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Ben Mijnheer - One of the best experts on this subject based on the ideXlab platform.
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transmission dosimetry with a liquid filled electronic portal imaging device
International Journal of Radiation Oncology Biology Physics, 1996Co-Authors: Marion Essers, M. Van Herk, Hugo Lanson, Ronald Boellaard, Ben MijnheerAbstract:PURPOSE: To assess the accuracy of transmission dose rate measurements for various phantom-detector geometries, performed with an electronic portal imaging device (EPID) and to compare these transmission dose rate values with exit dose rate data. METHODS AND MATERIALS: Transmission dose rate values on the central beam axis and beam profiles were measured with an EPID consisting of a matrix of liquid-filled Ionization Chambers. These data were compared with transmission and exit dose rate values, obtained using air-filled Ionization Chambers for a number of field sizes, phantom thickness, and phantom-detector distances. Various homogeneous and inhomogeneous phantoms were applied. RESULTS: The increase in dose rate with field size is larger for the EPID than in air, due to the larger amount of side scatter in the EPID. The difference has been taken into account by a deconvolution of the EPID images. An additional build-up layer on top of the commercial device is needed to reach dose maximum at the liquid Ionization Chambers for photon beam energies higher than about 4 MV. The transmission off-axis ratios (OAR) determined with the EPID and in air agreed within 2% for all tested cases, after deconvolution of the EPID signal. The agreement between the EPID-and exit-OAR decreased with increasing phantom-detector distance and the presence of inhomogeneities. For a phantom-detector distance of about 10 cm, the EPID- and exit-OARs agree within 2.5%. The difference could be up to 8% for an air inhomogeneity and a phantom-detector distance of 30 cm. CONCLUSIONS: The difference between EPID measurements and measurements in air can be explained by side scatter effects in the EPID and lack of adequate buildup, and can easily be taken into account. The loss of scatter compared with the situation at the exit side of the phantom explains the difference between transmission and exit dose values. At short phantom-detector distances, good agreement exists between transmission and exit dose rate. This implies that at this distance, the EPID can be used for simple comparison with exit dose calculations during patient treatments. At larger distances, more sophisticated conversion methods are required.
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transmission dosimetry with a liquid filled electronic portal imaging device
International Journal of Radiation Oncology Biology Physics, 1996Co-Authors: Marion Essers, M. Van Herk, Hugo Lanson, Ronald Boellaard, Ben MijnheerAbstract:PURPOSE: To assess the accuracy of transmission dose rate measurements for various phantom-detector geometries, performed with an electronic portal imaging device (EPID) and to compare these transmission dose rate values with exit dose rate data. METHODS AND MATERIALS: Transmission dose rate values on the central beam axis and beam profiles were measured with an EPID consisting of a matrix of liquid-filled Ionization Chambers. These data were compared with transmission and exit dose rate values, obtained using air-filled Ionization Chambers for a number of field sizes, phantom thickness, and phantom-detector distances. Various homogeneous and inhomogeneous phantoms were applied. RESULTS: The increase in dose rate with field size is larger for the EPID than in air, due to the larger amount of side scatter in the EPID. The difference has been taken into account by a deconvolution of the EPID images. An additional build-up layer on top of the commercial device is needed to reach dose maximum at the liquid Ionization Chambers for photon beam energies higher than about 4 MV. The transmission off-axis ratios (OAR) determined with the EPID and in air agreed within 2% for all tested cases, after deconvolution of the EPID signal. The agreement between the EPID-and exit-OAR decreased with increasing phantom-detector distance and the presence of inhomogeneities. For a phantom-detector distance of about 10 cm, the EPID- and exit-OARs agree within 2.5%. The difference could be up to 8% for an air inhomogeneity and a phantom-detector distance of 30 cm. CONCLUSIONS: The difference between EPID measurements and measurements in air can be explained by side scatter effects in the EPID and lack of adequate buildup, and can easily be taken into account. The loss of scatter compared with the situation at the exit side of the phantom explains the difference between transmission and exit dose values. At short phantom-detector distances, good agreement exists between transmission and exit dose rate. This implies that at this distance, the EPID can be used for simple comparison with exit dose calculations during patient treatments. At larger distances, more sophisticated conversion methods are required.
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dosimetric characteristics of a liquid filled electronic portal imaging device
International Journal of Radiation Oncology Biology Physics, 1995Co-Authors: Marion Essers, M. Van Herk, Bart R Hoogervorst, Hugo Lanson, Ben MijnheerAbstract:PURPOSE: To determine the characteristics of a commercial electronic portal imaging device (EPID), based on a two-dimensional matrix of liquid-filled Ionization Chambers, for transmission dose measurements during patient treatment. METHODS AND MATERIALS: Electronic portal imaging device measurements were performed in a cobalt-60 beam and two accelerator x-ray beams, and compared with measurements performed with a Farmer-type Ionization chamber in air in a miniphantom and in an extended water phantom. RESULTS: The warming up time of the EPID is about 1 h. The long-term stability of the detector is better than 1% under reference conditions for a period of about 3 months. The signal of the Ionization Chambers follows approximately the square root of the dose rate, although the relation becomes more linear for larger (> 1 Gy/min) dose rates. The signal can be transformed to dose rate with an accuracy of 0.6% (1 SD). The short-term influence of integrated dose on the sensitivity of the Ionization Chambers is small. The sensitivity increases about 0.5% for all Ionization Chambers after an absorbed dose of 8 Gy and returns to its original value in less than 5 min after stopping the irradiation. This small increase in sensitivity can be ascribed to the electrode distance of the Ionization Chambers in commercial EPIDs, which is 0.8 +/- 0.1 mm. The sensitivity increase depends on the electrode distance and is 4% for a 1.4 mm electrode distance. The scattering properties of the EPID Ionization Chambers were between those of an Ionization chamber in a miniphantom and in a water phantom. CONCLUSION: The matrix Ionization chamber EPID has characteristics that make it very suitable for dose rate measurements. It is therefore a very promising device for in vivo dosimetry purposes.
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dosimetric characteristics of a liquid filled electronic portal imaging device
International Journal of Radiation Oncology Biology Physics, 1995Co-Authors: Marion Essers, M. Van Herk, Bart R Hoogervorst, Hugo Lanson, Ben MijnheerAbstract:PURPOSE: To determine the characteristics of a commercial electronic portal imaging device (EPID), based on a two-dimensional matrix of liquid-filled Ionization Chambers, for transmission dose measurements during patient treatment. METHODS AND MATERIALS: Electronic portal imaging device measurements were performed in a cobalt-60 beam and two accelerator x-ray beams, and compared with measurements performed with a Farmer-type Ionization chamber in air in a miniphantom and in an extended water phantom. RESULTS: The warming up time of the EPID is about 1 h. The long-term stability of the detector is better than 1% under reference conditions for a period of about 3 months. The signal of the Ionization Chambers follows approximately the square root of the dose rate, although the relation becomes more linear for larger (> 1 Gy/min) dose rates. The signal can be transformed to dose rate with an accuracy of 0.6% (1 SD). The short-term influence of integrated dose on the sensitivity of the Ionization Chambers is small. The sensitivity increases about 0.5% for all Ionization Chambers after an absorbed dose of 8 Gy and returns to its original value in less than 5 min after stopping the irradiation. This small increase in sensitivity can be ascribed to the electrode distance of the Ionization Chambers in commercial EPIDs, which is 0.8 +/- 0.1 mm. The sensitivity increase depends on the electrode distance and is 4% for a 1.4 mm electrode distance. The scattering properties of the EPID Ionization Chambers were between those of an Ionization chamber in a miniphantom and in a water phantom. CONCLUSION: The matrix Ionization chamber EPID has characteristics that make it very suitable for dose rate measurements. It is therefore a very promising device for in vivo dosimetry purposes.
Marion Essers - One of the best experts on this subject based on the ideXlab platform.
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transmission dosimetry with a liquid filled electronic portal imaging device
International Journal of Radiation Oncology Biology Physics, 1996Co-Authors: Marion Essers, M. Van Herk, Hugo Lanson, Ronald Boellaard, Ben MijnheerAbstract:PURPOSE: To assess the accuracy of transmission dose rate measurements for various phantom-detector geometries, performed with an electronic portal imaging device (EPID) and to compare these transmission dose rate values with exit dose rate data. METHODS AND MATERIALS: Transmission dose rate values on the central beam axis and beam profiles were measured with an EPID consisting of a matrix of liquid-filled Ionization Chambers. These data were compared with transmission and exit dose rate values, obtained using air-filled Ionization Chambers for a number of field sizes, phantom thickness, and phantom-detector distances. Various homogeneous and inhomogeneous phantoms were applied. RESULTS: The increase in dose rate with field size is larger for the EPID than in air, due to the larger amount of side scatter in the EPID. The difference has been taken into account by a deconvolution of the EPID images. An additional build-up layer on top of the commercial device is needed to reach dose maximum at the liquid Ionization Chambers for photon beam energies higher than about 4 MV. The transmission off-axis ratios (OAR) determined with the EPID and in air agreed within 2% for all tested cases, after deconvolution of the EPID signal. The agreement between the EPID-and exit-OAR decreased with increasing phantom-detector distance and the presence of inhomogeneities. For a phantom-detector distance of about 10 cm, the EPID- and exit-OARs agree within 2.5%. The difference could be up to 8% for an air inhomogeneity and a phantom-detector distance of 30 cm. CONCLUSIONS: The difference between EPID measurements and measurements in air can be explained by side scatter effects in the EPID and lack of adequate buildup, and can easily be taken into account. The loss of scatter compared with the situation at the exit side of the phantom explains the difference between transmission and exit dose values. At short phantom-detector distances, good agreement exists between transmission and exit dose rate. This implies that at this distance, the EPID can be used for simple comparison with exit dose calculations during patient treatments. At larger distances, more sophisticated conversion methods are required.
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transmission dosimetry with a liquid filled electronic portal imaging device
International Journal of Radiation Oncology Biology Physics, 1996Co-Authors: Marion Essers, M. Van Herk, Hugo Lanson, Ronald Boellaard, Ben MijnheerAbstract:PURPOSE: To assess the accuracy of transmission dose rate measurements for various phantom-detector geometries, performed with an electronic portal imaging device (EPID) and to compare these transmission dose rate values with exit dose rate data. METHODS AND MATERIALS: Transmission dose rate values on the central beam axis and beam profiles were measured with an EPID consisting of a matrix of liquid-filled Ionization Chambers. These data were compared with transmission and exit dose rate values, obtained using air-filled Ionization Chambers for a number of field sizes, phantom thickness, and phantom-detector distances. Various homogeneous and inhomogeneous phantoms were applied. RESULTS: The increase in dose rate with field size is larger for the EPID than in air, due to the larger amount of side scatter in the EPID. The difference has been taken into account by a deconvolution of the EPID images. An additional build-up layer on top of the commercial device is needed to reach dose maximum at the liquid Ionization Chambers for photon beam energies higher than about 4 MV. The transmission off-axis ratios (OAR) determined with the EPID and in air agreed within 2% for all tested cases, after deconvolution of the EPID signal. The agreement between the EPID-and exit-OAR decreased with increasing phantom-detector distance and the presence of inhomogeneities. For a phantom-detector distance of about 10 cm, the EPID- and exit-OARs agree within 2.5%. The difference could be up to 8% for an air inhomogeneity and a phantom-detector distance of 30 cm. CONCLUSIONS: The difference between EPID measurements and measurements in air can be explained by side scatter effects in the EPID and lack of adequate buildup, and can easily be taken into account. The loss of scatter compared with the situation at the exit side of the phantom explains the difference between transmission and exit dose values. At short phantom-detector distances, good agreement exists between transmission and exit dose rate. This implies that at this distance, the EPID can be used for simple comparison with exit dose calculations during patient treatments. At larger distances, more sophisticated conversion methods are required.
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dosimetric characteristics of a liquid filled electronic portal imaging device
International Journal of Radiation Oncology Biology Physics, 1995Co-Authors: Marion Essers, M. Van Herk, Bart R Hoogervorst, Hugo Lanson, Ben MijnheerAbstract:PURPOSE: To determine the characteristics of a commercial electronic portal imaging device (EPID), based on a two-dimensional matrix of liquid-filled Ionization Chambers, for transmission dose measurements during patient treatment. METHODS AND MATERIALS: Electronic portal imaging device measurements were performed in a cobalt-60 beam and two accelerator x-ray beams, and compared with measurements performed with a Farmer-type Ionization chamber in air in a miniphantom and in an extended water phantom. RESULTS: The warming up time of the EPID is about 1 h. The long-term stability of the detector is better than 1% under reference conditions for a period of about 3 months. The signal of the Ionization Chambers follows approximately the square root of the dose rate, although the relation becomes more linear for larger (> 1 Gy/min) dose rates. The signal can be transformed to dose rate with an accuracy of 0.6% (1 SD). The short-term influence of integrated dose on the sensitivity of the Ionization Chambers is small. The sensitivity increases about 0.5% for all Ionization Chambers after an absorbed dose of 8 Gy and returns to its original value in less than 5 min after stopping the irradiation. This small increase in sensitivity can be ascribed to the electrode distance of the Ionization Chambers in commercial EPIDs, which is 0.8 +/- 0.1 mm. The sensitivity increase depends on the electrode distance and is 4% for a 1.4 mm electrode distance. The scattering properties of the EPID Ionization Chambers were between those of an Ionization chamber in a miniphantom and in a water phantom. CONCLUSION: The matrix Ionization chamber EPID has characteristics that make it very suitable for dose rate measurements. It is therefore a very promising device for in vivo dosimetry purposes.
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dosimetric characteristics of a liquid filled electronic portal imaging device
International Journal of Radiation Oncology Biology Physics, 1995Co-Authors: Marion Essers, M. Van Herk, Bart R Hoogervorst, Hugo Lanson, Ben MijnheerAbstract:PURPOSE: To determine the characteristics of a commercial electronic portal imaging device (EPID), based on a two-dimensional matrix of liquid-filled Ionization Chambers, for transmission dose measurements during patient treatment. METHODS AND MATERIALS: Electronic portal imaging device measurements were performed in a cobalt-60 beam and two accelerator x-ray beams, and compared with measurements performed with a Farmer-type Ionization chamber in air in a miniphantom and in an extended water phantom. RESULTS: The warming up time of the EPID is about 1 h. The long-term stability of the detector is better than 1% under reference conditions for a period of about 3 months. The signal of the Ionization Chambers follows approximately the square root of the dose rate, although the relation becomes more linear for larger (> 1 Gy/min) dose rates. The signal can be transformed to dose rate with an accuracy of 0.6% (1 SD). The short-term influence of integrated dose on the sensitivity of the Ionization Chambers is small. The sensitivity increases about 0.5% for all Ionization Chambers after an absorbed dose of 8 Gy and returns to its original value in less than 5 min after stopping the irradiation. This small increase in sensitivity can be ascribed to the electrode distance of the Ionization Chambers in commercial EPIDs, which is 0.8 +/- 0.1 mm. The sensitivity increase depends on the electrode distance and is 4% for a 1.4 mm electrode distance. The scattering properties of the EPID Ionization Chambers were between those of an Ionization chamber in a miniphantom and in a water phantom. CONCLUSION: The matrix Ionization chamber EPID has characteristics that make it very suitable for dose rate measurements. It is therefore a very promising device for in vivo dosimetry purposes.
H Palmans - One of the best experts on this subject based on the ideXlab platform.
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gradient corrections for reference dosimetry using farmer type Ionization Chambers in single layer scanned proton fields
Medical Physics, 2020Co-Authors: H Palmans, Joakim Medin, Petra Trnkova, S VatnitskyAbstract:Purpose: The local depth dose gradient and the displacement correction factor for Farmer-type Ionization Chambers are quantified for reference dosimetry at shallow depth in single-layer scanned proton fields. Method: Integrated radial profiles as a function of depth (IRPDs) measured at three proton therapy centers were smoothed by polynomial fits. The local relative depth dose gradient at measurement depths from 1 to 5 cm were derived from the derivatives of those fits. To calculate displacement correction factors, the best estimate of the effective point of measurement was derived from reviewing experimental and theoretical determinations reported in the literature. Displacement correction factors for the use of Farmer-type Ionization Chambers with their reference point (at the center of the cavity volume) positioned at the measurement depth were derived as a ratio of IRPD values at the measurement depth and at the effective point of measurement. Results: Depth dose gradients are as low as 0.1–0.4% per mm at measurement depths from 1 to 5 cm in the highest clinical proton energies (with residual ranges higher than 15 cm) and increase to 1% per mm at a residual range of 4 cm and become larger than 3% per mm for residual ranges lower than 2 cm. The literature review shows that the effective point of measurement of Farmer-type Ionization Chambers is, similarly as for carbon ion beams, located 0.75 times the cavity radius closer to the beam origin as the center of the cavity. If a maximum displacement correction of 2% is deemed acceptable to be included in calculated beam quality correction factors, Farmer-type ICs can be used at measurements depths from 1 to 5 cm for which the residual range is 4 cm or larger. If one wants to use the same beam quality correction factors as applicable to the conventional measurement point for scattered beams, located at the center of the SOBP, the relative standard uncertainty on the assumption that the displacement correction factor is unity can be kept below 0.5% for measurement depths of at least 2 cm and for residual ranges of 15 cm or higher. Conclusion: The literature review confirmed that for proton beams the effective point of measurement of Farmer-type Ionization Chambers is located 0.75 times the cavity radius closer to the beam origin as the center of the cavity. Based on the findings in this work, three options can be recommended for reference dosimetry of scanned proton beams using Farmer-type Ionization Chambers: (a) positioning the effective point of measurement at the measurement depth, (b) positioning the reference point at the measurement depth and applying a displacement correction factor, and (c) positioning the reference point at the measurement depth without applying a displacement correction factor. Based on limiting the acceptable uncertainty on the gradient correction factor to 0.5% and the maximum deviation of the displacement perturbation correction factor from unity to 2%, the first two options can be allowed for residual ranges of at least 4 cm while the third option only for residual ranges of at least 15 cm.
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reply to comment on lateral response heterogeneity of bragg peak Ionization Chambers for narrow beam photon and proton dosimetry
Physics in Medicine and Biology, 2019Co-Authors: Peter Kuess, T T Bohlen, Wolfgang Lechner, A Elia, Dietmar Georg, H PalmansAbstract:Shen (2019) commented on our paper 'Lateral response heterogeneity of Bragg peak Ionization Chambers for narrow-beam photon and proton dosimetry' regarding the impact of the low dose tail of the collimated x-ray beam we used to acquire individual response maps of large area Ionization Chambers. The behavior of this low dose tail was measured and compared to the simulations performed by Shen (2019). It was shown that the model of the tail by Shen (2019) is too simplistic and overestimates its effect.
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lateral response heterogeneity of bragg peak Ionization Chambers for narrow beam photon and proton dosimetry
Physics in Medicine and Biology, 2017Co-Authors: Peter Kuess, T T Bohlen, Wolfgang Lechner, A Elia, Dietmar Georg, H PalmansAbstract:Large area Ionization Chambers (LAICs) can be used to measure output factors of narrow beams. Dose area product measurements are proposed as an alternative to central-axis point dose measurements. Using such detectors requires detailed information on the uniformity of the response along the sensitive area. Eight LAICs were investigated in this study: four of type PTW-34070 (LAICThick) and four of type PTW-34080 (LAICThin). Measurements were performed in an x-ray unit using peak voltages of 100–200 kVp and a collimated beam of 3.1 mm (FWHM). The LAICs were moved with a step size of 5 mm to measure the chamber response at lateral positions. To account for beam positions where only a fraction of the beam impinged within the sensitive area of the LAICs, a corrected response was calculated which was the basis to calculate the relative response. The impact of a heterogeneous LAIC response, based on the obtained response maps was henceforth investigated for proton pencil beams and small field photon beams. A pronounced heterogeneity of the responses was observed in the investigated LAICs. The response of LAICThick generally decreased with increasing radius, resulting in a response correction of up to 5%. This correction was more pronounced and more diverse (up to 10%) for LAICThin. Considering a proton pencil beam the systematic offset for reference dosimetry was 2.4–4.1% for LAICThick and −9.5 to 9.4% for LAICThin. For relative dosimetry (e.g. integral depth-dose curves) systematic response variation by 0.8–1.9% were found. For a decreasing photon field size the systematic offset for absolute dose measurements showed a 2.5–4.5% overestimation of the response for 6 × 6 mm2 field sizes for LAICThick. For LAICThin the response varied even over a range of 20%. This study highlights the need for chamber-dependent response maps when using LAICs for absolute and relative dosimetry with proton pencil beams or small photon beams.
Klemens Zink - One of the best experts on this subject based on the ideXlab platform.
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monte carlo calculation of perturbation correction factors for air filled Ionization Chambers in clinical proton beams using topas geant
Zeitschrift Fur Medizinische Physik, 2021Co-Authors: Kiliansimon Baumann, Sina Kaupa, Constantin Bach, Rita Engenhartcabillic, Klemens ZinkAbstract:Abstract Introduction Current dosimetry protocols for clinical protons using air-filled Ionization Chambers assume that the perturbation correction factor is equal to unity for all Ionization Chambers and proton energies . Since previous Monte Carlo based studies suggest that perturbation correction factors might be significantly different from unity this study aims to determine perturbation correction factors for six plane-parallel and four cylindrical Ionization Chambers in proton beams at clinical energies. Materials and methods The dose deposited in the air cavity of the Ionization Chambers was calculated with the help of the Monte Carlo code TOPAS/Geant4 while specific constructive details of the Chambers were removed step by step. By comparing these dose values the individual perturbation correction factors p cel , p stem , p sleeve , p wall , p cav ⋅ p dis as well as the total perturbation correction factor p Q were derived for typical clinical proton energies between 80 and 250 MeV. Results The total perturbation correction factor p Q was smaller than unity for almost every Ionization chamber and proton energy and in some cases significantly different from unity (deviation larger than 1%). The maximum deviation from unity was 2.0% for cylindrical and 1.5% for plane-parallel Ionization Chambers. Especially the factor p wall was found to differ significantly from unity. It was shown that this is due to the fact that secondary particles, especially alpha particles and fragments, are scattered from the chamber wall into the air cavity resulting in an overresponse of the chamber. Conclusion Perturbation correction factors for Ionization Chambers in proton beams were calculated using Monte Carlo simulations . In contrast to the assumption of current dosimetry protocols the total perturbation correction factor p Q can be significantly different from unity. Hence, beam quality correction factors k Q , Q 0 that are calculated with the help of perturbation correction factors that are assumed to be unity come with a corresponding additional uncertainty.
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su g tep1 03 beam quality correction factors for linear accelerator with and without flattening filter
Medical Physics, 2016Co-Authors: D Czarnecki, B Poppe, P Von Voigtsrhetz, Klemens ZinkAbstract:Purpose: The impact of removing the flattening filter on absolute dosimetry based on IAEA's TPR-398 and AAPM's TG-51 was investigated in this study using Monte Carlo simulations. Methods: The EGSnrc software package was used for all Monte Carlo simulations performed in this work. Five different Ionization Chambers and nine linear accelerator heads have been modeled according to technical drawings. To generate a flattening filter free radiation field the flattening filter was replaced by a 2 mm thick aluminum layer. Dose calculation in a water phantom were performed to calculate the beam quality correction factor kQ as a function of the beam quality specifiers %dd(10)x, TPR20,10 and mean photon and electron energies at the point of measurement in photon fields with (WFF) and without flattening filter (FFF). Results: The beam quality correction factor as a function of %dd(10)x differs systematically between FFF and WFF beams for all investigated Ionization Chambers. The largest difference of 1.8% was observed for the largest investigated Farmer-type Ionization chamber with a sensitive volume of 0.69 cm3. For Ionization Chambers with a smaller nominal sensitive volume (0.015 – 0.3 cm3) the deviation was less than 0.4% between WFF and FFF beams for %dd(10)x > 62%. The specifier TPR20,10 revealed only a good correlation between WFF and FFF beams (< 0.3%) for low energies. Conclusion: The results confirm that %dd(10)x is a suitable beam quality specifier for FFF beams with an acceptable bias. The deviation depends on the volume of the Ionization chamber. Using %dd(10)x to predictkQ for a large volume chamber in a FFF photon field may lead to not acceptable errors according to the results of this study. This bias may be caused by the volume effect due to the inhomogeneous photon fields of FFF linear accelerators.
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monte carlo based perturbation and beam quality correction factors for thimble Ionization Chambers in high energy photon beams
Physics in Medicine and Biology, 2008Co-Authors: J Wulff, Johannes T Heverhagen, Klemens ZinkAbstract:This paper presents a detailed investigation into the calculation of perturbation and beam quality correction factors for Ionization Chambers in high-energy photon beams with the use of Monte Carlo simulations. For a model of the NE2571 Farmer-type chamber, all separate perturbation factors as found in the current dosimetry protocols were calculated in a fixed order and compared to the currently available data. Furthermore, the NE2571 Farmer-type and a model of the PTW31010 thimble chamber were used to calculate the beam quality correction factor kQ. The calculations of kQ showed good agreement with the published values in the current dosimetry protocols AAPM TG-51 and IAEA TRS-398 and a large set of published measurements. Still, some of the single calculated perturbation factors deviate from the commonly used ones; especially prepl deviates more than 0.5%. The influence of various sources of uncertainties in the simulations is investigated for the NE2571 model. The influence of constructive details of the chamber stem shows a negligible dependence on calculated values. A comparison between a full linear accelerator source and a simple collimated point source with linear accelerator photon spectra yields comparable results. As expected, the calculation of the overall beam quality correction factor is sensitive to the mean Ionization energy of graphite used. The measurement setup (source–surface distance versus source–axis distance) had no influence on the calculated values.
Hugo Lanson - One of the best experts on this subject based on the ideXlab platform.
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transmission dosimetry with a liquid filled electronic portal imaging device
International Journal of Radiation Oncology Biology Physics, 1996Co-Authors: Marion Essers, M. Van Herk, Hugo Lanson, Ronald Boellaard, Ben MijnheerAbstract:PURPOSE: To assess the accuracy of transmission dose rate measurements for various phantom-detector geometries, performed with an electronic portal imaging device (EPID) and to compare these transmission dose rate values with exit dose rate data. METHODS AND MATERIALS: Transmission dose rate values on the central beam axis and beam profiles were measured with an EPID consisting of a matrix of liquid-filled Ionization Chambers. These data were compared with transmission and exit dose rate values, obtained using air-filled Ionization Chambers for a number of field sizes, phantom thickness, and phantom-detector distances. Various homogeneous and inhomogeneous phantoms were applied. RESULTS: The increase in dose rate with field size is larger for the EPID than in air, due to the larger amount of side scatter in the EPID. The difference has been taken into account by a deconvolution of the EPID images. An additional build-up layer on top of the commercial device is needed to reach dose maximum at the liquid Ionization Chambers for photon beam energies higher than about 4 MV. The transmission off-axis ratios (OAR) determined with the EPID and in air agreed within 2% for all tested cases, after deconvolution of the EPID signal. The agreement between the EPID-and exit-OAR decreased with increasing phantom-detector distance and the presence of inhomogeneities. For a phantom-detector distance of about 10 cm, the EPID- and exit-OARs agree within 2.5%. The difference could be up to 8% for an air inhomogeneity and a phantom-detector distance of 30 cm. CONCLUSIONS: The difference between EPID measurements and measurements in air can be explained by side scatter effects in the EPID and lack of adequate buildup, and can easily be taken into account. The loss of scatter compared with the situation at the exit side of the phantom explains the difference between transmission and exit dose values. At short phantom-detector distances, good agreement exists between transmission and exit dose rate. This implies that at this distance, the EPID can be used for simple comparison with exit dose calculations during patient treatments. At larger distances, more sophisticated conversion methods are required.
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transmission dosimetry with a liquid filled electronic portal imaging device
International Journal of Radiation Oncology Biology Physics, 1996Co-Authors: Marion Essers, M. Van Herk, Hugo Lanson, Ronald Boellaard, Ben MijnheerAbstract:PURPOSE: To assess the accuracy of transmission dose rate measurements for various phantom-detector geometries, performed with an electronic portal imaging device (EPID) and to compare these transmission dose rate values with exit dose rate data. METHODS AND MATERIALS: Transmission dose rate values on the central beam axis and beam profiles were measured with an EPID consisting of a matrix of liquid-filled Ionization Chambers. These data were compared with transmission and exit dose rate values, obtained using air-filled Ionization Chambers for a number of field sizes, phantom thickness, and phantom-detector distances. Various homogeneous and inhomogeneous phantoms were applied. RESULTS: The increase in dose rate with field size is larger for the EPID than in air, due to the larger amount of side scatter in the EPID. The difference has been taken into account by a deconvolution of the EPID images. An additional build-up layer on top of the commercial device is needed to reach dose maximum at the liquid Ionization Chambers for photon beam energies higher than about 4 MV. The transmission off-axis ratios (OAR) determined with the EPID and in air agreed within 2% for all tested cases, after deconvolution of the EPID signal. The agreement between the EPID-and exit-OAR decreased with increasing phantom-detector distance and the presence of inhomogeneities. For a phantom-detector distance of about 10 cm, the EPID- and exit-OARs agree within 2.5%. The difference could be up to 8% for an air inhomogeneity and a phantom-detector distance of 30 cm. CONCLUSIONS: The difference between EPID measurements and measurements in air can be explained by side scatter effects in the EPID and lack of adequate buildup, and can easily be taken into account. The loss of scatter compared with the situation at the exit side of the phantom explains the difference between transmission and exit dose values. At short phantom-detector distances, good agreement exists between transmission and exit dose rate. This implies that at this distance, the EPID can be used for simple comparison with exit dose calculations during patient treatments. At larger distances, more sophisticated conversion methods are required.
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dosimetric characteristics of a liquid filled electronic portal imaging device
International Journal of Radiation Oncology Biology Physics, 1995Co-Authors: Marion Essers, M. Van Herk, Bart R Hoogervorst, Hugo Lanson, Ben MijnheerAbstract:PURPOSE: To determine the characteristics of a commercial electronic portal imaging device (EPID), based on a two-dimensional matrix of liquid-filled Ionization Chambers, for transmission dose measurements during patient treatment. METHODS AND MATERIALS: Electronic portal imaging device measurements were performed in a cobalt-60 beam and two accelerator x-ray beams, and compared with measurements performed with a Farmer-type Ionization chamber in air in a miniphantom and in an extended water phantom. RESULTS: The warming up time of the EPID is about 1 h. The long-term stability of the detector is better than 1% under reference conditions for a period of about 3 months. The signal of the Ionization Chambers follows approximately the square root of the dose rate, although the relation becomes more linear for larger (> 1 Gy/min) dose rates. The signal can be transformed to dose rate with an accuracy of 0.6% (1 SD). The short-term influence of integrated dose on the sensitivity of the Ionization Chambers is small. The sensitivity increases about 0.5% for all Ionization Chambers after an absorbed dose of 8 Gy and returns to its original value in less than 5 min after stopping the irradiation. This small increase in sensitivity can be ascribed to the electrode distance of the Ionization Chambers in commercial EPIDs, which is 0.8 +/- 0.1 mm. The sensitivity increase depends on the electrode distance and is 4% for a 1.4 mm electrode distance. The scattering properties of the EPID Ionization Chambers were between those of an Ionization chamber in a miniphantom and in a water phantom. CONCLUSION: The matrix Ionization chamber EPID has characteristics that make it very suitable for dose rate measurements. It is therefore a very promising device for in vivo dosimetry purposes.
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dosimetric characteristics of a liquid filled electronic portal imaging device
International Journal of Radiation Oncology Biology Physics, 1995Co-Authors: Marion Essers, M. Van Herk, Bart R Hoogervorst, Hugo Lanson, Ben MijnheerAbstract:PURPOSE: To determine the characteristics of a commercial electronic portal imaging device (EPID), based on a two-dimensional matrix of liquid-filled Ionization Chambers, for transmission dose measurements during patient treatment. METHODS AND MATERIALS: Electronic portal imaging device measurements were performed in a cobalt-60 beam and two accelerator x-ray beams, and compared with measurements performed with a Farmer-type Ionization chamber in air in a miniphantom and in an extended water phantom. RESULTS: The warming up time of the EPID is about 1 h. The long-term stability of the detector is better than 1% under reference conditions for a period of about 3 months. The signal of the Ionization Chambers follows approximately the square root of the dose rate, although the relation becomes more linear for larger (> 1 Gy/min) dose rates. The signal can be transformed to dose rate with an accuracy of 0.6% (1 SD). The short-term influence of integrated dose on the sensitivity of the Ionization Chambers is small. The sensitivity increases about 0.5% for all Ionization Chambers after an absorbed dose of 8 Gy and returns to its original value in less than 5 min after stopping the irradiation. This small increase in sensitivity can be ascribed to the electrode distance of the Ionization Chambers in commercial EPIDs, which is 0.8 +/- 0.1 mm. The sensitivity increase depends on the electrode distance and is 4% for a 1.4 mm electrode distance. The scattering properties of the EPID Ionization Chambers were between those of an Ionization chamber in a miniphantom and in a water phantom. CONCLUSION: The matrix Ionization chamber EPID has characteristics that make it very suitable for dose rate measurements. It is therefore a very promising device for in vivo dosimetry purposes.
