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198 Radiographic respiratory gating for lung radiotherapy without internal fiducials
P r o f f e r e d Papers
March 12 - 15
familiarity and confidence with the helical tomotherapy system. We then progressed to treating 4-5 patients consecutively, and are recently began treating 8-10 patients in a 5 hour time slot. Each 30 minute treatment time slot consists of an MVCT scan, patient repositioning as necessary, patient treatment according to in-house developed research protocol, and occasionaJJy a post-treatment MVCT scan. Outside the clinical time slot, a variety of research and development projects are conducted, along with machine characterization, servicing of the unit, and the performance of patient delivery quality assurance procedures. As of September, 2005 we have treated about 60 patients corresponding to about 1200 treatment fractions. Typical results will be presented from a variety of our in-house protocols. Conclusions: An image guided adaptive radiotherapy program using a helical tomotherapy system has been introduced at the CCI. A number of in-house protocols have been written, approved and are now accruing patients. Our initial observations are very encouraging, although we need to complete accrual to our in-house protocols and then evaluate the results before we can make definitive statements about the benefit to patients. 198 R A D I O G R A P H I C RESPIRATORY G A T I N G FOR RADIOTHERAPY W I T H O U T I N T E R N A L F I D U C I A L S
LUNG
S.S. Korreman 1, A. Boyer 2, E. Graves 2, H. Mostafavi 3, Q.T. Le2 1Copenhagen University Hospital, Copenhagen, Denmark, 2Stanford University School of Medicine, Stanford, California, 3Varian Gintzon Research Center, Mountain View, California, USA Objective: This study investigates the potential of a fluoroscopy based image-correlation technique disregarding internal fiducial markers to produce a respiratory trace suitable for gating of lung cancer patients. The motion of implanted internal fiducial markers serves as the measure of true tumor motion for assessment of the success of the method. Material and methods: Four lung cancer patients referred for CyberKnife radiosurgery had fiducial gold markers implanted in the tumor as part of an IRB approved study for dose escalation. Consecutive orthogonal fluoroscopic movies were recorded for each patient during breathing coaching, using the RPMT M gating system (Varian Medical Systems, Inc). The positions of the internal fiducial markers were measured within the video frames and used as the standard ("truth") of tumor motion. An external marker was synchronously tracked optically during the fluoroscopic acquisitions. A correlation technique was developed to compute a respiration trace from the fluoroscopic images. An analysis and comparison of the external optical marker gating with internaJ fluoroscopic gating was made based on receiver-operator characteristic (ROC) methods. The ROC technique was also used to compare correlation traces based on the internal fiducials with correlation traces made based on regions of interest (ROls) containing tumor images but independent of internal fiducials. Results: The image correlation respiratory trace was similar to the external marker optical trace in the phase regions of the respiratory cycle between inspiration and expiration peaks, where thresholds used for gating are typically defined. Successful gating was defined in terms of the position of the internal markers to be within a certain shift threshold corresponding to the beam aperture, for varying gating thresholds in the two traces tested. The image correlation trace was found to be significantly superior to the optical method in three out of four cases studied (p-values below 0.05). The image correlation method furthermore had a high discriminating value for all patients. No significant difference was found between gating based on image correlation with and without inclusion of internal fiducials in the ROIs. Conclusions: It was concluded that fluoroscopic gating based on correlation of native image features in 4ung fluoroscopic images performs adequately, and better than gating based on an external marker in respiratory gated radiotherapy for lung cancer.
$69
199 DEVELOPMENT OF A PROSTATE DEFORMATION MODEL TO ENABLE ACCURATE R E G I S T R A T I O N OF ENDORECTAL COIL MAGNETIC RESONANCE IMAGES ( E R C - M R I )
J. Hensel, C. M~nard, A. Kirilova, P. Chung, K. Brock Department of Radiation Oncology, Princess Margaret Hospital, Toronto, ON, Canada Objective: Magnetic resonance imaging (MRI) with the placement of an endorectal receiver coil (ERC) provides superior visualization of the prostate gland compared to the CT images traditionally used for radiotherapy treatment planning. In addition, increasing the volume of the ERC balloon further enhances structural resolution. This procedure does, however, cause severe deformation of the prostate gland and its adjacent structures, including the bladder and rectum. In order to integrate ERC images in the radiation treatment planning process, this deformation must be corrected. In this study, we aim to develop a deformable prostate model using a finite e~ement-based multi-organ deformable image registration method, 'Morfeus', developed at our institution. Material and Methods: Patients recently diagnosed with prostate cancer, and consented to receive external beam radiotherapy are enrolled in the study. Three gold fiducial markers are implanted in the prostate prior to imaging as per standard care in our institution. A series of axial MR images (FRFSE, Slice thickness: 2mm, TR: 6000 ms, TE: 13ms, ET: 11, FOV 18ram) are acquired with the ERC (Medrad MRInnervu) at various inflation volumes ranging from 100cc to 0cc. A final image series is also acquired after the ERC is removed. The prostate, bladder, and rectum volumes are segmented by a single observer. Each segmented organ volume is assigned different material properties (density, elastic modulus, and Poisson's ratio). Material properties are also assigned to a surrounding volume, representing the pelvis. In the deformable registration process between two image sets, the bladder and rectum are translated a priori according to the change in center of mass of the prostate volume. Using surface projections, Morfeus then deforms the bladder and rectum, thereby inferring the position and shape of the prostate gland. The accuracy of a deformation to a particular volume is determined by comparing the center of the fiducial markers in the inferred prostate to their positions in the actual prostate image. Results: The FRFSE pulse sequence, developed in this study, showed excellent anatomic resolution while permitting visualization of the fiducial markers as a signal void in all patients. These images were compared to CT and MRI GRE images, which clearly resolve the fiducial markers but provide limited anatomic detail. Results to date show good agreement of fiducial marker position when deforming the prostate from an ERC volume of 0cc to 100cc. For all fiducial markers (n=12), the mean absolute vector error in fiducial marker position is 2.0mm (min vector error~0.1mm, max vector error=4.2mm). Only two vector errors, in a single patient, were above the image resolution of 2.8ram (2.9mm, 4.2mm). The deformation process in Morfeus requires approximately 4-5 mins, performed on (Pentium 4, 2.80 GHz computer). Conclusion: Our deformation model developed using Morfeus can accurately predict the position of the prostate gland when the ERC balloon is inflated from 0cc to 100cc. Ongoing work will account for differing material properties of the peripheral and central zones of the prostate gJand, and will develop a deformation model to co-register treatment planning CT images to fully inflated ERC-MR images. The development of a successful deformable coregistration strategy is essential to integrate the superior information offered in ERC-MR images into the radiotherapy treatment planning process, thus improving tumor targeting and allowing escalated dose delivery.
2O0 3-D CONFORMAL RADIOTEHRAPY TREATMENT PLANNING FOR PROSTATE CANCER: BETTER ORGAN D E F I N I T I O N AND LESS DOSE TO THE PENILE BULB W I T H MRI-BASED V I R T U A L S I M U L A T I O N
G. Dipasquale, C. Ares, C. Jargy, P. Nouet, M. Rouzaud, R. Miralbell
March 12 - 15
familiarity and confidence with the helical tomotherapy system. We then progressed to treating 4-5 patients consecutively, and are recently began treating 8-10 patients in a 5 hour time slot. Each 30 minute treatment time slot consists of an MVCT scan, patient repositioning as necessary, patient treatment according to in-house developed research protocol, and occasionaJJy a post-treatment MVCT scan. Outside the clinical time slot, a variety of research and development projects are conducted, along with machine characterization, servicing of the unit, and the performance of patient delivery quality assurance procedures. As of September, 2005 we have treated about 60 patients corresponding to about 1200 treatment fractions. Typical results will be presented from a variety of our in-house protocols. Conclusions: An image guided adaptive radiotherapy program using a helical tomotherapy system has been introduced at the CCI. A number of in-house protocols have been written, approved and are now accruing patients. Our initial observations are very encouraging, although we need to complete accrual to our in-house protocols and then evaluate the results before we can make definitive statements about the benefit to patients. 198 R A D I O G R A P H I C RESPIRATORY G A T I N G FOR RADIOTHERAPY W I T H O U T I N T E R N A L F I D U C I A L S
LUNG
S.S. Korreman 1, A. Boyer 2, E. Graves 2, H. Mostafavi 3, Q.T. Le2 1Copenhagen University Hospital, Copenhagen, Denmark, 2Stanford University School of Medicine, Stanford, California, 3Varian Gintzon Research Center, Mountain View, California, USA Objective: This study investigates the potential of a fluoroscopy based image-correlation technique disregarding internal fiducial markers to produce a respiratory trace suitable for gating of lung cancer patients. The motion of implanted internal fiducial markers serves as the measure of true tumor motion for assessment of the success of the method. Material and methods: Four lung cancer patients referred for CyberKnife radiosurgery had fiducial gold markers implanted in the tumor as part of an IRB approved study for dose escalation. Consecutive orthogonal fluoroscopic movies were recorded for each patient during breathing coaching, using the RPMT M gating system (Varian Medical Systems, Inc). The positions of the internal fiducial markers were measured within the video frames and used as the standard ("truth") of tumor motion. An external marker was synchronously tracked optically during the fluoroscopic acquisitions. A correlation technique was developed to compute a respiration trace from the fluoroscopic images. An analysis and comparison of the external optical marker gating with internaJ fluoroscopic gating was made based on receiver-operator characteristic (ROC) methods. The ROC technique was also used to compare correlation traces based on the internal fiducials with correlation traces made based on regions of interest (ROls) containing tumor images but independent of internal fiducials. Results: The image correlation respiratory trace was similar to the external marker optical trace in the phase regions of the respiratory cycle between inspiration and expiration peaks, where thresholds used for gating are typically defined. Successful gating was defined in terms of the position of the internal markers to be within a certain shift threshold corresponding to the beam aperture, for varying gating thresholds in the two traces tested. The image correlation trace was found to be significantly superior to the optical method in three out of four cases studied (p-values below 0.05). The image correlation method furthermore had a high discriminating value for all patients. No significant difference was found between gating based on image correlation with and without inclusion of internal fiducials in the ROIs. Conclusions: It was concluded that fluoroscopic gating based on correlation of native image features in 4ung fluoroscopic images performs adequately, and better than gating based on an external marker in respiratory gated radiotherapy for lung cancer.
$69
199 DEVELOPMENT OF A PROSTATE DEFORMATION MODEL TO ENABLE ACCURATE R E G I S T R A T I O N OF ENDORECTAL COIL MAGNETIC RESONANCE IMAGES ( E R C - M R I )
J. Hensel, C. M~nard, A. Kirilova, P. Chung, K. Brock Department of Radiation Oncology, Princess Margaret Hospital, Toronto, ON, Canada Objective: Magnetic resonance imaging (MRI) with the placement of an endorectal receiver coil (ERC) provides superior visualization of the prostate gland compared to the CT images traditionally used for radiotherapy treatment planning. In addition, increasing the volume of the ERC balloon further enhances structural resolution. This procedure does, however, cause severe deformation of the prostate gland and its adjacent structures, including the bladder and rectum. In order to integrate ERC images in the radiation treatment planning process, this deformation must be corrected. In this study, we aim to develop a deformable prostate model using a finite e~ement-based multi-organ deformable image registration method, 'Morfeus', developed at our institution. Material and Methods: Patients recently diagnosed with prostate cancer, and consented to receive external beam radiotherapy are enrolled in the study. Three gold fiducial markers are implanted in the prostate prior to imaging as per standard care in our institution. A series of axial MR images (FRFSE, Slice thickness: 2mm, TR: 6000 ms, TE: 13ms, ET: 11, FOV 18ram) are acquired with the ERC (Medrad MRInnervu) at various inflation volumes ranging from 100cc to 0cc. A final image series is also acquired after the ERC is removed. The prostate, bladder, and rectum volumes are segmented by a single observer. Each segmented organ volume is assigned different material properties (density, elastic modulus, and Poisson's ratio). Material properties are also assigned to a surrounding volume, representing the pelvis. In the deformable registration process between two image sets, the bladder and rectum are translated a priori according to the change in center of mass of the prostate volume. Using surface projections, Morfeus then deforms the bladder and rectum, thereby inferring the position and shape of the prostate gland. The accuracy of a deformation to a particular volume is determined by comparing the center of the fiducial markers in the inferred prostate to their positions in the actual prostate image. Results: The FRFSE pulse sequence, developed in this study, showed excellent anatomic resolution while permitting visualization of the fiducial markers as a signal void in all patients. These images were compared to CT and MRI GRE images, which clearly resolve the fiducial markers but provide limited anatomic detail. Results to date show good agreement of fiducial marker position when deforming the prostate from an ERC volume of 0cc to 100cc. For all fiducial markers (n=12), the mean absolute vector error in fiducial marker position is 2.0mm (min vector error~0.1mm, max vector error=4.2mm). Only two vector errors, in a single patient, were above the image resolution of 2.8ram (2.9mm, 4.2mm). The deformation process in Morfeus requires approximately 4-5 mins, performed on (Pentium 4, 2.80 GHz computer). Conclusion: Our deformation model developed using Morfeus can accurately predict the position of the prostate gland when the ERC balloon is inflated from 0cc to 100cc. Ongoing work will account for differing material properties of the peripheral and central zones of the prostate gJand, and will develop a deformation model to co-register treatment planning CT images to fully inflated ERC-MR images. The development of a successful deformable coregistration strategy is essential to integrate the superior information offered in ERC-MR images into the radiotherapy treatment planning process, thus improving tumor targeting and allowing escalated dose delivery.
2O0 3-D CONFORMAL RADIOTEHRAPY TREATMENT PLANNING FOR PROSTATE CANCER: BETTER ORGAN D E F I N I T I O N AND LESS DOSE TO THE PENILE BULB W I T H MRI-BASED V I R T U A L S I M U L A T I O N
G. Dipasquale, C. Ares, C. Jargy, P. Nouet, M. Rouzaud, R. Miralbell