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Developments and implementation of a standardized data library for a transit dosimetry software, on Varian and Elekta linacs
Abstracts/Physica Medica 31 (2015) e23–e54
positioned safely for the patient or the material. EPID transit IVD consists in the measurement of the signal exiting form the patient, usually located far from the skin, then conversion of this signal into dose in order to, at the end of the process, compare the measured dose to the calculated dose. Two approaches exist to correlate measurement to calculated doses: prediction of portal dose image after the patient or retroprojection of the measured dose in the patient. Many scientific works demonstrate promising results with transit IVD either by replacing traditional punctual measurements or for applications on new techniques. In France, only few teams are using the transit IVD with success, maybe due to an investment cost still important. Conclusion: Expert’s group solicited by SFPM believe that transit IVD using EPID is equivalent to traditional IVD techniques. This technique is promising, still young, and needing some years to become mature in terms of measurements techniques, reconstruction algorithms and material concerns. Transit IVD can supply to professionals a tool to daily monitoring the quality of treatments, beyond in vivo results of patient’s measurements. Experts recommend the promotion of a medico-technique and economic evaluation of this technique and R&D structures should emerge to open path for development and evaluation. http://dx.doi.org/10.1016/j.ejmp.2015.10.032
31 DEVELOPMENTS AND IMPLEMENTATION OF A STANDARDIZED DATA LIBRARY FOR A TRANSIT DOSIMETRY SOFTWARE, ON VARIAN AND ELEKTA LINACS S. Celi a,b, V. Rousseau b, P. François c. a Service de Physique Médicale, Institut Curie, Paris, France; b DOSISOFT SA, Cachan, France; c Unité de Physique Médicale, CHU de Poitiers, Poitiers, France Introduction: In the context of an increasing use of complex techniques, in vivo dosimetry using an electronic portal imaging device (EPID) – the standard equipment of most modern accelerators – is an efficient alternative to conventional in vivo dosimetry. However, the setup of EPID dosimetry requires large libraries of measured data, which slows down the installation of such a system. With the aim to exempt users from lengthy measurements, the use a “golden” data set for some parameters has been previously tested for EPIGRAY (DOSIsoft). To create a fully standardized library, the method has been extended to all the measured parameters of EPIgray and tested for linacs from the most common vendors (Varian, Elekta). Methods: The data measured for 45 Varian and Elekta beams of quality indices 0.64 to 0.79 have been recorded, including the main library elements (Tissue Maximum Ratio with a finite scattering volume – fTMR, Conversion Factors – FC) and their components measured with an ionization chamber and the EPID. Series of calculated values were then determined from the trend curves of these parameters depending on the beam quality index. To refine the results, two types of corrections are applied: one to strike the eventual measurement errors and the other to eliminate outliers. In order to validate the different elements of the “golden” library, five libraries are created from the series of standard values calculated with different settings. Then, 30 patient treatment plans for Varian linacs and 20 treatment plans for Elekta linacs were re-calculated for each of these libraries, and results have been analyzed and benchmarked. Results: The values of the parameters calculated from a corrected database have a mean difference to the reference values of less than 2%. The use of a series of standard fTMRs was successful for both Varian and Elekta beams. For Varian linacs, the library including a complete set of standardized data gives acceptable results, although preliminary results indicate better performance when the values of the EPID are measured, not calculated from the database. Finally, the set of calculated EPID values does not give representative results for Elekta cases: the Elekta database seems to be still insufficient for a conclusive estimation. Conclusions: Though subject to some control measurements, a fully standardized library can be set up for Varian accelerators and achieve results comparable to those of a measured library. For Elekta accelerators, first satisfactory results were obtained for the series of calculated fTMRs, but more measured data are needed before widely using standard FCs. http://dx.doi.org/10.1016/j.ejmp.2015.10.033
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32 TRANSPARENT PHOTON DETECTOR FOR THE ONLINE MONITORING OF IMRT BEAMS R. Delorme a , R. Fabbro a , Y. Arnoud a , B. Boyer a , L. Gallin-Martel a , M.-L. Gallin-Martel a, O. Rossetto a, I. Fonteille a, J.-Y. Giraud b. a LPSC, Université Joseph Fourier Grenoble, Grenoble, France; b Centre Hospitalier Universitaire de Grenoble (CHUG), Inserm U836, Grenoble, France Introduction: An innovative Transparent Detector for Radiotherapy (TraDeRa) has been developed for radiotherapy quality assurance (QA). It consists in a pixelated matrix of ionization chambers with a patented electrodes design. Each electrode is connected to in-house designed specific integrated circuits, providing a map of beam intensity and shape, at the linac pulse-scale. The detector aims at real-time monitoring of modulated beam upstream to the patient during delivery sessions, with a field cover up to 40 × 40 cm2. The work described here was realized on a 1:4 scale prototype in order to evaluate the dynamic of measurements, to characterize the raDeRa ability for detecting leaf position errors and to determine the impact of the detector presence on the photon beam. Methods: An acquisition system without charges losses and an electronic calibration procedure were developed to homogenize the response of each electrode and associated readout channel, allowing significant reduction of acquisition biases. An analysis method which takes into account the statistical calibration and measurement errors provides the relevant intensity deviations between two accumulated images of different acquisitions. The measurements under irradiation were performed with a clinical 6 MV X-Ray beam and a low energy high intensity photon beam from the European Synchrotron Radiation Facility (ESRF). Dose calculations are performed with the Monte Carlo code PENELOPE, modeling the full accelerator head and the TraDeRa detector. Results: A 2% attenuation of the 6 MV beam was measured in the presence of TraDeRa and the preliminary simulation study showed no significant modification of the photon beam properties. TraDeRa detects error leaf position as small as 1 mm compared to reference field, for both static(about 10% of local over-response)and modulated fields(about 4% focal overresponse, see annex). In addition, measurements are stable over a large dynamic range from low intensity signals, as inter-leaves leaks, to very high intensities as obtained on the medical beamline of ESRF (dose rate thousand times larger than that of a conventional clinical linac). Conclusions: The 1:4 version of TraDeRa shows promising results for IMRT QA, allowing pulse-scale monitoring of the beam and high sensitivity for errors detection. The attenuation seems small enough not to hinder the irradiation while keeping the beam upstream to the patient under constant control. Nevertheless, further simulations will be necessary to convert the signal response of TraDeRa to dose response. In terms of dynamic, the detector is operational for every radiotherapy treatments, including the FFF modes with high dose rates(up to 2400 MU/min).The different versions ofTraDeRa already led to two patents [1,2]. A final prototype under development will include 1600 independent electrodes, half of them forming a high resolution area centered on the beam axis. The power supply, acquisition systems and data transmission will be embedded on the device, removing all external dependency and improving the compactness of the whole system. References [1] Brevet FR N° 11/53254. [2] Brevet FR N° 13/54339. http://dx.doi.org/10.1016/j.ejmp.2015.10.034
33 A NEW APPROACH FOR CALIBRATING HIGH ENERGY IMAGING SYSTEMS (EPIDS) IN ABSORBED DOSE TO WATER C. Boutry a, P. Dudouet a,b, D. Franck b. a Groupe Oncorad Garonne, Montauban, France; b Groupe Oncorad Garonne, Toulouse, France Introduction: During the past 10 years, the mathematical models proposed for converting an EPID signal into absorbed dose to water have evolved considerably by integrating correction functions of various degrees of complexity to take modern conditions of irradiation into account. As a whole, these models require prior knowledge of mechanical and dosimetric data for the radiation beams so that suitable correction factors can be applied. However, in spite of all this sophistication, the results obtained show discrepancies between the calculated and measured doses that may exceed
positioned safely for the patient or the material. EPID transit IVD consists in the measurement of the signal exiting form the patient, usually located far from the skin, then conversion of this signal into dose in order to, at the end of the process, compare the measured dose to the calculated dose. Two approaches exist to correlate measurement to calculated doses: prediction of portal dose image after the patient or retroprojection of the measured dose in the patient. Many scientific works demonstrate promising results with transit IVD either by replacing traditional punctual measurements or for applications on new techniques. In France, only few teams are using the transit IVD with success, maybe due to an investment cost still important. Conclusion: Expert’s group solicited by SFPM believe that transit IVD using EPID is equivalent to traditional IVD techniques. This technique is promising, still young, and needing some years to become mature in terms of measurements techniques, reconstruction algorithms and material concerns. Transit IVD can supply to professionals a tool to daily monitoring the quality of treatments, beyond in vivo results of patient’s measurements. Experts recommend the promotion of a medico-technique and economic evaluation of this technique and R&D structures should emerge to open path for development and evaluation. http://dx.doi.org/10.1016/j.ejmp.2015.10.032
31 DEVELOPMENTS AND IMPLEMENTATION OF A STANDARDIZED DATA LIBRARY FOR A TRANSIT DOSIMETRY SOFTWARE, ON VARIAN AND ELEKTA LINACS S. Celi a,b, V. Rousseau b, P. François c. a Service de Physique Médicale, Institut Curie, Paris, France; b DOSISOFT SA, Cachan, France; c Unité de Physique Médicale, CHU de Poitiers, Poitiers, France Introduction: In the context of an increasing use of complex techniques, in vivo dosimetry using an electronic portal imaging device (EPID) – the standard equipment of most modern accelerators – is an efficient alternative to conventional in vivo dosimetry. However, the setup of EPID dosimetry requires large libraries of measured data, which slows down the installation of such a system. With the aim to exempt users from lengthy measurements, the use a “golden” data set for some parameters has been previously tested for EPIGRAY (DOSIsoft). To create a fully standardized library, the method has been extended to all the measured parameters of EPIgray and tested for linacs from the most common vendors (Varian, Elekta). Methods: The data measured for 45 Varian and Elekta beams of quality indices 0.64 to 0.79 have been recorded, including the main library elements (Tissue Maximum Ratio with a finite scattering volume – fTMR, Conversion Factors – FC) and their components measured with an ionization chamber and the EPID. Series of calculated values were then determined from the trend curves of these parameters depending on the beam quality index. To refine the results, two types of corrections are applied: one to strike the eventual measurement errors and the other to eliminate outliers. In order to validate the different elements of the “golden” library, five libraries are created from the series of standard values calculated with different settings. Then, 30 patient treatment plans for Varian linacs and 20 treatment plans for Elekta linacs were re-calculated for each of these libraries, and results have been analyzed and benchmarked. Results: The values of the parameters calculated from a corrected database have a mean difference to the reference values of less than 2%. The use of a series of standard fTMRs was successful for both Varian and Elekta beams. For Varian linacs, the library including a complete set of standardized data gives acceptable results, although preliminary results indicate better performance when the values of the EPID are measured, not calculated from the database. Finally, the set of calculated EPID values does not give representative results for Elekta cases: the Elekta database seems to be still insufficient for a conclusive estimation. Conclusions: Though subject to some control measurements, a fully standardized library can be set up for Varian accelerators and achieve results comparable to those of a measured library. For Elekta accelerators, first satisfactory results were obtained for the series of calculated fTMRs, but more measured data are needed before widely using standard FCs. http://dx.doi.org/10.1016/j.ejmp.2015.10.033
e35
32 TRANSPARENT PHOTON DETECTOR FOR THE ONLINE MONITORING OF IMRT BEAMS R. Delorme a , R. Fabbro a , Y. Arnoud a , B. Boyer a , L. Gallin-Martel a , M.-L. Gallin-Martel a, O. Rossetto a, I. Fonteille a, J.-Y. Giraud b. a LPSC, Université Joseph Fourier Grenoble, Grenoble, France; b Centre Hospitalier Universitaire de Grenoble (CHUG), Inserm U836, Grenoble, France Introduction: An innovative Transparent Detector for Radiotherapy (TraDeRa) has been developed for radiotherapy quality assurance (QA). It consists in a pixelated matrix of ionization chambers with a patented electrodes design. Each electrode is connected to in-house designed specific integrated circuits, providing a map of beam intensity and shape, at the linac pulse-scale. The detector aims at real-time monitoring of modulated beam upstream to the patient during delivery sessions, with a field cover up to 40 × 40 cm2. The work described here was realized on a 1:4 scale prototype in order to evaluate the dynamic of measurements, to characterize the raDeRa ability for detecting leaf position errors and to determine the impact of the detector presence on the photon beam. Methods: An acquisition system without charges losses and an electronic calibration procedure were developed to homogenize the response of each electrode and associated readout channel, allowing significant reduction of acquisition biases. An analysis method which takes into account the statistical calibration and measurement errors provides the relevant intensity deviations between two accumulated images of different acquisitions. The measurements under irradiation were performed with a clinical 6 MV X-Ray beam and a low energy high intensity photon beam from the European Synchrotron Radiation Facility (ESRF). Dose calculations are performed with the Monte Carlo code PENELOPE, modeling the full accelerator head and the TraDeRa detector. Results: A 2% attenuation of the 6 MV beam was measured in the presence of TraDeRa and the preliminary simulation study showed no significant modification of the photon beam properties. TraDeRa detects error leaf position as small as 1 mm compared to reference field, for both static(about 10% of local over-response)and modulated fields(about 4% focal overresponse, see annex). In addition, measurements are stable over a large dynamic range from low intensity signals, as inter-leaves leaks, to very high intensities as obtained on the medical beamline of ESRF (dose rate thousand times larger than that of a conventional clinical linac). Conclusions: The 1:4 version of TraDeRa shows promising results for IMRT QA, allowing pulse-scale monitoring of the beam and high sensitivity for errors detection. The attenuation seems small enough not to hinder the irradiation while keeping the beam upstream to the patient under constant control. Nevertheless, further simulations will be necessary to convert the signal response of TraDeRa to dose response. In terms of dynamic, the detector is operational for every radiotherapy treatments, including the FFF modes with high dose rates(up to 2400 MU/min).The different versions ofTraDeRa already led to two patents [1,2]. A final prototype under development will include 1600 independent electrodes, half of them forming a high resolution area centered on the beam axis. The power supply, acquisition systems and data transmission will be embedded on the device, removing all external dependency and improving the compactness of the whole system. References [1] Brevet FR N° 11/53254. [2] Brevet FR N° 13/54339. http://dx.doi.org/10.1016/j.ejmp.2015.10.034
33 A NEW APPROACH FOR CALIBRATING HIGH ENERGY IMAGING SYSTEMS (EPIDS) IN ABSORBED DOSE TO WATER C. Boutry a, P. Dudouet a,b, D. Franck b. a Groupe Oncorad Garonne, Montauban, France; b Groupe Oncorad Garonne, Toulouse, France Introduction: During the past 10 years, the mathematical models proposed for converting an EPID signal into absorbed dose to water have evolved considerably by integrating correction functions of various degrees of complexity to take modern conditions of irradiation into account. As a whole, these models require prior knowledge of mechanical and dosimetric data for the radiation beams so that suitable correction factors can be applied. However, in spite of all this sophistication, the results obtained show discrepancies between the calculated and measured doses that may exceed