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| 2. |
- Karlsson, Mikael, et al.
(författare)
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Electron beam characteristics of the 50-MeV racetrack microtron
- 1992
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Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405. ; 19:2, s. 307-315
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Tidskriftsartikel (refereegranskat)abstract
- Electron beams in the MM50 racetrack microtron are generated by computer controlled scanning of a well-focused electron pencil beam. The treatment head is optimized to give a minimum of scatter between the source position and the collimator plane by a general minimization of all scattering material in the beam and by replacement of the air in the treatment head by helium, which has a much lower linear scattering power than air. A double-focused multileaf collimator with a 31-cm collimator to patient distance is used both for electron and photon collimation. In general, no extra electron collimation is needed for the standard SSD of 100 cm. To make irregular field collimation at a distance this far from the patient possible, a number of requirements have to be fulfilled regarding the virtual source position and the spatial and angular distribution of the initial electron beam. The virtual source position has been found to be at a fixed position for different irradiation parameters. This is important for the use of the light field in electron beam treatment but also for achieving a high degree of accuracy in the dosimetry. Scatter from the multileaf collimator has not been found to give any significant contribution to the radiation field or to the monitor output factor of the MM50. Experimental dose distribution data on the MM50 have been compared to data both from other types of treatment units and to Monte Carlo simulations.
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| 3. |
- Karlsson, Mikael, et al.
(författare)
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Photon beam characteristics on the MM50 racetrack microtron and a new approach for beam quality determination
- 1993
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Ingår i: Medical physics (Lancaster). - : Wiley. - 0094-2405. ; 20:1, s. 143-149
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Tidskriftsartikel (refereegranskat)abstract
- The photon beams of the MM50 racetrack microtron have special characteristics which make them more suitable than conventional photon beams for precision radiation therapy with good dosimetric control. The beam flattening is obtained by the scanning of an elementary beam instead of using a flattening filter. This will give a number of advantages such as the possibility to optimize field flattening to individual field forms and field sizes. The radiation quality is the same across the whole beam, which gives smaller changes in dose profiles with depth and also makes it easier to perform careful dose planning. Beam collimation is mainly performed by a multileaf collimator and the special design of the treatment head gives nearly ideal characteristics for dose determination in an arbitrary point in the treatment fields. The output factor has been shown to depend almost solely on scattering within the treatment field. The conventional methods for beam quality characterization have been found less suitable at high energies and a new method based on HVL measurements in water is proposed.
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| 4. |
- Laitano, RF, et al.
(författare)
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Correction factors for calibration of plane-parallel ionization chambers with a Co-60 gamma-ray beam
- 1993
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Ingår i: Physics in Medicine and Biology. - : IOP Publishing. - 0031-9155 .- 1361-6560. ; 38:1, s. 39-54
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Tidskriftsartikel (refereegranskat)abstract
- The appropriate correction factors have been determined to enable plane-parallel ionization chambers to be calibrated using a Co-60 photon beam with a known air kerma rate. These factors refer to different calibration conditions and to different types of plane-parallel chambers. The main purpose of these correction factors is to allow calibration procedures more widely practicable than those based on the use of an electron beam of sufficiently high energy. The condition required to apply the correction factors is that the characteristics of the chambers to be calibrated are the same as those reported for the chambers considered in this investigation. To this end the types of chambers investigated were among the most widely used plane-parallel chambers commercially available. The results of this work also give indications on the properties of the plane-parallel chambers with regard to the chamber-phantom matching.
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| 5. |
- Larsson, Mikael, 1963-, et al.
(författare)
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Optical properties of diffractive, bifocal intraocular lenses
- 1991
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Ingår i: Ophthalmic Lens Design and Fabrication, SPIE 1529(Societ of Photo-Optical Engineers, Bellingham WA, 1991).. - Bellingham WA. ; , s. 63-70
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Konferensbidrag (refereegranskat)abstract
- The resolution of diffractive, bifocal intraocular lenses was studied with regard to pupil displacement and diameter size through computer simulations, bench measurements and patient vision acuity measurements. Good agreement was obtained between these three methods of investigation. In particular we find that pupil displacements of the order of 1 mm reduce the resolution considerably for these lenses.
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| 6. |
- Nyström, H, et al.
(författare)
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Correction factors applied to plane-parallel ionization chambers
- 1993
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Ingår i: Physics in Medicine and Biology. - : IOP Publishing. - 0031-9155 .- 1361-6560. ; 38:2, s. 311-322
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Tidskriftsartikel (refereegranskat)abstract
- Plane-parallel ionization chambers are recommended by most dosimetry protocols' for use in low-energy electron beams. Such chambers are often calibrated at the clinic against a cylindrical chamber in a high-energy electron beam while no general procedures for Co-60 gamma-ray calibrations are yet adopted. If the perturbation due to the presence of the plane-parallel chamber in a water phantom at the calibration quality, i.e. Co-60, is known, a straightforward dose-to-water calibration would be possible. Furthermore, if the perturbation as a function of photon beam quality is known, the plane-parallel chamber might be used for all radiation qualities (both electrons and photons) used in megavoltage therapy. In this work the response of both plane-parallel and cylindrical chambers was studied and compared with ferrous sulphate dosimetry over a wide range of photon energies. Perturbation correction factors were determined for these chambers in photon beams. Also the influence of the aluminium central electrode of the NE2571 chamber was determined in the electron field relative to Co-60 gamma-radiation. Electron backscatter from the back wall of the chamber was shown to influence the signal in the chamber significantly. A procedure for calibration of Co-60 gamma-rays in water in plane-parallel chambers is discussed.
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| 7. |
- Nyström, H, et al.
(författare)
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Photon-beam quality specification by narrow-beam transmission measurements
- 1994
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Ingår i: Physics in Medicine and Biology. - : IOP Publishing. - 0031-9155 .- 1361-6560. ; 39:8, s. 1231-1245
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Tidskriftsartikel (refereegranskat)abstract
- Radiation quality specifications in megavoltage photon beams are usually based on depth-dose measurements performed under reference conditions. Stopping-power ratios and various correction factors are then related to parameters such as TPR10(20), which are extracted from depth-dose measurements. Stopping-power ratio determinations based on this concept were shown to be in error by more than 2% at high energies. Furthermore, electrons generated in the treatment bead can, at high energies, contribute to the dose at a depth of 10 cm and thus significantly affect the TPR10(20) ratio. This method was further shown to be inadequate when the dose in other parts of the field than the reference point was to be measured with ionization chamber dosimetry. A new standardized device for determining photon beam quality based on half value layer (HVL) measurements in water was developed and thoroughly investigated in both a low-energy, (4 MV) and a high-energy beam. A relation between HVL and stopping-power ratios water-to-air was determined by comparative measurements with air ionization chambers and liquid-filled ionization chambers together with Fricke dosimetry. Furthermore, different radiation quality gradients in the photon fields for different types of field-flattening systems, and field-compensating methods were discussed.
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| 8. |
- Nyström, Mikael
(författare)
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Effect of transverse inhomogeneity on deformations and stresses in an ice cover
- 1993
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Ingår i: The proceedings of the Third (1993) International Offshore and Polar Engineering Conference. - Golden : International Society of Offshore and Polar Engineers. - 1880653079 ; , s. 581-584
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Konferensbidrag (refereegranskat)abstract
- In this paper, the finite element method has been used as the numerical modelling tool. An assumption of an elastic material and transverse inhomogeneity was included. The stress distributions over the thickness of the ice cover were produced with a finite element program. These stress distributions were compared with calculations based on the assumption of a homogenous material. In the creeping mode the calculations were based on an assumption of an inhomogeneous material. The inhomogeneity was introduced both as a variation of Young's modulus and the shear modulus. Glen's law for creep was chosen to account for creep deformations, mainly because it is a simple and well documented equation. Results from these calculations were compared with experimental data from four-point beam tests
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| 9. |
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