ISSN:0094-2405    

DOI:10.1118/1.597814

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

This paper is the second of two that together present a novel approach to the problem of reconstructing vascular trees from a small number of projections. Previously, we described the reconstruction algorithm and how it effectively circumvents the matching or “correspondence problem” found in most photogrammetric or computer‐vision‐based approaches. The algorithm is fully automatic and assumes that the imaging geometry is known, the vascular tree is a connected structure, and that its center‐lines have been identified in three or more images. It employsconsistencyandconnectivityconstraints and comprises three steps: The first generates a connected structure representing the multiplicity of solutions that are consistent with the first two views; the second assigns a measure of agreement to each branch in this structure based on one or more additional projections; and the third step employs this measure to distinguish between those branches comprising the vasculature and the accompanying artifacts. This paper addresses the issue of validation via simulations and experiments. In addition to a clinical case, we examine the performance of the algorithm when applied to simulated projections of two 3‐D vascular models, both representative of the complexity faced in coronary and cerebral angiography. The results in each instance are impressive and demonstrate that adequate reconstructions may be obtained with as few as three distinct views.

ISSN:0094-2405    

DOI:10.1118/1.597815

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

When converting fractional (percentage) depth doses to tissue–phantom ratios, one must use a factor that accounts for the different source‐to‐point distances. Two minor correction factors are also involved. One is the ratio of total to primary dose at the two different distances from the source, for the same depth and field size. This factor is usually ignored. It was determined experimentally that this can introduce up to 1.5% error at 6 MV. The second correction factor reflects differences related to scattered photons and electrons at the depth of normalization in the two geometries. This correction is accounted for in published conversion procedures. It was found to be less than 1% provided the normalization depth is sufficient for electron equilibrium, which occurs first well beyond the depth of maximum dose. One may avoid electron‐equilibrium problems by using an interim normalization depth that provides electron equilibrium with some margin, renormalizing to a shallower depth if desired. With this precaution, the accuracy when measured fractional depth doses were converted to tissue–phantom ratios was comparable to that of directly measured tissue–phantom ratios even when the correction factors were ignored.

ISSN:0094-2405    

DOI:10.1118/1.597698

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

It has been shown that intensity‐modulated fields have the potential to deliver optimum dose distributions, i.e., high dose uniformity in the target and lower doses in the surrounding critical organs. One way to deliver such fields is by using dynamic multileaf collimation (DMLC). This capability is already available in research mode on some treatment machines. While much effort has been devoted to developing algorithms for DMLC, the mechanical reliability of this new treatment delivery mode has not been fully studied. In this work, we report a series of tests designed to investigate the mechanical aspects of DMLC and their implications on dosimetry. Specifically, these tests were designed to examine (1) the stability of leaf speed, (2) the effect of lateral disequilibrium on dose profiles between adjacent leaves, (3) the significance of acceleration and deceleration of leaf motion, (4) the effect of positional accuracy and rounded‐end of the leaves, and (5) create a simple test pattern that may serve as a basis for routine quality assurance checks. Results of these tests are presented. The implications on dosimetry and consideration for the design of leaf motion are discussed.

ISSN:0094-2405    

DOI:10.1118/1.597699

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

In recent years, the increased use of proton beams for clinical purposes has enhanced the demand for accurate absolute dosimetry for protons. As calorimetry is the most direct way to establish the absorbed dose and because water has recently been accepted as standard material for this type of beam, the importance of water calorimetry is obvious. In this work we report water calorimeter operation in an 85‐MeV proton beam and a comparison of the absorbed dose to water measured by ionometry with the dose resulting from water calorimetric measurements. To ensure a proper understanding of the heat defect for defined impurities in water for this type of radiation, a relative response study was first done in comparison with theoretical calculations of the heat defect. The results showed that pure hypoxic water and hydrogen‐saturated water yielded the same response with practically zero heat defect, in agreement with the model calculations. The absorbed dose inferred from these measurements was then compared with the dose derived from ionometry by applying the European Charged Heavy Particle Dosimetry (ECHED) protocol. Restricting the comparison to chambers recommended in the protocol, the calorimeter dose was found to be 2.6%±0.9% lower than the average ionometry dose. In order to estimate the significance of chamber‐dependent effects in this deviation, measurements were performed using a set of ten ionization chambers of five different types. The maximum internal deviation in the ionometry results amounted to 1.1%. We detected no systematic chamber volume dependence, but observed a small but systematic effect of the chamber wall thickness. The observed deviation between calorimetry and ionometry can be attributed to a combination of the value of (Wair/e)pfor protons, adopted in the ECHED protocol, the mass stopping power ratios of water to air for protons, and possibly small ionization chamber wall effects.

ISSN:0094-2405    

DOI:10.1118/1.597700

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

A modified method for the evaluation of bremsstrahlung spectra on the basis of transmission measurements is presented. The method is tested for two 4 MeV therapy accelerators. The calculated spectra are compared with other experimental results and Monte Carlo simulation.

ISSN:0094-2405    

DOI:10.1118/1.597701

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Radiotherapy x‐ray and electron beam surface doses are accurately measurable by use of a MOSFET detector system. The MOSFET (Metal Oxide Semiconductor Field Effect Transistor) is approximately 200‐μm in diameter and consists of a 0.5‐μm Al electrode on top of a 1‐μm SiO2and 300‐μm Si substrate. Results for % surface dose were within ±2% compared to the Attix chamber and within ±3% of TLD extrapolation results for normally incident beams. Detectors were compared using different energies, field size, and beam modifying devices such as block trays and wedges. Percentage surface dose for 10×10‐cm and 40×40‐cm field size for 6‐MV x rays at 100‐cm SSD using the MOSFET were 16% and 42% of maximum, respectively. Factors such as its small size, immediate retrieval of results, high accuracy attainable from low applied doses, and as the MOSFET records its dose history make it a suitableinvivodosimeter where surface and skin doses need to be determined. This can be achieved within part of the first fraction of dose (i.e., only 10 cGy is required.)

ISSN:0094-2405    

DOI:10.1118/1.597702

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Fluence modulation of multiple electron beams of various energies has been used to optimize the delivered dose distribution during electron beam radiation therapy. By maximizing the probability of achieving tumor control without causing severe complications electron beam fluence profiles have been optimized for superficial target volumes. It is possible to use several equiportal fluence‐modulated electron beams to modify the energy deposition with depth in a controlled manner making it possible to use the technique as an alternative to bolus. The technique was tested in two representative phantom geometries and in three clinical patient geometries using a set of five and two different energies. The local maxima in dose for the plans with five energies were typically lower than with the conventional or advanced bolus techniques. The principles for how the technique could be carried out in the future with a fourth generation radiotherapy accelerator are also indicated.

ISSN:0094-2405    

DOI:10.1118/1.597710

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

The response of thermoluminescent dosimeters to electron irradiations depends on the radiation dose, mean electron energy at the position of the dosimeter in phantom, and the size of the dosimeter. In this paper the semi‐empirical expression proposed by Holtetal. [Phys. Med. Biol.20, 559–570 (1975)] is combined with the calculated electron dose fraction to determine the thermoluminescent dosimetry (TLD) response as a function of the mean electron energy and the dosimeter size. The electron and photon dose fractions, defined as the relative contributions of electrons and bremsstrahlung photons to the total dose for a clinical electron beam, are calculated with Monte Carlo techniques using EGS4. Agreement between the calculated and measured TLD response is very good. We show that the considerable reduction in TLD response per unit dose at low electron energies, i.e., at large depths in phantom, is offset by an ever‐increasing relative contribution of bremsstrahlung photons to the total dose of clinical electron beams. This renders the TLD sufficiently reliable for dose measurements over the entire electron depth dose distribution despite the dependence of the TLD response on electron beam energy.

ISSN:0094-2405    

DOI:10.1118/1.597712

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY

摘要:

Electron energy spectra and angular distributions, including angular spreads, were measured using magnetic spectrometer techniques, at isocenter, for two clinical linear accelerators: one scanning beam machine, which achieves field flatness by scanning a pencil beam over the desired field at the patient plane, and one scattering foil machine, which disperses the electrons through a graded‐thickness scattering foil. All measurements were made at isocenter (in the patient plane), in air, 1 m from the nominal accelerator source. The energy measurements were confined to electrons traveling along the central axis; any widely scattered electrons were effectively neglected. The energy spectra of the scanning beam machine are all of nearly Gaussian shape and energy full‐width‐at‐half‐maximum intensity (FWHM) of about 5% of the peak mean energy (denoted〈E0〉*). The energy spectra of the scattering foil machine have a variety of forms as a function of energy, including even spectra with double peaks, and spectra which changed with time. The FWHM values ranged from 9%–22% of〈E0〉*. The angular spread measurements, at isocenter, yieldedσθx×〈E0〉*≊295mrad‐MeV for the scanning beam machine, and 346 mrad‐MeV for the scattering foil machine, whereσθxdenotes the standard deviation of the plane‐projected angular distribution. These angular spreads are 30%–40% smaller than angular spreads reported by others on a very similar machine using the penumbra method. Possible causes of this discrepancy are discussed.

ISSN:0094-2405    

DOI:10.1118/1.597817

出版商:American Association of Physicists in Medicine    

年代:1998

数据来源: WILEY