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Low-dose megavoltage cone-beam CT for dose-guided radiation therapy
Proceedings of the 45th Annual ASTRO Meeting
99
An Integrated Volumetric Imaging and Guidance System for Targeting of Soft-Tissue Structures in Radiotherapy
M.B. Sharpe,1,2 D.J. Moseley,4,1 T. Haycocks,1 J.H. Siewerdsen,4,3 D.A. Jaffray1,3 1 Radiation Medicine Program, Princess Margaret Hospital, Toronto, ON, Canada, 2Radiation Oncology, University of Toronto, Toronto, ON, Canada, 3Medical Biophysics and Radiation Oncology, University of Toronto, Toronto, ON, Canada, 4Medical Physics, Ontario Cancer Institute, Toronto, ON, Canada Purpose/Objective: Image guidance of radiotherapy procedures can significantly reduce the uncertainties associated with patient set-up and internal organ motion. This study assesses the geometric accuracy and precision of an integrated system for on-line treatment guidance using x-ray volume imaging (XVI) of soft-tissue structures. Key elements of the treatment guidance system, its geometric calibration, and validation of delivery precision will be reported. These results are prerequisites for forthcoming clinical application of this technology. Materials/Methods: Using a standard 3D virtual simulation procedure, a treatment plan was formulated to target a rigid spherical target placed within a visually opaque phantom. The phantom was then subjected to ten treatment sessions to assess the accuracy and precision of the image-guidance and treatment delivery procedures. For each session, the phantom was placed on the therapy table and images were acquired using a volumetric x-ray imaging system operating at kilo-voltage energies (1). Each volumetric dataset was reconstructed from a sequence of 285 cone-beam projections acquired over 360 degrees. The reconstructed volume spans a 263-mm diameter and a 263-mm length, with a 0.514-mm reconstructed resolution in all three dimensions. Using a dedicated interface developed for image acquisition and management, the image dataset was loaded directly into an active treatment planning session. Image fusion was performed to ascertain the couch movements required to correctly place the target at the prescribed isocentre. The XVI image fusion was viewed and manipulated with the option to concurrently display dose distributions and presegmented regions of interest. The measured corrections were then transferred to the treatment unit and the attending therapists were prompted for confirmation and approval. Finally, each correction was independently validated using a pair of orthogonal treatment beams and an electronic portal imaging system to record the spherical target position with respect to symmetric beam apertures. Results: An initial calibration cycle followed by repeated image-guidance sessions over a two-week period demonstrated the system can be used to relocate an unambiguous object to within less than 1mm (0.8 ⫾ 0.2mm) of the prescribed location. Treatment delivery could then proceed within the mechanical accuracy and precision of the delivery system, which was verified independently via a pair of orthogonal portal images (1.2 ⫾ 0.3mm). The total time to acquire the reconstructed image volume, perform fusion, evaluation, and implement the couch adjustment was under 15 minutes. Integration of a commercial planning system into the process was received favourably by physicians, physicists, and therapists because it provides a familiar set of tools for image interpretation and plan evaluation throughout the planning and delivery process. We anticipate this standardized toolset will simplify staff training for image-guided procedures. Conclusions: An x-ray volume imaging (XVI) system has been successfully integrated into a feasible on-line radiotherapy treatment guidance procedure. The system supports acquisition of cone-beam projections and CT reconstruction; with image fusion and assessment of geometric targeting supported by a commercially available treatment planning system. The integrated system permits the imaging and guidance process to be executed in less than 15 minutes. The excellent spatial resolution and delivery precision of the system are ideal for clinical implementation of high-precision localization and treatment of soft-tissue targets. This work was performed with support from the NIH/NIA (R33 AG19381) and the Synergy RP Consortium of Elekta Oncology System, and cooperation from Philips Radiation Oncology Systems. 1. D.A. Jaffray, J.H. Siewerdsen et al. IJROBP 2002;53:1337–1349.
100
Low-Dose Megavoltage Cone-Beam CT for Dose-Guided Radiation Therapy
J. Pouliot,1 P. Xia,1 M. Aubin,1 L. Verhey,1 A. Bani-Hashemi,2 F. Ghelmansarai,2 M. Mitschke,2 M. Svatos2 1 Radiation Oncology, University of California–San Francisco, San Francisco, CA, 2Siemens Oncology Care Systems, Concord, CA Purpose/Objective: The objective of this work is to demonstrate that Megavoltage Cone Beam CT (MVCT) can be performed with a very low exposure to the patient and with a quality sufficient to register 3D MVCT reconstructed image with the kilovoltage planning CT for patient alignment and dose verification purposes, hence opening the possibility to frequently perform MV cone beam CT on patients. Materials/Methods: Using a standard Linac, the 6 MV dose rate was lowered to expose an a-Si flat panel using from 0.04 to 0.08 monitor units (M.U.) per image. An acquisition mode was designed to produce a high signal to noise ratio without pulsing artifacts. Several acquisitions were performed with either an anthropomorphic head phantom or a frozen sheep head, using a combination of 90 to 180 images incremented by 1 to 2 degrees. For each experiment, MVCT reconstruction was performed to visualize the 3D bony anatomy and some soft-tissue details. A Torso phantom was also used to measure contrast resolution. The 3D registration can then be performed automatically with a mutual information algorithm or done manually from multiple 2D projections. In either case the displacement between the image sets is transformed into required translation and rotations to reposition the patient. Registrations with a Rando head phantom and a sheep head were performed with different threshold values to emphasize bone or soft-tissue. The portal images capturing the treatment beams were then back-projected into the patient volume using the same geometry to reconstruct the dose. Registration between the MVCT 3D image and the reconstructed dose could allow a direct evaluation of the dose delivered relative to the anatomy at the time of the treatment. Results: A number of 3D MVCT images were reconstructed with dose delivered ranging from 5 to 15 cGy. The acquisition was performed in the time required for half of a gantry rotation. The processing time was approximately 90 seconds for the reconstruction of a 2563 cube with 1.2mm voxel size. Implanted markers (1 mm ⫻ 3 mm) were easily visible for all exposure levels (see the two white spots on the Figure). The spatial and contrast resolution will be presented as a function of the kernel size, type of filtering and exposure level. Conclusions: This work shows how a conventional Linac with stable low dose rate and an a-Si imager can be used to obtain MVCT reconstructed images with as little as 5 M.U. and compare the patient position with the planning CT. We believe that the quality of these images, along with rapid acquisition and reconstruction times, suggests that clinically useful MVCT can be performed using a standard Linac and a flat panel imager.
S183
S184
I. J. Radiation Oncology
101
● Biology ● Physics
Volume 57, Number 2, Supplement, 2003
On-Line Volumetric CT-Guided Radiation Therapy
M. Oldham,1 D. Letourneau,1 L. Watt,2 P. Chen,1 A. Martinez,1 J. Wong1 1 Radiation Oncology, William Beaumont Hospital, Royal Oak, MI, 2Elekta Inc., Crawley, United Kingdom Purpose/Objective: Variations in patient set-up and organ motion are the limiting factors to achieving accurate radiation treatment. The new technique of on-line volumetric CT imaging holds promise to address these limitations and greatly improve treatment accuracy. However, the ability to pinpoint target location at the time of treatment must be supported by an efficient treatment process that preserves the validity of the information for the duration of the treatment. To do so, we have extended the capability of 3D virtual simulation on a personal computer (PC) platform for on-line image guidance. The presentation focuses on two contrasting applications: stereotactic intra-cranial radiation therapy and prostate radiation therapy. Materials/Methods: The volumetric CT imaging system consists of a kV-source/flat-panel combination mounted on the drum of a medical accelerator, with the imaging axis orthogonal to the therapy axis. The system is capable of cone-beam CT with image quality similar to that of conventional helical CT scanner. With stereotactic intra-cranial radiation therapy, geometric accuracy is paramount. With prostate treatment, rapid correction is key to minimize the risk of changing organ position. For both sites, acquisition and reconstruction of volumetric CT images were performed in 5 min after patient set-up. The CT data were then imported into the PC-based Exomio virtual simulation system. Image fusion, navigation and visualization tools on dual display of prescription and treatment CT images facilitate fast determination of translational and rotational variations (Figure 1). Corrections were facilitated using the fine adjustment of a radiosurgery position plate for intra-cranial treatment, and the treatment couch for prostate treatment. The data were recorded in the planning system for recalculation purposes. Results: For frameless and stereotactic intra-cranial radiation therapy, on-line volumetric image guidance enabled the determination of set-up errors greater than 1mm translation, and 1 degree rotation. This approaches the radiosurgical ideal. For pelvic imaging, limiting the scan length for verification reduced scattered radiation and improved image quality. Preliminary studies indicated that for prostate treatment, it would be possible to attain a PTV margin expansion to within 5 mm and to treat within a 20-min treatment slot. The treatment duration matches the preservation of the PTV margin. Conclusions: On-line x-ray volumetric CT imaging has the potential to significantly improve the treatment accuracy of both sterotactic and pelvic radiation treatments. As with all image guided treatment techniques, development of efficient procedures to utilize on-line data are paramount.
99
An Integrated Volumetric Imaging and Guidance System for Targeting of Soft-Tissue Structures in Radiotherapy
M.B. Sharpe,1,2 D.J. Moseley,4,1 T. Haycocks,1 J.H. Siewerdsen,4,3 D.A. Jaffray1,3 1 Radiation Medicine Program, Princess Margaret Hospital, Toronto, ON, Canada, 2Radiation Oncology, University of Toronto, Toronto, ON, Canada, 3Medical Biophysics and Radiation Oncology, University of Toronto, Toronto, ON, Canada, 4Medical Physics, Ontario Cancer Institute, Toronto, ON, Canada Purpose/Objective: Image guidance of radiotherapy procedures can significantly reduce the uncertainties associated with patient set-up and internal organ motion. This study assesses the geometric accuracy and precision of an integrated system for on-line treatment guidance using x-ray volume imaging (XVI) of soft-tissue structures. Key elements of the treatment guidance system, its geometric calibration, and validation of delivery precision will be reported. These results are prerequisites for forthcoming clinical application of this technology. Materials/Methods: Using a standard 3D virtual simulation procedure, a treatment plan was formulated to target a rigid spherical target placed within a visually opaque phantom. The phantom was then subjected to ten treatment sessions to assess the accuracy and precision of the image-guidance and treatment delivery procedures. For each session, the phantom was placed on the therapy table and images were acquired using a volumetric x-ray imaging system operating at kilo-voltage energies (1). Each volumetric dataset was reconstructed from a sequence of 285 cone-beam projections acquired over 360 degrees. The reconstructed volume spans a 263-mm diameter and a 263-mm length, with a 0.514-mm reconstructed resolution in all three dimensions. Using a dedicated interface developed for image acquisition and management, the image dataset was loaded directly into an active treatment planning session. Image fusion was performed to ascertain the couch movements required to correctly place the target at the prescribed isocentre. The XVI image fusion was viewed and manipulated with the option to concurrently display dose distributions and presegmented regions of interest. The measured corrections were then transferred to the treatment unit and the attending therapists were prompted for confirmation and approval. Finally, each correction was independently validated using a pair of orthogonal treatment beams and an electronic portal imaging system to record the spherical target position with respect to symmetric beam apertures. Results: An initial calibration cycle followed by repeated image-guidance sessions over a two-week period demonstrated the system can be used to relocate an unambiguous object to within less than 1mm (0.8 ⫾ 0.2mm) of the prescribed location. Treatment delivery could then proceed within the mechanical accuracy and precision of the delivery system, which was verified independently via a pair of orthogonal portal images (1.2 ⫾ 0.3mm). The total time to acquire the reconstructed image volume, perform fusion, evaluation, and implement the couch adjustment was under 15 minutes. Integration of a commercial planning system into the process was received favourably by physicians, physicists, and therapists because it provides a familiar set of tools for image interpretation and plan evaluation throughout the planning and delivery process. We anticipate this standardized toolset will simplify staff training for image-guided procedures. Conclusions: An x-ray volume imaging (XVI) system has been successfully integrated into a feasible on-line radiotherapy treatment guidance procedure. The system supports acquisition of cone-beam projections and CT reconstruction; with image fusion and assessment of geometric targeting supported by a commercially available treatment planning system. The integrated system permits the imaging and guidance process to be executed in less than 15 minutes. The excellent spatial resolution and delivery precision of the system are ideal for clinical implementation of high-precision localization and treatment of soft-tissue targets. This work was performed with support from the NIH/NIA (R33 AG19381) and the Synergy RP Consortium of Elekta Oncology System, and cooperation from Philips Radiation Oncology Systems. 1. D.A. Jaffray, J.H. Siewerdsen et al. IJROBP 2002;53:1337–1349.
100
Low-Dose Megavoltage Cone-Beam CT for Dose-Guided Radiation Therapy
J. Pouliot,1 P. Xia,1 M. Aubin,1 L. Verhey,1 A. Bani-Hashemi,2 F. Ghelmansarai,2 M. Mitschke,2 M. Svatos2 1 Radiation Oncology, University of California–San Francisco, San Francisco, CA, 2Siemens Oncology Care Systems, Concord, CA Purpose/Objective: The objective of this work is to demonstrate that Megavoltage Cone Beam CT (MVCT) can be performed with a very low exposure to the patient and with a quality sufficient to register 3D MVCT reconstructed image with the kilovoltage planning CT for patient alignment and dose verification purposes, hence opening the possibility to frequently perform MV cone beam CT on patients. Materials/Methods: Using a standard Linac, the 6 MV dose rate was lowered to expose an a-Si flat panel using from 0.04 to 0.08 monitor units (M.U.) per image. An acquisition mode was designed to produce a high signal to noise ratio without pulsing artifacts. Several acquisitions were performed with either an anthropomorphic head phantom or a frozen sheep head, using a combination of 90 to 180 images incremented by 1 to 2 degrees. For each experiment, MVCT reconstruction was performed to visualize the 3D bony anatomy and some soft-tissue details. A Torso phantom was also used to measure contrast resolution. The 3D registration can then be performed automatically with a mutual information algorithm or done manually from multiple 2D projections. In either case the displacement between the image sets is transformed into required translation and rotations to reposition the patient. Registrations with a Rando head phantom and a sheep head were performed with different threshold values to emphasize bone or soft-tissue. The portal images capturing the treatment beams were then back-projected into the patient volume using the same geometry to reconstruct the dose. Registration between the MVCT 3D image and the reconstructed dose could allow a direct evaluation of the dose delivered relative to the anatomy at the time of the treatment. Results: A number of 3D MVCT images were reconstructed with dose delivered ranging from 5 to 15 cGy. The acquisition was performed in the time required for half of a gantry rotation. The processing time was approximately 90 seconds for the reconstruction of a 2563 cube with 1.2mm voxel size. Implanted markers (1 mm ⫻ 3 mm) were easily visible for all exposure levels (see the two white spots on the Figure). The spatial and contrast resolution will be presented as a function of the kernel size, type of filtering and exposure level. Conclusions: This work shows how a conventional Linac with stable low dose rate and an a-Si imager can be used to obtain MVCT reconstructed images with as little as 5 M.U. and compare the patient position with the planning CT. We believe that the quality of these images, along with rapid acquisition and reconstruction times, suggests that clinically useful MVCT can be performed using a standard Linac and a flat panel imager.
S183
S184
I. J. Radiation Oncology
101
● Biology ● Physics
Volume 57, Number 2, Supplement, 2003
On-Line Volumetric CT-Guided Radiation Therapy
M. Oldham,1 D. Letourneau,1 L. Watt,2 P. Chen,1 A. Martinez,1 J. Wong1 1 Radiation Oncology, William Beaumont Hospital, Royal Oak, MI, 2Elekta Inc., Crawley, United Kingdom Purpose/Objective: Variations in patient set-up and organ motion are the limiting factors to achieving accurate radiation treatment. The new technique of on-line volumetric CT imaging holds promise to address these limitations and greatly improve treatment accuracy. However, the ability to pinpoint target location at the time of treatment must be supported by an efficient treatment process that preserves the validity of the information for the duration of the treatment. To do so, we have extended the capability of 3D virtual simulation on a personal computer (PC) platform for on-line image guidance. The presentation focuses on two contrasting applications: stereotactic intra-cranial radiation therapy and prostate radiation therapy. Materials/Methods: The volumetric CT imaging system consists of a kV-source/flat-panel combination mounted on the drum of a medical accelerator, with the imaging axis orthogonal to the therapy axis. The system is capable of cone-beam CT with image quality similar to that of conventional helical CT scanner. With stereotactic intra-cranial radiation therapy, geometric accuracy is paramount. With prostate treatment, rapid correction is key to minimize the risk of changing organ position. For both sites, acquisition and reconstruction of volumetric CT images were performed in 5 min after patient set-up. The CT data were then imported into the PC-based Exomio virtual simulation system. Image fusion, navigation and visualization tools on dual display of prescription and treatment CT images facilitate fast determination of translational and rotational variations (Figure 1). Corrections were facilitated using the fine adjustment of a radiosurgery position plate for intra-cranial treatment, and the treatment couch for prostate treatment. The data were recorded in the planning system for recalculation purposes. Results: For frameless and stereotactic intra-cranial radiation therapy, on-line volumetric image guidance enabled the determination of set-up errors greater than 1mm translation, and 1 degree rotation. This approaches the radiosurgical ideal. For pelvic imaging, limiting the scan length for verification reduced scattered radiation and improved image quality. Preliminary studies indicated that for prostate treatment, it would be possible to attain a PTV margin expansion to within 5 mm and to treat within a 20-min treatment slot. The treatment duration matches the preservation of the PTV margin. Conclusions: On-line x-ray volumetric CT imaging has the potential to significantly improve the treatment accuracy of both sterotactic and pelvic radiation treatments. As with all image guided treatment techniques, development of efficient procedures to utilize on-line data are paramount.