X Sun

Improvement of the cine‐CT based 4D‐CT imaging

An improved 4D-CT utility has been developed on the GE LightSpeed multislice CT (MSCT) and Discovery PET/CT scanners, which have the cine CT scan capability. Two new features have been added in this 4D-CT over the commercial Advantage 4D-CT from GE. One feature was a new tool for disabling parts of the respiratory signal with irregular respiration and improving the accuracy of phase determination for the respiratory signal from the Varian real-time positioning and monitoring (RPM) system before sorting of the cine CT images into the 4D-CT images. The second feature was to allow generation of the maximum-intensity-projection (MIP), average (AVG) and minimum-intensity-projection (mip) CT images from the cine CT images without a respiratory signal. The implementation enables the assessment of tumor motion in treatment planning with the MIP, AVG, and mip CT images on the GE MSCT and PET/CT scanners without the RPM and the Advantage 4D-CT with a GE Advantage windows workstation. Several clinical examples are included to illustrate this new application.

Thoracic target volume delineation using various maximum‐intensity projection computed tomography image sets for radiotherapy treatment planning

PURPOSE Four-dimensional computed tomography (4D-CT) is commonly used to account for respiratory motion of target volumes in radiotherapy to the thorax. From the 4D-CT acquisition, a maximum-intensity projection (MIP) image set can be created and used to help define the tumor motion envelope or the internal gross tumor volume (iGTV). The purpose of this study was to quantify the differences in automatically contoured target volumes for usage in the delivery of stereotactic body radiation therapy using MIP data sets generated from one of the four methods: (1) 4D-CT phase-binned (PB) based on retrospective phase calculations, (2) 4D-CT phase-corrected phase-binned (PC-PB) based on motion extrema, (3) 4D-CT amplitude-binned (AB), and (4) cine CT built from all available images. METHODS MIP image data sets using each of the four methods were generated for a cohort of 28 patients who had prior thoracic 4D-CT scans that exhibited lung tumor motion of at least 1 cm. Each MIP image set was automatically contoured on commercial radiation treatment planning system. Margins were added to the iGTV to observe differences in the final simulated planning target volumes (PTVs). RESULTS For all patients, the iGTV measured on the MIP generated from the entire cine CT data set (iGTVcine) was the largest. Expressed as a percentage of iGTVcine, 4D-CT iGTV (all sorting methods) ranged from 83.8% to 99.1%, representing differences in the absolute volume ranging from 0.02 to 4.20 cm3; the largest average and range of 4D-CT iGTV measurements was from the PC-PB data set. Expressed as a percentage of PTVcine (expansions applied to iGTVeine), the 4D-CT PTV ranged from 87.6% to 99.6%, representing differences in the absolute volume ranging from 0.08 to 7.42 cm3. Regions of the measured respiratory waveform corresponding to a rapid change of phase or amplitude showed an increased susceptibility to the selection of identical images for adjacent bins. Duplicate image selection was most common in the AB implementation, followed by the PC-PB method. The authors also found that the image associated with the minimum amplitude measurement did not always correlate with the image that showed maximum tumor motion extent. CONCLUSIONS The authors identified cases in which the MIP generated from a 4D-CT sorting process under-represented the iGTV by more than 10% or up to 4.2 cm3 when compared to the iGTVcine. They suggest utilization of a MIP generated from the full cine CT data set to ensure maximum inclusive tumor extent.

Dose calculation with respiration-averaged CT processed from cine CT without a respiratory surrogate

Dose calculation for thoracic radiotherapy is commonly performed on a free-breathing helical CT despite artifacts caused by respiratory motion. Four-dimensional computed tomography (4D-CT) is one method to incorporate motion information into the treatment planning process. Some centers now use the respiration-averaged CT (RACT), the pixel-by-pixel average of the ten phases of 4D-CT, for dose calculation. This method, while sparing the tedious task of 4D dose calculation, still requires 4D-CT technology. The authors have recently developed a means to reconstruct RACT directly from unsorted cine CT data from which 4D-CT is formed, bypassing the need for a respiratory surrogate. Using RACT from cine CT for dose calculation may be a means to incorporate motion information into dose calculation without performing 4D-CT. The purpose of this study was to determine if RACT from cine CT can be substituted for RACT from 4D-CT for the purposes of dose calculation, and if increasing the cine duration can decrease differences between the dose distributions. Cine CT data and corresponding 4D-CT simulations for 23 patients with at least two breathing cycles per cine duration were retrieved. RACT was generated four ways: First from ten phases of 4D-CT, second, from 1 breathing cycle of images, third, from 1.5 breathing cycles of images, and fourth, from 2 breathing cycles of images. The clinical treatment plan was transferred to each RACT and dose was recalculated. Dose planes were exported at orthogonal planes through the isocenter (coronal, sagittal, and transverse orientations). The resulting dose distributions were compared using the gamma index within the planning target volume (PTV). Failure criteria were set to 2%/1 mm. A follow-up study with 50 additional lung cancer patients was performed to increase sample size. The same dose recalculation and analysis was performed. In the primary patient group, 22 of 23 patients had 100% of points within the PTV pass y criteria. The average maximum and mean y indices were very low (well below 1), indicating good agreement between dose distributions. Increasing the cine duration generally increased the dose agreement. In the follow-up study, 49 of 50 patients had 100% of points within the PTV pass the y criteria. The average maximum and mean y indices were again well below 1, indicating good agreement. Dose calculation on RACT from cine CT is negligibly different from dose calculation on RACT from 4D-CT. Differences can be decreased further by increasing the cine duration of the cine CT scan.

New weighted maximum‐intensity‐projection images from cine CT for delineation of the lung tumor plus motion

PURPOSE In treatment planning of the lung tumor with 4D-CT, maximum-intensity-projection (MIP) images have been used for delineation of the gross tumor volume plus motion or iGTV, which can then be revised with the multiple phases of the 4D-CT images. Although majority of contouring can be performed with MIP, the MIP images are not recommended for delineation of iGTV if the tumor is near or connected to the diaphragm or other structures of a similar density due to insufficient contrast between the tumor and the surrounding tissues in the MIP images. To remedy this shortcoming, the authors developed a new weighted MIP (wMIP) from cine CT without respiratory gating for contouring the iGTV. METHODS The wMIP images are obtained by keeping one phase of the cine CT images with the largest tumor in the overlap region of the tumor and the diaphragm. Outside the overlap region, the wMIP images are identical to the MIP images. Both MIP and wMIP images are obtained without respiratory gating from cine CT. RESULTS The authors demonstrated in a study of seven patients that wMIP can achieve 92% of the iGTV from 4D-CT. The maximum surface separation of the two iGTVs between wMIP and 4D-CT was 1.7 mm and six out of the seven studies had less than 1 mm in surface separation between the iGTVs of wMIP and 4D-CT. CONCLUSIONS This development has the potential of enabling many CT scanners capable of cine CT to assess the respiratory motion of a lung tumor without 4D-CT.PURPOSE In treatment planning of the lung tumor with 4D-CT, maximum-intensity-projection (MIP) images have been used for delineation of the gross tumor volume plus motion or iGTV, which can then be revised with the multiple phases of the 4D-CT images. Although majority of contouring can be performed with MIP, the MIP images are not recommended for delineation of iGTV if the tumor is near or connected to the diaphragm or other structures of a similar density due to insufficient contrast between the tumor and the surrounding tissues in the MIP images. To remedy this shortcoming, the authors developed a new weighted MIP (wMIP) from cine CT without respiratory gating for contouring the iGTV. METHODS The wMIP images are obtained by keeping one phase of the cine CT images with the largest tumor in the overlap region of the tumor and the diaphragm. Outside the overlap region, the wMIP images are identical to the MIP images. Both MIP and wMIP images are obtained without respiratory gating from cine CT. RESULTS The authors demonstrated in a study of seven patients that wMIP can achieve 92% of the iGTV from 4D-CT. The maximum surface separation of the two iGTVs between wMIP and 4D-CT was 1.7 mm and six out of the seven studies had less than 1 mm in surface separation between the iGTVs of wMIP and 4D-CT. CONCLUSIONS This development has the potential of enabling many CT scanners capable of cine CT to assess the respiratory motion of a lung tumor without 4D-CT.

SU‐GG‐J‐137: New Prospective Gated CBCT

Purpose: Design and feasibility of a new prospective gated CBCT for gated treatment IGRT and body SRT treatments. Method and Materials: 4D cone beam CT (4D CBCT) can be used to assess tumor motion for IGRT and body SRT treatments. However, acquisition time for 4D CBCT is normally longer than 3 minutes. For gated treatments, it is sufficient to measure the tumor motion in the range of the gate. We propose using prospective gated CBCT for assessing the tumor motion for the gated treatment to save time in CBCTdata acquisition. The same monitoring system (RPM, Varian Medical Systems, Palo Alto, CA) is used to gate the radiotherapy treatment and the CBCT acquisition. Both x‐ray and gantry rotation start when the condition of gating is met such as the breathing amplitude falls in the threshold setting for therapy. The patient can be coached with audio prompting during data acquisition and treatment. The acquisition time is between 1 to 2 min and can be accelerated with a faster gantry rotation. Only one CBCT reconstruction is needed. We applied this approach on a dynamic phantom with 30% duty cycle of beam on time. Results: The gated acquisition was feasible and the image acquired with the gated acquisition of the phantom demonstrated the gated image of the phantom with the specified duty cycle. Conclusion: We have designed a new prospective CBCT and demonstrated its feasibility with a dynamic phantom. This system uses the same thresholds for imaging and treatment demonstrating position and residual motion within the gate on each day. The prospective CBCT is shorter in acquisition time than 4D CBCT. This system can be applied for improving the quality of gated treatment IGRT and body SRT treatments.

SU‐E‐J‐116: New RPM Amplitude Averaging Enlarges Gross Tumor Volume in 4DCT

Purpose: To determine if new RPM amplitude averaging enlarges NSCLC iGTV delineation in the 4D‐CT simulation. Methods: We selected 24 NSCLC patients with respiratory‐induced lung motion of 1 cm or greater and generated two 4D‐CTs (from one scan) for each patient using a GE 8‐slice PET/CT scanner with a Varian RPM respiratory surrogate. For each patient both 4D‐CTs used phase‐corrected phase‐binning (PC‐PB) for image selection; however, for phase determination one 4D‐CT assigned phases based on the RPM amplitude at the median time point of the image acquisition (the contemporary technique), and the other 4D‐CT assigned phases based on the average RPM amplitude recorded over the image acquisition time. We developed software to assign phase and select images as described to create maximum intensity projections (MIPs) of the 4D‐CTs. These two MIPs and the cine MIP (CMIP; generated from all acquired images before image selection in 4D‐CT) were sent to the Pinnacle 9.0 treatment planning software for auto‐contouring to obtain internal gross tumor volumes (iGTVs). Auto‐contouring was done at the lung window level with a threshold of 800 and occasional, consistent manual intervention to prune regions that included non‐tumor tissue. Results: On average, iGTVs generated using the median time point RPMs and averaged RPM amplitudes were 91.9% and 94.1% of the iGTV generated from the CMIP, respectively (a 2.2% difference). The mean increase in iGTV when switching to RPM averaging was 0.37 cc with a p‐value of 0.0402 and corresponds to a mean percent increase in iGTV of 2.40%. Conclusions: This work demonstrates that new RPM amplitude averaging enlarges NSCLC iGTVs generated from MIPs of 4D‐CT. The presenting author, Luke Hunter, received funding support as a recipient of the Hertz Foundation Applied Science Fellowship.

SU-FF-J-175: Segmentation of Moving Targets in PET: Threshold Dependence On Lesion Size, Motion Extent, and Signal-To-Background

Purpose: To determine optimal PET activity concentration thresholds for lung lesions as a function of lesion size, motion extent, and signal‐to‐background ratio. Method and Materials: The background tank of the NEMA IEC phantom was filled with water and the 6 spheres were kept empty. The phantom was placed on a motion platform positioned on the PET/CT scanner bed. The platform moves sinusoidally at 0, 1, 2, and 3 cm amplitudes and a 4‐second period. Cine CT scans were taken at each amplitude and minimum intensity projections were formed to determine the “motion envelope” of the empty spheres. This volume served as the reference volume. The 6 spheres and background tank were filled with activity to create two signal‐to‐background ratios (SBR): Infinite (no background) and 10:1. At each motion extent and SBR, the phantom was scanned for 6 minutes in 3‐D mode. Thresholds of varying percentages of maximum activity concentration were applied to the PETimages to determine which threshold best fit the reference CT contour for each motion condition, sphere size, and SBR. Optimization was performed by minimizing the separation between the CT and PET threshold surfaces. Results: For all spheres in the “no background” condition, the optimal threshold dropped substantially to less than 15% of maximum activity concentration beyond 1 cm of motion. For SBR of 10:1, thresholds for all spheres and all motion extents increased, especially for smaller spheres where the maximum value was degraded due to motion. Conclusion: Though motion and size had a measurable impact on the optimal threshold, SBR seemed to be the dominant factor in optimal threshold determination. Additional analysis will include scans at finer motion amplitudes (every 0.5 cm), repeated PETimaging at each amplitude, and optimal threshold analysis for over 30 patient studies.

SU‐GG‐J‐121: Lung Tumor Setup Using 2D‐3D Registration of Respiratory‐Gated Oblique X‐Rays and the Planning 4D‐CT

Purpose: To investigate the feasibility of using intensity based 2D‐3D registration to directly setup on respiratory gated lungtumors using x‐ray source angles tailored for each patient. Method and Materials: A 2D‐3D registration software was developed in‐house which renders digitally reconstructed radiographs(DRR) in real time with a consumer graphics card. The user was then able to conveniently view DRRs at various source angles to choose the angles where the tumor is less obstructed by surrounding structures during the planning phase. The 2D‐3D software, using mutual information, simultaneously aligned DRRs from the end‐expiration planning 4D‐CT with four end‐expiration projection x‐rays (lateral, Anterior‐Posterior (AP), and 2 oblique) on a sub‐image based on the projection of the tumor. For the analysis, the x‐ray projection data was taken from three lung patients in a separate research protocol for 4D cone‐beam CT. The 2D‐3D results were compared with 3D‐3D alignment of the end‐expiration 4D cone‐beam CT with the end‐expiration planning 4D‐CT on a region around the tumor.Respiration phase information was deduced from a commercial external fiducial system (Real‐time Position Management, Varian Medical Systems). The volumes of the three tumors were 14.0 cc, 45.8 cc, and 17.9 cc. The extents of motion in the Superior‐Inferior (SI) direction were 2 mm, 4 mm, and 11 mm. Results: The mean differences between the 3D‐3D and 2D‐3D registrations were −0.1 mm, 0.1 mm, and 1.6 mm in the lateral, AP, and SI directions respectively. The standard deviations were 2.2 mm, 1.8 mm, and 1.0 mm. Conclusion: We have demonstrated the feasibility of direct lungtumor setup with per‐patient x‐ray source angles. The preferred oblique x‐ray angles were anterior and posterior angles on the same lateral side as the tumor. Future work will investigate the optimal number of x‐rays and the lower limit of the tumors size.

SU‐GG‐J‐131: Motion Assessment Using Phase‐ and Amplitude‐Binned 4D‐CT: Comparison with Cine CT

Purpose: To determine the accuracy of tumor motion extent measurements derived from phase‐ and amplitude‐binned 4D‐CT maximum intensity projections (MIPs). Method and Materials: Twelve stage I patients (13 lesions) received cine CT scans for 4D‐CT radiotherapy simulation. All lesions were in the middle of the lung parenchyma and not adjacent to surrounding tissue. Two 4D‐CT image sets were constructed: One sorted by phase, the other sorted by amplitude. MIPs were formed from each 4D‐CT image set and from unsorted cine CT data. Lesions were auto‐segmented on each image set using a threshold of −225 HU. Using the auto‐segmented contours as a guide, three‐dimensional motion extent was assessed visually on both 4D‐CT MIPs and compared with motion extent assessed on MIP from cine CT. The cine MIP was considered the reference image set because it includes all images at each couch position and therefore more accurately samples the respiratory pattern. Results: For phase‐binned 4D‐CT, motion extent was accurately identified on 12 of 13 lesions. For amplitude‐binned 4D‐CT, motion extent was accurately identified on 11 of 13 lesions. The two patients whose motion extent was missed on amplitude‐binned 4D‐CT demonstrated a substantial discrepancy near the superior edge of the tumor. Upon further investigation, this discrepancy was found to be caused by lag between the respiratory surrogate and internal anatomy.Conclusion: Both phase binning and amplitude binning were unable to accurately identify the motion extent for all 13 lesions. Though generally regarded as more artifact‐free, amplitude‐binned 4D‐CT demonstrated increased sensitivity to the correlation between respiratory surrogate and internal anatomy which caused inaccurate assessments of motion extent. MIP processed directly from cine CT should be used in place of MIP processed from 4D‐CT (either phase‐ or amplitude‐binned) wherever possible.

SU‐GG‐J‐138: New Weighted Maximum Intensity Projection (MIP) Images for Assessing Tumor Motion in the Thorax Without a Respiratory Surrogate

Purpose: MIP image is effective for encompassing the tumor motion except in the area where the tumor is near the diaphragm. This is due to the fact that the tumor and the diaphragm may occupy the same space at two different times in respiration. We developed a new weighted MIP image to eliminate this constraint and advanced the application of cine CTimaging for assessing the tumor motion without a respiratory surrogate. Method and Materials: We selected three patient studies from the conventional cine‐CT based 4D‐CT imaging on a GE 8‐slice CT. These three patients each had a lungtumor near the diaphragm, and their MIP images were compromised by the diaphragm obscuring the tumor. We first identified the obscured region of 2 to 6 cm for weighted MIP processing. In each cine acquisition of 2 cm coverage, we identified one image of 2.5 mm thickness corresponding to the end‐inspiration (EI) phase, associated with the least diaphragm or the largest lung region in the image. Selection of one image automatically groups in the other 7 images acquired at the same time from the 8‐slice CT, reducing the selection time. We then submitted the selected images of EI phase along with the cine CTimages outside the obscured region for MIP processing, resulting in a new weighted MIP image that shows good contrast for encompassing tumor motion. Results: The contrast of tumor in MIP was improved with weighted MIP for all three patients when the tumor in near the diaphragm. Conclusion: A new weighted MIP has been designed to further advance the application of cine CTimaging for assessing tumor motion without a respiratory surrogate when the tumor is near the diaphragm.