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PO-0862: Comparison of two commercially available real-time tumor tracking solutions in a phantom study
S86 Materials and Methods: Two treatment plans, one obtained using photons and one using protons, were compared for three brain tumor cases. In the optimization, a standard constant RBE equal to 1.1 was assumed. In the plan evaluation, both a constant RBE and a variable RBE depending on dose, linear energy transfer (LET), and the tissue specific parameter α/β of photons, were used to calculate the resulting biologically equivalent dose. The comparison of the plans was based on dose distributions, DVHs, and normal tissue complication probabilities (NTCPs). Results: RBE distributions accounting for the dependence of RBE on dose, LET, and α/β ratio of photons, were obtained. Generally, lower RBE was predicted in the tumor and higher RBE in adjacent normal tissue than the commonly assumed RBE equal to 1.1. Despite that, generally the proton plans showed considerably lower equivalent doses to normal tissue than the photon plans. However, in small volumes in critical structures close to the tumor, the increased RBE gave rise to hot spots leading to predicted worse NTCPs compared to those obtained with photons. Figure. RBE distribution.
ESTRO 33, 2014 scan and selection of a blood vessel (or tumor) structure to be tracked in subsequent real-time 2D MRI series. (2) Generation of a library of 2D image templates oriented parallel to the subsequent 2D MRI series by reslicing and resampling the 3D MRI scan. (3) 3D tracking of the selected structure in each real-time 2D image by finding the template and template position that yield the highest normalized cross correlation coefficient with the image. Since the tracked structure has a known 3D position relative to each template, the selection and 2D localization of a specific template translates into quantification of both the throughplane and in-plane position of the structure. As a proof of principle, 3D tracking of liver blood vessel structures was performed in two 5.4Hz axial, sagittal, and coronal real-time 2D MRI series for five healthy volunteers (six series of 30 seconds per volunteer). In each 2D MRI series, 3D tracking was performed for an anteriorly and a posteriorly located liver blood vessel. Validation tests were carried out to support the tracking algorithm and included quantification of the breathing induced 3D liver motion, liver motion directionality and liver deformation. Results: Axial, sagittal, andcoronal 2D MRI series successfully yielded the 3D respiratory motion for all volunteers. The motion directionality and amplitude were very similar when measured directly as in-plane motion or estimated indirectly as through-plane motion (see table 1 and figure 1). The mean peak-to-peak breathing amplitude was 1.6 mm (left-right), 11.0 mm (cranio-caudal), and 2.5 mm (anterior-posterior) (Table 1). The mean cranio-caudal breathing motion amplitude was invariably larger (2.3 - 3.3mm) in the cranial part of the liver than in caudal parts. No systematic trend of breathing induced liver deformation was found between the anterior and posterior liver regions.
Conclusions: Disregarding RBE variations might lead to a lower effect in the tumor and a higher effect in normal tissues than expected leading to too optimistic estimates for the proton plans. In order to exploit the full potential of protons, not only the improved dose distribution but also the biological effectiveness needs to be taken into account and optimized. With such an approach, better proton treatment plans should be obtained.
POSTER: PHYSICS TRACK: INTRAFRACTION MOTION MANAGEMENT PO-0861 Three-dimensional liver motion tracking using 2D real-time MRI L. Brix1, S. Ringgaard2, T. Sangild Sørensen3, P. Rugaard Poulsen4 1 Region Midt, Procurement & Clinical Engineering, Aarhus N, Denmark 2 Aarhus University Hospital, MR Research Centre, Aarhus N, Denmark 3 Aarhus University, Computer Science, Aarhus N, Denmark 4 Aarhus University Hospital Nørrebrogade, Oncology, Aarhus C, Denmark Purpose/Objective: Combined magnetic resonance imaging (MRI) systems and linear accelerators for radiotherapy (MR-linacs) are currently under development. MRI is non-invasive and possesses the ability to acquire and reconstruct images in real-time with high soft tissue contrast. However, in order to fully exploit the advantages of MRlinacs, new tracking methods are required to obtain fast real-time spatial target localization. This study presents and evaluates a method for tracking 3D respiratory liver motion in 2D real-time MRI image series with high temporal and spatial resolution. Materials and Methods: The proposed method for 3D tracking in 2D realtime MRI series has three steps: (1) Recording of a volumetric 3D MRI
Conclusions: A method for 3D tracking in 2D MRI series was developed and demonstrated for liver tracking in volunteers. Implementation of the tracking algorithm for real-time use on the image stream from the MRI scanner would allow real-time 3D localization with integrated MR-linac systems. PO-0862 Comparison of two commercially available real-time tumor tracking solutions in a phantom study T. Depuydt1, M. Croisé1, T. Lacornerie2, K. Poels1, D. Verellen1, G. Storme1, M. De Ridder1 1 Universitair Ziekenhuis Brussel, Radiotherapy, Brussels, Belgium 2 Centre Oscar Lambret, Département Universitaire de Radiothérapie, Lille, France Purpose/Objective: To make a head-to-head comparison of two commercially available real-time tumor tracking (RTTT) systems, the Accuray CyberKnife system(CK) and the BrainLab/MHI Vero system, using a moving phantom.
ESTRO 33, 2014 Materials and Methods: The CK system is a 6MV linac mounted on a robot arm gantry with multiple joints offering a large degree of freedom for moving the beam. The Vero system is a 6MV linac mounted on an Oring gantry with a gimbals system, allowing pan and tilt rotation of the therapeutic beam. Both systems feature hybrid tumor tracking using an optical surrogate breathing motion signal and a correspondence model (CM) predicting internal tumor motion from the external surrogate. A 1D moving phantom was assembled to reproduce motion signals acquired from patients.The external motion and internal motion could be driven with a different signal and additionally a base-line drift could be superimposed on the internal motion to mimic realistic patient behavior. Radiochromic film was inserted in the moving wooden cylinder of the phantom (Quasar, Modus Medical Inc.) and fiducial markers were inserted inside or in close proximity of a polystyrene tumor object. To compare dynamic behavior of both systems at high target speeds, one single vertical beam was tracking the moving tumor object. The frequency of a 40 mm amplitude sinus motion was increased from 0 to 30 breaths-per-minute (bpm), reaching a maximum speed of 63 mm/s. The blurring of the dose fall-off in the direction of the motion was quantified taking the 20%-80% isodose distance. In a second experiment, treatment plans of 21 fixed-cone beams on CK and 9 MLC-collimated beams were delivered both statically and dynamically with reproduction of 3 different sets of patient motion data by the phantom. The experiments with motion were executed with and without a linear base-line drift (0.5 mm/min), and with and without rebuilding/updating of the CM. The measured dose distributions were compared with Monte-Carlo calculated dose, using gamma evaluation (2 mm, 3%).
S87 tumor baseline shift is of crucial importance for the success of the treatment. Mid-ventilation strategy [1] aims to solve the problem with the help of 4DCT and 4DCBCT. The determination of PTV margins thanks the van Herk recipe [2] is supposed to take into account systematic and random errors. The aim of this study is to evaluate the intra-fraction tumour baseline shift and verify the relevance of the PTV margins for patients treated with 4DCBCT image guidance. Materials and Methods: A total of 115 fractions of 16 patients with early stage NSCLC were retrospectively analysed. All were treated in frameless and free breathing conditions. The treatment margins were calculated according to van Herk recipe in the mid-ventilation phase. Fractionation scheme was 8x7.5 Gy. Dose was delivered with single or two arcs VMAT technique. During each fraction a first 4DCBCT was performed to determine the tumour baseline shift compared to planning 4DCT, a second 4DCBCT (post-correction) allowed us to verify the resulting localization accuracy and a third 4DCBCT (post-treatment) was performed after the end of the irradiation to control the intra-fraction tumour baseline shift. For each patient and each fraction, the tumour motion amplitude (obtained from 4DCT images) and the intra-fraction tumour baseline shift obtained from post-correction and post-treatment 4DCBCT data were assessed. Results: The averaged 3D vector data of the tumour motion amplitude and of the intra-fraction tumour baseline shift are presented in table 1 with the data of the margin sizes in each direction.
Results: Up to a frequency of 20 bpm (43 mm/s max. speed) the dosefall of was not altered by more than 0.9 mm and 0.6 mm for CK and Vero respectively (Fig. 1). Beyond 20 bpm differences were seen between both systems. The dose-fall off width increased by 4.5 mm on CK and 0.8 mm on Vero for 30 bpm. For treatment plan delivery, the gamma passrates were 100% for CK and 99.6%for Vero for a static target and on average 97.3% and 99.3% respectively for a the dynamic target without base-line drifting. With drifting but no updating/rebuilding of the CM, this was reduced to below 65% for both. Using rebuilding/updating, high pass-rates were re-established for both systems to 95.8% and 97.5% respectively. Conclusions: A first direct comparison study between CK and Vero was conducted. In general similar performance was seen for both systems based on delivered dose to moving targets. For tracking of high target speeds above 43 mm/s, less penumbra blurring was seen for Vero compared to CK.
The tumour motion amplitude was exceeding the applied treatment margins in four cases. This is justified by the hypothesis underlying the mid-ventilation strategy. In 115 fractions, the 3D vector of the intra-fraction tumour baseline shift (mean 2.8 ± 1.7 mm) is always inferior to the tumour motion amplitude (mean 7.6 ± 5.2 mm) exceptin one case. Most of the patient showed an intra-fraction tumour baseline shift inferior to 6.0 ± 1.8 mm. In all cases, the mean time between the post-correction 4DCBCT and the posttreatment 4DCBCT was 14.9 ± 4.9 min. The maximum time was 29.0 ± 4.0 min. Conclusions: We have developed a robust and efficient clinical protocol for frameless stereotactic NSCLC patients’ workflow. According to our clinical series specificities, we have demonstrated that the tumour intrafraction baseline shift is of lesser importance compared to the tumour motion amplitude. Our treatment time is in the range of reported literature [3]. As a consequence, the general mid-ventilation strategy coupled with van Herk recipe and 4DCBCT-based IGRT is an accurate and efficient way of treating NSCLC lung tumours. [1] Wolthaus, IJROBP 65(5),2006 [2] van Herk, IJROBP 47(4),2000 [3] Sonke, IJROBP 74(2), 2009
PO-0863 Intra-fraction baseline shift and margin analysis based on 4DCBCT during frameless lung stereotactic radiotherapy S. Thengumpallil1, J.F. Germond1, N. Peguret2, M. Ozsahin2, J. Bourhis2, F. Bochud1, R. Moeckli1 1 Lausanne University Hospital, Institute of Radiation Physics, Lausanne, Switzerland 2 Lausanne University Hospital, Radio-Oncology Department, Lausanne, Switzerland Purpose/Objective: Frameless stereotactic lung treatments require high precision in the evaluation of the tumour volume motion. Intra-fraction
PO-0864 Four-dimensional measurement of lung tumours and implanted gold markers by 320-slice CT scan Y. Iizuka1, Y. Matsuo1, T. Shiozumi2, N. Ueki1, T. Kishi1, S. Kozawa3, T. Takakura3, M. Nakamura1, T. Mizowaki1, M. Hiraoka1 1 Kyoto University Graduate School of Medicine, Department of Radiation Oncology and Image Applied therapy, Kyoto, Japan 2 Kyoto University Faculty of Medicine, Department of Medicine, Kyoto, Japan 3 Kyoto University Hospital, Clinical Radiology Service Division, Kyoto, Japan Purpose/Objective: Fiducial markers implanted in or near a target are often used as internal surrogates for motion management. The tumour position is estimated based on the position of the markers. We used more than two markers to predict tumour position for dynamic tracking stereotactic body radiotherapy (SBRT). Although the relative distance between the tumour and markers varies according to the breathing cycle, it is difficult to assess the variation. We can obtain images as long
ESTRO 33, 2014 scan and selection of a blood vessel (or tumor) structure to be tracked in subsequent real-time 2D MRI series. (2) Generation of a library of 2D image templates oriented parallel to the subsequent 2D MRI series by reslicing and resampling the 3D MRI scan. (3) 3D tracking of the selected structure in each real-time 2D image by finding the template and template position that yield the highest normalized cross correlation coefficient with the image. Since the tracked structure has a known 3D position relative to each template, the selection and 2D localization of a specific template translates into quantification of both the throughplane and in-plane position of the structure. As a proof of principle, 3D tracking of liver blood vessel structures was performed in two 5.4Hz axial, sagittal, and coronal real-time 2D MRI series for five healthy volunteers (six series of 30 seconds per volunteer). In each 2D MRI series, 3D tracking was performed for an anteriorly and a posteriorly located liver blood vessel. Validation tests were carried out to support the tracking algorithm and included quantification of the breathing induced 3D liver motion, liver motion directionality and liver deformation. Results: Axial, sagittal, andcoronal 2D MRI series successfully yielded the 3D respiratory motion for all volunteers. The motion directionality and amplitude were very similar when measured directly as in-plane motion or estimated indirectly as through-plane motion (see table 1 and figure 1). The mean peak-to-peak breathing amplitude was 1.6 mm (left-right), 11.0 mm (cranio-caudal), and 2.5 mm (anterior-posterior) (Table 1). The mean cranio-caudal breathing motion amplitude was invariably larger (2.3 - 3.3mm) in the cranial part of the liver than in caudal parts. No systematic trend of breathing induced liver deformation was found between the anterior and posterior liver regions.
Conclusions: Disregarding RBE variations might lead to a lower effect in the tumor and a higher effect in normal tissues than expected leading to too optimistic estimates for the proton plans. In order to exploit the full potential of protons, not only the improved dose distribution but also the biological effectiveness needs to be taken into account and optimized. With such an approach, better proton treatment plans should be obtained.
POSTER: PHYSICS TRACK: INTRAFRACTION MOTION MANAGEMENT PO-0861 Three-dimensional liver motion tracking using 2D real-time MRI L. Brix1, S. Ringgaard2, T. Sangild Sørensen3, P. Rugaard Poulsen4 1 Region Midt, Procurement & Clinical Engineering, Aarhus N, Denmark 2 Aarhus University Hospital, MR Research Centre, Aarhus N, Denmark 3 Aarhus University, Computer Science, Aarhus N, Denmark 4 Aarhus University Hospital Nørrebrogade, Oncology, Aarhus C, Denmark Purpose/Objective: Combined magnetic resonance imaging (MRI) systems and linear accelerators for radiotherapy (MR-linacs) are currently under development. MRI is non-invasive and possesses the ability to acquire and reconstruct images in real-time with high soft tissue contrast. However, in order to fully exploit the advantages of MRlinacs, new tracking methods are required to obtain fast real-time spatial target localization. This study presents and evaluates a method for tracking 3D respiratory liver motion in 2D real-time MRI image series with high temporal and spatial resolution. Materials and Methods: The proposed method for 3D tracking in 2D realtime MRI series has three steps: (1) Recording of a volumetric 3D MRI
Conclusions: A method for 3D tracking in 2D MRI series was developed and demonstrated for liver tracking in volunteers. Implementation of the tracking algorithm for real-time use on the image stream from the MRI scanner would allow real-time 3D localization with integrated MR-linac systems. PO-0862 Comparison of two commercially available real-time tumor tracking solutions in a phantom study T. Depuydt1, M. Croisé1, T. Lacornerie2, K. Poels1, D. Verellen1, G. Storme1, M. De Ridder1 1 Universitair Ziekenhuis Brussel, Radiotherapy, Brussels, Belgium 2 Centre Oscar Lambret, Département Universitaire de Radiothérapie, Lille, France Purpose/Objective: To make a head-to-head comparison of two commercially available real-time tumor tracking (RTTT) systems, the Accuray CyberKnife system(CK) and the BrainLab/MHI Vero system, using a moving phantom.
ESTRO 33, 2014 Materials and Methods: The CK system is a 6MV linac mounted on a robot arm gantry with multiple joints offering a large degree of freedom for moving the beam. The Vero system is a 6MV linac mounted on an Oring gantry with a gimbals system, allowing pan and tilt rotation of the therapeutic beam. Both systems feature hybrid tumor tracking using an optical surrogate breathing motion signal and a correspondence model (CM) predicting internal tumor motion from the external surrogate. A 1D moving phantom was assembled to reproduce motion signals acquired from patients.The external motion and internal motion could be driven with a different signal and additionally a base-line drift could be superimposed on the internal motion to mimic realistic patient behavior. Radiochromic film was inserted in the moving wooden cylinder of the phantom (Quasar, Modus Medical Inc.) and fiducial markers were inserted inside or in close proximity of a polystyrene tumor object. To compare dynamic behavior of both systems at high target speeds, one single vertical beam was tracking the moving tumor object. The frequency of a 40 mm amplitude sinus motion was increased from 0 to 30 breaths-per-minute (bpm), reaching a maximum speed of 63 mm/s. The blurring of the dose fall-off in the direction of the motion was quantified taking the 20%-80% isodose distance. In a second experiment, treatment plans of 21 fixed-cone beams on CK and 9 MLC-collimated beams were delivered both statically and dynamically with reproduction of 3 different sets of patient motion data by the phantom. The experiments with motion were executed with and without a linear base-line drift (0.5 mm/min), and with and without rebuilding/updating of the CM. The measured dose distributions were compared with Monte-Carlo calculated dose, using gamma evaluation (2 mm, 3%).
S87 tumor baseline shift is of crucial importance for the success of the treatment. Mid-ventilation strategy [1] aims to solve the problem with the help of 4DCT and 4DCBCT. The determination of PTV margins thanks the van Herk recipe [2] is supposed to take into account systematic and random errors. The aim of this study is to evaluate the intra-fraction tumour baseline shift and verify the relevance of the PTV margins for patients treated with 4DCBCT image guidance. Materials and Methods: A total of 115 fractions of 16 patients with early stage NSCLC were retrospectively analysed. All were treated in frameless and free breathing conditions. The treatment margins were calculated according to van Herk recipe in the mid-ventilation phase. Fractionation scheme was 8x7.5 Gy. Dose was delivered with single or two arcs VMAT technique. During each fraction a first 4DCBCT was performed to determine the tumour baseline shift compared to planning 4DCT, a second 4DCBCT (post-correction) allowed us to verify the resulting localization accuracy and a third 4DCBCT (post-treatment) was performed after the end of the irradiation to control the intra-fraction tumour baseline shift. For each patient and each fraction, the tumour motion amplitude (obtained from 4DCT images) and the intra-fraction tumour baseline shift obtained from post-correction and post-treatment 4DCBCT data were assessed. Results: The averaged 3D vector data of the tumour motion amplitude and of the intra-fraction tumour baseline shift are presented in table 1 with the data of the margin sizes in each direction.
Results: Up to a frequency of 20 bpm (43 mm/s max. speed) the dosefall of was not altered by more than 0.9 mm and 0.6 mm for CK and Vero respectively (Fig. 1). Beyond 20 bpm differences were seen between both systems. The dose-fall off width increased by 4.5 mm on CK and 0.8 mm on Vero for 30 bpm. For treatment plan delivery, the gamma passrates were 100% for CK and 99.6%for Vero for a static target and on average 97.3% and 99.3% respectively for a the dynamic target without base-line drifting. With drifting but no updating/rebuilding of the CM, this was reduced to below 65% for both. Using rebuilding/updating, high pass-rates were re-established for both systems to 95.8% and 97.5% respectively. Conclusions: A first direct comparison study between CK and Vero was conducted. In general similar performance was seen for both systems based on delivered dose to moving targets. For tracking of high target speeds above 43 mm/s, less penumbra blurring was seen for Vero compared to CK.
The tumour motion amplitude was exceeding the applied treatment margins in four cases. This is justified by the hypothesis underlying the mid-ventilation strategy. In 115 fractions, the 3D vector of the intra-fraction tumour baseline shift (mean 2.8 ± 1.7 mm) is always inferior to the tumour motion amplitude (mean 7.6 ± 5.2 mm) exceptin one case. Most of the patient showed an intra-fraction tumour baseline shift inferior to 6.0 ± 1.8 mm. In all cases, the mean time between the post-correction 4DCBCT and the posttreatment 4DCBCT was 14.9 ± 4.9 min. The maximum time was 29.0 ± 4.0 min. Conclusions: We have developed a robust and efficient clinical protocol for frameless stereotactic NSCLC patients’ workflow. According to our clinical series specificities, we have demonstrated that the tumour intrafraction baseline shift is of lesser importance compared to the tumour motion amplitude. Our treatment time is in the range of reported literature [3]. As a consequence, the general mid-ventilation strategy coupled with van Herk recipe and 4DCBCT-based IGRT is an accurate and efficient way of treating NSCLC lung tumours. [1] Wolthaus, IJROBP 65(5),2006 [2] van Herk, IJROBP 47(4),2000 [3] Sonke, IJROBP 74(2), 2009
PO-0863 Intra-fraction baseline shift and margin analysis based on 4DCBCT during frameless lung stereotactic radiotherapy S. Thengumpallil1, J.F. Germond1, N. Peguret2, M. Ozsahin2, J. Bourhis2, F. Bochud1, R. Moeckli1 1 Lausanne University Hospital, Institute of Radiation Physics, Lausanne, Switzerland 2 Lausanne University Hospital, Radio-Oncology Department, Lausanne, Switzerland Purpose/Objective: Frameless stereotactic lung treatments require high precision in the evaluation of the tumour volume motion. Intra-fraction
PO-0864 Four-dimensional measurement of lung tumours and implanted gold markers by 320-slice CT scan Y. Iizuka1, Y. Matsuo1, T. Shiozumi2, N. Ueki1, T. Kishi1, S. Kozawa3, T. Takakura3, M. Nakamura1, T. Mizowaki1, M. Hiraoka1 1 Kyoto University Graduate School of Medicine, Department of Radiation Oncology and Image Applied therapy, Kyoto, Japan 2 Kyoto University Faculty of Medicine, Department of Medicine, Kyoto, Japan 3 Kyoto University Hospital, Clinical Radiology Service Division, Kyoto, Japan Purpose/Objective: Fiducial markers implanted in or near a target are often used as internal surrogates for motion management. The tumour position is estimated based on the position of the markers. We used more than two markers to predict tumour position for dynamic tracking stereotactic body radiotherapy (SBRT). Although the relative distance between the tumour and markers varies according to the breathing cycle, it is difficult to assess the variation. We can obtain images as long