摘要:

Analytic models for calculation of scatter distributions from flattening filters in therapy photon beams are presented. It is shown that the amount of scatter with high atomic number filters can vary from 2% in 4‐MV beams to 10% for 24‐MV beams. The use of low atomic number filters can increase the amount of scatter by a factor of 2. The dependence on the opening angle of the primary collimator is quite large since a larger opening angle requires a thicker filter, which increases the scattered fraction of the filtered beam. The scatter makes the filter act as an extended source of extra‐focal radiation. The source distribution for monomedia filters is shown to be almost triangular. Integration in the calculation‐point's eye view over the visible part of the filter scatter source yields the scatter fraction of the total energy fluence incident upon the patient. The experimentally well‐known “tilt” of dose profiles for asymmetrical fields is explained by the model. For complete modeling of head scatter distributions in treatment planning, the model presented must be combined with models also describing the scatter from the collimators, auxiliary modulators such as wedges and compensating filters, and collimator backscatter to the beam monitor.

ISSN:0269-3127    

DOI:10.1118/1.597205

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A dose calculation algorithm has been developed for photon beams with intensity modulation generated by dynamic jaw or multileaf collimations. First, an in‐air fluence distribution is constructed based on the dynamic motion of the jaws or leaves, taking into account the variation of output with field size defined by the jaws. The fluence distribution is then convolved with the appropriate pencil beam kernel to give correction factors which are used to calculate the dose distribution for an intensity‐modulated photon field. The proposed algorithm is strictly valid in homogeneous media only, patient heterogeneity correction is accounted for in an approximate manner. Dose distributions at several depths and for several field sizes were calculated for 6‐ and 15‐MV x‐ray beams for a set of standard wedges produced by dynamic jaws. Measurements were made with film and an ion chamber. Comparisons between calculated and measured data show good agreement (within 2%) for both dose profiles and wedge factors. Similar calculations and measurements were also made for a 25‐MV intensity‐modulated photon field produced by dynamic motion of a multileaf collimator. Agreement between calculations and measurements is also good (within 3%). The “tongue‐and‐groove” effect associated with a multileaf collimator design is also examined using a ring‐shaped field produced by matching two component fields. The computation time for a dynamic‐collimated field is the same as that for an irregular field shaped by conventional blocks. The algorithm is applicable to any pattern of jaw or multileaf motions. The strengths and remaining problems of the algorithm are discussed.

ISSN:0269-3127    

DOI:10.1118/1.597206

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In electron beams, the dose in phantom under the central shielding depends on electron‐beam energy, depth in phantom, and shield area and thickness. In our experiments, all shield thicknesses were larger than the range of electrons in the shield material. At a given depth, the dose under the shield never exceeds the open field value; however, it can attain quite a large proportion of the open field value despite a shield thickness which exceeds the range of electrons in the shield material. The effects of shield area on the dose under the central shields were studied in detail and dose distributions are given as a function of shield lateral dimensions and electron‐beam energy. It is shown that in clinical use of central shielding, the best approach to dose estimation under the shield is direct measurement in phantom under conditions of the actual clinical setup.

ISSN:0269-3127    

DOI:10.1118/1.597207

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This report is an extension of the 1983 AAPM protocol, popularly known as the TG‐21 Protocol. It deals with the calibration of plane‐parallel ionization chambers and their use in calibrating therapy electron beams. A hierarchy of methods is presented. The first is to calibrate the plane‐parallel chamber in a high energy electron beam against a cylindrical chamber which has anNgascylvalue that has been obtained from a NIST traceable60Co beam calibration. The second method, which is recommended for implementation by the ADCLs is an in‐air calibration against a NIST‐traceable calibrated cylindrical chamber in a Cobalt‐60 beam to obtain a plane‐parallel‐chamber calibration factor in terms of exposure or air kerma. The third method places the two chambers in a phantom in a Cobalt‐60 beam, and leads to anNgasppvalue for the plane‐parallel chamber. This report also givesNgaspp/(NxAion)ppandNgaspp/(NkAion)ppvalues for five commonly used commercially available plane‐parallel chambers: the Capintec PS‐033, the Exradin P‐11, the Holt, the NACP and the PTW‐Markus. The calculation of theseNgasratios introduces aKcompfactor which is also calculated for the five parallel plate chambers. The use of the plane‐parallel chambers follows the 1983 AAPM protocol for absorbed dose calibrations of electrons, except that new energy‐dependentPreplvalues are given for the Capintec PS‐033 and PTW‐Markus chambers consistent with the consensus of reports in the literature. For all the chambers, however,Preplis unity for 20 MeV electrons. This report does not address the issue of the use of plane‐parallel chambers in calibrating photon beams.

ISSN:0269-3127    

DOI:10.1118/1.597359

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.597208

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The limit at which quantization noise becomes dominant in video‐based real‐time portal imaging has been studied. Quantization noise due to truncation in integer frame averaging is shown to be dominant over the input analog‐to‐digital converter (A/D) quantization noise, unless image addition is used in video‐based real‐time portal imaging systems. Portal images acquired with the Newvicon camera by averaging more than 64 frames are found to be dominated by the quantization noise due to truncation. It has shown that the signal‐to‐noise ratio (SNR) is limited to 886.8 when using an 8‐bit A/D with digital frame averaging, but higher values can be achieved with digital frame addition. It is also shown that digital frame addition together with 16‐bit processing can achieve higher contrast resolution than digital frame averaging and 8‐bit processing.

ISSN:0269-3127    

DOI:10.1118/1.597360

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Gd2O2S phosphor screens between 250 and 1000 mg/cm2thick were evaluated for use in megavoltage imaging systems. The phosphor layers were placed on brass plates ranging from 1 to 5 mm thick, each with and without an optical back reflector (white paint). Light output and spatial resolution were measured at 6‐ and 23‐MV x‐ray energies. Light output was found to increase linearly with phosphor thickness up to 500 mg/cm2, reaching a plateau at 1000 mg/cm2. Spatial resolution [modulation transfer function (MTF)] decreased exponentially with phosphor thickness up to 750 mg/cm2, where a minimum was reached. The variation of MTF with phosphor thickness was found to obey a simple empirical relation.

ISSN:0269-3127    

DOI:10.1118/1.597209

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Using fiber optic manufacturing techniques, it is possible to produce a radiographic grid that discriminates against scattered radiation in two dimensions. Such grids consist of septa composed of glass with a high lead content; the interspace material is air, so that approximately 80% of the grid area is open. In this way, effective high ratio grids can be produced with relatively low Bucky factors. The performance of samples of such grid material is characterized in terms of both scatter rejection and dose efficiency for application in digital mammography in both slot–beam and area–beam geometry. For area beams, five‐ to tenfold improved scatter rejection relative to conventional grids was observed. In slot configurations, such grids could provide improved SNR/dose performance and more effective utilization of the heat loading capability of the x‐ray source.

ISSN:0269-3127    

DOI:10.1118/1.597236

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Several authors have proposed variations of the iterative filtered backprojection (IFBP) reconstruction algorithms claiming fast initial convergence rates. We have found that these algorithms are trying to minimize an unusual squared‐error criterion in a suboptimal way. As a result, existing IFBP algorithms are inefficient in the minimization of the criterion, and may become unstable at higher iteration numbers. We show that existing IFBP algorithms can be modified to use the steepest descent technique by simply optimizing the step size at each iteration. Further gains in convergence rates can be achieved with conjugate gradient IFBP algorithms derived from the same criterion. The steepest descent and conjugate gradient IFBP algorithms are guaranteed to converge, unlike some IFBP algorithms, and will do so in fewer iterations than existing IFBP algorithms.

ISSN:0269-3127    

DOI:10.1118/1.597210

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The unique geometry of electron‐beam CT (EBCT) scanners produces radiation dose profiles with widths which can be considerably different from the corresponding nominal scan width. Additionally, EBCT scanners produce both complex (multiple‐slice) and narrow (3 mm) radiation profiles. This work describes the measurement of the axial dose distribution from EBCT within a scattering phantom using film dosimetry methods, which offer increased convenience and spatial resolution compared to thermoluminescent dosimetry (TLD) techniques. Therapy localization film was cut into 8×220 mm strips and placed within specially constructed light‐tight holders for placement within the cavities of a CT Dose Index (CTDI) phantom. The film was calibrated using a conventional overhead x‐ray tube with spectral characteristics matched to the EBCT scanner (130 kVp, 10 mm Al HVL). The films were digitized at five samples per mm and calibrated dose profiles plotted as a function ofz‐axis position. Errors due to angle‐of‐incidence and beam hardening were estimated to be less than 5% and 10%, respectively. The integral exposure under film dose profiles agreed with ion‐chamber measurements to within 15%. Exposures measured along the radiation profile differed from TLD measurements by an average of 5%. The film technique provided acceptable accuracy and convenience in comparison to conventional TLD methods, and allowed high spatial‐resolution measurement of EBCT radiation dose profiles.

ISSN:0269-3127    

DOI:10.1118/1.597200

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY