- Email: [email protected]
VMAT techniques
Abstracts / Physica Medica 52 (2018) 99–187
bed. In France, the most used IORT treatment system is the INTRABEAM one, manufactured by the Zeiss Company, which is delivered with a database used to assess the absolute dose distribution. This work aims at establishing traceability to a dosimetry reference independent from the manufacturer, as recommended in the report published in 2016 by the French National Authority for Health on the evaluation of such treatment. Methods. Several types of IORT low-energy photon sources are commercially available and any metrology laboratory cannot afford to be equipped with all of them. So a multi-step methodology, applicable to any of these sources, was developed both to establish and transfer a dosimetry reference. This methodology removes the requirement to bring together, within the same laboratory, the source and the dedicated, but non-transportable, primary standard Free-Air-Chamber (FAC). Firstly, the photon energy spectrum of the commercial source is characterized at the hospital, then the spectrum is reproduced at the primary laboratory using a conventional X-ray generator. The reproduced beam is then characterized, in terms of air kerma rate, using a primary FAC and also used to calibrate a transfer cavity ionization chamber, which is in turn used to calibrate the INTRABEAMÒ system. Finally, a Monte Carlo model of the source is developed to calculate a conversion factor, from air kerma to absorbed dose to water under reference conditions. Results. This methodology was applied to the INTRABEAMÒ system of a hospital associated with a 4 cm spherical applicator. The absorbed dose to water was established at a depth of 1 cm in water. The Zeiss Company recently changed its dosimetry procedure, going from the TARGIT method to the non-TARGIT one. Our value is higher by 33% and 16% respectively than those given by the TARGIT and non-TARGIT methods. Conclusions. The present study shows, for IORT X-ray sources, the importance of making available to end-users independent dosimetry measurements traceable to national standards. https://doi.org/10.1016/j.ejmp.2018.06.472
[P173] Proposal of a comprehensive pre-treatment QA procedure in IMRT/VMAT techniques Abdulaziz Alhazmi a, Chiara Gianoli a, Juliana Martins a,*, Sebastian Neppl b, Stella Veloza a, Michael Reiner b, Claus Belka c, Katia Parodi a a
Ludwig-Maximilians-Universität München, Department of Medical Physics, Munich, Germany b University Hospital, LMU Munich, Department of Radiation Oncology, Munich, Germany c Klinikum Großhadern LMU, LMU Munich, Department of Radiation Oncology, Munich, Germany ⇑ Corresponding author. Purpose. The abstract aims to propose a comprehensive pretreatment QA procedure, independent of TPS calculation and Linac control system, to detect errors related to dose calculation and/or dose delivery in state-of-art IMRT/VMAT techniques. Methods. The proposed pre-treatment QA procedure consists of the subsequent application of two independent methodologies: one relevant to dose calculation QA and the other to dose delivery QA. The dose calculation QA relies on validating the analytical TPS dose distribution with a more accurate equipment-specific MonteCarlo (MC) dose simulation [1]. A Geant4 (MC) model is developed to be used for 3D dose distribution simulations. The dose delivery QA relies on an in-house developed, planspecific, 3D non-transit dosimetry approach based on EPID technology. The methodology reconstructs a 3D dose distribution in a
149
virtual cylindrical water phantom based on calibrated and backprojected EPID images [2]. The accuracy of the reconstructed 3D dose distribution has been investigated in this work. The sensitivity to geometric and dosimetric errors between the original treatment plan and intentionally modified treatment plans has been assessed in terms of controlled translational shifts of the MLC, rotational shifts of the gantry and over-dosages. Quantification is based on gamma evaluation with acceptance criteria of (3 mm, 3%). The QA evaluation of the dose calculation is performed between the TPS dose distribution and the MC dose simulation. Afterwards, the dose delivery QA evaluation is performed between the TPS calculation and the reconstructed 3D dose distribution in the virtual water phantom, as sufficiently accurate in homogeneous (water) media. Results. The initial adequacy of the two presented methodologies for the proposal of a comprehensive pre-treatment QA procedure is promising. An overview of the results under controlled changes and/or errors will be presented and the interpretation of detected errors in real clinical scenarios will be discussed. Conclusions. The proposed comprehensive pre-treatment QA procedure provides independent validation methods with the potential to quantitatively support the decision making process in presence of errors related to dose calculation or dose delivery. References 1. Kry, Stephen F, et al. Int J Radiat Oncol Biol Phys. 2013;85.1: e95–e100. 2. Alhazmi A et al. Med Phys 2017;44.6:2887. https://doi.org/10.1016/j.ejmp.2018.06.473
[P174] Study for the amendment of sensitivity quenching in carbon-ion dose images acquired with Gafchromic films Grazia Gambarini a, Giorgio Barzon a, Daniela Bettega a, Lorenzo Bettinelli a, Gabriele Camoni a, Mauro Carrara b,*, Alfredo Mirandola c, Mario Ciocca c a Università Degli Studi DI Milano, National Institute of Nuclear Physics (Infn), Section of Milan, Department of Physics, Milan, Italy b Fondazione Irccs Istituto Nazionale Dei Tumori, Department of Diagnostic Imaging and Radiotherapy, Medical Physics, Milan, Italy c Centro Nazionale DI Adroterapia Oncologoca (Cnao), Medical Physics Unit, Pavia, Italy ⇑ Corresponding author.
Purpose. Gafchromic films could be valid detectors for in-phantom dose controls in hadrontherapy but the quenching of their sensitivity with increasing the linear energy transfer (LET) of radiation brings a limit to their utilisation. This effect is manifest in the case of exposures to protons but it is really noticeable for carbon ions, whose LET is high also at the beam-entrance position. A method for correcting dose images acquired with films irradiated with proton beams has been initially proposed and tested with high approximations [1] and then developed with more exhaustive procedure [2]. In this work, studies aimed at applying the method, in his final structure, to carbon ions are carried out. Methods. Gafchromic EBT3 films that were suitably placed in a solid-water phantom and exposed to carbon ion beams at the synchrotron of CNAO (Pavia, Italy). Optical analyses were performed with conventional scanner or laboratory-made instrument for light-transmittance detection. The method for amending dose images evaluates, in each image point, a correction coefficient
bed. In France, the most used IORT treatment system is the INTRABEAM one, manufactured by the Zeiss Company, which is delivered with a database used to assess the absolute dose distribution. This work aims at establishing traceability to a dosimetry reference independent from the manufacturer, as recommended in the report published in 2016 by the French National Authority for Health on the evaluation of such treatment. Methods. Several types of IORT low-energy photon sources are commercially available and any metrology laboratory cannot afford to be equipped with all of them. So a multi-step methodology, applicable to any of these sources, was developed both to establish and transfer a dosimetry reference. This methodology removes the requirement to bring together, within the same laboratory, the source and the dedicated, but non-transportable, primary standard Free-Air-Chamber (FAC). Firstly, the photon energy spectrum of the commercial source is characterized at the hospital, then the spectrum is reproduced at the primary laboratory using a conventional X-ray generator. The reproduced beam is then characterized, in terms of air kerma rate, using a primary FAC and also used to calibrate a transfer cavity ionization chamber, which is in turn used to calibrate the INTRABEAMÒ system. Finally, a Monte Carlo model of the source is developed to calculate a conversion factor, from air kerma to absorbed dose to water under reference conditions. Results. This methodology was applied to the INTRABEAMÒ system of a hospital associated with a 4 cm spherical applicator. The absorbed dose to water was established at a depth of 1 cm in water. The Zeiss Company recently changed its dosimetry procedure, going from the TARGIT method to the non-TARGIT one. Our value is higher by 33% and 16% respectively than those given by the TARGIT and non-TARGIT methods. Conclusions. The present study shows, for IORT X-ray sources, the importance of making available to end-users independent dosimetry measurements traceable to national standards. https://doi.org/10.1016/j.ejmp.2018.06.472
[P173] Proposal of a comprehensive pre-treatment QA procedure in IMRT/VMAT techniques Abdulaziz Alhazmi a, Chiara Gianoli a, Juliana Martins a,*, Sebastian Neppl b, Stella Veloza a, Michael Reiner b, Claus Belka c, Katia Parodi a a
Ludwig-Maximilians-Universität München, Department of Medical Physics, Munich, Germany b University Hospital, LMU Munich, Department of Radiation Oncology, Munich, Germany c Klinikum Großhadern LMU, LMU Munich, Department of Radiation Oncology, Munich, Germany ⇑ Corresponding author. Purpose. The abstract aims to propose a comprehensive pretreatment QA procedure, independent of TPS calculation and Linac control system, to detect errors related to dose calculation and/or dose delivery in state-of-art IMRT/VMAT techniques. Methods. The proposed pre-treatment QA procedure consists of the subsequent application of two independent methodologies: one relevant to dose calculation QA and the other to dose delivery QA. The dose calculation QA relies on validating the analytical TPS dose distribution with a more accurate equipment-specific MonteCarlo (MC) dose simulation [1]. A Geant4 (MC) model is developed to be used for 3D dose distribution simulations. The dose delivery QA relies on an in-house developed, planspecific, 3D non-transit dosimetry approach based on EPID technology. The methodology reconstructs a 3D dose distribution in a
149
virtual cylindrical water phantom based on calibrated and backprojected EPID images [2]. The accuracy of the reconstructed 3D dose distribution has been investigated in this work. The sensitivity to geometric and dosimetric errors between the original treatment plan and intentionally modified treatment plans has been assessed in terms of controlled translational shifts of the MLC, rotational shifts of the gantry and over-dosages. Quantification is based on gamma evaluation with acceptance criteria of (3 mm, 3%). The QA evaluation of the dose calculation is performed between the TPS dose distribution and the MC dose simulation. Afterwards, the dose delivery QA evaluation is performed between the TPS calculation and the reconstructed 3D dose distribution in the virtual water phantom, as sufficiently accurate in homogeneous (water) media. Results. The initial adequacy of the two presented methodologies for the proposal of a comprehensive pre-treatment QA procedure is promising. An overview of the results under controlled changes and/or errors will be presented and the interpretation of detected errors in real clinical scenarios will be discussed. Conclusions. The proposed comprehensive pre-treatment QA procedure provides independent validation methods with the potential to quantitatively support the decision making process in presence of errors related to dose calculation or dose delivery. References 1. Kry, Stephen F, et al. Int J Radiat Oncol Biol Phys. 2013;85.1: e95–e100. 2. Alhazmi A et al. Med Phys 2017;44.6:2887. https://doi.org/10.1016/j.ejmp.2018.06.473
[P174] Study for the amendment of sensitivity quenching in carbon-ion dose images acquired with Gafchromic films Grazia Gambarini a, Giorgio Barzon a, Daniela Bettega a, Lorenzo Bettinelli a, Gabriele Camoni a, Mauro Carrara b,*, Alfredo Mirandola c, Mario Ciocca c a Università Degli Studi DI Milano, National Institute of Nuclear Physics (Infn), Section of Milan, Department of Physics, Milan, Italy b Fondazione Irccs Istituto Nazionale Dei Tumori, Department of Diagnostic Imaging and Radiotherapy, Medical Physics, Milan, Italy c Centro Nazionale DI Adroterapia Oncologoca (Cnao), Medical Physics Unit, Pavia, Italy ⇑ Corresponding author.
Purpose. Gafchromic films could be valid detectors for in-phantom dose controls in hadrontherapy but the quenching of their sensitivity with increasing the linear energy transfer (LET) of radiation brings a limit to their utilisation. This effect is manifest in the case of exposures to protons but it is really noticeable for carbon ions, whose LET is high also at the beam-entrance position. A method for correcting dose images acquired with films irradiated with proton beams has been initially proposed and tested with high approximations [1] and then developed with more exhaustive procedure [2]. In this work, studies aimed at applying the method, in his final structure, to carbon ions are carried out. Methods. Gafchromic EBT3 films that were suitably placed in a solid-water phantom and exposed to carbon ion beams at the synchrotron of CNAO (Pavia, Italy). Optical analyses were performed with conventional scanner or laboratory-made instrument for light-transmittance detection. The method for amending dose images evaluates, in each image point, a correction coefficient