Utrecht Intertitial Applicator Shifts and Organ Movements in 3D CT-Based HDR Brachytherapy of Cervical Cancer

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.

S836

International Journal of Radiation Oncology  Biology  Physics

The prescribed dose of HDR brachytherapy was 7 Gy4 fractions. 6 patients received brachytherapy only for two fractions as their individual reasons. We got the permission of the patients for the investigation. The dose of brachytherapy were calculated and transformed to EQD2 values in Gya/b, biologically weighted dose normalized to 2 Gy fractionation. The used a/b values for target volumes and OAR were 10 and 3 Gy, respectively. We adopted the Zephyr Patient Transport Sled included Lithotomy Stirrups. After the applicator implanted, CT imaging was achieved for oncologist contouring CTVhr, CTVir, and OAR, including bladder, rectum, sigmoid colon and small intestines, which were performed according to GYN GEC ESTRO recommendations. The treatment plan was optimized according to the CTVhr, and executed in the case of total dose of CTV-hr D90 controlled by 80 to 85 Gya/b, and OAR dose constrained by 90 Gya/ b3 for bladder, 75 Gya/b3 for others. After the treatment, CT imaging was repeated. Two CT imaging were matched by pelvic structures. In both imaging, we defined the tandem by the tip and the base as the marker point, and evaluated applicator shift, including X (left as negative and right as positive) , Y (head as negative and foot as positive) and Z (anterior as negative and posterior as positive). Based on the repeated CT imaging, oncologist contoured the target volume and OAR again. We combined the treatment plan with the repeated CT imaging and evaluated the total dose of the target and OAR. We evaluated the change range for the doses of CTV-hr D90, D2cc for OAR. Results: The average applicator shift were 0.10 mm to -0.16mm for X, 1.49 mm to 2.14 mm for Y, and 2.3mm to 1.9 mm for Z. The change of average physical doses and EQD2 values in Gya/b range for CTV-hr D90 decreased by 2.55% and 3.5%, bladder D2cc decreased by 5.94 % and 8.77%, rectum D2cc decreased by 2.94% and 4%, sigmoid colon D2cc decreased by 3.38% and 3.72%, and small intestines D2cc increased by 3.72% and 10.94%. Conclusions: Applicator shifts and organ movements induced the total dose inaccurately. Although we adopted the Zephyr Patient Transport Sled included Lithotomy Stirrups, which reduced the movement of patients, the doses of target volume and OAR were variated inevitably. Applicator shifts and organ movements during the process of treatment could not be ignored. Author Disclosure: H.G. Cheng: None.

Results: The TSS and PSS patients had average  SD weight and volume of 69.815.6 and 75.221.1 kg and 13.012.6 and 13.315.2 cm3, respectively. The rates of repeat CBCT acquisition were 20% (33/161) and 28% (42/151) in the TSS and PSS, respectively. The maximum repeat of CBCT occurred in the first session in both systems: TSS was 25% and PSS was 50%. The systematic error, {x, y, z, 3Dvector shift: (x2+y2+z2) 1/2 [mm]}, was {0.53.7, 2.32.5, 0.73.5, 7.13.1} and {0.43.6, 0.74.0, 0.05.5, 9.24.2} and random setup error were (5.1, 3.0, 3.5, 3.9) and (4.6, 4.8, 5.4, 5.3) mm for TSS and PSS, respectively. The systematic error along y axis was significantly larger for the TSS (p< 0.05). The 3D vector was significantly larger for the PSS (p< 0.05). The ratio [%] of cases with shifts more than 5 mm were (20, 12.5, 25, 70) and (22.5, 22.5, 37.5, 87.5) and more than 10 mm was (2.5, 5, 0, 15) and (2.5, 7.5, 15, 32.5) for the TSS and PSS, respectively. From the first to the fourth session, the BPL had a significantly larger error compared to the TSS for 3D vector (p< 0.05). Setup time was significantly longer for the TSS than for the BPL (TSS; 16 min, PSS; 13 min) (p< 0.05). Treatment times were similar in both systems (TSS; 39 min, PSS; 39 min). Conclusions: The setup time was shorter but more shifts were needed to compensate for inter-fractional setup error for the PSS than for the TSS. Adequate accuracy can be achieved with either system thus allowing SBRT for varied patient habitus and comfort. Author Disclosure: Y. Ueda: E. Research Grant; JSPS Core-to-Core Program (No.23003). I.J. Das: None. H.R. Cardenes: None. T. Teshima: None.

3594 Setup Evaluation of 2 Immobilization Systems for Stereotactic Body Radiation Therapy Y. Ueda,1,2 I.J. Das,1 H.R. Cardenes,1 and T. Teshima2; 1Indiana University School of Medicine, Indianapolis, IN, 2Osaka Medical Center for Cancer and Cardiovascular Diseases, Osaka, Japan Purpose/Objective(s): Immobilization is essential and critical for stereotactic body radiation therapy (SBRT) in reducing motion and setup accuracies. Two commonly used commercial immobilization systems are investigated for the efficacies and accuracies in SBRT. Materials/Methods: The setup accuracy is analyzed retrospectively under IRB exempt status for 80 patients that were divided equally (40 patients each) in True Stereotactic System (TSS) and Pseudo Stereotactic System (PSS). Patients were treated between 3-5 fractions. Each system have an abdominal compression plate that was applied to limit the diaphragmatic excursion visualized under fluoroscopy to 0.5 cm. Patients were simulated in either of the immobilization devices depending on the body size and patients comfort using 4D CT. Target volume drawn was based on phase binning providing maximum intensity projection for the PTV. Skin marks were used for initial setup at treatment but cone beam CT (CBCT) was acquired before each treatment session for setup verification. Couch shifts for the inter-fractional were analyzed to evaluate the initial setup accuracy of each immobilization system. If a large error (>0.5 cm) occurred in any direction, an additional CBCT was acquired for verification after localization. The positioning and treatment times were also analyzed for 16 patients (6; TSS, 10; PSS).

3595 Adaptive Motion Mapping in Pancreatic SBRT Patients Using Fourier Transforms B. Jones, T. Schefter, and M. Miften; University of Colorado School of Medicine, Aurora, CO Purpose/Objective(s): 4DCT is commonly used to quantify and account for pancreatic tumor motion during SBRT, yet there are often significant differences between 4DCT and the motion observed during treatment. The purpose of this study was to develop an adaptive protocol to predict which patients will exhibit significant deviations, and to adapt the radiation therapy plan to ensure coverage of the target. Materials/Methods: Motion data was analyzed from 15 patients treated with 30 Gy in 5 fractions to pancreatic tumors. Patients were simulated and treated under free-breathing, and fiducial markers were implanted to assist with localization using cone-beam CT (CBCT). A template matching algorithm was used to identify the fiducial markers in the CBCT projections, which were used to reconstruct the 3D trajectory of the tumor throughout the acquisition. The Fourier transform was used to obtain the frequency spectrum of the tumor trajectory in the superior-inferior (SI) direction. We developed a metric (termed “Spectral Coherence”, SC) which describes the relative contribution of multiple motion frequencies to respiration. Trajectories with high SC are dominated by a single breathing rate, while low SC denotes a more heterogeneous frequency distribution. The tumor trajectory was also used to compute the deviation at each point in time from the expected tumor location (defined by 4DCT) and coverage of these tumors (using several target volume margins). Results: Using 4DCT, the range of motion exhibited by these tumors was 1.30.6 mm, 0.90.6 mm, and 3.71.0 mm in the anterior-posterior (AP), left-right (LR), and SI directions. During CBCT acquisition, the phaseresolved trajectory was in error from the expected 4DCT location by an average of 1.50.4 mm (0.70.2, 0.70.3, and 1.00.4 mm in the AP, LR, and SI directions), with average errors as high as 3.5 mm per fraction. SC was highly correlated with mean trajectory error (p < 10-10), and explained over 50% of the tumor motion trajectory variation observed in these patients. The SC observed in a single CBCT predicted the mean SC for that patient with an average error of 10%. When adapting the treatment based on motion frequency mapping, target volume margins were expanded by 2 mm in patients with low SC, and decreased by 2 mm in