| 1. |
- Blad, Börje, et al.
(författare)
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Beam Adjustments for Unflattened X-ray Beam Modes for an Elekta Synergy Linear Accelerator
- 2012
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Ingår i: IFMBE Proceedings. - Berlin, Heidelberg : Springer Berlin Heidelberg. - 1680-0737. - 9783642235085 ; 37, s. 683-685
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Konferensbidrag (refereegranskat)abstract
- A flattening filter free x-ray beam mode was added to an Elekta linear accelerator by placing a flat copper plate in the filter carousel and recalibrating the electron steering servo, gun servo and dosimetry system. Machine configurations were saved onto a separate hard disk in order to remain separate from the clinical configuration. Profile measurements with a Schuster BMS diode array show that the beam is stable and start up performance is similar to the normal filtered beam.
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| 2. |
- 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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| 3. |
- Dalaryd, Mårten, et al.
(författare)
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A Monte Carlo study of a flattening filter-free linear accelerator verified with measurements.
- 2010
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Ingår i: Physics in Medicine and Biology. - : IOP Publishing. - 1361-6560 .- 0031-9155. ; 55:23, s. 7333-7344
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Tidskriftsartikel (refereegranskat)abstract
- A Monte Carlo model of an Elekta Precise linear accelerator has been built and verified by measured data for a 6 and 10 MV photon beam running with and without a flattening filter in the beam line. In this study the flattening filter was replaced with a 6 mm thick copper plate, provided by the linac vendor, in order to stabilize the beam. Several studies have shown that removal of the filter improves some properties of the photon beam, which could be beneficial for radiotherapy treatments. The investigated characteristics of this new beam included output, spectra, mean energy, half value layer and the origin of scattered photons. The results showed an increased dose output per initial electron at the central axis of 1.76 and 2.66 for the 6 and 10 MV beams, respectively. The number of scattered photons from the accelerator head was reduced by (31.7 ± 0.03)% (1 SD) for the 6 MV beam and (47.6 ± 0.02)% for the 10 MV beam. The photon energy spectrum of the unflattened beam was softer compared to a conventional beam and did not vary significantly with the off-axis distance, even for the largest field size (0-20 cm off-axis).
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| 4. |
- 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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| 5. |
- Dalaryd, Mårten
(författare)
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Dosimetric effects of removing the flattening filter in radiotherapy treatment units
- 2015
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The aim of this work was to investigate the dosimetric effects of removing the flattening filter from conventional C-arm medical linear accelerators. In conventional linear accelerators used for radiotherapy, a flattening filter is positioned in the beam line to provide a uniform lateral dose profile at a specified depth in water. However, for some radiotherapy treatments, a uniform lateral dose profile is not necessary, e.g. stereotactic treatments with small fields or treatments with intensity modulated fields. In this work, a comprehensive set of measurements and Monte Carlo simulations for a modified Elekta Precise linear accelerator, operating with and without a flattening filter, were performed and the differences were evaluated. For an Elekta Precise linac, it was found that by removing the flattening filter the dose could be delivered approximately twice as fast as when the flattening filter is in the beam line, under certain conditions. The scatter produced in the treatment head was reduced by 30 %–45 % when the flattening filter was removed and the variation of scattered radiation with field size was also reduced. Removal of the flattening filter resulted in a softer photon energy spectra which leads to a steeper absorbed dose fall-off with depth and less lateral variation across the field. By increasing the acceleration potential of the linac, the depth–dose profiles become more similar to those of the equivalent conventional photon beam and thus the output will also be increased. The suitability of two beam quality measures, TPR20,10 and %dd(10)x, in predicting water to air mass collision stopping-power ratios sw,air for flattening filter-free photon beams was also investigated. These quality measures are used in reference dosimetry for the determination of absorbed dose in water. It was shown that the relationship between TPR20,10 and sw,air used in a current international code of practice for reference dosimetry, overestimates the stopping-power ratio by approximately 0.3 % for flattening filter-free photon beams, while the relationship between %dd(10)x and sw,air, used in the North American code of practice is more accurate. A new beam quality metric, consisting of both TPR20,10 and TPR10,5 was evaluated. It was found that this new beam quality specifier more accurately predicted stopping power ratios for flattening filter-free photon beams. A beam quality specifier defined by the first two moments (describing the mean and variance) of the spectral distribution was also investigated and found to accurately predict stopping-power ratios for beams without a flattening filter.
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| 6. |
- Kragl, Gabriele, et al.
(författare)
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Dosimetric characteristics of 6 and 10 MV unflattened photon beams
- 2009
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Ingår i: Radiotherapy and Oncology. - : Elsevier BV. - 1879-0887 .- 0167-8140. ; 93:1, s. 141-146
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Tidskriftsartikel (refereegranskat)abstract
- Purpose: To determine dosimetric properties of unflattened megavoltage photon beams. Materials and methods: Dosimetric data including depth dose, profiles, output factors and phantom scatter factors from three different beam qualities provided by Elekta Precise linacs, operated with and without flattening filter were examined. Additional measurements of leaf transmission, leakage radiation and surface dose were performed. In flattening filter free (FFF) mode a 6-mm thick copper filter was placed into the beam to stabilize it. Results: Depths of dose maxima for flattened and unflattened beams did not deviate by more than 2 mm and penumbral widths agreed within 1 mm. In FFF mode the collimator exchange effect was found to be on average 0.3% for rectangular fields. Between maximum and minimum field size head scatter factors of unflattened beams showed on average 40% and 56% less variation for 6 and 10 MV beams than conventional beams. Phantom scatter factors for FFF beams differed up to 4% from the published reference data. For field sizes smaller than 15 cm, surface doses relative to the dose at d(max) increased for unflattened beams with maximum differences of 7% at 6 MV and 25% at 10 MV for a 5 x 5 cm(2) field. For a 30 x 30 cm(2) field, relative surface dose decreased by about 10% for FFF beams. Leaf transmission on the central axis was 0.3% and 0.4% lower for unflattened 6 and 10 MV beams, respectively. Leakage radiation was reduced by 52% for 6 MV and by 65% for 10 MV unflattened beams. Conclusions: The results of the study were independently confirmed at two radiotherapy centres. Phantom scatter reference data need to be reconsidered for medical accelerators operated without a flattening filter. (C) 2009 Elsevier Ireland Ltd. All rights reserved. Radiotherapy and Oncology 93 (2009) 141-146
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| 7. |
- Kragl, Gabriele, et al.
(författare)
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Flattening filter free beams in SBRT and IMRT: Dosimetric assessment of peripheral doses
- 2011
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Ingår i: Zeitschrift für Medizinische Physik. - : Elsevier BV. - 1876-4436 .- 0939-3889. ; 21:2, s. 91-101
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Tidskriftsartikel (refereegranskat)abstract
- Purpose: Recently, there has been a growing interest in operating medical linear accelerators without a flattening filter Due to reduced scatter, leaf transmission and radiation head leakage a reduction of out-of-field dose is expected for flattening filter free beams. The aim of the present study was to determine the impact of unflattened beams on peripheral dose for advanced treatment techniques with a large number of MUs. Material and methods: An Elekta Precise linac was modified to provide 6 and 10 MV photon beams without a flattening filter Basic beam data were collected and implemented into the TPS Oncentra Masterplan (Nucletron). Leakage radiation, which predominantly contributes to peripheral dose at larger distances from the field edge, was measured using a Farmer type ionisation chamber SBRT (lung) and IMRT (prostate, head&neck) treatment plans were generated for 6 and 10 MV for both flattened and unflattened beams. All treatment plans were delivered to the relevant anatomic region of an anthropomorphic phantom which was extended by a solid water slab phantom. Dosimetric measurements were performed with TLD-700 rods, radiochromic films and a Farmer type ionisation chamber The detectors were placed within the slab phantom and positioned along the isocentric longitudinal axis. Results: Using unflattened beams results in a reduction of treatment head leakage by 52% for 6 and 65% for 10 MV. Thus, peripheral doses were in general smaller for treatment plans calculated with unflattened beams. At about 20 cm distance from the field edge the dose was on average reduced by 23 and 31% for the 6 and 10 MV SBRT plans. For the IMRT plans (10 MV) the average reduction was 16% for the prostate and 18% for the head&neck case, respectively. For all examined cases, the relative deviation between peripheral doses of flattened and unflattened beams was found to increase with increasing distance from the field. Conclusions: Removing the flattening filter lead to reduced peripheral doses for advanced treatment techniques. The relative difference between peripheral doses of flattened and unflattened beams was more pronounced when the nominal beam energy was increased. Patients may benefit by decreased exposure of normal tissue to scattered dose outside the field.
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