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OC-0236 SET-UP ACCURACY AND INTRA-FRACTIONAL MOTION IN VMAT PROSTATE TREATMENTS
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OC-0235 INTER AND INTRAFRACTION VARIATIONS OF BREATHING MOTION AND COMPARISON BETWEEN 3D AND 4D SETUP CORRECTIONS USING CBCT F. van den Boomen1, M. Bierings1, D. Schuring1 1 Catharina Hospital, Department of Radiotherapy, Eindhoven, The Netherlands Purpose/Objective: Respiration-correlated cone-beam CT (4D CBCT) can be used to verify the mid-ventilation position and breathing amplitude of lung tumours. Traditionally, position verification of these tumours is performed using convential 3D cone-beam CT, where the CBCT scan is matched on an average CT scan. In this work, a comparison between setup corrections determined with conventional 3D CBCT and with 4D CBCT is made. Furthermore, the interfraction variation of the peak-to-peak (PtP) amplitude of the tumour is determined. For a selected group of patients, intrafraction variations are also determined Materials and Methods: CBCT data for 50 patients with lung tumours, each receiving an average of 8 scans, was included in this study. All patients received a respiration-correlated planning CT, from which the breathing amplitude and mid-ventilation position of the tumour was determined. Prior to treatment, a cone-beam CT scan was made using an Elekta XVI system, and both a 3D and 4D CT dataset were reconstructed. The setup correction based on the 3D CBCT was determined by matching the 3D reconstruction on the average planning CT. For the 4D CBCT dataset the setup correction and PtP amplitude were calculated by matching the 10 reconstructed breathing phases to the mid-ventilation planning scan and calculating the average displacement using the Elekta Symmetry software. The 3D and 4D setup corrections for each fraction were compared to each other, and the PtP amplitude was compared to the values determined on the original planning CT. Results: For all patients, the average 4D setup correction is plotted against the correction determined with 3D CBCT (see figure A). No statistically significant differences were found between the 3D and 4D setup corrections, the standard deviation of the average difference between 3D and 4D corrections being 1 mm or less in all directions. The average vector difference between 3D and 4D shifts was equal to 1.5 ± 1.0 mm (max. 7 mm). For larger motion amplitudes, 4D matches can however be more easily verified as for these patients the tumour in the 3D CBCT shows significant blurring due to motion. In figure B the average PtP amplitude of the tumour motion is plotted against the pre-treatment amplitude for all patients. The error bars reflect the standard deviation of the PtP amplitude over all fractions. The daily variations of the tumour motion compared to the values on the pre-treatment scan were within acceptable limits (average SD of 1.2 mm). The pre-treatment scan appears to slightly overestimate the PtP amplitude during treatment in most cases. Intrafraction variation of the PtP amplitude was small.
Conclusions: Although 3D and 4D CBCT matches resulted in similar setup corrections, for large tumor motion the matches could be more easily interpreted using 4D cone-beam CT. Variations in the daily PtP amplitude of the tumour were small. On average, the pre-treatment 4D scan slightly overestimates the daily tumour motion during treatment. OC-0236 SET-UP ACCURACY AND INTRA-FRACTIONAL MOTION IN VMAT PROSTATE TREATMENTS A. Tini1, A. Leong1, S. Lang1, M. Kovacs1, C. Winter1, C. Oehler2, D. Zwahlen2 1 University Hospital Zürich, Radio-Oncology, Zurich, Switzerland 2 Kantonsspital Graubünden, Radio-Oncology, Chur, Switzerland Purpose/Objective: The set-up accuracy and intra-fractional motion (IFM) of patients receiving volumetric intensity modulated arc therapy (VMAT) for localized prostate cancer were evaluated through combination of two- and three-dimensional imaging techniques. Materials and Methods: Ten patients were recruited for the study after three gold fiducial markers were implanted into the prostate. Prescribed daily doses to PTV were 2 and 3 Gy with total doses between 57 Gy in 19 fractions and 74 Gy in 37 fractions. Patients were positioned with the use of head, knee and foot supports. Additionally, rectum balloons inflated with 50 ml of air were utilised to increase prostate position consistency and reduce rectal toxicity. Patients were instructed to fill their bladder by drinking 400 ml of water one hour prior to treatment. Once per week, paired 2D kV orthogonal images followed by 3D conebeam CT (CBCT) were used to match upon fiducial markers. A post-treatment CBCT was then acquired to quantify target volume IFM. Pre-treatment kV and CBCT image matches were compared to evaluate their congruence. kV image pairs alone were used for matching on other treatment days. Results: Comparison of 2D-2D and 3D-3D pre-treatment image matches resulted in mean vertical, longitudinal and lateral discrepancies of 0.06 mm (SD 1.41), -0.26 mm (SD 1.51) and -0.16 mm (SD 0.85), respectively. The maximum deviation in all directions was 3 mm. Mean IFM as measured by pre- and post-treatment CBCT was 0.61 mm (SD 2.26), 0.51 mm (SD 2.41) and -0.14 mm (SD 1.42) in the vertical, longitudinal and lateral planes, respectively. The maximum deviation observed was 8.0 mm in the longitudinal direction. A systematic error of 1.39 mm was observed in the vertical direction, a
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potential result of patient relaxation. IFM was smallest in lateral direction (less than 3.0 mm).Mean treatment beam-on time was 1.14 min per fraction. Average treatment duration per patient per day including all imaging procedures was 13 min. Conclusions: 2D-2D and 3D-3D pre-treatment marker matches showed good congruence, confirming the accuracy and reliability of 2D kV imaging alone. The monitoring and quantification of IFM is essential for development of treatment protocols using hypofractionation for localized prostate cancer. OC-0237 POSITION VARIABILITY OF THE LARYNX DURING RADIOTHERAPY OF STAGE 1/2 LARYNGEAL CANCER M. Buijs1, P. Remeijer1, S. Ali1 1 The Netherlands Cancer Institute - Antoni van Leeuwenhoek Hospital, Radiotherapy, Amsterdam, The Netherlands Purpose/Objective: At our clinic patients with stage 1/2 laryngeal cancer are optionally treated with radiotherapy. A dose of 60 Gy is given in 25 fractions with a 2D 6 MV treatment plan based on two lateral fields, which are set around standard anatomical landmarks. Our standard protocol was to perform a weekly visual validation on portal images of the treatment fields. In 2009, for a few patients with stage 1/2 laryngeal cancer, CBCT images were acquired during radiotherapy as well, and registered using the vertebrae. In these patients a large difference in the position of the larynx was found. Therefore a study was performed to determine the position variability of the larynx in relation to rigid bony structures. Materials and Methods: 10 patients treated for laryngeal cancer were selected for this study. Registrations was performed on 85 CBCT scans with a clipbox on rigid bony structures (vertebrae) and with a clipbox on the larynx for CBCT to planning CT. Position errors were collected for LR, CC and AP-direction for both registration methods and compared. The registrations were performed by two observers to establish the match accuracy of both registration methods. Results: Linear regression analysis of the set up errors measured with registration on the larynx versus the vertebrae shows a poor agreement for the LR (slope=0.496, 95%CI 0.281-0.711, R2= 0.203) and CC direction (slope=0.556, 95%CI 0.368-0.744 R2= 0.294) and a moderate agreement for the AP direction (slope=1.14, 95%CI 1.0011.279, R2= 0.761), see graphs 1, 2 and 3. Residual random and systematic set up errors after corrections based on registration in bony structures instead of registration on the larynx are estimated for LR-direction to be σ=1.4 mm and ∑=1.3 mm, for CC σ=2.0 mm and ∑=2.4 mm, and for AP σ=0.9 and ∑=0.6 mm. Linear regression analysis of the observers shows very good agreement between the two observers for both methods; the slopes are for all directions between 0.96-1.02, R2>0.8. The match accuracy for registration on the vertebrae is <1.9 mm in all directions for the random error and varies for the systematic error from -0.1-1.5 mm. The match accuracy is for registration on the larynx is in all directions for the random error <1.1 mm and varies for the systematic error from 0.4-1.10 mm. Conclusions: Clipbox registration on the vertebrae underestimates the set up error of the larynx in the LR and CC-direction and is not influenced by observer variation. Therefore, measurement and correction of setup errors based on the vertebrae is not recommended for larynx treatments, especially when using more advanced radiotherapy techniques such as IMRT or VMAT.
OC-0238 EVALUATION THE EFFECTIVENESS OF IMMOBILIZATION SYSTEMS BY DAILY MEGAVOLTAGE COMPUTED TOMOGRAPHY IN TOMOTHERAPY K. Cheng1, V. Wu2 1 Hong Kong Sanatorium & Hospital, Division of Radiotherapy, Happy Valley, Hong Kong (SAR) China 2 The Hong Kong Polytechnic University, Department of Health Technology and Informatics, Hung Hom, Hong Kong (SAR) China Purpose/Objective: Through evaluating the effectiveness of some common immobilization systems in radiotherapy, this study aims to identify optimum immobilization system for different body regions. It also aims at providing valuable reference for oncologists in deciding planning target margin by calculating clinical target volume-planning target volume margin (CTV-PTV margin). Materials and Methods: 4867 tomotherapy daily treatment fractions' set up data from 212 patients were collected. Based on the pretreatment megavoltage computed tomography (MVCT), setup corrections were recorded and analyzed. Systematic error, random error and 3D vector were calculated and compared between the different immobilization systems. The CTV-PTV margins for various regions were also calculated. Results: Calculated systematic error, random error, 3D vector and CTV-PTV margins were shown in Table 1. There were no significant difference, which greater than 1mm, was detected between selected immobilization systems used in the head and neck (H&N), chest, abdomen and pelvic regions. Significant differences were found in breast groups, which shown prone immobilization system was less stable than supine one in this study. 3D vectors of setup variations with larger magnitudes also occurred more frequently in prone breast group showing that it provided less stability. Table 1. Calculated systematic errors, random errors, 3D vectors and CTV-PTV margins.
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OC-0235 INTER AND INTRAFRACTION VARIATIONS OF BREATHING MOTION AND COMPARISON BETWEEN 3D AND 4D SETUP CORRECTIONS USING CBCT F. van den Boomen1, M. Bierings1, D. Schuring1 1 Catharina Hospital, Department of Radiotherapy, Eindhoven, The Netherlands Purpose/Objective: Respiration-correlated cone-beam CT (4D CBCT) can be used to verify the mid-ventilation position and breathing amplitude of lung tumours. Traditionally, position verification of these tumours is performed using convential 3D cone-beam CT, where the CBCT scan is matched on an average CT scan. In this work, a comparison between setup corrections determined with conventional 3D CBCT and with 4D CBCT is made. Furthermore, the interfraction variation of the peak-to-peak (PtP) amplitude of the tumour is determined. For a selected group of patients, intrafraction variations are also determined Materials and Methods: CBCT data for 50 patients with lung tumours, each receiving an average of 8 scans, was included in this study. All patients received a respiration-correlated planning CT, from which the breathing amplitude and mid-ventilation position of the tumour was determined. Prior to treatment, a cone-beam CT scan was made using an Elekta XVI system, and both a 3D and 4D CT dataset were reconstructed. The setup correction based on the 3D CBCT was determined by matching the 3D reconstruction on the average planning CT. For the 4D CBCT dataset the setup correction and PtP amplitude were calculated by matching the 10 reconstructed breathing phases to the mid-ventilation planning scan and calculating the average displacement using the Elekta Symmetry software. The 3D and 4D setup corrections for each fraction were compared to each other, and the PtP amplitude was compared to the values determined on the original planning CT. Results: For all patients, the average 4D setup correction is plotted against the correction determined with 3D CBCT (see figure A). No statistically significant differences were found between the 3D and 4D setup corrections, the standard deviation of the average difference between 3D and 4D corrections being 1 mm or less in all directions. The average vector difference between 3D and 4D shifts was equal to 1.5 ± 1.0 mm (max. 7 mm). For larger motion amplitudes, 4D matches can however be more easily verified as for these patients the tumour in the 3D CBCT shows significant blurring due to motion. In figure B the average PtP amplitude of the tumour motion is plotted against the pre-treatment amplitude for all patients. The error bars reflect the standard deviation of the PtP amplitude over all fractions. The daily variations of the tumour motion compared to the values on the pre-treatment scan were within acceptable limits (average SD of 1.2 mm). The pre-treatment scan appears to slightly overestimate the PtP amplitude during treatment in most cases. Intrafraction variation of the PtP amplitude was small.
Conclusions: Although 3D and 4D CBCT matches resulted in similar setup corrections, for large tumor motion the matches could be more easily interpreted using 4D cone-beam CT. Variations in the daily PtP amplitude of the tumour were small. On average, the pre-treatment 4D scan slightly overestimates the daily tumour motion during treatment. OC-0236 SET-UP ACCURACY AND INTRA-FRACTIONAL MOTION IN VMAT PROSTATE TREATMENTS A. Tini1, A. Leong1, S. Lang1, M. Kovacs1, C. Winter1, C. Oehler2, D. Zwahlen2 1 University Hospital Zürich, Radio-Oncology, Zurich, Switzerland 2 Kantonsspital Graubünden, Radio-Oncology, Chur, Switzerland Purpose/Objective: The set-up accuracy and intra-fractional motion (IFM) of patients receiving volumetric intensity modulated arc therapy (VMAT) for localized prostate cancer were evaluated through combination of two- and three-dimensional imaging techniques. Materials and Methods: Ten patients were recruited for the study after three gold fiducial markers were implanted into the prostate. Prescribed daily doses to PTV were 2 and 3 Gy with total doses between 57 Gy in 19 fractions and 74 Gy in 37 fractions. Patients were positioned with the use of head, knee and foot supports. Additionally, rectum balloons inflated with 50 ml of air were utilised to increase prostate position consistency and reduce rectal toxicity. Patients were instructed to fill their bladder by drinking 400 ml of water one hour prior to treatment. Once per week, paired 2D kV orthogonal images followed by 3D conebeam CT (CBCT) were used to match upon fiducial markers. A post-treatment CBCT was then acquired to quantify target volume IFM. Pre-treatment kV and CBCT image matches were compared to evaluate their congruence. kV image pairs alone were used for matching on other treatment days. Results: Comparison of 2D-2D and 3D-3D pre-treatment image matches resulted in mean vertical, longitudinal and lateral discrepancies of 0.06 mm (SD 1.41), -0.26 mm (SD 1.51) and -0.16 mm (SD 0.85), respectively. The maximum deviation in all directions was 3 mm. Mean IFM as measured by pre- and post-treatment CBCT was 0.61 mm (SD 2.26), 0.51 mm (SD 2.41) and -0.14 mm (SD 1.42) in the vertical, longitudinal and lateral planes, respectively. The maximum deviation observed was 8.0 mm in the longitudinal direction. A systematic error of 1.39 mm was observed in the vertical direction, a
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potential result of patient relaxation. IFM was smallest in lateral direction (less than 3.0 mm).Mean treatment beam-on time was 1.14 min per fraction. Average treatment duration per patient per day including all imaging procedures was 13 min. Conclusions: 2D-2D and 3D-3D pre-treatment marker matches showed good congruence, confirming the accuracy and reliability of 2D kV imaging alone. The monitoring and quantification of IFM is essential for development of treatment protocols using hypofractionation for localized prostate cancer. OC-0237 POSITION VARIABILITY OF THE LARYNX DURING RADIOTHERAPY OF STAGE 1/2 LARYNGEAL CANCER M. Buijs1, P. Remeijer1, S. Ali1 1 The Netherlands Cancer Institute - Antoni van Leeuwenhoek Hospital, Radiotherapy, Amsterdam, The Netherlands Purpose/Objective: At our clinic patients with stage 1/2 laryngeal cancer are optionally treated with radiotherapy. A dose of 60 Gy is given in 25 fractions with a 2D 6 MV treatment plan based on two lateral fields, which are set around standard anatomical landmarks. Our standard protocol was to perform a weekly visual validation on portal images of the treatment fields. In 2009, for a few patients with stage 1/2 laryngeal cancer, CBCT images were acquired during radiotherapy as well, and registered using the vertebrae. In these patients a large difference in the position of the larynx was found. Therefore a study was performed to determine the position variability of the larynx in relation to rigid bony structures. Materials and Methods: 10 patients treated for laryngeal cancer were selected for this study. Registrations was performed on 85 CBCT scans with a clipbox on rigid bony structures (vertebrae) and with a clipbox on the larynx for CBCT to planning CT. Position errors were collected for LR, CC and AP-direction for both registration methods and compared. The registrations were performed by two observers to establish the match accuracy of both registration methods. Results: Linear regression analysis of the set up errors measured with registration on the larynx versus the vertebrae shows a poor agreement for the LR (slope=0.496, 95%CI 0.281-0.711, R2= 0.203) and CC direction (slope=0.556, 95%CI 0.368-0.744 R2= 0.294) and a moderate agreement for the AP direction (slope=1.14, 95%CI 1.0011.279, R2= 0.761), see graphs 1, 2 and 3. Residual random and systematic set up errors after corrections based on registration in bony structures instead of registration on the larynx are estimated for LR-direction to be σ=1.4 mm and ∑=1.3 mm, for CC σ=2.0 mm and ∑=2.4 mm, and for AP σ=0.9 and ∑=0.6 mm. Linear regression analysis of the observers shows very good agreement between the two observers for both methods; the slopes are for all directions between 0.96-1.02, R2>0.8. The match accuracy for registration on the vertebrae is <1.9 mm in all directions for the random error and varies for the systematic error from -0.1-1.5 mm. The match accuracy is for registration on the larynx is in all directions for the random error <1.1 mm and varies for the systematic error from 0.4-1.10 mm. Conclusions: Clipbox registration on the vertebrae underestimates the set up error of the larynx in the LR and CC-direction and is not influenced by observer variation. Therefore, measurement and correction of setup errors based on the vertebrae is not recommended for larynx treatments, especially when using more advanced radiotherapy techniques such as IMRT or VMAT.
OC-0238 EVALUATION THE EFFECTIVENESS OF IMMOBILIZATION SYSTEMS BY DAILY MEGAVOLTAGE COMPUTED TOMOGRAPHY IN TOMOTHERAPY K. Cheng1, V. Wu2 1 Hong Kong Sanatorium & Hospital, Division of Radiotherapy, Happy Valley, Hong Kong (SAR) China 2 The Hong Kong Polytechnic University, Department of Health Technology and Informatics, Hung Hom, Hong Kong (SAR) China Purpose/Objective: Through evaluating the effectiveness of some common immobilization systems in radiotherapy, this study aims to identify optimum immobilization system for different body regions. It also aims at providing valuable reference for oncologists in deciding planning target margin by calculating clinical target volume-planning target volume margin (CTV-PTV margin). Materials and Methods: 4867 tomotherapy daily treatment fractions' set up data from 212 patients were collected. Based on the pretreatment megavoltage computed tomography (MVCT), setup corrections were recorded and analyzed. Systematic error, random error and 3D vector were calculated and compared between the different immobilization systems. The CTV-PTV margins for various regions were also calculated. Results: Calculated systematic error, random error, 3D vector and CTV-PTV margins were shown in Table 1. There were no significant difference, which greater than 1mm, was detected between selected immobilization systems used in the head and neck (H&N), chest, abdomen and pelvic regions. Significant differences were found in breast groups, which shown prone immobilization system was less stable than supine one in this study. 3D vectors of setup variations with larger magnitudes also occurred more frequently in prone breast group showing that it provided less stability. Table 1. Calculated systematic errors, random errors, 3D vectors and CTV-PTV margins.