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Lung tumor motion measured using retrospective 4D CT and correlated with tumor location and attachment
S288
I. J. Radiation Oncology
● Biology ● Physics
Volume 60, Number 1, Supplement, 2004
breathing motion of close to the same magnitude as the relative reduction in external breathing motion. Deviations between external and internal motion reductions were smaller than 25 percent points. For patients exhibiting regular breathing, phase gating provided the largest potential for internal motion reduction, especially when there was a phase shift between external and internal motion. Within gating thresholds corresponding to 40% dutycycles, phase gating motion reductions were up to twice as large as amplitude gating reductions. In cases of irregular breathing, amplitude gating still gave a stable limitation of internal movement. Examples of amplitude gating and phase gating for two cases of regular and irregular breathing, respectively, are shown in the figures. The figures represent scatter plots of external anterior-posterior motion versus internal superior-inferior motion. The horizontal lines indicate the ranges of internal breathing motion within gating intervals corresponding to a dutycycle of 40%. Conclusions: Respiratory gating using external marker monitoring provides consistent reduction of internal (intra-session) lung tumor breathing motion. The choice between phase and amplitude gating approaches should be based on patients individual breathing patterns.
1053
Lung Tumor Motion Measured Using Retrospective 4D CT and Correlated with Tumor Location and Attachment
E. D. Brandner, A. Wu, H. Chen, D. Heron, S. Kalnicki, S. Burton Radiation Oncology, UPMC Cancer Centers, Pittsburgh, PA Purpose/Objective: To measure lung tumor motion using retrospective 4D CT and correlate the motion with tumor position and attachment. Materials/Methods: Lung patients were scanned using retrospective 4D CT to account for respiratory motion of the tumor. For 4D CT, multiple images are acquired at each slice location and then sorted according to the phase of the respiratory cycle in which the image was acquired. In order to acquire the 4D CT, each patient was positioned on the CT couch with his/her arms in a wingboard and feet tied together. While acquiring a scout CT and a helical CT (not yet the 4D CT), the patient’s breathing cycle is observed using the “RPM computer” (Varian Medical Systems). The respiratory cycle is observed using a marker block taped to the patient’s abdomen about 5 cm below the xiphoid. The motion of the marker block is relayed to the RPM computer by an infrared camera. An audible breathing coach is programmed with the patient’s breathing cycle time and started. The patient is asked if the rhythm is comfortable and if not, slight adjustments are made to the time until a comfortable rhythm is achieved. The CT slice spacing is set to 2.5 mm so that motions as small as 2.5 mm in the superior-inferior (S-I) direction can be observed. The volume to be scanned is determined from the scout image. The number of images is limited to 1500. The user programs the scanner to remain at each slice location acquiring multiple images for at least one full breathing cycle of the patient. All of these images are synchronized with the breathing cycle and sorted according to the breathing phase at which they were acquired. A 3D image is created for each breathing phase. The location of the tumor was identified by the lung lobe in which the tumor resided. Any attachment observed superiorly, inferiorly, anteriorly, posteriorly, medially, or laterally was recorded even if the attachment was only via a distinguishable vessel. Tumor displacement was measured by locating each edge of the tumor on the phases representing its most superior and most inferior positions. If the Medial-Lateral (M-L) displacement and the Anterior-Posterior (A-P) displacements were less than 3 mm, the superior and the inferior edges of the tumor were recorded on the phases representing its most superior and most inferior positions. The average displacement of the inferior and superior edges was identified as the tumor displacement. Tumors with A-P or M-L displacements of more than 3 mm were contoured before identifying their displacements. To contour the tumors, the 3D images representing its most superior and most inferior positions were exported to the planning computer (Eclipse, Varian Medical Systems). The tumor was contoured on both 3D images. The planning computer was then used to identify the center of the tumor on each phase using a built in center of mass calculation. These centers at the superior and inferior positions were recorded and their S-I difference represents the S-I displacement of the tumor. Results: Lung tumor motion has been measured in 10 patients and associated with tumor position and attachment. Tumors located in the superior and inferior lobes of the lungs have been observed. Superior, inferior, anterior, posterior, lateral, and medial attachments have all been observed. Conclusions: The tumors that moved more than 3 mm were all located in the inferior lobes. Of these tumors, those attached posteriorly were observed to move the most. Tumors with extensive attachment moved the least. Tumor S-I motions from 0 to 13.8 mm were observed.
Proceedings of the 46th Annual ASTRO Meeting
1054
Recalculation of Dose Changes Due to Breathing Movement Assessed From Respiratory-Correlated Cone Beam CT
J. Sonke,1 J. Balter,2 M. van Herk,1 L. Zijp,1 M. Kessler,2 D. McShan,2 R. Kashani2 1
Radiotherapie, The Netherlands Cancer Institute , Amsterdam, Netherlands, 2Radiation Oncology, University of Michigan, Ann Arbor, MI Purpose/Objective: The advent of “4D” CT methods, combined with deformable alignment tools, makes it possible to reassess dose distributions to properly account for the influence of breathing. Respiratory-correlated cone beam CT (RCCBCT) integrated with a linear accelerator permits acquisition of a “4D” patient model immediately prior to treatment, and thus provides a wealth of information for adapting the patient model to reflect changes in tumor configuration and breathing amplitude and periodicity. An infrastructure was developed to use RCCBCT to estimate changes in delivered dose as compared to a standard (static) plan based on pre-treatment CT scanning. Materials/Methods: Using the treatment planning CT scan as a patient model, the target and standard PTV expansion are defined according to routine clinical practice. At the start of treatment (and any period thereafter), an RCCBCT data set is acquired and sorted into 8 breathing phases. The local anatomy is aligned using a deformable alignment tool that iteratively adjusts the positions of 30 control points to maximize the mutual information between the reference phase (exhale) and the deformed (via thin plate splines) test phase (e.g. inhale). The resulting optimal transformations from exhale through inhale phases provide maps to relate regional anatomic changes during breathing. The treatment planning target (CTV) is placed in the frame of this path in two steps. First, an initial alignment is performed to match the skeletons of the treatment planning scan and the reference phase of the RCCBCT (removing setup error, which is to be analyzed separately). Second, phase offset of the target is selected so that placing the initial target in the breathing path mapped from RCCBCT maximizes coverage of the target through its traversal of all breathing phases (measuring and then removing variation in baseline target position, to simulate target-based positioning). Once the treatment planning CT model is thus optimally “placed” in the RCCBCT, the measured transformation through the breathing cycle is used to deform the density map from this scan to various sampled phases of breathing. Dose is calculated in space on the deformed density maps (the RCCBCT is not used for dose calculation as it does not span the entire patient). The doses are then summed relative to the anatomy from the treatment planning scan by applying the inverse of the deformation maps to the appropriate phase. Results: This infrastructure was developed and applied to patient data acquired during routine treatment of lung cancer on a RCCBCT-equipped linear accelerator. Initial treatment planning models were shown (by alignment) to improperly represent the mean position of the target during breathing at treatment, leading to potential gross target miss when positioning is done using skeletal anatomy. Both the amplitude of movement and baseline position at exhale were shown to vary over the course of treatment (from analysis of 4D data taken prior to, at the start of, and near completion of therapy). Calculated doses including breathing showed minor increased volume of involved lung irradiated to high doses, and significant decreases (30%) in minimum PTV dose as compared to pre-treatment estimates via conventional (static) dose calculation. PTV margins were shown to be sufficient to account for breathing variations (CTV dose was maintained), however without the inclusion of setup variation. Conclusions: Implementation of this infrastructure makes it possible to reassess delivered dose due to breathing changes during therapy. Early cases studied indicate the limitations of pre-treatment movement estimates for planning, and call for predictive or adaptive methods of incorporating breathing into conformal and IMRT plans to maximize lung sparing while ensuring target coverage. This work was supported in part by NIH P01-CA59827 and Elekta Medical Systems
S289
I. J. Radiation Oncology
● Biology ● Physics
Volume 60, Number 1, Supplement, 2004
breathing motion of close to the same magnitude as the relative reduction in external breathing motion. Deviations between external and internal motion reductions were smaller than 25 percent points. For patients exhibiting regular breathing, phase gating provided the largest potential for internal motion reduction, especially when there was a phase shift between external and internal motion. Within gating thresholds corresponding to 40% dutycycles, phase gating motion reductions were up to twice as large as amplitude gating reductions. In cases of irregular breathing, amplitude gating still gave a stable limitation of internal movement. Examples of amplitude gating and phase gating for two cases of regular and irregular breathing, respectively, are shown in the figures. The figures represent scatter plots of external anterior-posterior motion versus internal superior-inferior motion. The horizontal lines indicate the ranges of internal breathing motion within gating intervals corresponding to a dutycycle of 40%. Conclusions: Respiratory gating using external marker monitoring provides consistent reduction of internal (intra-session) lung tumor breathing motion. The choice between phase and amplitude gating approaches should be based on patients individual breathing patterns.
1053
Lung Tumor Motion Measured Using Retrospective 4D CT and Correlated with Tumor Location and Attachment
E. D. Brandner, A. Wu, H. Chen, D. Heron, S. Kalnicki, S. Burton Radiation Oncology, UPMC Cancer Centers, Pittsburgh, PA Purpose/Objective: To measure lung tumor motion using retrospective 4D CT and correlate the motion with tumor position and attachment. Materials/Methods: Lung patients were scanned using retrospective 4D CT to account for respiratory motion of the tumor. For 4D CT, multiple images are acquired at each slice location and then sorted according to the phase of the respiratory cycle in which the image was acquired. In order to acquire the 4D CT, each patient was positioned on the CT couch with his/her arms in a wingboard and feet tied together. While acquiring a scout CT and a helical CT (not yet the 4D CT), the patient’s breathing cycle is observed using the “RPM computer” (Varian Medical Systems). The respiratory cycle is observed using a marker block taped to the patient’s abdomen about 5 cm below the xiphoid. The motion of the marker block is relayed to the RPM computer by an infrared camera. An audible breathing coach is programmed with the patient’s breathing cycle time and started. The patient is asked if the rhythm is comfortable and if not, slight adjustments are made to the time until a comfortable rhythm is achieved. The CT slice spacing is set to 2.5 mm so that motions as small as 2.5 mm in the superior-inferior (S-I) direction can be observed. The volume to be scanned is determined from the scout image. The number of images is limited to 1500. The user programs the scanner to remain at each slice location acquiring multiple images for at least one full breathing cycle of the patient. All of these images are synchronized with the breathing cycle and sorted according to the breathing phase at which they were acquired. A 3D image is created for each breathing phase. The location of the tumor was identified by the lung lobe in which the tumor resided. Any attachment observed superiorly, inferiorly, anteriorly, posteriorly, medially, or laterally was recorded even if the attachment was only via a distinguishable vessel. Tumor displacement was measured by locating each edge of the tumor on the phases representing its most superior and most inferior positions. If the Medial-Lateral (M-L) displacement and the Anterior-Posterior (A-P) displacements were less than 3 mm, the superior and the inferior edges of the tumor were recorded on the phases representing its most superior and most inferior positions. The average displacement of the inferior and superior edges was identified as the tumor displacement. Tumors with A-P or M-L displacements of more than 3 mm were contoured before identifying their displacements. To contour the tumors, the 3D images representing its most superior and most inferior positions were exported to the planning computer (Eclipse, Varian Medical Systems). The tumor was contoured on both 3D images. The planning computer was then used to identify the center of the tumor on each phase using a built in center of mass calculation. These centers at the superior and inferior positions were recorded and their S-I difference represents the S-I displacement of the tumor. Results: Lung tumor motion has been measured in 10 patients and associated with tumor position and attachment. Tumors located in the superior and inferior lobes of the lungs have been observed. Superior, inferior, anterior, posterior, lateral, and medial attachments have all been observed. Conclusions: The tumors that moved more than 3 mm were all located in the inferior lobes. Of these tumors, those attached posteriorly were observed to move the most. Tumors with extensive attachment moved the least. Tumor S-I motions from 0 to 13.8 mm were observed.
Proceedings of the 46th Annual ASTRO Meeting
1054
Recalculation of Dose Changes Due to Breathing Movement Assessed From Respiratory-Correlated Cone Beam CT
J. Sonke,1 J. Balter,2 M. van Herk,1 L. Zijp,1 M. Kessler,2 D. McShan,2 R. Kashani2 1
Radiotherapie, The Netherlands Cancer Institute , Amsterdam, Netherlands, 2Radiation Oncology, University of Michigan, Ann Arbor, MI Purpose/Objective: The advent of “4D” CT methods, combined with deformable alignment tools, makes it possible to reassess dose distributions to properly account for the influence of breathing. Respiratory-correlated cone beam CT (RCCBCT) integrated with a linear accelerator permits acquisition of a “4D” patient model immediately prior to treatment, and thus provides a wealth of information for adapting the patient model to reflect changes in tumor configuration and breathing amplitude and periodicity. An infrastructure was developed to use RCCBCT to estimate changes in delivered dose as compared to a standard (static) plan based on pre-treatment CT scanning. Materials/Methods: Using the treatment planning CT scan as a patient model, the target and standard PTV expansion are defined according to routine clinical practice. At the start of treatment (and any period thereafter), an RCCBCT data set is acquired and sorted into 8 breathing phases. The local anatomy is aligned using a deformable alignment tool that iteratively adjusts the positions of 30 control points to maximize the mutual information between the reference phase (exhale) and the deformed (via thin plate splines) test phase (e.g. inhale). The resulting optimal transformations from exhale through inhale phases provide maps to relate regional anatomic changes during breathing. The treatment planning target (CTV) is placed in the frame of this path in two steps. First, an initial alignment is performed to match the skeletons of the treatment planning scan and the reference phase of the RCCBCT (removing setup error, which is to be analyzed separately). Second, phase offset of the target is selected so that placing the initial target in the breathing path mapped from RCCBCT maximizes coverage of the target through its traversal of all breathing phases (measuring and then removing variation in baseline target position, to simulate target-based positioning). Once the treatment planning CT model is thus optimally “placed” in the RCCBCT, the measured transformation through the breathing cycle is used to deform the density map from this scan to various sampled phases of breathing. Dose is calculated in space on the deformed density maps (the RCCBCT is not used for dose calculation as it does not span the entire patient). The doses are then summed relative to the anatomy from the treatment planning scan by applying the inverse of the deformation maps to the appropriate phase. Results: This infrastructure was developed and applied to patient data acquired during routine treatment of lung cancer on a RCCBCT-equipped linear accelerator. Initial treatment planning models were shown (by alignment) to improperly represent the mean position of the target during breathing at treatment, leading to potential gross target miss when positioning is done using skeletal anatomy. Both the amplitude of movement and baseline position at exhale were shown to vary over the course of treatment (from analysis of 4D data taken prior to, at the start of, and near completion of therapy). Calculated doses including breathing showed minor increased volume of involved lung irradiated to high doses, and significant decreases (30%) in minimum PTV dose as compared to pre-treatment estimates via conventional (static) dose calculation. PTV margins were shown to be sufficient to account for breathing variations (CTV dose was maintained), however without the inclusion of setup variation. Conclusions: Implementation of this infrastructure makes it possible to reassess delivered dose due to breathing changes during therapy. Early cases studied indicate the limitations of pre-treatment movement estimates for planning, and call for predictive or adaptive methods of incorporating breathing into conformal and IMRT plans to maximize lung sparing while ensuring target coverage. This work was supported in part by NIH P01-CA59827 and Elekta Medical Systems
S289