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Deep Inspiration Breath Hold Radiation Therapy Reduces Heart Radiation Dose in Gastric Radiation Therapy
S834
International Journal of Radiation Oncology Biology Physics
Conclusions: IPSA obtains a desired dose to the HRCTV-D90 with a relatively higher dose to the HRCTV-D100 while holding the doses to the OARs to their limiting values, thus offering a significant benefit with image-based brachytherapy for the treatment of cervical cancer. Author Disclosure: N. Bhandare: None.
with optically stimulated luminescent detectors (OSL). All these modifications lead to a significant dose reduction of at least 50% up to a reduction of 90% in comparison with the default protocol doses while still preserving a proper image quality for positioning. Materials/Methods: Output measurements of the CBCT X-ray tube were performed at 100 kVp and 120 kVp. Measurements were based on the AAPM Task Group 61 protocol using a Farmer-type ionization chamber calibrated for the corresponding beam quality. Once the outputs were determined at the reference point, the correlation between OSL’s number of counts and the resultant dose was achieved. An anthropomorphic adult phantom with heterogeneous densities and 271 plugs for OSL detectors covering the whole body was used. Default scan protocols of the head and neck, breast, chest and pelvis were investigated. The mean dose and the maximum dose to the organs were collected over multiple scans to assure the reproducibility of the given method. Results: The maximum dose to any given organ never exceeded 3.0 cGy for all the default protocols. For the particular head and neck cases, the maximum dose never reached more than 0.4 cGy. Radio-sensitive organs such as lungs, breasts, bone marrow, stomach and intestines received between 1.0 cGy and 2.0 cGy per scan. In the chest and pelvis protocols, the thyroid gland and the testis received more dose than the other organs (2.0 - 3.0 cGy) due to their proximity to the surface and the low attenuation of the beam before reaching the OSLs. Conclusions: With the new modifications made to the default scan parameters, a considerable organ dose reduction of at least 50% can be achieved for every protocol without compromising image quality for patient positioning. With our in-house protocols, no organ received more than 1.5 cGy per scan. These results are therefore in compliance with the ALARA principle. Also, a new ultra-low dose scanning protocol for the chest was introduced giving only 0.1 cGy to the lungs and 0.2 cGy to the heart and thyroid respectively. This significant dose reduction is desirable especially for young lymphoma patients. Author Disclosure: E. Letourneau: None. M. Hinse: None. F. Vallejo Castaneda: None.
3588 Assessment of Setup Variability for Prostate Patients With Rectal Balloon by 3D CBCT and Real-Time Surface Imaging J.L. Peng, D.G. Mcdonald, M.S. Ashenafi, A.G. Ellis, and K.N. Vanek; Medical University of South Carolina, Charleston, SC Purpose/Objective(s): Inter- and intra-fraction positional uncertainty of prostate patients immobilized with rectal balloons was assessed by offline analysis of consecutive daily kV CBCT. Patients were divided into two groups characterized by setup scenario: (1) using 3 fiducial skin marks; and (2) using real-time 3D surface imaging (AlignRT, VisionRT Inc). It is the first study to evaluate prostate setup errors using a rectal balloon for immobilization and surface imaging for setup guidance. Materials/Methods: In this prospective study, ten prostate patients were immobilized internally using rectal balloon and setup using either skin marks or 3D surface images. kV CBCT was then used for online set-up correction and rigid registration with 3 internal implanted makers. For the 3D surface imaging group, following the CBCT, new reference 3D surface images were acquired. These new reference images were used to setup each remaining daily treatment. The required daily shifts for each patient, as determined by CBCT, were compared. Finally, using the real-time tracking features of the 3D surface imaging system, the intra-fractional motion that occurred while delivery each plan were analyzed. Results: The mean inter-fractional motion for the 3D surfaces imaging group was smaller (3.9 1.6 mm) than that of the skin marks group (6.8 3.3 mm) (p < 0.005). The 90th percentile and the maximum displacements were 6.2 mm and 8.4 mm, respectively, for the 3D surface imaging group and 9.2 mm and 10.7 mm for the skin marks group. The intra-fractional errors, as tracked by the 3D surface imaging in real time were within 3 mm in three axes. It was noted that the most common positional difference between the 3D surface imaging and CBCT was in the longitudinal direction, and when, present, was approximately 3-5 mm. This may be due to the differences in the rotations of the femur, though more investigation is needed. Thus, re-acquiring the reference surface image could improve the correlation with the CBCTs. Conclusions: Surface imaging for prostate setup using a rectal balloon for immobilization appears promising. By aligning the pelvis and thigh surface with stereo-photogrammetry, greater congruence with CBCT can be achieved than when patients are set up by skin marks alone. In addition, the real-time tracking feature allows for verification of patient position throughout entire treatment. In the future, a correlation of whole pelvis surface and prostate bed must be quantitatively established. Author Disclosure: J.L. Peng: K. Advisory Board; Clinical Physics Advisor (non-paid),VisonRT Inc. D.G. Mcdonald: None. M.S. Ashenafi: None. A.G. Ellis: None. K.N. Vanek: None.
3589 Organ Dose Reduction While Using In-House CBCT Patient-Specific Protocols Based on OSL Dosimetry E. Letourneau, M. Hinse, and F. Vallejo Castaneda; Centre Integre de Cancerologie de Laval, Laval, QC, Canada Purpose/Objective(s): In radiation therapy, dose contributions coming from planning and patient positioning images can seem negligible compared to the treatment dose. However, radiation-induced complications are numerous even at low dose. In order to diminish undesirable effects to the patient, one must concretely apply the ALARA principle and minimize dose to organs at risk (OAR) while not compromising treatment quality. In this study, appropriate adjustments to cone beam computed tomography (CBCT) imaging protocol parameters were performed. This was achieved after measuring the dose to organs in an anthropomorphic phantom filled
3590 Deep Inspiration Breath Hold Radiation Therapy Reduces Heart Radiation Dose in Gastric Radiation Therapy M. Hojgaard,1 M.C. Aznar,1,2 D.A. Schut,1 L. Specht,1,3 A.N. Pedersen,1 and P.M. Petersen1,3; 1Department of Oncology, Section for Radiotherapy, Rigshospitalet, Copenhagen, Denmark, 2Niels Bohr Institute, Faculty of Sciences, University of Copenhagen, Copenhagen, Denmark, 3Department of Haematology, Rigshospitalet, Copenhagen, Denmark Purpose/Objective(s): Gastric lymphoma is a rare neoplasm and is curatively treated with radiation therapy. The long term survival requires attention to organs at risk (OAR) to minimize long term toxicity. In this study we investigated whether respiratory controlled radiation therapy (RT) could decrease the dose to OAR in the treatment of the stomach, comparing treatment plans with free breathing (FB), deep inspiration breath-hold (DIBH) and expiration breath-hold (EBH). Materials/Methods: 9 patients, who previously participated in a prospective trial on breathing controlled RT in breast cancer, had scans in FB, DIBH and EBH. A Volumetric-Modulated Arc Therapy plan was done for each patient, using the scans done in FB, DIBH and EBH, respectively. We simulated a planning for aggressive gastric lymphoma with 2 Gy x 20. Planning target volume (PTV) coverage (% of PTV receiving 95% dose level) (V95%) and mean doses to the organs at risk (heart, lungs, kidneys, bowel cavity and liver) were calculated for each patient in both FB, DIBH and EBH. Wilcoxon’s signed rank test for paired data was used for statistical analysis for groupwise comparison of FB, EBH and DIBH. Results: Volumes and dose characteristics for the 3 breathing modalities are shown in Table 1. Doses for left lung/right lung not shown (no significant differences between treatment modalities). Conclusions: RT in DIBH compared to RT in FB and EBH reduces dose to the heart without compromising the dose to the target in patients treated to the stomach. Based on this analysis breathing controlled RT with DIBH will be compared to conventional FB in a prospective trial of gastric lymphomas.
Volume 90 Number 1S Supplement 2014 Scientific Abstract 3590; Table
Parameter
FB (median, range)
Poster Viewing Abstracts S835
See legend under ’’Results’’ All doses in Gy DIBH (median, range)
EBH (median, range)
P-value*
97 (93-100) >.2/>.2/ >.2 PTV volume (ccm) 474 (345-845) 458 (270-827) 593 (307-820) >.2/>.2/ >.2 Mean heart dose 7.2 (5.3-8.7) 4.8 (1.3-7.3) 8.7 (3.0-10) <.001/.2/ .005-.01 Mean left kidney 11 (6.1-27) 10 (6.6-16) 13 (5.6-15) >.2/>.2/ dose >.2 Mean right kidney 3.6 (1.0-6.1) 3.7 (1.3-13) 4.8 (2.8-11) >.2/.2/>.2 dose Mean liver dose 9.6 (6.8-12) 8.9 (4.6-13) 10 (5.9-15) >.2/>.2/ >.2 Mean bowel cavity 17 (11-24) 20 (12-24) 22 (11-27) >.2/.1/>.2 dose PTV V95%
99 (96-100)
98 (90-100)
*P-values for Wilcoxon’s signed rank test in the following order: FB vs DIBH/FB vs EBH/DIBH vs EBH. Free breathing (FB); Expiration breath hold (EBH); Deep inspiration breath hold (DIBH).
Author Disclosure: M. Hojgaard: None. M.C. Aznar: None. D.A. Schut: None. L. Specht: None. A.N. Pedersen: None. P.M. Petersen: None.
3591 Lung 4D-CBCT Reconstruction Using Prior Information and LimitedAngle Projections: Phantom and Patient Studies Y. Zhang,1 F. Yin,1 T. Pan,2 I. Vergalasova,1 and L. Ren1; 1Duke University, Durham, NC, 2The University of Texas, MD Anderson Cancer Center, Houston, TX Purpose/Objective(s): Current application of 4D-CBCT in clinics is limited due to its long scan time and high imaging dose. This study is to develop and clinically evaluate a 4D-CBCT reconstruction method using limited-angle projections to reduce scan time and dose. Materials/Methods: A motion-modeling and free-form deformation (MMFD) method is proposed to reconstruct 4D-CBCT images using only orthogonal limited-angle projections. In this method, each phase of the onboard 4D-CBCT is viewed as a deformation of a prior CT image. To solve the deformation field, the MM-FD method applies a principal component analysis based motion-modeling to generate a coarse estimation of the deformation field, followed by a constrained free-form deformation algorithm to further fine-tune the field. Data acquired from a dynamic thorax physical phantom and three lung cancer patients were used to evaluate the MM-FD method. 4D-CBCT reconstructions from a full sample of projections were used as the ground truth. The accuracy of the MM-FD method was evaluated by calculating the volume percentage difference (VPD) and center-of-mass shift (COMS) of the reconstructed tumor volume. The accuracy of the FDK, MM-only and FD-only methods was also evaluated for comparison. Results: Results showed that MM-FD was substantially more accurate than FDK, MM-only and FD-only methods. For the phantom study using orthogonal 30 projections, the average (standard deviation) VPD values for MM-only, FD-only and MM-FD methods were 223.3% (2.5%), 29.2% (24.9%) and 6.6% (2.6%), respectively. The corresponding COMS values were 2.0mm (1.6mm), 3.5mm (2.7mm) and 1.1mm (0.4mm), respectively. For the patient study, the average (standard deviation) VPD values for MM-only, FD-only and MM-FD methods were 36.0% (32.2%), 21.4% (13.9%) and 9.6% (6.1%). The corresponding COMS values were 2.2mm (1.5mm), 1.6mm (0.6mm) and 1.1mm (0.5mm). The MM-FD method yielded similar reconstruction accuracy using different scan directions. The average (standard deviation) VPD and COMS were 6.6% (2.6%) and 1.1mm (0.5mm) using orthogonal 30 projections from anterior-posterior and left-lateral directions, and 8.3% (2.5%) and 1.3mm (0.4mm) using projections from left-anterior-oblique and left-posterior-oblique directions. The MM-FD method was less
accurate when using projections of lower sampling frequency. The average (standard deviation) VPD and COMS were 6.6% (2.6%) and 1.1mm (0.4mm) using 1/projection, and 23.1% (5.3%) and 1.2mm (0.4mm) using 4/projection. Conclusions: The MM-FD method can accurately reconstruct 4D-CBCT images using only limited-angle projections and therefore shows great potential to reduce the scan time and dose for 4D-CBCT imaging. Author Disclosure: Y. Zhang: A. Employee; Duke University Medical Center. F. Yin: A. Employee; Duke University Medical Center. E. Research Grant; Varian Medical Systems, National Institute of Health. Q. Patent/License Fee/Copyright; Patent. S. Leadership; AAPM. T. Pan: A. Employee; The University of Texas, MD Anderson Cancer Center. E. Research Grant; Cancer Prevention Research Institute of Texas. I. Vergalasova: A. Employee; Duke University Medical Center. L. Ren: A. Employee; Duke University Medical Center. E. Research Grant; Varian Medical Systems, National Institute of Health.
3592 Intrafraction Position Management PosteCone Beam CT Using Stereoscopic X-Ray Verification for Stereotactic Body Radiation Therapy Lung Treatment C. Abing, J. Wochos, C. Driscoll, P. Conway, and D. Gold; Gundersen Health System, La Crosse, WI Purpose/Objective(s): To use existing technology at our institution to evaluate the efficacy of using stereoscopic x-rays for intra-fraction position verification after initial imaging and positioning performed with a conebeam CT (CBCT). Materials/Methods: Forty-four fractions of data were collected over the course of nine patients. Stereotactic body radiation therapy (SBRT) target position verification was performed by CBCT. Immediately afterwards, an initial set of stereoscopic x-rays were taken. Using the x-ray computed shifts as a baseline; further sets of x-rays were taken every 2-3 beams, and compared with the baseline values. If a deviation of 2mm or greater was detected in any of the 3 translational degrees of freedom, treatment was stopped and an additional CBCT was performed to reaffirm proper target position. This process was repeated until the completion of each patient’s treatment. Results: There was one fraction where the patient’s position had changed greater than 2mm within 3 translational degrees of freedom; 4mm in the superior/inferior direction. There were 5 instances where the total isocenter displacement (vector) changed more than 2mm. Image acquisition and evaluation of the CBCT and stereoscopic x-ray images was five minutes and twenty seconds respectively. Conclusions: The method proved to be efficient and cost effective utilizing existing technology and treatment equipment. It was also accurate and precise. Because the system is independent of the treatment machine, stereoscopic x-rays can be taken when the gantry is at each of the four cardinal angles during the patient’s treatment; thus limiting interruptions. This method has increased our confidence that our target is consistently at the prescribed position throughout the treatment, as well as increased confidence in our immobilization system. Our institution has made this procedure part of all SBRT lung treatments. Author Disclosure: C. Abing: None. J. Wochos: None. C. Driscoll: None. P. Conway: None. D. Gold: None.
3593 Utrecht Intertitial Applicator Shifts and Organ Movements in 3D CT-Based HDR Brachytherapy of Cervical Cancer H.G. Cheng; China-Japan Union Hospital of JiLin University, Changchun, China Purpose/Objective(s): To investigate Utrecht intertitial applicator shifts, and its effects of organ movements on DVH parameters during 3D CTbased HDR brachytherapy of cervical cancer. Materials/Methods: 13 cervical cancer patients underwent brachytherapy after external beam radiation therapy for total dose to 45 Gy/25 fractions.
International Journal of Radiation Oncology Biology Physics
Conclusions: IPSA obtains a desired dose to the HRCTV-D90 with a relatively higher dose to the HRCTV-D100 while holding the doses to the OARs to their limiting values, thus offering a significant benefit with image-based brachytherapy for the treatment of cervical cancer. Author Disclosure: N. Bhandare: None.
with optically stimulated luminescent detectors (OSL). All these modifications lead to a significant dose reduction of at least 50% up to a reduction of 90% in comparison with the default protocol doses while still preserving a proper image quality for positioning. Materials/Methods: Output measurements of the CBCT X-ray tube were performed at 100 kVp and 120 kVp. Measurements were based on the AAPM Task Group 61 protocol using a Farmer-type ionization chamber calibrated for the corresponding beam quality. Once the outputs were determined at the reference point, the correlation between OSL’s number of counts and the resultant dose was achieved. An anthropomorphic adult phantom with heterogeneous densities and 271 plugs for OSL detectors covering the whole body was used. Default scan protocols of the head and neck, breast, chest and pelvis were investigated. The mean dose and the maximum dose to the organs were collected over multiple scans to assure the reproducibility of the given method. Results: The maximum dose to any given organ never exceeded 3.0 cGy for all the default protocols. For the particular head and neck cases, the maximum dose never reached more than 0.4 cGy. Radio-sensitive organs such as lungs, breasts, bone marrow, stomach and intestines received between 1.0 cGy and 2.0 cGy per scan. In the chest and pelvis protocols, the thyroid gland and the testis received more dose than the other organs (2.0 - 3.0 cGy) due to their proximity to the surface and the low attenuation of the beam before reaching the OSLs. Conclusions: With the new modifications made to the default scan parameters, a considerable organ dose reduction of at least 50% can be achieved for every protocol without compromising image quality for patient positioning. With our in-house protocols, no organ received more than 1.5 cGy per scan. These results are therefore in compliance with the ALARA principle. Also, a new ultra-low dose scanning protocol for the chest was introduced giving only 0.1 cGy to the lungs and 0.2 cGy to the heart and thyroid respectively. This significant dose reduction is desirable especially for young lymphoma patients. Author Disclosure: E. Letourneau: None. M. Hinse: None. F. Vallejo Castaneda: None.
3588 Assessment of Setup Variability for Prostate Patients With Rectal Balloon by 3D CBCT and Real-Time Surface Imaging J.L. Peng, D.G. Mcdonald, M.S. Ashenafi, A.G. Ellis, and K.N. Vanek; Medical University of South Carolina, Charleston, SC Purpose/Objective(s): Inter- and intra-fraction positional uncertainty of prostate patients immobilized with rectal balloons was assessed by offline analysis of consecutive daily kV CBCT. Patients were divided into two groups characterized by setup scenario: (1) using 3 fiducial skin marks; and (2) using real-time 3D surface imaging (AlignRT, VisionRT Inc). It is the first study to evaluate prostate setup errors using a rectal balloon for immobilization and surface imaging for setup guidance. Materials/Methods: In this prospective study, ten prostate patients were immobilized internally using rectal balloon and setup using either skin marks or 3D surface images. kV CBCT was then used for online set-up correction and rigid registration with 3 internal implanted makers. For the 3D surface imaging group, following the CBCT, new reference 3D surface images were acquired. These new reference images were used to setup each remaining daily treatment. The required daily shifts for each patient, as determined by CBCT, were compared. Finally, using the real-time tracking features of the 3D surface imaging system, the intra-fractional motion that occurred while delivery each plan were analyzed. Results: The mean inter-fractional motion for the 3D surfaces imaging group was smaller (3.9 1.6 mm) than that of the skin marks group (6.8 3.3 mm) (p < 0.005). The 90th percentile and the maximum displacements were 6.2 mm and 8.4 mm, respectively, for the 3D surface imaging group and 9.2 mm and 10.7 mm for the skin marks group. The intra-fractional errors, as tracked by the 3D surface imaging in real time were within 3 mm in three axes. It was noted that the most common positional difference between the 3D surface imaging and CBCT was in the longitudinal direction, and when, present, was approximately 3-5 mm. This may be due to the differences in the rotations of the femur, though more investigation is needed. Thus, re-acquiring the reference surface image could improve the correlation with the CBCTs. Conclusions: Surface imaging for prostate setup using a rectal balloon for immobilization appears promising. By aligning the pelvis and thigh surface with stereo-photogrammetry, greater congruence with CBCT can be achieved than when patients are set up by skin marks alone. In addition, the real-time tracking feature allows for verification of patient position throughout entire treatment. In the future, a correlation of whole pelvis surface and prostate bed must be quantitatively established. Author Disclosure: J.L. Peng: K. Advisory Board; Clinical Physics Advisor (non-paid),VisonRT Inc. D.G. Mcdonald: None. M.S. Ashenafi: None. A.G. Ellis: None. K.N. Vanek: None.
3589 Organ Dose Reduction While Using In-House CBCT Patient-Specific Protocols Based on OSL Dosimetry E. Letourneau, M. Hinse, and F. Vallejo Castaneda; Centre Integre de Cancerologie de Laval, Laval, QC, Canada Purpose/Objective(s): In radiation therapy, dose contributions coming from planning and patient positioning images can seem negligible compared to the treatment dose. However, radiation-induced complications are numerous even at low dose. In order to diminish undesirable effects to the patient, one must concretely apply the ALARA principle and minimize dose to organs at risk (OAR) while not compromising treatment quality. In this study, appropriate adjustments to cone beam computed tomography (CBCT) imaging protocol parameters were performed. This was achieved after measuring the dose to organs in an anthropomorphic phantom filled
3590 Deep Inspiration Breath Hold Radiation Therapy Reduces Heart Radiation Dose in Gastric Radiation Therapy M. Hojgaard,1 M.C. Aznar,1,2 D.A. Schut,1 L. Specht,1,3 A.N. Pedersen,1 and P.M. Petersen1,3; 1Department of Oncology, Section for Radiotherapy, Rigshospitalet, Copenhagen, Denmark, 2Niels Bohr Institute, Faculty of Sciences, University of Copenhagen, Copenhagen, Denmark, 3Department of Haematology, Rigshospitalet, Copenhagen, Denmark Purpose/Objective(s): Gastric lymphoma is a rare neoplasm and is curatively treated with radiation therapy. The long term survival requires attention to organs at risk (OAR) to minimize long term toxicity. In this study we investigated whether respiratory controlled radiation therapy (RT) could decrease the dose to OAR in the treatment of the stomach, comparing treatment plans with free breathing (FB), deep inspiration breath-hold (DIBH) and expiration breath-hold (EBH). Materials/Methods: 9 patients, who previously participated in a prospective trial on breathing controlled RT in breast cancer, had scans in FB, DIBH and EBH. A Volumetric-Modulated Arc Therapy plan was done for each patient, using the scans done in FB, DIBH and EBH, respectively. We simulated a planning for aggressive gastric lymphoma with 2 Gy x 20. Planning target volume (PTV) coverage (% of PTV receiving 95% dose level) (V95%) and mean doses to the organs at risk (heart, lungs, kidneys, bowel cavity and liver) were calculated for each patient in both FB, DIBH and EBH. Wilcoxon’s signed rank test for paired data was used for statistical analysis for groupwise comparison of FB, EBH and DIBH. Results: Volumes and dose characteristics for the 3 breathing modalities are shown in Table 1. Doses for left lung/right lung not shown (no significant differences between treatment modalities). Conclusions: RT in DIBH compared to RT in FB and EBH reduces dose to the heart without compromising the dose to the target in patients treated to the stomach. Based on this analysis breathing controlled RT with DIBH will be compared to conventional FB in a prospective trial of gastric lymphomas.
Volume 90 Number 1S Supplement 2014 Scientific Abstract 3590; Table
Parameter
FB (median, range)
Poster Viewing Abstracts S835
See legend under ’’Results’’ All doses in Gy DIBH (median, range)
EBH (median, range)
P-value*
97 (93-100) >.2/>.2/ >.2 PTV volume (ccm) 474 (345-845) 458 (270-827) 593 (307-820) >.2/>.2/ >.2 Mean heart dose 7.2 (5.3-8.7) 4.8 (1.3-7.3) 8.7 (3.0-10) <.001/.2/ .005-.01 Mean left kidney 11 (6.1-27) 10 (6.6-16) 13 (5.6-15) >.2/>.2/ dose >.2 Mean right kidney 3.6 (1.0-6.1) 3.7 (1.3-13) 4.8 (2.8-11) >.2/.2/>.2 dose Mean liver dose 9.6 (6.8-12) 8.9 (4.6-13) 10 (5.9-15) >.2/>.2/ >.2 Mean bowel cavity 17 (11-24) 20 (12-24) 22 (11-27) >.2/.1/>.2 dose PTV V95%
99 (96-100)
98 (90-100)
*P-values for Wilcoxon’s signed rank test in the following order: FB vs DIBH/FB vs EBH/DIBH vs EBH. Free breathing (FB); Expiration breath hold (EBH); Deep inspiration breath hold (DIBH).
Author Disclosure: M. Hojgaard: None. M.C. Aznar: None. D.A. Schut: None. L. Specht: None. A.N. Pedersen: None. P.M. Petersen: None.
3591 Lung 4D-CBCT Reconstruction Using Prior Information and LimitedAngle Projections: Phantom and Patient Studies Y. Zhang,1 F. Yin,1 T. Pan,2 I. Vergalasova,1 and L. Ren1; 1Duke University, Durham, NC, 2The University of Texas, MD Anderson Cancer Center, Houston, TX Purpose/Objective(s): Current application of 4D-CBCT in clinics is limited due to its long scan time and high imaging dose. This study is to develop and clinically evaluate a 4D-CBCT reconstruction method using limited-angle projections to reduce scan time and dose. Materials/Methods: A motion-modeling and free-form deformation (MMFD) method is proposed to reconstruct 4D-CBCT images using only orthogonal limited-angle projections. In this method, each phase of the onboard 4D-CBCT is viewed as a deformation of a prior CT image. To solve the deformation field, the MM-FD method applies a principal component analysis based motion-modeling to generate a coarse estimation of the deformation field, followed by a constrained free-form deformation algorithm to further fine-tune the field. Data acquired from a dynamic thorax physical phantom and three lung cancer patients were used to evaluate the MM-FD method. 4D-CBCT reconstructions from a full sample of projections were used as the ground truth. The accuracy of the MM-FD method was evaluated by calculating the volume percentage difference (VPD) and center-of-mass shift (COMS) of the reconstructed tumor volume. The accuracy of the FDK, MM-only and FD-only methods was also evaluated for comparison. Results: Results showed that MM-FD was substantially more accurate than FDK, MM-only and FD-only methods. For the phantom study using orthogonal 30 projections, the average (standard deviation) VPD values for MM-only, FD-only and MM-FD methods were 223.3% (2.5%), 29.2% (24.9%) and 6.6% (2.6%), respectively. The corresponding COMS values were 2.0mm (1.6mm), 3.5mm (2.7mm) and 1.1mm (0.4mm), respectively. For the patient study, the average (standard deviation) VPD values for MM-only, FD-only and MM-FD methods were 36.0% (32.2%), 21.4% (13.9%) and 9.6% (6.1%). The corresponding COMS values were 2.2mm (1.5mm), 1.6mm (0.6mm) and 1.1mm (0.5mm). The MM-FD method yielded similar reconstruction accuracy using different scan directions. The average (standard deviation) VPD and COMS were 6.6% (2.6%) and 1.1mm (0.5mm) using orthogonal 30 projections from anterior-posterior and left-lateral directions, and 8.3% (2.5%) and 1.3mm (0.4mm) using projections from left-anterior-oblique and left-posterior-oblique directions. The MM-FD method was less
accurate when using projections of lower sampling frequency. The average (standard deviation) VPD and COMS were 6.6% (2.6%) and 1.1mm (0.4mm) using 1/projection, and 23.1% (5.3%) and 1.2mm (0.4mm) using 4/projection. Conclusions: The MM-FD method can accurately reconstruct 4D-CBCT images using only limited-angle projections and therefore shows great potential to reduce the scan time and dose for 4D-CBCT imaging. Author Disclosure: Y. Zhang: A. Employee; Duke University Medical Center. F. Yin: A. Employee; Duke University Medical Center. E. Research Grant; Varian Medical Systems, National Institute of Health. Q. Patent/License Fee/Copyright; Patent. S. Leadership; AAPM. T. Pan: A. Employee; The University of Texas, MD Anderson Cancer Center. E. Research Grant; Cancer Prevention Research Institute of Texas. I. Vergalasova: A. Employee; Duke University Medical Center. L. Ren: A. Employee; Duke University Medical Center. E. Research Grant; Varian Medical Systems, National Institute of Health.
3592 Intrafraction Position Management PosteCone Beam CT Using Stereoscopic X-Ray Verification for Stereotactic Body Radiation Therapy Lung Treatment C. Abing, J. Wochos, C. Driscoll, P. Conway, and D. Gold; Gundersen Health System, La Crosse, WI Purpose/Objective(s): To use existing technology at our institution to evaluate the efficacy of using stereoscopic x-rays for intra-fraction position verification after initial imaging and positioning performed with a conebeam CT (CBCT). Materials/Methods: Forty-four fractions of data were collected over the course of nine patients. Stereotactic body radiation therapy (SBRT) target position verification was performed by CBCT. Immediately afterwards, an initial set of stereoscopic x-rays were taken. Using the x-ray computed shifts as a baseline; further sets of x-rays were taken every 2-3 beams, and compared with the baseline values. If a deviation of 2mm or greater was detected in any of the 3 translational degrees of freedom, treatment was stopped and an additional CBCT was performed to reaffirm proper target position. This process was repeated until the completion of each patient’s treatment. Results: There was one fraction where the patient’s position had changed greater than 2mm within 3 translational degrees of freedom; 4mm in the superior/inferior direction. There were 5 instances where the total isocenter displacement (vector) changed more than 2mm. Image acquisition and evaluation of the CBCT and stereoscopic x-ray images was five minutes and twenty seconds respectively. Conclusions: The method proved to be efficient and cost effective utilizing existing technology and treatment equipment. It was also accurate and precise. Because the system is independent of the treatment machine, stereoscopic x-rays can be taken when the gantry is at each of the four cardinal angles during the patient’s treatment; thus limiting interruptions. This method has increased our confidence that our target is consistently at the prescribed position throughout the treatment, as well as increased confidence in our immobilization system. Our institution has made this procedure part of all SBRT lung treatments. Author Disclosure: C. Abing: None. J. Wochos: None. C. Driscoll: None. P. Conway: None. D. Gold: None.
3593 Utrecht Intertitial Applicator Shifts and Organ Movements in 3D CT-Based HDR Brachytherapy of Cervical Cancer H.G. Cheng; China-Japan Union Hospital of JiLin University, Changchun, China Purpose/Objective(s): To investigate Utrecht intertitial applicator shifts, and its effects of organ movements on DVH parameters during 3D CTbased HDR brachytherapy of cervical cancer. Materials/Methods: 13 cervical cancer patients underwent brachytherapy after external beam radiation therapy for total dose to 45 Gy/25 fractions.