摘要:

The convolution/superposition method of dose calculation has the potential to become the preferred technique for radiotherapy treatment planning. When this approach is used for therapeutic x‐ray beams, the dose spread kernels are usually aligned parallel to the central axis of the incident beam. While this reduces the computational burden, it is more rigorous to tilt the kernel axis to align it with the diverging beam rays that define the incident direction of primary photons. We have assessed the validity of the parallel kernel approximation by computing dose distributions using parallel and tilted kernels for monoenergetic photons of 2, 6, and 10 MeV; source‐to‐surface distances (SSDs) of 50, 80, and 100 cm; and for field sizes of 5×5, 15×15, and 30×30 cm2. Over most of the irradiated volume, the parallel kernel approximation yields results that differ from tilted kernel calculations by 3% or less for SSDs greater than 80 cm. Under extreme conditions of a short SSD, a large field size and high incident photon energy, the parallel kernel approximation results in discrepancies that may be clinically unacceptable. For 10‐MeV photons, we have observed that the parallel kernel approximation can overestimate the dose by up to 4.4% of the maximum on the central axis for a field size of 30×30 cm2applied with a SSD of 50 cm. Very localized dose underestimations of up to 27% of the maximum dose occurred in the penumbral region of a 30×30‐cm2field of 10‐MeV photons applied with a SSD of 50 cm.

ISSN:0269-3127    

DOI:10.1118/1.596955

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

X‐ray spectral reconstruction from transmission data was investigated by direct resolution of the matrix systemA*F=Tusing spectral algebra. Theoretical, numerical, and physical conditions were studied to obtain the physical solution of the problem. It is shown why the least‐square‐fitting methods may not give good results for high energy photon spectra above several MeV. The proposed method does not need a preshaped starting spectrum. Simulation studies have been made with arbitrary but realistic spectra of different shapes and maximum energies simulating the reality and using different materials as attenuator like carbon, aluminum, and even water. Conditions on the attenuator choice to obtain the physical solution of the system are given. The reconstruction method provides good results in the radiotherapy energy range despite measurement errors. The computation is quick and implementable on a personal computer with a mathematical coprocessor.

ISSN:0269-3127    

DOI:10.1118/1.596956

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In a recent paper [Med. Phys.19, 1441 (1992)] it was shown that the field size dependence of on‐axis relative output from a linear accelerator can be well described by a model comprising two components. One component represents radiation emanating from a point source and the other represents radiation from a field size dependent distributed source. In this communication the application of the model is extended to off‐axis situations and demonstrates that a good description of the dependence of off‐axis output on collimator configuration can be obtained. Of particular clinical relevance in this regard is the beam profile of half blocked fields which are adequately reproduced. The model has also been applied to the study of profiles generated by simulated dynamic wedging.

ISSN:0269-3127    

DOI:10.1118/1.596957

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Tomotherapy, literally “slice therapy,” is a proposal for the delivery of radiation therapy with intensity‐modulated strips of radiation. The proposed method employs a linear accelerator, or another radiation‐emitting device, which would be mounted on a ring gantry like a CT scanner. The patient would move through the bore of the gantry simultaneously with gantry rotation. The intensity modulation would be performed by temporally modulated multiple independent leaves that open and close across the slit opening. At any given time, any leaf would be (1) closed, covering a portion of the slit, (2) open, allowing radiation through, or (3) changing between these states. This method would result in the delivery of highly conformal radiation. Overall treatment times should be comparable with contemporary treatment delivery times. The ring gantry would make it convenient to mount a narrow multisegmented megavoltage detector system for beam verification and a CT scanner on the treatment unit. Such a treatment unit could become a powerful tool for treatment planning, conformal treatment, and verification using tomographic images. The physical properties of this treatment delivery are evaluated and the fundamental design specifications are justified.

ISSN:0269-3127    

DOI:10.1118/1.596958

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Electronic equilibrium conditions are studied in a homogeneous medium irradiated by monoenergetic photons with Compton scattering as a predominant process. Based on the concept of straight charged particle tracks, a geometrical model for spatial distribution of Compton electrons is developed in the limit of primary photon interactions. The model is applied to examine conditions of electronic equilibrium in collimated photon beams and to define equilibrium phase diagrams which establish correlation between various degrees of electronic equilibrium and primary dose. The diagrams predict that in a single direction (longitudinal or lateral) partial electronic equilibrium can be observed in radiation fields of dimensions smaller than the maximum range of secondary electrons. Associated macroscopic effects appear as a variation of the primary dose build‐up rate with beam radius and depth in phantom. These effects are observed in the case of both primary and total absorbed dose as judged by the Monte Carlo generated data in waterlike material (1–8 MeV photons).

ISSN:0269-3127    

DOI:10.1118/1.596959

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The treatment plans for stereotactic radiosurgery employ small, circular, noncoplanar fields applied in a series of arcs, or with synchronous rotation of the accelerator gantry and patient support assembly. Primary or metastatic brain tumors and arterial‐venous malformations are localized in relation to a stereotactic head frame using CT, MRI, and angiography. As x‐ray doses in the range of 20–40 Gy are delivered in a single treatment, it is critical that the dose distribution produced by the accelerator accurately reflect the one developed by the treatment planning computer. Until the advent of Fricke‐infused gels, whose NMR characteristics are changed by irradiation, there was no practical method for assessing the accuracy of x‐ray beam positioning on a target that was localized by both CT and MRI. A stereotactic head frame was attached to a hollow glass head filled with a Fricke‐infused gel. A 2‐mm target point at approximately the center of this manikin was localized by CT and MRI. The head frame was then mounted to the patient support assembly of a linear accelerator, and given a dose of 40 Gy to the isocenter from 6‐MV x rays using a modified version of the dynamic stereotactic radiosurgery plan developed in Montreal. Subsequent MRI showed the target point at the center of the dose distribution, thus confirming the accuracy of the stereotactic radiosurgery procedure. This demonstrated the unique characteristics of the Fricke‐infused gel for the simultaneous localization of x‐ray beams in three dimensions.

ISSN:0269-3127    

DOI:10.1118/1.597126

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The dosimetry of arced electron beams is of increasing importance because of the increased capabilities of modern linear accelerators. A practical pencil beam algorithm has been developed for arc electron beams and is capable of using computed tomography information for heterogeneity corrections. For homogeneous phantoms, the maximum dose and bremsstrahlung components are predicted very accurately, that is, within 1% of the maximum dose. However, the depth of maximum dose (treatment depth) is predicted to be deeper than measurement, as much as 0.7 cm deeper. For a heterogeneous lung phantom, the discrepancies are as high as 30%, but the accuracy of dose calculation is consistent with conventional stationary pencil beam algorithms. It was concluded that improvements in the dose prediction are possible with more accurate calculations of the pencil beam widths and the incorporation of range straggling into the algorithm.

ISSN:0269-3127    

DOI:10.1118/1.596960

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A new prototype 6‐MV flattening filter was designed by the manufacturer for use in a popular dual energy linear accelerator. To satisfy the contract demands, this filter was designed to produce a beam whose characteristics matched precisely with those of the 6‐MV beam produced from a single photon peak energy unit from the same manufacturer and already in operation in the department. A single set of 6‐MV dosimetric files for both units can now be maintained. The new filter has forced percent depth values over a wide clinical range of field sizes and depths to agree within 1.3%. Beam profiles now agree to within 1% over the useful area. For wedges with similar wedge angles, transmission factors now agree to within 1%. Standard acceptance testing performance specifications provided by the manufacturer were not adequate for clinical beam matching. The purchase contract for these units included our own specifications, which were more rigid and pertinent to our goal. Details of the effort are discussed.

ISSN:0269-3127    

DOI:10.1118/1.596961

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A commercial megavoltage constancy meter was tested for routine checks of the x‐ray output of an orthovoltage therapy unit. A practical method of using the meter for both output and energy checks was developed and is presented as part of an orthovoltage quality assurance program.

ISSN:0269-3127    

DOI:10.1118/1.596962

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

ISSN:0269-3127    

DOI:10.1118/1.596963

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY