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| 3. |
- Knöös, Tommy, et al.
(författare)
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Which depth dose data should be used for dose planning when wedge filters are used to modify the photon beam?
- 1991
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Ingår i: Physics in Medicine and Biology. - : IOP Publishing. - 1361-6560 .- 0031-9155. ; 36:2, s. 255-267
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Tidskriftsartikel (refereegranskat)abstract
- The use of beam data for open photon fields when calculating absorbed dose distributions for beams with wedge filters has been studied. The depth doses for beams with wedge filters are changed through beam hardening and the dose maximum can be shifted; both these changes result in errors in the final dose calculations of several per cent if open beam data are used. The errors are larger for 6 MV than for 18 MV x-rays. The depth of measurement for determining the wedge factor and the influence of other beam modifying devices are discussed. It is recommended that the reference depth be used instead of the dose maximum for these kinds of measurements since the influence of contaminating electrons in the beam will then be avoided and the wedge factor will be correct at a clinically relevant depth.
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| 4. |
- Ljungberg, Michael, et al.
(författare)
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Comparison of Dual-Window and Convolution Scatter Correction Techniques using the Monte Carlo Method
- 1990
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Ingår i: Physics in Medicine and Biology. - : IOP Publishing. - 1361-6560 .- 0031-9155. ; 35:8, s. 1099-1110
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Tidskriftsartikel (refereegranskat)abstract
- In order to improve the accuracy of SPECT measurements, correction methods have to be used. Two different scatter correction techniques-the dual-window (DW) technique and the convolution (CV) technique-were compared using projection data, simulated by the Monte Carlo method. Comparison with measured data was also made to validate the accuracy of the Monte Carlo code. The main goal was to investigate how well the estimated scatter distributions, using these two scatter correction techniques, fit the simulated, true scatter distribution of the projection data obtained in the photo-peak window. The scatter distributions predicted by the CV technique were found to be consistently lower than those simulated by the Monte Carlo method in the part of the scatter distribution corresponding to the locations of the sources. The DW technique gives lower estimates of the scatter distribution. However, the scatter distribution estimated by the DW technique and the simulated scatter distribution bear a close resemblance to each other.
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| 5. |
- Nilsson, M, et al.
(författare)
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Application of the Fano theorem in inhomogeneous media using a convolution algorithm
- 1992
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Ingår i: Physics in Medicine and Biology. - : IOP Publishing. - 1361-6560 .- 0031-9155. ; 37:1, s. 69-83
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Tidskriftsartikel (refereegranskat)abstract
- For photon fields, separate dose distribution kernels have been generated for charged particles produced in the first interaction, for single and multiple scattered photons, including bremsstrahlung and annihilation. These kernels are applied in absorbed dose calculations for radiotherapy treatment planning using a convolution technique. The vast amount of kernel data required for 3D calculations can be accurately generated out of a small subset, due to rotational symmetry. Due to the discrete sampling of both the irradiated object and the dose distribution kernels, application of the Fano theorem and O'Connor's scaling theorem is not possible without difficulties. The scaling process has been thoroughly investigated and a density dependent correction factor applied to the central kernel value has been derived. The calculated absorbed dose distributions were found to agree well with Monte Carlo calculated data using the EGS4 program.
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| 7. |
- Olsson, L E, et al.
(författare)
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MR imaging of absorbed dose distributions for radiotherapy using ferrous sulphate gels
- 1990
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Ingår i: Physics in Medicine and Biology. - : IOP Publishing. - 0031-9155 .- 1361-6560. ; 35:12, s. 31-1623
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Tidskriftsartikel (refereegranskat)abstract
- The measurement of absorbed dose distributions using dosemeter gel and magnetic resonance imaging (MRI) in a standard geometry has been investigated. Absorbed depth-dose curves and profiles measured with this new technique show good agreement with corresponding measurements using diodes. This was proven in a 60Co beam as well as an electron beam. The dosemeter gel is made of agarose and ferrous sulphate solution. The dose response is linear (r = 0.9996) in the investigated dose interval, 0-40 Gy. The sensitivity is a factor of about six higher compared to ordinary ferrous sulphate solution, known as 'Fricke'. This is a true 3D dose measurement technique which will have a number of applications in radiation therapy, since it is possible to mould the gel to arbitrary geometries, mix different radiation qualities and integrate the absorbed dose from different kinds of fields.
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| 8. |
- 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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| 9. |
- 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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| 10. |
- 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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