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A new method for image guided radiation therapy using computed tomography
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I. J. Radiation Oncology
● Biology ● Physics
Volume 60, Number 1, Supplement, 2004
and from 3.09 to 4.45 (mean 3.71) for IMRT. At the 20% isodose level values ranged from 5.15 to 10.45 (mean 8.07) for the CyberKnife and from 12.1 to 32.56 (mean 18.68) for IMRT. Conclusions: The ratio between the volumes encompassed by the mid-low isodose surfaces to the volume encompassed by the reference isodose surface is in general greater for IMRT than for the CyberKnife. This behavior can be explained observing that IMRT is a coplanar technique while the CyberKnife uses a non-coplanar approach to the target volume. This results in a greater irradiated volume but in a lower volume which receives intermediate dose values. Also the conformity index is better for the CyberKnife than in the IMRT case. The homogeneity index resulted in general better for IMRT. All these facts can be expressed by saying that the dose gradient passing from out-target to in-target points is in general higher with the CyberKnife than with coplanar IMRT. The observed behavior allows CyberKnife users to treat lesions with a hypo-fractionated schedule compared to IMRT, because organs at risk receive in general a lower dose with the CyberKnife.
2467
A New Method for Image Guided Radiation Therapy Using Computed Tomography
T. Jenkins, M. Wolfe, Y. Feng, H. Mota, C. Sibata, R. R. Allison Radiation Oncology, The Brody School of Medicine, ECU, Greenville, NC Purpose/Objective: Image guided radiation therapy (IGRT) is a technique for increasing the precision of radiation delivery that couples diagnostic imaging with the treatment procedure. In some radiation oncology facilities, computed tomography (CT) scanners on rails are being installed in the treatment room with linear accelerators (LINAC’s) so that patients can be imaged and then treated on the same table without changing the patient setup. This allows clinicians to “see-inside” their patients on a regular basis during the course of therapy and correct for any changes that may occur. Current CT-IGRT procedures rely on the placement of external radiopaque markers on the patient’s skin. These markers are needed to locate the setup isocenter in the CT scan and are used as a reference from which any shifts are made. Unfortunately, these markers can suffer from several sources of error including uncertainty in placement, patient motion, laser inaccuracies, and variation in image registration. We have developed a system that does not require fiducial markers and thus simplifies the IGRT process while increasing accuracy. Materials/Methods: A system was designed to map CT images to their actual location and orientation in the treatment room. Custom-milled “anchor points” were mounted to three walls and the ceiling. These anchor points allowed precise radial distances to be measured in any direction. Custom software was then created that could determine the x, y, and z coordinate of any point in the room based on it’s distance from each of these anchor points. A coordinate system fixed to the room was thus established. Next, phantom measurements were used to determine a matrix transform that could convert any point in the CT volume into its associated room coordinate. Because a mathematical relationship was established between CT pixels and their exact location in the treatment room, fiducial markers were no longer needed to calibrate the system on a per patient basis. A custom IGRT phantom was developed to test the accuracy of our system. Results: This calibration method has simplified our daily CT-IGRT workflow. Patients are now placed on the treatment table the same as for conventional radiation therapy. Before treating, the table is rotated by 180 degrees to the imaging position and a CT scan is made. The images are sent automatically via DICOM to custom software where they are compared with the original treatment plan. There is no need at this point to register the images as before with external markers. The IGRT team simply selects a new isocenter and the software automatically calculates the needed table shift and rotation based on the calibration matrix. Next, the table is placed into treatment position using these shifts and the treatment proceeds as normal. Our research indicates that isocenters can be reliably located to better than 0.5 mm in the image plane and 0.8 mm in the scan direction. Conclusions: CT based IGRT can be a cumbersome process if the images are registered during each procedure using fiducial markers. In contrast, the anchor point system and custom software allow for rapid, reproducible, and accurate patient setup. Our experience has shown this method to be easier and more reliable than the use of fiducial markers.
I. J. Radiation Oncology
● Biology ● Physics
Volume 60, Number 1, Supplement, 2004
and from 3.09 to 4.45 (mean 3.71) for IMRT. At the 20% isodose level values ranged from 5.15 to 10.45 (mean 8.07) for the CyberKnife and from 12.1 to 32.56 (mean 18.68) for IMRT. Conclusions: The ratio between the volumes encompassed by the mid-low isodose surfaces to the volume encompassed by the reference isodose surface is in general greater for IMRT than for the CyberKnife. This behavior can be explained observing that IMRT is a coplanar technique while the CyberKnife uses a non-coplanar approach to the target volume. This results in a greater irradiated volume but in a lower volume which receives intermediate dose values. Also the conformity index is better for the CyberKnife than in the IMRT case. The homogeneity index resulted in general better for IMRT. All these facts can be expressed by saying that the dose gradient passing from out-target to in-target points is in general higher with the CyberKnife than with coplanar IMRT. The observed behavior allows CyberKnife users to treat lesions with a hypo-fractionated schedule compared to IMRT, because organs at risk receive in general a lower dose with the CyberKnife.
2467
A New Method for Image Guided Radiation Therapy Using Computed Tomography
T. Jenkins, M. Wolfe, Y. Feng, H. Mota, C. Sibata, R. R. Allison Radiation Oncology, The Brody School of Medicine, ECU, Greenville, NC Purpose/Objective: Image guided radiation therapy (IGRT) is a technique for increasing the precision of radiation delivery that couples diagnostic imaging with the treatment procedure. In some radiation oncology facilities, computed tomography (CT) scanners on rails are being installed in the treatment room with linear accelerators (LINAC’s) so that patients can be imaged and then treated on the same table without changing the patient setup. This allows clinicians to “see-inside” their patients on a regular basis during the course of therapy and correct for any changes that may occur. Current CT-IGRT procedures rely on the placement of external radiopaque markers on the patient’s skin. These markers are needed to locate the setup isocenter in the CT scan and are used as a reference from which any shifts are made. Unfortunately, these markers can suffer from several sources of error including uncertainty in placement, patient motion, laser inaccuracies, and variation in image registration. We have developed a system that does not require fiducial markers and thus simplifies the IGRT process while increasing accuracy. Materials/Methods: A system was designed to map CT images to their actual location and orientation in the treatment room. Custom-milled “anchor points” were mounted to three walls and the ceiling. These anchor points allowed precise radial distances to be measured in any direction. Custom software was then created that could determine the x, y, and z coordinate of any point in the room based on it’s distance from each of these anchor points. A coordinate system fixed to the room was thus established. Next, phantom measurements were used to determine a matrix transform that could convert any point in the CT volume into its associated room coordinate. Because a mathematical relationship was established between CT pixels and their exact location in the treatment room, fiducial markers were no longer needed to calibrate the system on a per patient basis. A custom IGRT phantom was developed to test the accuracy of our system. Results: This calibration method has simplified our daily CT-IGRT workflow. Patients are now placed on the treatment table the same as for conventional radiation therapy. Before treating, the table is rotated by 180 degrees to the imaging position and a CT scan is made. The images are sent automatically via DICOM to custom software where they are compared with the original treatment plan. There is no need at this point to register the images as before with external markers. The IGRT team simply selects a new isocenter and the software automatically calculates the needed table shift and rotation based on the calibration matrix. Next, the table is placed into treatment position using these shifts and the treatment proceeds as normal. Our research indicates that isocenters can be reliably located to better than 0.5 mm in the image plane and 0.8 mm in the scan direction. Conclusions: CT based IGRT can be a cumbersome process if the images are registered during each procedure using fiducial markers. In contrast, the anchor point system and custom software allow for rapid, reproducible, and accurate patient setup. Our experience has shown this method to be easier and more reliable than the use of fiducial markers.