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Clinical Evaluation of a Novel 4D-CBCT Reconstruction Scheme Based on Simultaneous Motion Estimation and Image Reconstruction
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1101 Clinical Evaluation of a Novel 4D-CBCT Reconstruction Scheme Based on Simultaneous Motion Estimation and Image Reconstruction J. Wang,1 J. Dang,1 L. Ouyang,1 X. Gu,1 and T. Pan2; 1University of Texas Southwestern Medical Center, Dallas, TX, 2University of Texas MD Anderson Cancer Center, Houston, TX Purpose/Objective(s): In the present 4D cone-beam CT (4D-CBCT) process, image reconstruction, and motion estimation are performed as two sequential steps. In such a process, motion estimation accuracy is limited by the image quality of reconstructed 4D-CBCT, which is often degraded due to the limited number of projections at each phase. We have recently proposed a novel 4D-CBCT image reconstruction scheme that is able to perform simultaneous motion estimation and image reconstruction (SMEIR). The purpose of this work is to optimize and characterize the performance of the SMEIR through patient evaluation studies. Materials/Methods: The SMEIR algorithm consists of two steps: (1) motioncompensated image reconstruction; and (2) motion model estimation from projections directly. In the motion-compensated image reconstruction, we utilized the projections from all of the other phases to reconstruct a reference phase 4D-CBCT by explicitly considering the motion model between different phases. In the motion model estimation, we obtained the updated inverse consistent motion model from the projections directly, overcoming the limitation of estimating the motion model from reconstructed images. A lung cancer patient was scanned for 4.5 minutes and total 1679 CBCT projections were acquired. The projections were grouped into 10 phases according to the respiration signal. 4DCBCT at each phase was reconstructed by total variation (TV) minimization. To evaluate the performance of the SMEIR algorithm on the conventional 1-minute CBCT scan, the projections at each phase were downsampled by different factors ranging from 2 to 10. Four-dimensional-CBCT images were reconstructed from the downsampled projections by using the standard FDK, TV, and SMEIR. Using the 4D-CBCT reconstructed from fully sampled projections, relative error of image and error of tumor motion trajectory were analyzed to quantify the performance of different reconstruction algorithms. Results: The SMEIR algorithm outperforms FDK and TV in both image reconstruction accuracy and tumor tracking trajectory accuracy. When the average number of projections at each phase decreases to 19, relative image reconstruction errors for FDK, TV, and SMEIR are 38.80%, 14.85%, and 10.05%, respectively; the maximum tumor tracking errors for FDK, TV, and SMEIR are 2.77 mm, 1.94 mm, and 0.54 mm, respectively. Conclusions: The patient study results show that the SMEIR algorithm can achieve 1-mm tumor tracking accuracy even when the average number of projections at each phase reduces to 19. These results suggest that the SMEIR algorithm enable the use of conventional 1-minute CBCT for accurate of motion modeling and 4D-CBCT image reconstruction. Author Disclosure: J. Wang: None. J. Dang: None. L. Ouyang: None. X. Gu: None. T. Pan: None.
1102 Hybrid Dual Energy (HDE) Fluoroscopy for Real-Time Tracking of Lung Tumors J.C. Roeske, R. Patel, M. Campana, J. Panfil, M. Surucu, A. Block, and M. Harkenrider; Loyola University Medical Center, Maywood, IL Purpose/Objective(s): Dual energy (DE) imaging uses a combination of high and low energy kV planar x-ray images to remove bone and produce soft tissue enhanced images. Previous studies have shown that DE imaging improves the localization of lung tumors on planar kV x-ray images, compared to single energy imaging. Thus, combining DE imaging with fluoroscopy may enable real-time tumor motion tracking. However, such an approach would require a “fast switching” x-ray generator (FSDE) to produce alternating high and low energy x-ray images. We describe here a Hybrid DE (HDE) approach that can be used with existing imaging hardware to enhance lung tumor visualization for real-time motion tracking. Materials/Methods: The HDE algorithm uses a single image set obtained prior to intra-fraction imaging to provide bone suppression on subsequent
International Journal of Radiation Oncol Biology Physics fluoroscopic images. Briefly, a 60 kVp and 120 kVp image set are obtained using respiratory gating, and are combined to produce a bone-weighted image. This image highlights the ribs and other skeletal bones, and suppresses the surrounding soft tissue. Next a standard fluoroscopic image sequence is obtained. Logarithmic subtraction is performed on a frame-byframe basis using the previously obtained bone-weighted image. This subtraction results in a fluoroscopic image sequence in which the ribs are suppressed, providing improved visualization of the tumor. To validate this approach, both high and low energy fluoroscopic image sets were obtained on 3 patients (total of 9 fluoroscopic sets) using an on-board imager. To simulate FSDE, the high and low energy fluoroscopic images were aligned based on respiratory phase, and frame-by-frame subtraction was performed to produce a soft tissue fluoroscopic image set. HDE fluoroscopy sequences were produced using only the high-energy sequence and a single bone-weighted image. A template-based motion-tracking algorithm was used on both FSDE and HDE image sequences and the tracking coordinates, as well as the peak-to-side lobe ratio (PSR - a quantitative measure of the template match), were compared. Results: A total of 1304 fluoroscopic frames were evaluated and there was no significant difference in the location of the tumor center using HDE vs FSDE fluoroscopy. The average differences in the tumor centroid coordinates were -0.05 +/- 0.29 mm and 0.02 +/- 0.27 mm in the x- and ydirections, respectively. The absolute maximum differences in x-y coordinates were 0.74 mm and 1.17 mm, respectively. The PSR values were also comparable with average values of 3.63 +/- 0.80 vs 3.62 +/- 0.84 (p Z 0.36) for FSDE and HDE fluoroscopy, respectively. Conclusions: The HDE approach provides improved visualization of lung tumors on fluoroscopic imaging without the need for any additional hardware. Moreover, HDE provides a means for real-time motion tracking of lung tumors, producing results that are comparable to FSDE fluoroscopy. Author Disclosure: J.C. Roeske: E. Research Grant; Varian Medical Systems. R. Patel: E. Research Grant; Varian Medical Systems. M. Campana: None. J. Panfil: None. M. Surucu: None. A. Block: None. M. Harkenrider: None.
1103 Analyzing Tumorlets in Prostate LDR Brachytherapy: Possible Lessons for Focal Therapy D.A. Todor,1 M.S. Anscher,1 M.P. Hagan,1 F. Siebert,2 K. Sebastian,2 and T. Catron1; 1Virginia Commonwealth University, Richmond, VA, 2 Universita¨tsklinikum Schleswig-Holstein, Kiel, Germany Purpose/Objective(s): Prostate cancer is a multi-focal disease but in the PSA era is characterized by relatively small volumes and number of tumor foci. We hypothesize that when treating whole prostate glands, it is dose delivered to tumor foci (tumorlets) that actually matters. We aim to identify the relationship between dosimetric and radiobiological parameters at the whole prostate and tumorlet levels. By examining pre-plans and post-implant plans from two different institutions, using two different isotopes, we also aim to assess the importance of spatial dose distributions. Materials/Methods: Plans from 72 patients treated with I-125 and Pd-103 seed implants were analyzed, 34 of which were treated at Institution A and 38 at Institution B. Pre- and post-implant plans were exported as DICOM files to a software platform built to create in-silico prostate tumorlets. Four thousand five hundred tumorlets of random position and morphology were created for each case in 9 classes of volumes ranging from 0.1 to 10 cm3. Each tumorlet was superimposed over the 3D dose distribution and positional, dosimetric, and radiobiological quantities were computed. Two different modalities to aggregate voxel level BEDs were employed and analyzed: EUBED and gBEUD. A treatment reference was established using BED of a 78 Gy external beam regimen (2 Gy/fx). “Probabilities of cure” (PoC) were computed as the ratio between the number of tumorlets with gBEUD or EUBED the equivalent EBRT reference treatment and total number of tumorlets. Results: The gBEUD seems to match well the currently employed BED based on D90 in pre-plans, but was consistently less in post-implant plans, pointing out to the fact that, unlike BED, gBEUD is sensitive to both
1101 Clinical Evaluation of a Novel 4D-CBCT Reconstruction Scheme Based on Simultaneous Motion Estimation and Image Reconstruction J. Wang,1 J. Dang,1 L. Ouyang,1 X. Gu,1 and T. Pan2; 1University of Texas Southwestern Medical Center, Dallas, TX, 2University of Texas MD Anderson Cancer Center, Houston, TX Purpose/Objective(s): In the present 4D cone-beam CT (4D-CBCT) process, image reconstruction, and motion estimation are performed as two sequential steps. In such a process, motion estimation accuracy is limited by the image quality of reconstructed 4D-CBCT, which is often degraded due to the limited number of projections at each phase. We have recently proposed a novel 4D-CBCT image reconstruction scheme that is able to perform simultaneous motion estimation and image reconstruction (SMEIR). The purpose of this work is to optimize and characterize the performance of the SMEIR through patient evaluation studies. Materials/Methods: The SMEIR algorithm consists of two steps: (1) motioncompensated image reconstruction; and (2) motion model estimation from projections directly. In the motion-compensated image reconstruction, we utilized the projections from all of the other phases to reconstruct a reference phase 4D-CBCT by explicitly considering the motion model between different phases. In the motion model estimation, we obtained the updated inverse consistent motion model from the projections directly, overcoming the limitation of estimating the motion model from reconstructed images. A lung cancer patient was scanned for 4.5 minutes and total 1679 CBCT projections were acquired. The projections were grouped into 10 phases according to the respiration signal. 4DCBCT at each phase was reconstructed by total variation (TV) minimization. To evaluate the performance of the SMEIR algorithm on the conventional 1-minute CBCT scan, the projections at each phase were downsampled by different factors ranging from 2 to 10. Four-dimensional-CBCT images were reconstructed from the downsampled projections by using the standard FDK, TV, and SMEIR. Using the 4D-CBCT reconstructed from fully sampled projections, relative error of image and error of tumor motion trajectory were analyzed to quantify the performance of different reconstruction algorithms. Results: The SMEIR algorithm outperforms FDK and TV in both image reconstruction accuracy and tumor tracking trajectory accuracy. When the average number of projections at each phase decreases to 19, relative image reconstruction errors for FDK, TV, and SMEIR are 38.80%, 14.85%, and 10.05%, respectively; the maximum tumor tracking errors for FDK, TV, and SMEIR are 2.77 mm, 1.94 mm, and 0.54 mm, respectively. Conclusions: The patient study results show that the SMEIR algorithm can achieve 1-mm tumor tracking accuracy even when the average number of projections at each phase reduces to 19. These results suggest that the SMEIR algorithm enable the use of conventional 1-minute CBCT for accurate of motion modeling and 4D-CBCT image reconstruction. Author Disclosure: J. Wang: None. J. Dang: None. L. Ouyang: None. X. Gu: None. T. Pan: None.
1102 Hybrid Dual Energy (HDE) Fluoroscopy for Real-Time Tracking of Lung Tumors J.C. Roeske, R. Patel, M. Campana, J. Panfil, M. Surucu, A. Block, and M. Harkenrider; Loyola University Medical Center, Maywood, IL Purpose/Objective(s): Dual energy (DE) imaging uses a combination of high and low energy kV planar x-ray images to remove bone and produce soft tissue enhanced images. Previous studies have shown that DE imaging improves the localization of lung tumors on planar kV x-ray images, compared to single energy imaging. Thus, combining DE imaging with fluoroscopy may enable real-time tumor motion tracking. However, such an approach would require a “fast switching” x-ray generator (FSDE) to produce alternating high and low energy x-ray images. We describe here a Hybrid DE (HDE) approach that can be used with existing imaging hardware to enhance lung tumor visualization for real-time motion tracking. Materials/Methods: The HDE algorithm uses a single image set obtained prior to intra-fraction imaging to provide bone suppression on subsequent
International Journal of Radiation Oncol Biology Physics fluoroscopic images. Briefly, a 60 kVp and 120 kVp image set are obtained using respiratory gating, and are combined to produce a bone-weighted image. This image highlights the ribs and other skeletal bones, and suppresses the surrounding soft tissue. Next a standard fluoroscopic image sequence is obtained. Logarithmic subtraction is performed on a frame-byframe basis using the previously obtained bone-weighted image. This subtraction results in a fluoroscopic image sequence in which the ribs are suppressed, providing improved visualization of the tumor. To validate this approach, both high and low energy fluoroscopic image sets were obtained on 3 patients (total of 9 fluoroscopic sets) using an on-board imager. To simulate FSDE, the high and low energy fluoroscopic images were aligned based on respiratory phase, and frame-by-frame subtraction was performed to produce a soft tissue fluoroscopic image set. HDE fluoroscopy sequences were produced using only the high-energy sequence and a single bone-weighted image. A template-based motion-tracking algorithm was used on both FSDE and HDE image sequences and the tracking coordinates, as well as the peak-to-side lobe ratio (PSR - a quantitative measure of the template match), were compared. Results: A total of 1304 fluoroscopic frames were evaluated and there was no significant difference in the location of the tumor center using HDE vs FSDE fluoroscopy. The average differences in the tumor centroid coordinates were -0.05 +/- 0.29 mm and 0.02 +/- 0.27 mm in the x- and ydirections, respectively. The absolute maximum differences in x-y coordinates were 0.74 mm and 1.17 mm, respectively. The PSR values were also comparable with average values of 3.63 +/- 0.80 vs 3.62 +/- 0.84 (p Z 0.36) for FSDE and HDE fluoroscopy, respectively. Conclusions: The HDE approach provides improved visualization of lung tumors on fluoroscopic imaging without the need for any additional hardware. Moreover, HDE provides a means for real-time motion tracking of lung tumors, producing results that are comparable to FSDE fluoroscopy. Author Disclosure: J.C. Roeske: E. Research Grant; Varian Medical Systems. R. Patel: E. Research Grant; Varian Medical Systems. M. Campana: None. J. Panfil: None. M. Surucu: None. A. Block: None. M. Harkenrider: None.
1103 Analyzing Tumorlets in Prostate LDR Brachytherapy: Possible Lessons for Focal Therapy D.A. Todor,1 M.S. Anscher,1 M.P. Hagan,1 F. Siebert,2 K. Sebastian,2 and T. Catron1; 1Virginia Commonwealth University, Richmond, VA, 2 Universita¨tsklinikum Schleswig-Holstein, Kiel, Germany Purpose/Objective(s): Prostate cancer is a multi-focal disease but in the PSA era is characterized by relatively small volumes and number of tumor foci. We hypothesize that when treating whole prostate glands, it is dose delivered to tumor foci (tumorlets) that actually matters. We aim to identify the relationship between dosimetric and radiobiological parameters at the whole prostate and tumorlet levels. By examining pre-plans and post-implant plans from two different institutions, using two different isotopes, we also aim to assess the importance of spatial dose distributions. Materials/Methods: Plans from 72 patients treated with I-125 and Pd-103 seed implants were analyzed, 34 of which were treated at Institution A and 38 at Institution B. Pre- and post-implant plans were exported as DICOM files to a software platform built to create in-silico prostate tumorlets. Four thousand five hundred tumorlets of random position and morphology were created for each case in 9 classes of volumes ranging from 0.1 to 10 cm3. Each tumorlet was superimposed over the 3D dose distribution and positional, dosimetric, and radiobiological quantities were computed. Two different modalities to aggregate voxel level BEDs were employed and analyzed: EUBED and gBEUD. A treatment reference was established using BED of a 78 Gy external beam regimen (2 Gy/fx). “Probabilities of cure” (PoC) were computed as the ratio between the number of tumorlets with gBEUD or EUBED the equivalent EBRT reference treatment and total number of tumorlets. Results: The gBEUD seems to match well the currently employed BED based on D90 in pre-plans, but was consistently less in post-implant plans, pointing out to the fact that, unlike BED, gBEUD is sensitive to both