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[P251] Dosimetric impact of intermediate dose calculation on heterogeneous region radiotherapy planning
Abstracts / Physica Medica 52 (2018) 99–187
Comparing the data across the departments revealed no systematic differences based on the absorbed dose protocol each department followed. There may be a slight difference in the determination of relative output of the different applicators depending on the ion chamber used for output factor measurements, but with limited data available this remains inconclusive. The 90% and 85% dose to water range of x-rays agree between the departments within 1 mm for similar beam qualities, even though different methods were used to determine the depth dose deposition. Conclusions. A national comparison of kV radiotherapy units was conducted, and overall the units were found very similar. No systematic differences were observed based on the calibration protocol, but the available data hint that the choice of ion chamber for calibration measurements may have a small effect on the determination of output factors. https://doi.org/10.1016/j.ejmp.2018.06.530
[P251] Dosimetric impact of intermediate dose calculation on heterogeneous region radiotherapy planning Ugur Akbas a,*, Canan Koksal a, Nazmiye Donmez Kesen a, Gizem Kaval b, Sule Karaman b, Nergiz Dagoglu b, Hatice Bilge a, Musa Altun b a
Istanbul University Oncology Institute, Medical Physics, Istanbul, Turkey b Istanbul University Medical Faculty, Radiation Oncology, Istanbul, Turkey ⇑ Corresponding author. Purpose. In radiotherapy planning of heterogeneous regions, differences which negatively affect the quality of the plan in terms of homogeneity and conformity may occur between the optimal dose volume histogram (DVH) and the final calculated DVH. The intermediate dose calculation (IDC) module, which is included in the Analytical Anisotropic Algorithm (AAA), is used even though the planning period is slightly increased in radiotherapy plans of lung tumors with low tissue density to reduce these differences. The purpose of this study is to examine the impact of IDC on radiotherapy planning for the heterogeneous maxillary sinus region. Methods. In this study, the homogeneity index (HI) and the conformity index (CI) of the intensity modulated radiotherapy plans prepared using Eclipse-TPS and AAA v8.9 of 12 patients with maxillary sinus cancer were calculated. The original plans were reoptimized with the same optimization criteria by using AAA v15.1 with and without IDC. HI and CI values of the new plans were calculated after re-optimization. The HI and CI values, maximum doses and the critical organ doses were compared between the original and new plans. Results. AAA v15.1 with IDC increased the plan quality in terms of HI, CI and maximum dose values. The HI values (ideal value=0) were found to be 0.093, 0.090 and 0.067; the CI values (ideal value=1) were found to be 1.149, 1.142 and 1.055; the maximum doses were found to be 108.1%, 107.8% and 105.3% for AAA v8.9, AAA v15.1 without IDC and AAA v15.1 with IDC, respectively. The differences between AAA v15.1 with IDC and the others are statistically significant for HI values, CI values and maximum doses but there is no significant difference for the doses received by critical organs. Conclusions. The use of the IDC increases the quality of intensity modulated radiotherapy plans in heterogeneous regions such as head and neck. This work was supported by Scientific Research Projects Coordination Unit of Istanbul University (Project Number: I.U.BAP-23057) https://doi.org/10.1016/j.ejmp.2018.06.531
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[P252] Energy calibration of semi-conductor detectors for computed tomography Stephan Rosendahl a,*, Ludwig Büermann b a
Physikalisch-Technische Bundesanstalt, Dosimetrie für Strahlentherapie und Röntgendiagnostik, Braunschweig, Germany b Physikalisch-Technische Bundesanstalt, 6.2 - Dosimetry for Radiation Therapy and Diagnostic Radiology, Braunschweig, Germany ⇑ Corresponding author. Purpose. Semiconductor detectors are widely used in computed tomography (CT). Their high spatial resolution and sensitivity give them an advantage compared to ionization chambers. Unfortunately, their air kerma response is strongly energy-dependent. The aim of this study is the accurate calibration of semiconductor detectors with respect to CT scanner specific spectra at different values of the tube voltage. Methods. The air kerma response of several semiconductor detectors of the same type (Doseprofiler, RTI) has been measured free-inair using ISO-N X-ray qualities. The machine-specific, kermaaveraged calibration factors for different tube potentials are using the known X-ray fluence spectra of a GE Optima CT 660. The results are validated by comparing dose measurements made with the Doseprofiler, which was placed inside the isocenter of the CT, with measurements made using a calibrated ionization chamber with known energy response. In anthropomorphic phantoms, the beamhardening characteristics of the phantom and the CT source must be considered. The simulation of X-ray fluence spectra inside the inhomogeneous object is very complex, as the angle-dependent material composition of both the phantom and the X-ray source of the CT must be incorporated. Hence, direct calibration with calibrated ionization chambers inside the phantom has been performed, which is the more direct approach. Results. It has been shown that the measured response of the semiconductor detectors varies by 80% for different ISO-N X-ray modalities. Consequently, the calculated CT-specific calibration factors free-in-air in total change by 24% for tube voltages between 80 kV and 140 kV. Dose measurements with the ionization chamber and Doseprofiler showed good agreement, with the results diverging by less than 2% after correction. The in-phantom calibration with an ionization chamber lead to comparable results with differences of 20%-24% in the calibration factors for values of the tube voltage between 80 kV and 140 kV and for different anthropomorphic phantoms. Conclusions. Semiconductor detectors have the advantage of high spatial resolution and sensitivity. However, the air kerma response of these detectors is typically very poor. Hence spectral-corrected calibration factors or cross-calibration with ionization chambers is absolutely necessary in order to achieve the highest accuracy in the absolute dose measurement for all tube potentials. https://doi.org/10.1016/j.ejmp.2018.06.532
[P254] Local gamma index analysis - new approach Zbisław Tabor a, Monika Tulik b, Damian Kabat b, Anna Wydra c,* a Cracow University of Technology, Faculty of Physics, Mathematics and Computer Science, Krakow, Poland b Center of Oncology, Maria Skłodowksa-Curie Memorial Institute, Kraków Branch, Kraków, Poland c Optinav Sp. Z O.O., Medical Engineering, Słupsk, Poland ⇑ Corresponding author.
Purpose. The gamma index analysis is a well-known approach in radiotherapy. The weakest point of widely accepted gamma index method seems to be choosing global approach. Choosing one nor-
Comparing the data across the departments revealed no systematic differences based on the absorbed dose protocol each department followed. There may be a slight difference in the determination of relative output of the different applicators depending on the ion chamber used for output factor measurements, but with limited data available this remains inconclusive. The 90% and 85% dose to water range of x-rays agree between the departments within 1 mm for similar beam qualities, even though different methods were used to determine the depth dose deposition. Conclusions. A national comparison of kV radiotherapy units was conducted, and overall the units were found very similar. No systematic differences were observed based on the calibration protocol, but the available data hint that the choice of ion chamber for calibration measurements may have a small effect on the determination of output factors. https://doi.org/10.1016/j.ejmp.2018.06.530
[P251] Dosimetric impact of intermediate dose calculation on heterogeneous region radiotherapy planning Ugur Akbas a,*, Canan Koksal a, Nazmiye Donmez Kesen a, Gizem Kaval b, Sule Karaman b, Nergiz Dagoglu b, Hatice Bilge a, Musa Altun b a
Istanbul University Oncology Institute, Medical Physics, Istanbul, Turkey b Istanbul University Medical Faculty, Radiation Oncology, Istanbul, Turkey ⇑ Corresponding author. Purpose. In radiotherapy planning of heterogeneous regions, differences which negatively affect the quality of the plan in terms of homogeneity and conformity may occur between the optimal dose volume histogram (DVH) and the final calculated DVH. The intermediate dose calculation (IDC) module, which is included in the Analytical Anisotropic Algorithm (AAA), is used even though the planning period is slightly increased in radiotherapy plans of lung tumors with low tissue density to reduce these differences. The purpose of this study is to examine the impact of IDC on radiotherapy planning for the heterogeneous maxillary sinus region. Methods. In this study, the homogeneity index (HI) and the conformity index (CI) of the intensity modulated radiotherapy plans prepared using Eclipse-TPS and AAA v8.9 of 12 patients with maxillary sinus cancer were calculated. The original plans were reoptimized with the same optimization criteria by using AAA v15.1 with and without IDC. HI and CI values of the new plans were calculated after re-optimization. The HI and CI values, maximum doses and the critical organ doses were compared between the original and new plans. Results. AAA v15.1 with IDC increased the plan quality in terms of HI, CI and maximum dose values. The HI values (ideal value=0) were found to be 0.093, 0.090 and 0.067; the CI values (ideal value=1) were found to be 1.149, 1.142 and 1.055; the maximum doses were found to be 108.1%, 107.8% and 105.3% for AAA v8.9, AAA v15.1 without IDC and AAA v15.1 with IDC, respectively. The differences between AAA v15.1 with IDC and the others are statistically significant for HI values, CI values and maximum doses but there is no significant difference for the doses received by critical organs. Conclusions. The use of the IDC increases the quality of intensity modulated radiotherapy plans in heterogeneous regions such as head and neck. This work was supported by Scientific Research Projects Coordination Unit of Istanbul University (Project Number: I.U.BAP-23057) https://doi.org/10.1016/j.ejmp.2018.06.531
171
[P252] Energy calibration of semi-conductor detectors for computed tomography Stephan Rosendahl a,*, Ludwig Büermann b a
Physikalisch-Technische Bundesanstalt, Dosimetrie für Strahlentherapie und Röntgendiagnostik, Braunschweig, Germany b Physikalisch-Technische Bundesanstalt, 6.2 - Dosimetry for Radiation Therapy and Diagnostic Radiology, Braunschweig, Germany ⇑ Corresponding author. Purpose. Semiconductor detectors are widely used in computed tomography (CT). Their high spatial resolution and sensitivity give them an advantage compared to ionization chambers. Unfortunately, their air kerma response is strongly energy-dependent. The aim of this study is the accurate calibration of semiconductor detectors with respect to CT scanner specific spectra at different values of the tube voltage. Methods. The air kerma response of several semiconductor detectors of the same type (Doseprofiler, RTI) has been measured free-inair using ISO-N X-ray qualities. The machine-specific, kermaaveraged calibration factors for different tube potentials are using the known X-ray fluence spectra of a GE Optima CT 660. The results are validated by comparing dose measurements made with the Doseprofiler, which was placed inside the isocenter of the CT, with measurements made using a calibrated ionization chamber with known energy response. In anthropomorphic phantoms, the beamhardening characteristics of the phantom and the CT source must be considered. The simulation of X-ray fluence spectra inside the inhomogeneous object is very complex, as the angle-dependent material composition of both the phantom and the X-ray source of the CT must be incorporated. Hence, direct calibration with calibrated ionization chambers inside the phantom has been performed, which is the more direct approach. Results. It has been shown that the measured response of the semiconductor detectors varies by 80% for different ISO-N X-ray modalities. Consequently, the calculated CT-specific calibration factors free-in-air in total change by 24% for tube voltages between 80 kV and 140 kV. Dose measurements with the ionization chamber and Doseprofiler showed good agreement, with the results diverging by less than 2% after correction. The in-phantom calibration with an ionization chamber lead to comparable results with differences of 20%-24% in the calibration factors for values of the tube voltage between 80 kV and 140 kV and for different anthropomorphic phantoms. Conclusions. Semiconductor detectors have the advantage of high spatial resolution and sensitivity. However, the air kerma response of these detectors is typically very poor. Hence spectral-corrected calibration factors or cross-calibration with ionization chambers is absolutely necessary in order to achieve the highest accuracy in the absolute dose measurement for all tube potentials. https://doi.org/10.1016/j.ejmp.2018.06.532
[P254] Local gamma index analysis - new approach Zbisław Tabor a, Monika Tulik b, Damian Kabat b, Anna Wydra c,* a Cracow University of Technology, Faculty of Physics, Mathematics and Computer Science, Krakow, Poland b Center of Oncology, Maria Skłodowksa-Curie Memorial Institute, Kraków Branch, Kraków, Poland c Optinav Sp. Z O.O., Medical Engineering, Słupsk, Poland ⇑ Corresponding author.
Purpose. The gamma index analysis is a well-known approach in radiotherapy. The weakest point of widely accepted gamma index method seems to be choosing global approach. Choosing one nor-