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EP-1457 CTV-PTV MARGINS WHEN USING KV A PRIORI ALIGMENT AND VERIFICATION FOR HEAD AND NECK CANCER PATIENTS
S556
amplitudes selected to correspond phantom velocity in the range 0 – 15 mm/sec. Each marker displacement is evaluated by correction shift estimate using manual match at markers endpoint relative to amplitude at 0 mm. Results: Fiducial marker detection using centre of mass can be verified within 3.7 mm for phantom velocity up to 15 mm/sec on 10 bins 4D CT. The deviation between actual physical positions and detected centre off mass strongly depends on phantom velocity during image acquisition of the phase bins. No correlation of marker detection is found among fiducial markers centre of mass and marker rotation relative to superior-inferior axis, CT slice thickness or NiTi stent length. Marker detection on gated IGRT using BrainLAB ExacTrac software can be verified within 1 mm. No correlation if found between detection accuracy and phantom velocity during gated IGRT, slice thickness and marker rotation. Conclusions: The safety margin for respiratory gated IGRT with GM and NiTi stent fiducial markers will be in the range of 1 – 4 mm. Safety margin correlates to the marker velocity during acquisition of the CT phase selected for treatment planning. EP-1457 CTV-PTV MARGINS WHEN USING KV A PRIORI ALIGMENT AND VERIFICATION FOR HEAD AND NECK CANCER PATIENTS E. van Dieren1, H. Richards1, S. Koch1 1 Medisch Spectrum Twente, radiotherapy, Enschede, The Netherlands Purpose/Objective: With the advent of a priori kV imaging in radiotherapy, systematic and random errors in patient positioning are largely eliminated. Thus, margins are defined only by intrafraction motion, possible deformations and matching errors, possibly leading to tighter margins than historically used. The goal of this study was to establish the intrafraction motion of patients irradiated for cancers of the head and neck, measured using a posteriori verification. Materials and Methods: For patients irradiated for Head & Neck cancers, a priori setup correction was performed using the Varian On Board Imager with orthogonal images and DRRs, on which PTV(s) were superimposed. Weekly, a posteriori EPID (61 patients, 280 images) or kV verification (50 patients, 175 images) was performed, which allow to assess the intrafraction (I.F.) movement in both groups. Based on the I.F. movements, a CTV-PTV margin was defined as the distance which covers 95% of I.F. movements. Patient characteristics in and between groups were compared in relation to I.F. movement. Additionally, results of a routine geometric test for stereotactic treatment were used. Assuming similar table accuracy, I.F. movement results were deconvolved using the results of the geometric test. Treatment planning was performed using the new margins and a historically used margin of 5 mm. Results: In group 1 (EPID), the margin was found to be 5 mm, for group 2 (kV) 3.5 mm. The two groups were similar in terms of prescribed dose, CTV volume, and fixation technique. The only difference was the used verification. For both groups, the margin reduced by 1 mm for patients treated with a curative dose only. Using the geometric test results, deconvolution of I.F. movements reduces margins by additionally 0.5 mm. The largest 5% of I.F. movement was associated with rotations (also 5%) and was usually seen for treatment with a palliative intent. Planning examples show beneficial effects in dose and DVH when the margin is reduced: less overlap allows more sparing of critical structures. Conclusions: This study shows that a generic margin for radiotherapy of head and neck cancer patients is not accurate. A reduction is feasable, especially for patients treated with a curative intent. This study also shows that for patients with head and neck cancer, actual margins can be smaller than previously assessed on the basis of a posteriori EPIDs. Since both groups were similar, it may be that MV is an unsuitable instrument for verification of high accuracy treatments. Geometric inaccuracy of the treatment table may even be larger when the full patient weight is on the treatment table. Its effect on margins warrants introduction of a (6 DoF) table with higher accuracy. Rotations, deformations, and user errors still remain to be investigated. Until then, using these margins in clinical practice is hazardous. We're currently contemplating replacing orthogonal images by CBCT. However, such a study is difficult to perform because of the larger radiation dose and longer linac time.
ESTRO 31
EP-1458 A COMPARISON OF THREE REGISTRATION METHODS USING 4D PLANNING CT AND ON-BOARD 4D CONE-BEAM CT FOR LUNG VMAT K. Nakagawa1, Y. Masutani1, A. Haga1, H. Yamashita1, W. Takahashi1, A. Sakumi1, S. Kida1, N. Saotome1, K. Ohtomo1, K. Yoda2 1 University of Tokyo Hospital, Dept. of Radiology, Tokyo, Japan 2 Elekta KK, Research Physics Dept., Tokyo, Japan Purpose/Objective: To establish a lung tumor registration workflow using 4D planning CT and 4D cone-beam CT. Materials and Methods: A large bore 16-multislice CT, Aquillion LB (Toshiba, Japan), was employed to scan a lung tumor patient with Anzai belt (Anzai Medical, Japan) to obtain 10-phase respiratory correlated CT data for 4D treatment planning. A body frame (Elekta, Sweden) was used to constrain the patient respiratory movement. Immediately before treatment, 10-phase 4D cone-beam CT (CBCT) data were acquired using a kV x-ray imager, XVI, equipped with a Synergy linac (Elekta, UK). Three different registration techniques have been performed for comparison: 1) A PTV was defined by adding a 5 mm margin to an ITV created by using 10 CTVs, each of which was defined on each of the 10-phase planning CT data. The PTV ROI was exported to the XVI workstation. Patient couch positioning was manually performed by comparing the PTV ROI to a moving tumor by repeatedly showing each phase image of the 4D CBCT on the XVI display monitor. 2) 3D maximum intensity projection (MIP) image was calculated by 4D 10phase planning CT to create an integrated tumor trace. Similar 3D MIP image was also created by 4D 10-phase CBCT. The two MIP images were manually registered to derive the patient couch positioning data in a separate workstation, and the resulting repositioning data was input to the XVI workstation. 3) 3D averaged image was calculated by using 4D planning CT data and it was exported to the XVI workstation. The same operation was done for 4D CBCT data using a built-in functionality in the XVI workstation. Then usual built-in gray-valuebased 3D registration was automatically calculated. Results: The resulting correction vectors were (0.4, 0.1, -0.5) in cm for the PTV ROI based manual registration, (0.6, 0.1, -0.3) in cm for the MIP based manual registration, and (0.32, 0.10, -0.42) in cm for the average based automatic registration. Figure 1 shows an overlaid surface rendering image of the 3D MIP of the 4D planning CT (in green) and 3D MIP of the 4D CBCT (in purple).
Conclusions: We have compared three different registration methods, and small but significant differences were observed. Among them, the PTV ROI based manual registration is the easiest solution but the ROI may not be the best reference because of a possible rotational error between the planning CT and the CBCT. The 3D MIP or 3D average images calculated by each 4D data set have more spatial information thereby possibly allowing a more accurate tumor registration. It is anticipated that a 6-degrees-of-freedom couch may facilitate stereotactic lung treatment using 4D planning CT and 4D CBCT to compensate any rotational error of the patient body. It is also expected that using identical couch tops for planning and treatment will become more important in this context. EP-1459 A STUDY OF A GATED INTENSITY-MODULATED RADIOTHERAPY (IMRT) ESOPHAGUS CANCER CASE BASED ON 3D DOSE MEASUREMENTS S. Stevens1, P. Dvorak1, M. McQuaid1, A. Richmond1 1 The London Clinic, Medical Physics, London, United Kingdom
amplitudes selected to correspond phantom velocity in the range 0 – 15 mm/sec. Each marker displacement is evaluated by correction shift estimate using manual match at markers endpoint relative to amplitude at 0 mm. Results: Fiducial marker detection using centre of mass can be verified within 3.7 mm for phantom velocity up to 15 mm/sec on 10 bins 4D CT. The deviation between actual physical positions and detected centre off mass strongly depends on phantom velocity during image acquisition of the phase bins. No correlation of marker detection is found among fiducial markers centre of mass and marker rotation relative to superior-inferior axis, CT slice thickness or NiTi stent length. Marker detection on gated IGRT using BrainLAB ExacTrac software can be verified within 1 mm. No correlation if found between detection accuracy and phantom velocity during gated IGRT, slice thickness and marker rotation. Conclusions: The safety margin for respiratory gated IGRT with GM and NiTi stent fiducial markers will be in the range of 1 – 4 mm. Safety margin correlates to the marker velocity during acquisition of the CT phase selected for treatment planning. EP-1457 CTV-PTV MARGINS WHEN USING KV A PRIORI ALIGMENT AND VERIFICATION FOR HEAD AND NECK CANCER PATIENTS E. van Dieren1, H. Richards1, S. Koch1 1 Medisch Spectrum Twente, radiotherapy, Enschede, The Netherlands Purpose/Objective: With the advent of a priori kV imaging in radiotherapy, systematic and random errors in patient positioning are largely eliminated. Thus, margins are defined only by intrafraction motion, possible deformations and matching errors, possibly leading to tighter margins than historically used. The goal of this study was to establish the intrafraction motion of patients irradiated for cancers of the head and neck, measured using a posteriori verification. Materials and Methods: For patients irradiated for Head & Neck cancers, a priori setup correction was performed using the Varian On Board Imager with orthogonal images and DRRs, on which PTV(s) were superimposed. Weekly, a posteriori EPID (61 patients, 280 images) or kV verification (50 patients, 175 images) was performed, which allow to assess the intrafraction (I.F.) movement in both groups. Based on the I.F. movements, a CTV-PTV margin was defined as the distance which covers 95% of I.F. movements. Patient characteristics in and between groups were compared in relation to I.F. movement. Additionally, results of a routine geometric test for stereotactic treatment were used. Assuming similar table accuracy, I.F. movement results were deconvolved using the results of the geometric test. Treatment planning was performed using the new margins and a historically used margin of 5 mm. Results: In group 1 (EPID), the margin was found to be 5 mm, for group 2 (kV) 3.5 mm. The two groups were similar in terms of prescribed dose, CTV volume, and fixation technique. The only difference was the used verification. For both groups, the margin reduced by 1 mm for patients treated with a curative dose only. Using the geometric test results, deconvolution of I.F. movements reduces margins by additionally 0.5 mm. The largest 5% of I.F. movement was associated with rotations (also 5%) and was usually seen for treatment with a palliative intent. Planning examples show beneficial effects in dose and DVH when the margin is reduced: less overlap allows more sparing of critical structures. Conclusions: This study shows that a generic margin for radiotherapy of head and neck cancer patients is not accurate. A reduction is feasable, especially for patients treated with a curative intent. This study also shows that for patients with head and neck cancer, actual margins can be smaller than previously assessed on the basis of a posteriori EPIDs. Since both groups were similar, it may be that MV is an unsuitable instrument for verification of high accuracy treatments. Geometric inaccuracy of the treatment table may even be larger when the full patient weight is on the treatment table. Its effect on margins warrants introduction of a (6 DoF) table with higher accuracy. Rotations, deformations, and user errors still remain to be investigated. Until then, using these margins in clinical practice is hazardous. We're currently contemplating replacing orthogonal images by CBCT. However, such a study is difficult to perform because of the larger radiation dose and longer linac time.
ESTRO 31
EP-1458 A COMPARISON OF THREE REGISTRATION METHODS USING 4D PLANNING CT AND ON-BOARD 4D CONE-BEAM CT FOR LUNG VMAT K. Nakagawa1, Y. Masutani1, A. Haga1, H. Yamashita1, W. Takahashi1, A. Sakumi1, S. Kida1, N. Saotome1, K. Ohtomo1, K. Yoda2 1 University of Tokyo Hospital, Dept. of Radiology, Tokyo, Japan 2 Elekta KK, Research Physics Dept., Tokyo, Japan Purpose/Objective: To establish a lung tumor registration workflow using 4D planning CT and 4D cone-beam CT. Materials and Methods: A large bore 16-multislice CT, Aquillion LB (Toshiba, Japan), was employed to scan a lung tumor patient with Anzai belt (Anzai Medical, Japan) to obtain 10-phase respiratory correlated CT data for 4D treatment planning. A body frame (Elekta, Sweden) was used to constrain the patient respiratory movement. Immediately before treatment, 10-phase 4D cone-beam CT (CBCT) data were acquired using a kV x-ray imager, XVI, equipped with a Synergy linac (Elekta, UK). Three different registration techniques have been performed for comparison: 1) A PTV was defined by adding a 5 mm margin to an ITV created by using 10 CTVs, each of which was defined on each of the 10-phase planning CT data. The PTV ROI was exported to the XVI workstation. Patient couch positioning was manually performed by comparing the PTV ROI to a moving tumor by repeatedly showing each phase image of the 4D CBCT on the XVI display monitor. 2) 3D maximum intensity projection (MIP) image was calculated by 4D 10phase planning CT to create an integrated tumor trace. Similar 3D MIP image was also created by 4D 10-phase CBCT. The two MIP images were manually registered to derive the patient couch positioning data in a separate workstation, and the resulting repositioning data was input to the XVI workstation. 3) 3D averaged image was calculated by using 4D planning CT data and it was exported to the XVI workstation. The same operation was done for 4D CBCT data using a built-in functionality in the XVI workstation. Then usual built-in gray-valuebased 3D registration was automatically calculated. Results: The resulting correction vectors were (0.4, 0.1, -0.5) in cm for the PTV ROI based manual registration, (0.6, 0.1, -0.3) in cm for the MIP based manual registration, and (0.32, 0.10, -0.42) in cm for the average based automatic registration. Figure 1 shows an overlaid surface rendering image of the 3D MIP of the 4D planning CT (in green) and 3D MIP of the 4D CBCT (in purple).
Conclusions: We have compared three different registration methods, and small but significant differences were observed. Among them, the PTV ROI based manual registration is the easiest solution but the ROI may not be the best reference because of a possible rotational error between the planning CT and the CBCT. The 3D MIP or 3D average images calculated by each 4D data set have more spatial information thereby possibly allowing a more accurate tumor registration. It is anticipated that a 6-degrees-of-freedom couch may facilitate stereotactic lung treatment using 4D planning CT and 4D CBCT to compensate any rotational error of the patient body. It is also expected that using identical couch tops for planning and treatment will become more important in this context. EP-1459 A STUDY OF A GATED INTENSITY-MODULATED RADIOTHERAPY (IMRT) ESOPHAGUS CANCER CASE BASED ON 3D DOSE MEASUREMENTS S. Stevens1, P. Dvorak1, M. McQuaid1, A. Richmond1 1 The London Clinic, Medical Physics, London, United Kingdom