| 1. |
- 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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| 2. |
- 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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| 3. |
- Ceberg, Crister, et al.
(författare)
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Prediction of stopping-power ratios in flattening-filter free beams.
- 2010
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Ingår i: Medical Physics. - : Wiley. - 0094-2405. ; 37:3, s. 1164-1168
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Tidskriftsartikel (refereegranskat)abstract
- PURPOSE: In recent years, there has been an increasing interest in flattening-filter free (FFF) beams. However, since the removal of the flattening filter will affect both the mean and the variance of the energy spectrum, current beam-quality specifiers may not be adequate for reference dosimetry in such beams. The purpose of this work was to investigate an alternative, more general beam-quality specifier. METHODS: The beam-quality specifier used in this work was a combination of the kerma-weighted mean and the coefficient of variation of the linear attenuation coefficient in water. These parameters can in theory be determined from narrow-beam transmission measurements using a miniphantom "in-air," which is a measurement condition well suited also to small and nonstandard fields. The relation between the Spencer-Attix stopping-power ratios and this novel beam-quality specifier was described by a simple polynomial. For reference, the authors used Monte Carlo calculated spectra and stopping-power data for nine different beams, with and without flattening filter. RESULTS: The polynomial coefficients were obtained by least-squares optimization. For all beams included in this investigation, the average of the differences between the predicted and the Monte Carlo calculated stopping-power ratios was 0.02 +/- 0.17% (1 SD) (including TomoTherapy and CyberKnife example beams). CONCLUSIONS: An alternative dual-parameter beam-quality specifier was investigated. The evaluation suggests that it can be used successfully to predict stopping-power ratios in FFF as well as conventional beams, regardless of filtration.
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| 4. |
- 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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| 5. |
- Dalaryd, Mårten, et al.
(författare)
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Combining tissue-phantom ratios to provide a beam-quality specifier for flattening filter free photon beams.
- 2014
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Ingår i: Medical Physics. - : Wiley. - 0094-2405. ; 41:11
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Tidskriftsartikel (refereegranskat)abstract
- There are currently several commercially available radiotherapy treatment units without a flattening filter in the beam line. Unflattened photon beams have an energy and lateral fluence distribution that is different from conventional beams and, thus, their attenuation properties differ. As a consequence, for flattening filter free (FFF) beams, the relationship between the beam-quality specifier TPR20,10 and the Spencer-Attix restricted water-to-air mass collision stopping-power ratios, L̄/ρair (water), may have to be refined in order to be used with equivalent accuracy as for beams with a flattening filter. The purpose of this work was twofold. First, to study the relationship between TPR20,10 and L̄/ρair (water) for FFF beams, where the flattening filter has been replaced by a metal plate as in most clinical FFF beams. Second, to investigate the potential of increasing the accuracy in determining L̄/ρair (water) by adding another beam-quality metric, TPR10,5. The relationship between L̄/ρair (water) and %dd(10)x for beams with and without a flattening filter was also included in this study.
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| 6. |
- Johnsson, Stefan, et al.
(författare)
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Off-axis primary-dose measurements using a mini-phantom
- 1997
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Ingår i: Medical Physics. - : Wiley. - 0094-2405. ; 24:5, s. 763-767
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Tidskriftsartikel (refereegranskat)abstract
- The characterization of the incident photon beam is usually divided into its dependence on collimator setting (head-scatter factor) and off-axis position (primary off-axis ratio). These parameters are normally measured "in air" with a build-up cap thick enough to generate full dose build-up at the depth of dose maximum. In order to prevent any influence from contaminating electrons, it has been recommended that head-scatter measurements are carried out using a mini-phantom rather than a conventional build-up cap. Due to the volume of the mini-phantom, the effects from attenuation and scatter are not negligible. In relative head-scatter measurements these effects cancel and the head scatter is thus a good representation of the variation of the incident photon beam with collimator setting. However, in off-axis measurements, attenuation and scatter conditions vary due to beam softening and do not cancel in the calculation of the primary off-axis ratio. The purpose of the present work was to estimate the effects from attenuation and phantom scatter in order to determine their influence on primary off-axis ratio measurements. We have characterized the off-axis beam-softening effect by means of narrow-beam transmission measurements to obtain the effective attenuation coefficient as a function of off-axis position. We then used a semi-analytical expression for the phantom-scatter calculation that depends solely on this attenuation coefficient. The derived formalism for relative "in air" measurements using a mini-phantom is clear and consistent, which enables the user to separately calculate the effects from scatter and attenuation. For the investigated beam qualities, 6 and 18 MV, our results indicate that the effects from attenuation and scatter in the mini-phantom nearly cancel (the combined effect is less than 1%) within 12.5 cm from the central beam axis. Thus, no correction is needed when the primary off-axis ratio is measured with a mini-phantom.
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| 7. |
- Petersson, Kristoffer, et al.
(författare)
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Beam commissioning and measurements validating the beam model in a new TPS that converts helical tomotherapy plans to step-and-shoot IMRT plans.
- 2011
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Ingår i: Medical Physics. - : Wiley. - 0094-2405. ; 38:1, s. 40-46
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Tidskriftsartikel (refereegranskat)abstract
- A new type of treatment planning system called SHAREPLAN has been studied, which enables the transfer of treatment plans generated for helical tomotherapy delivery to plans that can be delivered on C-arm linacs. The purpose is to ensure continuous patient treatment during periods of unscheduled downtime for the TomoTherapy unit, particularly in clinics without a backup unit. The purpose of this work was to verify that the plans generated in this novel planning system are deliverable and accurate. The work consists primarily of beam commissioning, verification of the beam model, and measurements verifying that generated plans are deliverable with sufficient accuracy.
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| 8. |
- Petersson, Kristoffer, et al.
(författare)
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Conversion of helical tomotherapy plans to step-and-shoot IMRT plans-Pareto front evaluation of plans from a new treatment planning system
- 2011
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Ingår i: Medical Physics. - : Wiley. - 0094-2405. ; 38:6, s. 3130-3138
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Tidskriftsartikel (refereegranskat)abstract
- Purpose: The resulting plans from a new type of treatment planning system called SharePlan (TM) have been studied. This software allows for the conversion of treatment plans generated in a TomoTherapy system for helical delivery, into plans deliverable on C-arm linear accelerators (linacs), which is of particular interest for clinics with a single TomoTherapy unit. The purpose of this work was to evaluate and compare the plans generated in the SharePlan system with the original TomoTherapy plans and with plans produced in our clinical treatment planning system for intensity-modulated radiation therapy (IMRT) on C-arm linacs. In addition, we have analyzed how the agreement between SharePlan and TomoTherapy plans depends on the number of beams and the total number of segments used in the optimization. Methods: Optimized plans were generated for three prostate and three head-and-neck (H&N) cases in the TomoTherapy system, and in our clinical treatment planning systems (TPS) used for IMRT planning with step-and-shoot delivery. The TomoTherapy plans were converted into step-and-shoot IMRT plans in SharePlan. For each case, a large number of Pareto optimal plans were created to compare plans generated in SharePlan with plans generated in the Tomotherapy system and in the clinical TPS. In addition, plans were generated in SharePlan for the three head-and-neck cases to evaluate how the plan quality varied with the number of beams used. Plans were also generated with different number of beams and segments for other patient cases. This allowed for an evaluation of how to minimize the number of required segments in the converted IMRT plans without compromising the agreement between them and the original TomoTherapy plans. Results: The plans made in SharePlan were as good as or better than plans from our clinical system, but they were not as good as the original TomoTherapy plans. This was true for both the head-and-neck and the prostate cases, although the differences between the plans for the latter were small. The evaluation of the head-and-neck cases also showed that the plans generated in SharePlan were improved when more beams were used. The SharePlan Pareto front came close to the front for the TomoTherapy system when a sufficient number of beams were added. The results for plans generated with varied number of beams and segments demonstrated that the number of segments could be minimized with maintained agreement between SharePlan and TomoTherapy plans when 10-19 beams were used. Conclusions: This study showed (using Pareto front evaluation) that the plans generated in SharePlan are comparable to plans generated in other TPSs. The evaluation also showed that the plans generated in SharePlan could be improved with the use of more beams. To minimize the number of segments needed in a plan with maintained agreement between the converted IMRT plans and the original TomoTherapy plans, 10-19 beams should be used, depending on target complexity. SharePlan has proved to be useful and should thereby be a time-saving complement as a backup system for clinics with a single TomoTherapy system installed alongside conventional C-arm linacs. (C) 2011 American Association of Physicists in Medicine. [DOI: 10.1118/1.3592934]
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