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First results of performing 3D EPID-based in-vivo dosimetry for prostate treated by rapidarc
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Abstracts / Physica Medica 30 (2014) e45ee74
EVALUATION OF A PHANTOM RELATED GAMMA INDEX THRESHOLD FOR VMAT QA , M. A. Negri, A. Scaggion, M.A. Rossato, D. Canonico, R. Zandona Paiusco. Veneto Institute of Oncology IOV-IRCCS, Medical Physics Department, Via Gattamelata 64, 35128 Padova, Italy Background: VMAT pre-treatment QA are usually performed comparing measured and calculated dose distribution in phantom by means of gamma index (GI) metric. This procedure is based on the assumption that the differences between measured and calculated dose distribution in any phantom reflect those in patient. The aim of this study was to assess the relation between gamma index passing rate (GP%) evaluated for different phantoms geometry and patients. A phantom related GP% acceptance level is established based on a new strategy that takes into account the acceptance level defined for patients Materials and methods: 30 H&N cancer patient were selected. The data contained in the MLC and the gantry log files provided by the Varian Linac were used to create RTPLAN DICOM files simulating the delivered plan. Mathematical phantoms were created reproducing geometries of the widely available QA systems such as planar systems, cylindrical surface systems and a3D cylindrical measurement systems. Both planned and delivered plans dose distribution were calculated in patients and in phantoms. Mean GI and GP% correlations between phantoms as well as between phantoms and patients were investigated. ROC analysis was applied to define the GP% thresholds for different phantoms based on the patients acceptance level. Results: The study shows that the results of the gamma analysis strongly depends on the geometry of the dosimeter used for the pre-treatment verification. In case of planar geometries the inaccuracies detected with the phantom do not absolutely reflect those occurring in patients. Moreover the GI map changes with the measurements depth. Conversely, in volumetric and cylindrical 2D geometry, the above mentioned correlations are fully confirmed. Finally the ROC analysis showed that the GP% on different phantom have a different meaning in term of dose differences on patient. Discussion: The analysis strengthens that planar dosimetric system is not adequately representative of dose differences in patient. Our finding clearly shows that the choice of the phantom deeply influences the GI values. A general criterion for the acceptance of a treatment plan, as usually done, can not be used. Our study supplies a method to get the optimal geometry related GI threshold. FIRST RESULTS OF PERFORMING 3D EPID-BASED IN-VIVO DOSIMETRY FOR PROSTATE TREATED BY RAPIDARC E. Villaggi, A.U.S.L. Piacenza Purpose. To investigate the feasibility of using electronic portal imaging device (EPID) transit dosimetry and to analyze 3D in-vivo dose-volume histograms (DVHs) in prostate cases treated by volumetric modulated arc techniques (VMAT). Methods and materials: Eleven patients were prospectively enorolled on a in-vivo quality assurance protocol in which dosimetric EPID images were acquired during first VMAT treatment fraction. Portal Dose Images were acquired on a Varian aSi1000 EPID attached to a Varian ClinacCD linear. Treatments were delivered using the linear accelerator’s 6 MV x-ray beam and a Varian 120 Dynamic MultiLeaf Collimator. The Varian Eclipse treatment planning system was used to planning RapidArc prostate treatments in a single arc delivery. Transit dosimetry calculations were performed using Dosimetry Check software (Math Resolutions LLC). Pretreatment patient-specific quality assurance was previously performed using local 3% 3 mm gamma criterion to detect technical sources of systematic error. 2D and 3D dose distributions, point doses, gamma distributions (with 5% 3mm criterion) and DVH statistics were compared. Results: Absolute differences of Reference Point doses between in vivo dosimetry and Eclipse calculation averaged 2,6%. Volume and area in sagittal, axial and coronal plane through the isocenter with gamma < 1 are reported in table 1, where statistics is relative to ten patients. Planned DVH parameters were compared with in-vivo results: median dose (D50), near-
maximum dose (D2) and near-minimum dose (D98) in PTV, as recommended in ICRU Report 83. OAR significant values were also compared. One case was excluded from statistics because very poor match was observed between prediction and measurement during the transit dosimetry session. Further investigation attributed this to internal anatomic changes and a replanning was constructed. Conclusion: EPID-based in vivo dosimetry could play an important role in the total chain of verification procedures in a radiotherapy department, through clinical parameters given by 3DVH in terms of PTV coverage and OAR overdoses[1]. More research is needed to assess optimal values for alert criteria for 3D verification of VMAT treatments of various treatment sites[2]. References: [1] Sean L. Berry et al, Initial Clinical Experience Performing Patient Treatment Verification With an Electronic Portal Imaging Device Transit Dosimeter, Int J Radiation Oncol Biol Phys, Vol. 88, No 1, pp. 204-209, 2014 [2] Ben Mijnheer et al, In vivo dosimetry in external beam radiotherapy, Medical Physics 40 (7), 2013 Table 1 Summary of EPID transit dosimetry results.
Gamma-distribution
Isocenter Point Dose (Difference) PTV Doses(Measured minus Calculated)
Rectum Doses(Measured minus Calculated)
Bladder(Measured minus Calculated)
VolumeGamma < 1 Threshold 10% VolumeGamma < 1No threshold 1-percentile AreaGamma < 1 transversal plane AreaGamma < 1coronal plane AreaGamma < 1sagittal plane
Mean Dose DnearMIN (D2) DnearMAX(D98) Median Dose (D50) Mean Dose V75Gy V70Gy V60Gy MeanDose
Average (range)%
Standard Dviation
82.9(75.7 - 89.2)
3.6%
96.4(94.4 - 98.1)
1.2%
1.6(1.3 - 2.3) 89.1(74.2 - 98.7)
0.3 6.8%
96.6(89.0 - 100)
3.8%
96.6(92.8 - 99.9)
2.2%
2.6(-1.5 - +5.8)
2.4%
2.3(-1.1 - +4.8) 3.9(0.4 - +5 .6) -2.2(-9.2 - +3.8) 2.8(-0.9 - +4.8) 2.3(-2.6 - +7.3) 2.3(-0.3 - +10.1) 1.3(0.0 - +6.8) 2.3(-0.2 - +6.3) -6.3(-8.4 - -3.2)
1.7% 1.7% 3.4% 1.7% 3.1% 3.0% 2.0% 1.8% 2.0%
MLC POSITIONAL ACCURACY EVALUATION THROUGH THE PICKET FENCE TEST ON EBT2 FILMS AND A 3D DOSIMETRIC PHANTOM Maritina Rouchota, Ioannis Floros, Christina Armpilia, Maria Lyra, Christos Antypas. 1st Department of Radiology, Medical Physics Unit, Aretaieion Hospital, University of Athens, Greece Background: The accuracy of MLC positions during radiotherapy is substantial as even small positional deviations can translate in considerable dose delivery errors. This becomes crucial when radiosensitive organs are located near the treated volume and especially during IMRT, where dose gradients can become steep. A test commonly conducted to measure the positional accuracy of the MLCs is the picket fence test. Materials and Methods: Two alterations of the picket test fence test were performed for the MLC positional inspection. The first alteration was performed on a radiochromic film, consisting of a series of step-and-shoot measurements, creating narrow bands at specified intervals. The width of the narrow bands in the resulting image is checked for discrepancies. The second alteration was performed on the Delta4 diode array phantom, according to a DICOM RT plan created in the Delta4 software. Leaves make a series of major stops, at each stop three measurements are considered around the diodes. The gap width and the MLC dispositions are calculated through the software and compared to the actual values. Gravity effects that cause sag and backlash in the MLC carriage and support assemblies
Abstracts / Physica Medica 30 (2014) e45ee74
EVALUATION OF A PHANTOM RELATED GAMMA INDEX THRESHOLD FOR VMAT QA , M. A. Negri, A. Scaggion, M.A. Rossato, D. Canonico, R. Zandona Paiusco. Veneto Institute of Oncology IOV-IRCCS, Medical Physics Department, Via Gattamelata 64, 35128 Padova, Italy Background: VMAT pre-treatment QA are usually performed comparing measured and calculated dose distribution in phantom by means of gamma index (GI) metric. This procedure is based on the assumption that the differences between measured and calculated dose distribution in any phantom reflect those in patient. The aim of this study was to assess the relation between gamma index passing rate (GP%) evaluated for different phantoms geometry and patients. A phantom related GP% acceptance level is established based on a new strategy that takes into account the acceptance level defined for patients Materials and methods: 30 H&N cancer patient were selected. The data contained in the MLC and the gantry log files provided by the Varian Linac were used to create RTPLAN DICOM files simulating the delivered plan. Mathematical phantoms were created reproducing geometries of the widely available QA systems such as planar systems, cylindrical surface systems and a3D cylindrical measurement systems. Both planned and delivered plans dose distribution were calculated in patients and in phantoms. Mean GI and GP% correlations between phantoms as well as between phantoms and patients were investigated. ROC analysis was applied to define the GP% thresholds for different phantoms based on the patients acceptance level. Results: The study shows that the results of the gamma analysis strongly depends on the geometry of the dosimeter used for the pre-treatment verification. In case of planar geometries the inaccuracies detected with the phantom do not absolutely reflect those occurring in patients. Moreover the GI map changes with the measurements depth. Conversely, in volumetric and cylindrical 2D geometry, the above mentioned correlations are fully confirmed. Finally the ROC analysis showed that the GP% on different phantom have a different meaning in term of dose differences on patient. Discussion: The analysis strengthens that planar dosimetric system is not adequately representative of dose differences in patient. Our finding clearly shows that the choice of the phantom deeply influences the GI values. A general criterion for the acceptance of a treatment plan, as usually done, can not be used. Our study supplies a method to get the optimal geometry related GI threshold. FIRST RESULTS OF PERFORMING 3D EPID-BASED IN-VIVO DOSIMETRY FOR PROSTATE TREATED BY RAPIDARC E. Villaggi, A.U.S.L. Piacenza Purpose. To investigate the feasibility of using electronic portal imaging device (EPID) transit dosimetry and to analyze 3D in-vivo dose-volume histograms (DVHs) in prostate cases treated by volumetric modulated arc techniques (VMAT). Methods and materials: Eleven patients were prospectively enorolled on a in-vivo quality assurance protocol in which dosimetric EPID images were acquired during first VMAT treatment fraction. Portal Dose Images were acquired on a Varian aSi1000 EPID attached to a Varian ClinacCD linear. Treatments were delivered using the linear accelerator’s 6 MV x-ray beam and a Varian 120 Dynamic MultiLeaf Collimator. The Varian Eclipse treatment planning system was used to planning RapidArc prostate treatments in a single arc delivery. Transit dosimetry calculations were performed using Dosimetry Check software (Math Resolutions LLC). Pretreatment patient-specific quality assurance was previously performed using local 3% 3 mm gamma criterion to detect technical sources of systematic error. 2D and 3D dose distributions, point doses, gamma distributions (with 5% 3mm criterion) and DVH statistics were compared. Results: Absolute differences of Reference Point doses between in vivo dosimetry and Eclipse calculation averaged 2,6%. Volume and area in sagittal, axial and coronal plane through the isocenter with gamma < 1 are reported in table 1, where statistics is relative to ten patients. Planned DVH parameters were compared with in-vivo results: median dose (D50), near-
maximum dose (D2) and near-minimum dose (D98) in PTV, as recommended in ICRU Report 83. OAR significant values were also compared. One case was excluded from statistics because very poor match was observed between prediction and measurement during the transit dosimetry session. Further investigation attributed this to internal anatomic changes and a replanning was constructed. Conclusion: EPID-based in vivo dosimetry could play an important role in the total chain of verification procedures in a radiotherapy department, through clinical parameters given by 3DVH in terms of PTV coverage and OAR overdoses[1]. More research is needed to assess optimal values for alert criteria for 3D verification of VMAT treatments of various treatment sites[2]. References: [1] Sean L. Berry et al, Initial Clinical Experience Performing Patient Treatment Verification With an Electronic Portal Imaging Device Transit Dosimeter, Int J Radiation Oncol Biol Phys, Vol. 88, No 1, pp. 204-209, 2014 [2] Ben Mijnheer et al, In vivo dosimetry in external beam radiotherapy, Medical Physics 40 (7), 2013 Table 1 Summary of EPID transit dosimetry results.
Gamma-distribution
Isocenter Point Dose (Difference) PTV Doses(Measured minus Calculated)
Rectum Doses(Measured minus Calculated)
Bladder(Measured minus Calculated)
VolumeGamma < 1 Threshold 10% VolumeGamma < 1No threshold 1-percentile AreaGamma < 1 transversal plane AreaGamma < 1coronal plane AreaGamma < 1sagittal plane
Mean Dose DnearMIN (D2) DnearMAX(D98) Median Dose (D50) Mean Dose V75Gy V70Gy V60Gy MeanDose
Average (range)%
Standard Dviation
82.9(75.7 - 89.2)
3.6%
96.4(94.4 - 98.1)
1.2%
1.6(1.3 - 2.3) 89.1(74.2 - 98.7)
0.3 6.8%
96.6(89.0 - 100)
3.8%
96.6(92.8 - 99.9)
2.2%
2.6(-1.5 - +5.8)
2.4%
2.3(-1.1 - +4.8) 3.9(0.4 - +5 .6) -2.2(-9.2 - +3.8) 2.8(-0.9 - +4.8) 2.3(-2.6 - +7.3) 2.3(-0.3 - +10.1) 1.3(0.0 - +6.8) 2.3(-0.2 - +6.3) -6.3(-8.4 - -3.2)
1.7% 1.7% 3.4% 1.7% 3.1% 3.0% 2.0% 1.8% 2.0%
MLC POSITIONAL ACCURACY EVALUATION THROUGH THE PICKET FENCE TEST ON EBT2 FILMS AND A 3D DOSIMETRIC PHANTOM Maritina Rouchota, Ioannis Floros, Christina Armpilia, Maria Lyra, Christos Antypas. 1st Department of Radiology, Medical Physics Unit, Aretaieion Hospital, University of Athens, Greece Background: The accuracy of MLC positions during radiotherapy is substantial as even small positional deviations can translate in considerable dose delivery errors. This becomes crucial when radiosensitive organs are located near the treated volume and especially during IMRT, where dose gradients can become steep. A test commonly conducted to measure the positional accuracy of the MLCs is the picket fence test. Materials and Methods: Two alterations of the picket test fence test were performed for the MLC positional inspection. The first alteration was performed on a radiochromic film, consisting of a series of step-and-shoot measurements, creating narrow bands at specified intervals. The width of the narrow bands in the resulting image is checked for discrepancies. The second alteration was performed on the Delta4 diode array phantom, according to a DICOM RT plan created in the Delta4 software. Leaves make a series of major stops, at each stop three measurements are considered around the diodes. The gap width and the MLC dispositions are calculated through the software and compared to the actual values. Gravity effects that cause sag and backlash in the MLC carriage and support assemblies