI:10.1118/1.597193

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The production of off‐focus x rays in the head of a 6 MV linac has been investigated using theegs4 Monte Carlo code. The purpose of the study was to identify the sources of off‐focus radiation and the relative contribution for each source. Even though a particular energy and linac were modeled, the broad conclusions are expected to be general since the effects of head scatter are similar for most conventional head designs, regardless of manufacturer, energy, and model. The head components that were modeled include the exit window of the accelerating structure, target, beam stopper, flattening filter, monitor chamber, primary and secondary collimators, and air. Monoenergetic 6 MeV electrons were followed through the exit window, target, and beam stopper until all energy was expended. Primary‐ and higher‐order x rays produced throughout the head were followed until they were either absorbed or passed through a plane at the isocenter. Sites of off‐focus radiation were found to be diffusely distributed throughout the head, with the most intense sources being the primary collimator, flattening filter, and beam stopper. Data analysis shows that the collimator effect is determined primarily by the volume of the extended head‐scatter source that is exposed to the point of measurement through the collimating system. The results of this study provide a rationale for developing extended source models to calculate the collimator factor for fields defined by arbitrary collimation. An additional advantage is an improvement in the agreement between measured and calculated isodose distributions.

ISSN:0094-2405    

DOI:10.1118/1.597194

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The central dose received from high energy photon beams that are obliquely incident on an absorber is markedly different from that absorbed when such beams are normally incident at the surface. In this work, we calculate the dose differences using a convolution of energy deposition kernels in a phantom presenting an oblique entrance surface. The dose distributions were calculated with oblique incident 1.25, 3, and 6 MeV monoenergetic and polyenergetic photon beams angled from 0° to 80°. In order to study the dose variations with surface obliquity, we introduced an obliquity factor. For each energy, the obliquity factor was calculated as a function of depth and field size. We found that, ignoring the electron contamination from the air and from the treatment machine head, the influence of obliquity can be described in terms of upstream and downstream contribution of the electrons set in motion by the primary photons. In actual beams, especially for large field sizes, the electron contamination becomes significant and tends to reduce the influence of surface obliquity. Results indicate that the obliquity factor is highly dependent on the beam energy and depth, and are in good agreement with our experimental results measured for 10 and 25 MV x‐ray beams. In this paper a theoretical explanation of these dose variations due to oblique incidence of the beam is presented.

ISSN:0094-2405    

DOI:10.1118/1.597195

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A simple modification is presented for two commercially available hyperthermia applicators which dramatically improves the regulation and dynamic control of the temperature at the bolus/tissue interface. This alteration requires the addition of a variable speed pump, bubble trap, simple heat exchanger, and a few minor changes to the existing system. With this modified design, the water within the bolus is directly circulated and temperature controlled. The convective nature of the circulating system ensures uniform temperature throughout the extended bolus and increases the thermal energy transfer at the bolus/tissue interface. This modification also provides significantly improved flexibility in controlling the treatment temperature distributions since the bolus/tissue interface temperature can now be dynamically varied during a treatment, in addition to adjusting the applicator power output and frequency.

ISSN:0094-2405    

DOI:10.1118/1.597196

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A set of standard wedge filters has been modified for use with half‐collimated beams of a 6 MV linear accelerator. The position of the standard size wedge filter has been shifted as far to one side of the wedge plate to ensure optimum half‐collimated field coverage (up to 20×30 cm) required in certain clinical situations. Dosimetric parameters were normalized at 1.5 cm depth and at an off‐axis reference point (3.5 cm from the central axis of the collimator at 100 cm SSD. The shapes of the wedged profile and isodose curves of the modified wedges remained similar to those of standard wedges. Data presented include wedge transmission factors, wedge angles, beam profiles, and isodose distributions. The clinical advantages of using modified wedge filters (larger field size, larger transmission, and smaller weight) over standard large wedges is discussed.

ISSN:0094-2405    

DOI:10.1118/1.597197

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Yttrium‐90 (90Y), a pure beta emitter, is an attractive radionuclide for radioimmunotherapy of cancer. Therapeutic management requires quantitative imaging to measure the pharmacokinetics of the radionuclide in the patient for radiation dosimetric calculations. The bremsstrahlung emissions can be utilized to acquire an image of beta sources using a gamma camera. Quantitation of90Y by bremsstrahlung imaging is difficult because of poor image quality that results from septal penetration and scatter secondary to the broad bremsstrahlung energies. In this work, quantitative methods for bremsstrahlung imaging of90Y sources that involved the use of (a) a Wiener filter to deconvolve the septal penetration and scatter while suppressing image noise, and (b) the geometric mean of the conjugate view (GM) and effective point source (EPS) methods to quantify activities were investigated. An abdominal phantom was prepared with90Y activities in the liver, spleen, tumors, and background volumes that were similar to those observed in patient studies. A twofold improvement in resolution recovery for full width at tenth maximum of the line spread function at 11 cm depth in water was achieved using Wiener restoration. Definition of the organ and tumor edges was greatly enhanced and cross talk between adjacent sources was suppressed after Wiener restoration. These improvements in image quality led to more accurate estimation of organ and tumor activities. Using the optimum attenuation correction method for GM and EPS quantitation of filtered bremsstrahlung images, estimates of individual activities (≤17% error) and cumulated activities (≤8% error) in all of the sources were accurate except for a tumor of 2 cm diameter. The results of this study provide the basis for a method to quantify beta source distribution and demonstrate the potential use of bremsstrahlung imaging in clinical settings.

ISSN:0094-2405    

DOI:10.1118/1.597198

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A new image‐registration technique that matches multiple structures on complementary imaging data sets (e.g., CT and MRI) has been developed and tested with both phantom and patient data. The algorithm assumes a rigid‐body transformation and is suitable for correlating structures within the cranium or at the skull base. The basic premise of the new technique is that an optimum transformation is achieved when the relative volume lying outside of the intersection between a structure and its transformed counterpart is a minimum. This relative volume is calculated numerically using a random sampling approach, and a binary searching algorithm was used to step through the nine‐dimensional parameter space consisting of three rotation angles, three scaling factors and three components of a translation vector. For the nine tests using phantom data, the automated structure‐matching technique was able to predict the correct rotation angles to within ±1°. The expected clinical performance of the new technique was assessed by comparing results obtained with the new method to those obtained using other techniques for 12 patients who were treated with charged particles at Lawrence Berkeley Laboratory (LBL) and who had image‐registration studies performed as part of their treatment plan. For 9 of the 12 patients considered, the new structure‐matching technique produced a significantly better registration than the older methods, as measured by the resultant average relative volume lying outside of the intersection between any structure and its transformed counterpart. For the other three patients, results were not significantly different for the new structure‐matching method and the older techniques.

ISSN:0094-2405    

DOI:10.1118/1.597187

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A method of unfolding patient movement between biplane radiographs is presented. This method requires biplane projections of two points with a known separation to determine a translation motion or biplane projections of two or three point pairs to determine a general patient motion. Such a point pair can be provided by two ends of a line source or two radiopaque marks, with a fixed distance between them. The point pairs can be taped on skin, implanted in the target volume, or embedded in the bony structures. After obtaining the point locations on each film, the three‐dimensional coordinates of points of interest at the instant of each view can be reconstructed, with an error of less than 0.5 mm. The motion parameters, characterized by a pseudotranslation vector and three axial rotational angles for the region of interest relative to the machine coordinate system, are also provided. In comparing with some other reconstruction algorithms, the point pair method can significantly improve source localization or point reconstruction in the presence of patient motion. Computer simulations and phantom experiments demonstrates the feasibility and accuracy of the method.

ISSN:0094-2405    

DOI:10.1118/1.597188

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Two approaches to estimate the optimal radiographic magnification for a TV camera‐based portal imaging system and portal films have been used. The first approach optimizes signal transfer while the second optimizes signal‐to‐noise ratio (SNR) transfer. In order to perform these optimization calculations, the physical characteristics of the imaging system (modulation transfer function and noise power spectrum) as well as the sizes of the radiation sources of our medical linear accelerators have been measured. Using these data, the optimal magnification considering signal transfer alone (Msignal) has been calculated to range between 2.0 and 2.3 for the TV camera‐based imaging system and is about 1.0 for portal films. Conversely, the optimal magnification considering SNR transfer (MSNR) has been calculated to range between 1.5 and 1.7 for the TV camera‐based imaging system and is about 1.0 for portal films. The results suggest that most portal imaging systems are operated close to their optimal radiographic magnification.

ISSN:0094-2405    

DOI:10.1118/1.597189

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY