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EP-1725: Evolution of QA program for dynamic MLC in volumetric modulatedarc therapy based on EPID portal dosimetry
S258 distance leads to the preservation of edges having high contrast and the smoothness of noisy voxels having low contrast. The introduction of weighted relaxation map makes it possible to adjust the effects of the updates that would have been made to the unchanged or changed regions of the prior images. The performance of the proposed algorithm was evaluated by the cheese phantom with locally-different plugs and compared with filtered backprojection (FBP) and CS-based algorithms. Results: MVCT images generated using the proposed algorithm with sinogram taken by the coarse mode showed spatial resolution comparable to images reconstructed at the fine mode. In particular, the proposed algorithm led to less artifact and higher contrast-to-noise ratio in comparison with CS and FBP algorithms by preserving unchanged parts of the prior images and by updating locally-different features between prior and current images. Conclusions: The technique allows us to effectively take advantage previously obtained MVCT and thus provides high quality MVCT images with sinogram acquired by the coarse mode. This can result in the reduction of the patient dose in addition to the fast scanning time in this system. EP-1725 Evolution of QA program for dynamic MLC in volumetric modulated arc therapy based on EPID portal dosimetry R. Tortosa Oliver1, N. Chinillach Ferrando1, P. Soler Catalan1, J.C. Morales Marco1, F. Andreu Martinez1 1 Hospital IMED Elche, Department of Radiophysics, Elche (Alicante), Spain
ESTRO 33, 2014 Conclusions: The quality assurance program for dMLC in VMAT treatments based on portal dosimetry is a powerful tool that provides complete information about positioning-speed dMLC and confidence in arc therapy techniques. It is important to remark that it is very low time consuming. EP-1726 Application of a CT metal artifact reduction method using monoenergetic imaging to RT planning E. Bär1, S. Kuchenbecker2, F. Sterzing3, A. Schwahofer1 1 German Cancer Research Center (DKFZ), Medical Physics in Radiation Oncology, Heidelberg, Germany 2 German Cancer Research Center (DKFZ), Medical Physics in Radiology, Heidelberg, Germany 3 University of Heidelberg, Department of RadioOncology, Heidelberg, Germany Purpose/Objective: Metal artifacts in CT images are one of the main problems in radiation oncology as they introduce uncertainties to target and organ at risk delineation as well as dose calculation. This study investigates metal artifact reduction based on monoenergetic imaging.
Purpose/Objective: Volumetric modulated arc therapy (VMAT) delivers the dose to the target with a volumetric approach using accurate and fast dynamic multileaf collimators (dMLC), variable dose-rate and variable gantry speed. Quality assurance of leaf positioning is critical for the correct delivery of the treatment. Radiotherapy Oncology Department from our hospital has established a weekly program to check it. We present the results of different tests for the first six months since VMAT treatments started.
Materials and Methods: 1) A phantom study (Gammex RMI 462) investigated metal artifacts caused by metals with different densities: Aluminium 2.7 g/cm3 (Al), Titanium 4.5 g/cm3 (Ti), Steel 7.9 g/cm3 (St), Tungsten 19.3 g/cm3 (W). Data were collected using a clinical DECT scanner (Siemens Somatom Definition Flash) with tube voltages of 100 kVp and 140 kVp (Sn). For each tube voltage a volume was reconstructed. Based on the two volumes a voxel by voxel alpha blending was performed (Siemens Syngo Software) to obtain monoenergetic data sets at 66, 85 and 105 keV. The evaluation was done regarding optical properties of the images, CT values (HU) and dosimetrically by creating static field plans. A data set without metal substitute serves asreference (DECT Ref). 2) A head & neck patient with dental fillings (Amalgam 10 g/cm 3) was scanned with a single energy CT (SECT) scan and an additional DECT scan. Monoenergetic extrapolation was performed as described above and evaluated the same way.
Materials and Methods: A Varian DHX clinac (Varian, Palo Alto, Ca) with Millennium 120-MLC is used. Attached to the linac,there is amorphous silicon based EPID calibrated for dosimetric purposes. The following tests were planned to evaluate VMAT performance: - Test 1.1 - Picket Fence Test during VMAT delivery. - Test 1.2 - Picket Fence Test during VMAT delivery with intentional errors - Test 2 – Dose-rate versus Gantry speed during VMAT delivery - Test 3 – Control of Leaf speed during VMAT delivery.
Results: 1) A pure visual assessment of all data shows small reductions of artifacts in the data sets with Al and Ti perturbation at Mono 105 keV.For metals with higher densities such as St or W, no artifact reduction is observed. Looking at the CT numbers, no improvement with the monoenergetic filter can be detected, even not for Al and Ti. Dose was measured 7 cm behind the isocenter of the static field (isocenter in the middle of the metal insert). Small improvements (around 1%) can be seen in table 1. However, the dose uncertainty is in the order of 10% 20% (Al / W), thus improvements are statistically insignificant.
All tests were done with photons of 6 MV. We have selected a sourcedetector distance (SDD) of 100 cm and a dose-rate of 600 MU/min. These choices allow us to prevent saturation and to establish a compromise between resolution and signal-noise ratio. Integrated images are analyzed by gamma comparison (3%, 3 mm) using Portal DosimetryTM software provided by Varian. Results: Images from the proposed tests are shown in Figure 1:
2) For Amalgam with a density between St and W, monoenergetic data sets of the patient do not show substantial artifact reduction (figure 1). Local dose uncertainties around the metal artifact determined with static field approaches are ~5%. This cannot be observed in complex dose distributions like in IMRT. Here, resulting dose errors from variations in CT numbers can be compensated for by beams from other directions.
Fig 1. Images from Test 1.1(a), Test 1.2 (b), Test 2 (c) and Test 3 (d) The evolution of picket fence test performed during VMAT delivery indicates that gamma comparison is within 99 % and picket fence with intentional errors is reproduced within 99.5% average. For dose-rate versus gantry speed test similar results have been found. 98.5% of cases from Test 2 and 98% of cases from Test 3 have passed the gamma evaluation (3%, 3 mm).
Conclusions: A clear dependency of artifact reduction to the density of the metal was proven. As expected, the higher the densities the more distinctive are the artifacts (due to higher absorption and scatter of
ESTRO 33, 2014 Conclusions: The quality assurance program for dMLC in VMAT treatments based on portal dosimetry is a powerful tool that provides complete information about positioning-speed dMLC and confidence in arc therapy techniques. It is important to remark that it is very low time consuming. EP-1726 Application of a CT metal artifact reduction method using monoenergetic imaging to RT planning E. Bär1, S. Kuchenbecker2, F. Sterzing3, A. Schwahofer1 1 German Cancer Research Center (DKFZ), Medical Physics in Radiation Oncology, Heidelberg, Germany 2 German Cancer Research Center (DKFZ), Medical Physics in Radiology, Heidelberg, Germany 3 University of Heidelberg, Department of RadioOncology, Heidelberg, Germany Purpose/Objective: Metal artifacts in CT images are one of the main problems in radiation oncology as they introduce uncertainties to target and organ at risk delineation as well as dose calculation. This study investigates metal artifact reduction based on monoenergetic imaging.
Purpose/Objective: Volumetric modulated arc therapy (VMAT) delivers the dose to the target with a volumetric approach using accurate and fast dynamic multileaf collimators (dMLC), variable dose-rate and variable gantry speed. Quality assurance of leaf positioning is critical for the correct delivery of the treatment. Radiotherapy Oncology Department from our hospital has established a weekly program to check it. We present the results of different tests for the first six months since VMAT treatments started.
Materials and Methods: 1) A phantom study (Gammex RMI 462) investigated metal artifacts caused by metals with different densities: Aluminium 2.7 g/cm3 (Al), Titanium 4.5 g/cm3 (Ti), Steel 7.9 g/cm3 (St), Tungsten 19.3 g/cm3 (W). Data were collected using a clinical DECT scanner (Siemens Somatom Definition Flash) with tube voltages of 100 kVp and 140 kVp (Sn). For each tube voltage a volume was reconstructed. Based on the two volumes a voxel by voxel alpha blending was performed (Siemens Syngo Software) to obtain monoenergetic data sets at 66, 85 and 105 keV. The evaluation was done regarding optical properties of the images, CT values (HU) and dosimetrically by creating static field plans. A data set without metal substitute serves asreference (DECT Ref). 2) A head & neck patient with dental fillings (Amalgam 10 g/cm 3) was scanned with a single energy CT (SECT) scan and an additional DECT scan. Monoenergetic extrapolation was performed as described above and evaluated the same way.
Materials and Methods: A Varian DHX clinac (Varian, Palo Alto, Ca) with Millennium 120-MLC is used. Attached to the linac,there is amorphous silicon based EPID calibrated for dosimetric purposes. The following tests were planned to evaluate VMAT performance: - Test 1.1 - Picket Fence Test during VMAT delivery. - Test 1.2 - Picket Fence Test during VMAT delivery with intentional errors - Test 2 – Dose-rate versus Gantry speed during VMAT delivery - Test 3 – Control of Leaf speed during VMAT delivery.
Results: 1) A pure visual assessment of all data shows small reductions of artifacts in the data sets with Al and Ti perturbation at Mono 105 keV.For metals with higher densities such as St or W, no artifact reduction is observed. Looking at the CT numbers, no improvement with the monoenergetic filter can be detected, even not for Al and Ti. Dose was measured 7 cm behind the isocenter of the static field (isocenter in the middle of the metal insert). Small improvements (around 1%) can be seen in table 1. However, the dose uncertainty is in the order of 10% 20% (Al / W), thus improvements are statistically insignificant.
All tests were done with photons of 6 MV. We have selected a sourcedetector distance (SDD) of 100 cm and a dose-rate of 600 MU/min. These choices allow us to prevent saturation and to establish a compromise between resolution and signal-noise ratio. Integrated images are analyzed by gamma comparison (3%, 3 mm) using Portal DosimetryTM software provided by Varian. Results: Images from the proposed tests are shown in Figure 1:
2) For Amalgam with a density between St and W, monoenergetic data sets of the patient do not show substantial artifact reduction (figure 1). Local dose uncertainties around the metal artifact determined with static field approaches are ~5%. This cannot be observed in complex dose distributions like in IMRT. Here, resulting dose errors from variations in CT numbers can be compensated for by beams from other directions.
Fig 1. Images from Test 1.1(a), Test 1.2 (b), Test 2 (c) and Test 3 (d) The evolution of picket fence test performed during VMAT delivery indicates that gamma comparison is within 99 % and picket fence with intentional errors is reproduced within 99.5% average. For dose-rate versus gantry speed test similar results have been found. 98.5% of cases from Test 2 and 98% of cases from Test 3 have passed the gamma evaluation (3%, 3 mm).
Conclusions: A clear dependency of artifact reduction to the density of the metal was proven. As expected, the higher the densities the more distinctive are the artifacts (due to higher absorption and scatter of