| 1. |
- Sjogren, R, et al.
(författare)
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Influence of electron contamination on in vivo surface dosimetry for high-energy photon beams
- 1998
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Ingår i: Medical physics (Lancaster). - 0094-2405. ; 25:6, s. 916-921
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Tidskriftsartikel (refereegranskat)abstract
- The influence of the electron contamination at in vivo dosimetry with diodes on the patient surface has been investigated by introducing different accessories in the beam path and by changing the field size and SSD. The results show a clear correlation between the electron contamination at an effective measuring depth of the diode and the signal from the patient diode. When the electron contamination is taken into account the agreement between the diode values and the absorbed dose is greatly improved. More accurate in vivo dosimetry with less error margins is therefore possible if better predictions of the electron contamination in high-energy photon beams can be performed. (C) 1998 American Association of Physicists in Medicine. [S0094-2405(98)00606-3].
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| 2. |
- Sjogren, R, et al.
(författare)
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Methods for the determination of effective monitor chamber thickness
- 1999
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Ingår i: Medical physics (Lancaster). - 0094-2405. ; 26:9, s. 1871-1873
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Tidskriftsartikel (refereegranskat)abstract
- There are a number of models, both analytical and Monte Carlo, which are used to describe the fluence from the treatment head of accelerators. One common problem in these simulations is to find relevant information about details in the treatment head. A complex unit in the treatment head for which reliable data is seldom given is the monitor chamber. In this work two methods are described for obtaining this information by analyzing the increased scattering of an electron beam when the monitor chamber is introduced in the beam. It was found that the effective thickness of the electrodes in a monitor chamber can be determined with sufficient accuracy by using experimental results combined with Fermi-Eyges theory or Monte Carlo simulations. (C) 1999 American Association of Physicists in Medicine. [S0094-2405(99)01009-3].
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| 3. |
- Karlsson, M G, et al.
(författare)
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Treatment head design for multileaf collimated high-energy electrons
- 1999
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Ingår i: Medical physics (Lancaster). - 0094-2405. ; 26:10, s. 2161-2167
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Tidskriftsartikel (refereegranskat)abstract
- This paper describes how a conventional treatment head can be modified for use of multileaf collimated electron beams. Automatic and dynamic beam delivery are possible for both electrons and photons by using the computer controlled multileaf collimator (MLC) for both photon and electron beams. Thereby, the electron beams can be mixed more freely into the treatment to take advantage of the specific depth modulation characteristics of electrons. The investigation was based on Monte Carlo calculations using the software package BEAM. The physical parameters used in this optimization were the beam penumbra and the virtual/effective point source position. These parameters are essential for shaping beams, beam matching and for dosimetry calculations. The optimization was carried out by modifying a number of parameters: replacing the air atmosphere in the treatment head with helium, adding a helium bag below the MLC, changing the position of the scattering foils, modifying the monitor chamber, and adjusting the position of the MLC. The beam characteristics for some of these designs were found to fulfil our criteria for clinically useful beams down to at least 9 MeV. (C) 1999 American Association of Physicists in Medicine. [S0094-2405(99)00610-0].
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| 4. |
- Ahnesjö, Anders, et al.
(författare)
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Modeling transmission and scatter for photon beam attenuators
- 1995
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Ingår i: Medical Physics. - : Wiley. - 0094-2405. ; 22:11, s. 1711-1720
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Tidskriftsartikel (refereegranskat)abstract
- The development of treatment planning methods in radiation therapy requires dose calculation methods that are both accurate and general enough to provide a dose per unit monitor setting for a broad variety of fields and beam modifiers. The purpose of this work was to develop models for calculation of scatter and transmission for photon beam attenuators such as compensating filters, wedges, and block trays. The attenuation of the beam is calculated using a spectrum of the beam, and a correction factor based on attenuation measurements. Small angle coherent scatter and electron binding effects on scattering cross sections are considered by use of a correction factor. Quality changes in beam penetrability and energy fluence to dose conversion are modeled by use of the calculated primary beam spectrum after passage through the attenuator. The beam spectra are derived by the depth dose effective method, i.e., by minimizing the difference between measured and calculated depth dose distributions, where the calculated distributions are derived by superposing data from a database for monoenergetic photons. The attenuator scatter is integrated over the area viewed from the calculation point of view using first scatter theory. Calculations are simplified by replacing the energy and angular-dependent cross-section formulas with the forward scatter constant r2(0) and a set of parametrized correction functions. The set of corrections include functions for the Compton energy loss, scatter attenuation, and secondary bremsstrahlung production. The effect of charged particle contamination is bypassed by avoiding use of dmax for absolute dose calibrations. The results of the model are compared with scatter measurements in air for copper and lead filters and with dose to a water phantom for lead filters for 4 and 18 MV. For attenuated beams, downstream of the buildup region, the calculated results agree with measurements on the 1.5% level. The accuracy was slightly less in situations where the scatter component is very large, as for very large fields with very short filter to detector distances. The implementation of the model into treatment planning systems is discussed.
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| 5. |
- Bjärngard, Bengt E, et al.
(författare)
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Quality control of measured x-ray beam data
- 1997
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Ingår i: Medical Physics. - : Wiley. - 0094-2405. ; 24:9, s. 1441-1444
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Tidskriftsartikel (refereegranskat)abstract
- The purpose of this study was to examine whether the quality of measured x-ray beam data can be judged from how well the data agree with a semiempirical formula. Tissue-phantom ratios (TPR) and output factors for several accelerators in the energy range 4-25 MV were fitted to the formula, separating the dose contributions from primary and phantom-scattered photons. The former was described by exponential attenuation in water, with beam hardening, and the latter by the scatter-to-primary dose ratio using two parameters related to the probability and the directional distribution of the scattered photons. Electron disequilibrium was not considered. Two approaches were evaluated. In one, the attenuation and hardening coefficients were determined from measurements in a narrow-beam geometry; in the other, they were extracted by the fitting procedure. Measured and fitted data agreed within +/- 2% in both cases. The differences were randomly distributed and had a standard deviation of typically 0.7%. Singular points with errors were easily identified. Systematic errors were revealed by increased standard deviation. However, when the attenuation was derived by the fitting algorithm, the attenuation coefficient deviated significantly from the experimental value. It is concluded that the semiempirical formula can serve to evaluate and verify beam data measured in water and that the physically most accurate description requires that the attenuation and hardening coefficients be determined in a narrow-beam geometry. The attenuation coefficient is an excellent measure of both the primary and the scatter dose component, i.e., of beam quality.
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| 6. |
- Bjärngard, Bengt E, et al.
(författare)
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Tissue-phantom ratios from percentage depth doses
- 1996
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Ingår i: Medical Physics. - : Wiley. - 0094-2405. ; 23:5, s. 629-634
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Tidskriftsartikel (refereegranskat)abstract
- 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.
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| 7. |
- Cahn, R N, et al.
(författare)
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Detective quantum efficiency dependence on x-ray energy weighting in mammography
- 1999
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Ingår i: Medical physics. - Medical physics : Wiley. - 0094-2405. ; 26:12, s. 2680-2683
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Tidskriftsartikel (refereegranskat)abstract
- An evaluation of the dependence of detective quantum efficiency (DQE) on the incident energy spectrum has been made for mammography. The DQE dependence on the energy spectrum has been evaluated for energy-integrating detectors, photon-counting detectors, and detectors that measure the energy of each photon. To isolate the effect of the x-ray energy spectrum the detector has been assumed to be ideal, i.e., all noise sources are assumed to be zero except for quantum fluctuations. The result shows that the improvement in DQE, if the energy-integrating detector is compared to a single-photon counting detector, is of the order of 10%. Comparing the energy-integrating detector and the detector measuring the energy for each photon the improvement is around 30% using a molybdenumanodespectrum typical in mammography. It is shown that the optimal weight factors to combine the data in the case the energy is measured are very well approximated if the weight factors are proportional to E−3." style="position: relative;" tabindex="0" id="MathJax-Element-1-Frame" class="MathJax">E−3. Another conclusion is that in calculating the DQE, a detector should be compared to one that uses ideal energy weighting for each photon since this provides the best signal-to-noise ratio. This has generally been neglected in the literature.
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| 8. |
- Ceberg, Crister, et al.
(författare)
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The effects of divergence and nonuniformity on the x-ray pencil-beam dose kernel
- 1996
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Ingår i: Medical Physics. - : Wiley. - 0094-2405. ; 23:9, s. 1531-1535
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Tidskriftsartikel (refereegranskat)abstract
- The scattered-photon part of pencil-beam dose kernels for high-energy x-ray beams can be derived experimentally by differentiating the broad-beam scatter-to-primary dose ratio as a function of radius. Formally, this requires a uniform and parallel beam, and the procedure is complicated by the nonideal, actual beam conditions: the primary dose profile is not uniform, the beam quality is not constant, and the distance to the source is not infinite. The experimentally determined scatter-to-primary ratios can be corrected for these effects before they are differentiated to give the pencil-beam kernels. The correction factors were calculated and shown to reach as much as 5% of the true scatter-to-primary ratio. The effect on the pencil beam was evaluated and corrected pencil beams were determined.
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| 9. |
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| 10. |
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