4DCT Image Sets Research Articles

Real time 4D IMRT treatment planning based on a dynamic virtual patient model: Proof of concept

To develop a novel four-dimensional (4D) intensity modulated radiation therapy (IMRT) treatment planning methodology based on dynamic virtual patient models. The 4D model-based planning (4DMP) is a predictive tracking method which consists of two main steps: (1) predicting the 3D deformable motion of the target and critical structures as a function of time during treatment delivery; (2) adjusting the delivery beam apertures formed by the dynamic multi-leaf collimators (DMLC) to account for the motion. The key feature of 4DMP is the application of a dynamic virtual patient model in motion prediction, treatment beam adjustment, and dose calculation. A lung case was chosen to demonstrate the feasibility of the 4DMP. For the lung case, a dynamic virtual patient model (4D model) was first developed based on the patient's 4DCT images. The 4D model was capable of simulating respiratory motion of different patterns. A model-based registration method was then applied to convert the 4D model into a set of deformation maps and 4DCT images for dosimetric purposes. Based on the 4D model, 4DMP treatment plans with different respiratory motion scenarios were developed. The quality of 4DMP plans was then compared with two other commonly used 4D planning methods: maximum intensity projection (MIP) and planning on individual phases (IP). Under regular periodic motion, 4DMP offered similar target coverage as MIP with much better normal tissue sparing. At breathing amplitude of 2 cm, the lung V20 was 23.9% for a MIP plan and 16.7% for a 4DMP plan. The plan quality was comparable between 4DMP and IP: PTV V97 was 93.8% for the IP plan and 93.6% for the 4DMP plan. Lung V20 of the 4DMP plan was 2.1% lower than that of the IP plan and Dmax to cord was 2.2 Gy higher. Under a real time irregular breathing pattern, 4DMP had the best plan quality. PTV V97 was 90.4% for a MIP plan, 88.6% for an IP plan and 94.1% for a 4DMP plan. Lung V20 was 20.1% for the MIP plan, 17.8% for the IP plan and 17.5% for the 4DMP plan. The deliverability of the real time 4DMP plan was proved by calculating the maximum leaf speed of the DMLC. The 4D model-based planning, which applies dynamic virtual patient models in IMRT treatment planning, can account for the real time deformable motion of the tumor under different breathing conditions. Under regular motion, the quality of 4DMP plans was comparable with IP and superior to MIP. Under realistic motion in which breathing amplitude and period change, 4DMP gave the best plan quality of the three 4D treatment planning techniques.

Initial experience and clinical comparison of two image guidance methods for SBRT treatment: 4DCT versus respiratory‐triggered imaging

For Stereotactic Body Radiation Therapy (SBRT) treatment of lung and liver, we quantified the differences between two image guidance methods: 4DCT and ExacTrac respiratory‐triggered imaging. Five different patients with five liver lesions and one lung lesion for a total of 19 SBRT delivered fractions were studied. For the 4DCT method, a manual registration process was used between the 4DCT image sets from initial simulation and treatment day to determine the required daily image‐guided corrections. We also used the ExacTrac respiratory‐triggered imaging capability to verify the target positioning, and calculated the differences in image guidance shifts between these two methods. The mean (standard deviation) of the observed differences in image‐guided shifts between 4DCT and ExacTrac respiratory‐triggered image guidance was left/right (L/R)=0.4(2.0)mm, anterior/posterior (A/P)=1.4(1.7) mm, superior/inferior (S/I)=2.2(2.0) mm, with no difference larger than 5.0 mm in any given direction for any individual case. The largest error occurred in the S/I direction, with a mean of 2.2 mm for the six lesions. This seems reasonable, because respiratory motion and the resulting imaging uncertainties are most pronounced in this S/I direction. Image guidance shifts derived from ExacTrac triggered imaging at two extreme breathing phases (i.e., full exhale vs. full inhale), agreed well (less than 2.0 mm) with each other. In summary, two very promising image guidance methods of 4DCT and ExacTrac respiratory‐triggered imaging were presented and the image guidance shifts were comparable for the patients evaluated in this study.PACS number: 87.55.ne

SU-GG-T-585: The Impact of Free Breathing versus Average 4D CT Image Data on External Beam Radiotherapy Planning for Liver Tumors

PURPOSE: To investigate the dosimetric impact of using two different 4DCT image data set for the treatment planning on tumor and surrounding normal tissues in liver patients. MATERIALS AND METHODS: Ten patients with primary liver malignancies underwent radiotherapy (4 3D-conformal and 6 IMRT) treatment. A set of 4DCT images was obtained for target delineation and an average CT (AvCT) was generated by averaging over 10 phases of the complete breathing cycle. Another set of free breathing (FB) CT was acquired for treatment planning. The internal GTV was contoured based on 4DCT, then copied to FB and AvCT image sets. Organs at risk (OAR) DVHs were generated for the liver, left and right kidneys, stomach and duodenum. RESULTS: SI motion of liver ranged from 0.5 to 2.25 cm, with a median of 1.25 cm. The GTV volume ranged from 77 to 966 cc, and the median was 257 cc. When the same beam arrangements and prescription doses were applied to FBCT and AVCT, no statistically significant difference was observed for the minimum and maximum GTV dosage, prescription dose coverage for the GTV/CTV/PTV, or OAR DVHs (V20 for liver and kidneys, V50 for stomach and duodenum) (p<0.001). The maximum difference observed was ∼4% minimum GTV dose and ∼13% GTV prescription dose coverage, which we believe was partly due to change of air distribution in the GI tract between FBCT and AVCT. CONCLUSIONS: There was no statistically significant difference for target coverage and OAR DVHs observed when the plan generated with FBCT was applied to AvCT. Assuming that the patient's breathing pattern is almost identical at simulation as throughout the treatment course, we believe Average CT planning is a more reproducible representation of the patient's anatomy and tumor and therefore will yield a more accurate plan that could potentially improve tumor coverage.

SU‐GG‐J‐01: 3D‐3D Deformable Registration Based on Bifurcations of Tubular Organs for the 4D Computed Tomography (4DCT) Image Sets

Purpose: To develop an accurate feature based 3D‐3D deformable registration method for patient‐specific motion model used in external beam radiation treatment of lung cancers based on a 4D computed tomography (4DCT) image set by utilizing unique features of the bifurcations of tubular organs. Method and Materials: Each 4DCT set consisted of 10 phases of 3DCT volumes during one breath cycle. A 3D tubular organ segmentation was first performed on each of the phases to extract the centerlines of bronchial trees, estimate radius of bronchial trees and automatically detect the bifurcation points by applying learning algorithm with specially designed filters. A novel deformable registration method was applied to minimize the distances of the corresponding bifurcation points between a target phase and a reference phase (e.g., between 0% phase and 50% phase) to capture the transformation between different phases. The results were evaluated by using volume and distance based estimators. Results: The learning method was trained and tested with positive and negative examples. The generalization error of the learning method was estimated using bootstrapping with the mean error rate 4.6%. The detailed quantitative and qualitative registration results are shown in the supporting materials. The mean distance estimator yielded results ranging from 1.93 mm to 4.46 mm between the corresponding points between the 0% phase images and the 50% phase images after the deformable registration. The root‐mean‐square error ranged from 1.99 mm to 5.13 mm. Conclusions: A novel and accurate 3D‐3D registration method based on the bifurcations of the tubular organs was developed to capture the transformation between the 3D CT images in the 4D computed tomography (4DCT) image sets. These preliminary results show that the proposed method is robust, fast and accurate for the deformable registration of the tubular organ in the lung.

WE‐C‐204B‐06: Tumor Specific Reconstruction of Motion Probability Density Function from 4DCT

Purpose: The actual delivered dose to a moving tumor can deviate from prescribed dose not only during each fraction, but during each IMRT field. This deviation can be anticipated and incorporated into a treatment plan if the tumor specific motion probability density function (PDF) is accurately identified. In this study, a novel technique in determining the actual, tumor specific PDF of a moving gross tumor volume (GTV) is described and confirmed through phantom experiments using a quantitative approach of 4DCT imaging. Methods and Materials: We hypothesize that a PDF of the GTV can be obtained through weighted sums of phase‐sorted 4DCT images. Experimental validation is performed using a lung phantom attached to a programmable motion platform. The ground truth motion PDF of the targets was calculated through convolution of the motion pattern and the GTV dimension along the direction of motion. These PDF convolutions are compared to the normalized, reconstructed CT numbers obtained from averaging ten phase sorted 4DCT images after background subtraction. Results: The PDF reconstructed from averaged, weighted sets of 4DCT images was almost identical to the ground truth PDF in all comparisons made. Through sums of phase sorted 4DCT images, the resulting CT number represents a relative probability of finding some portion of the GTV in each geometric location (or voxel). In physical phantom reconstructions, the Pearson correlation coefficient between CT‐based PDF and the ground truth PDF was greater than .97 in all cases considered. An actual patient PDF was also evaluated in frontal and sagittal planes. Conclusions: Combining phase sorted CT images is an effective method in obtaining the actual tumor PDF as seen by an IMRT field. With the patient specific PDF, treatment planning to ensure proper target coverage can be achieved through intra‐field modulation, dose rate, and monitoring initial beam‐on phase.

Four-dimensional dosimetry validation and study in lung radiotherapy using deformable image registration and Monte Carlo techniques

Thoracic cancer treatment presents dosimetric difficulties due to respiratory motion and lung inhomogeneity. Monte Carlo and deformable image registration techniques have been proposed to be used in four-dimensional (4D) dose calculations to overcome the difficulties. This study validates the 4D Monte Carlo dosimetry with measurement, compares 4D dosimetry of different tumor sizes and tumor motion ranges, and demonstrates differences of dose-volume histograms (DVH) with the number of respiratory phases that are included in 4D dosimetry. BEAMnrc was used in dose calculations while an optical flow algorithm was used in deformable image registration and dose mapping. Calculated and measured doses of a moving phantom agreed within 3% at the center of the moving gross tumor volumes (GTV). 4D CT image sets of lung cancer cases were used in the analysis of 4D dosimetry. For a small tumor (12.5 cm3) with motion range of 1.5 cm, reduced tumor volume coverage was observed in the 4D dose with a beam margin of 1 cm. For large tumors and tumors with small motion range (around 1 cm), the 4D dosimetry did not differ appreciably from the static plans. The dose-volume histogram (DVH) analysis shows that the inclusion of only extreme respiratory phases in 4D dosimetry is a reasonable approximation of all-phase inclusion for lung cancer cases similar to the ones studied, which reduces the calculation in 4D dosimetry.

Evaluation of Fiducial Marker Migration and Respiratory-Induced Motion for Image Guided Radiotherapy in Whole Breast Irradiation.

Abstract Purpose: Image guided radiation therapy (IGRT) may be beneficial to improve set up accuracy and reduce treatment margins, therefore improving efficacy while decreasing long term toxicity of whole breast irradiation (WBI). In order to assess if intraparenchymal fiducials can facilitate the use of IGRT, this study quantifies fiducial motion over the normal course of whole breast irradiation.Materials and Methods: On a prospective IRB-approved protocol, fiducials were placed in fourteen patients who were then treated with 3D conformal WBI. Three or four intraparenchymal gold fiducial markers were placed in each patient at the periphery of the surgical bed intraoperatively. Free breathing 4D CT image sets were obtained at pre-treatment simulation and then every two weeks over a six week course of treatment. Each fiducial marker was contoured at end-inspiration and end-expiration on all 4D CT image sets. The effects of respiration induced motion were assessed by comparing the positions of the fiducials between inspiration and expiration. Overall fiducial migration was determined by comparing the relative positions of each marker to one another from the pre-treatment and post-treatment 4D CT image sets.Results: The 4D CT image sets were acquired over a period of 55±4 days with a range of 49 to 60 days. The respiration induced fiducial motion, as measured by the average change in the fiducials' center of mass, was 0.8±0.6 mm with a range of 0 to 2.2 mm. The average fiducial displacement over the course of treatment as determined from relative seed position from the image sets was 1.5±1.2 mm with a range of 0.1 to 3.3 mm for all individual fiducials with 71% of the fiducials moving closer to each other and 29% away from each other.Conclusions: The preliminary results of using gold fiducials for breast IGRT are promising. Fiducial position was very stable during treatment, and there was very little respiration induced motion. Using IGRT, there is the potential to improve the accuracy of daily set-up and to reduce margins in WBI. Better tumor bed localization and reduced margin size will decrease the volume of normal tissue treated, which may translate into improved local control by ensuring accurate coverage of the target volumes throughout the treatment course and improvement in cosmesis and other long term toxicities. Citation Information: Cancer Res 2009;69(24 Suppl):Abstract nr 4113.

Four-dimensional deformable image registration using trajectory modeling

A four-dimensional deformable image registration (4D DIR) algorithm, referred to as 4D local trajectory modeling (4DLTM), is presented and applied to thoracic 4D computed tomography (4DCT) image sets. The theoretical framework on which this algorithm is built exploits the incremental continuity present in 4DCT component images to calculate a dense set of parameterized voxel trajectories through space as functions of time. The spatial accuracy of the 4DLTM algorithm is compared with an alternative registration approach in which component phase to phase (CPP) DIR is utilized to determine the full displacement between maximum inhale and exhale images. A publically available DIR reference database (http://www.dir-lab.com) is utilized for the spatial accuracy assessment. The database consists of ten 4DCT image sets and corresponding manually identified landmark points between the maximum phases. A subset of points are propagated through the expiratory 4DCT component images. Cubic polynomials were found to provide sufficient flexibility and spatial accuracy for describing the point trajectories through the expiratory phases. The resulting average spatial error between the maximum phases was 1.25 mm for the 4DLTM and 1.44 mm for the CPP. The 4DLTM method captures the long-range motion between 4DCT extremes with high spatial accuracy.

Implementation and evaluation of various demons deformable image registration algorithms on a GPU

Online adaptive radiation therapy (ART) promises the ability to deliver an optimal treatment in response to daily patient anatomic variation. A major technical barrier for the clinical implementation of online ART is the requirement of rapid image segmentation. Deformable image registration (DIR) has been used as an automated segmentation method to transfer tumor/organ contours from the planning image to daily images. However, the current computational time of DIR is insufficient for online ART. In this work, this issue is addressed by using computer graphics processing units (GPUs). A gray-scale-based DIR algorithm called demons and five of its variants were implemented on GPUs using the compute unified device architecture (CUDA) programming environment. The spatial accuracy of these algorithms was evaluated over five sets of pulmonary 4D CT images with an average size of 256 × 256 × 100 and more than 1100 expert-determined landmark point pairs each. For all the testing scenarios presented in this paper, the GPU-based DIR computation required around 7 to 11 s to yield an average 3D error ranging from 1.5 to 1.8 mm. It is interesting to find out that the original passive force demons algorithms outperform subsequently proposed variants based on the combination of accuracy, efficiency and ease of implementation.

A radiobiological analysis of the effect of 3D versus 4D image-based planning in lung cancer radiotherapy

Dose distributions generated on a static anatomy may differ significantly from those delivered to temporally varying anatomy such as for abdominal and thoracic tumors, due largely in part to the unavoidable organ motion and deformation effects stemming from respiration. In this work, the degree of such variation for three treatment techniques, namely static conventional, gating and target tracking radiotherapy, was investigated. The actual delivered dose was approximated by planning all the phases of a 4DCT image set. Data from six (n = 6) previously treated lung cancer patients were used for this study with tumor motion ranging from 2 to 10 mm. Complete radiobiological analyses were performed to assess the clinical significance of the observed discrepancies between the 3D and 4DCT image-based dose distributions. Using the complication-free tumor control probability (P+) objective, we observed small differences in P+ between the 3D and 4DCT image-based plans (<2.0% difference on average) for the gating and static conventional regimens and higher differences in P+ (4.0% on average) for the tracking regimen. Furthermore, we observed, as a general trend, that the 3D plan underestimated the P+ values. While it is not possible to draw any general conclusions from a small patient cohort, our results suggest that there exists a patient population in which 4D planning does not provide any additional benefits beyond that afforded by 3D planning for static conventional or gated radiotherapy. This statement is consistent with previous studies based on physical dosimetric evaluations only. The higher differences observed with the tracking technique suggest that individual patient plans should be evaluated on a case-by-case basis to assess if 3D or 4D imaging is appropriate for the tracking technique.

ANALYSIS OF 4D TUMOR TRACKING RADIOTHERAPY FOR MOBILE TARGETS USING RADIOBIOLOGICAL ASSESSMENT

Purpose: To show that radiobiological analysis offers a better clinical assessment of treatment outcome compared to physical dosimetric analysis and to apply this radiobiological analysis to characterize possible benefits of tumor tracking over conventional radiotherapy. Materials: 4DCT data from six (n=6) patients with tumor motion in the range of 2-10mmwere used. Ten image sets corresponding to various phases of the respiratory cycle were derived from each 4DCT image set. A single clinician contoured the target and organs at risk on all ten phase-dependent image sets. Two plans were developed per patient based on tracking and conventional radiotherapy. In the latter, larger margins were used to account for organ motion. First, a reference image set was chosen and a plan developed based on a composite target. The plan parameters were then exported to the remaining phases and 9 other plans were created. By applying a validated deformable image registration algorithm, a deformation field was derived between each phase and the reference image which was then used to deform the dose from each of the phases to the reference phase where a weighted sum of the dose was computed to constitute the 4D dose from which a 4D DVH was developed.Tracking was characterized by reduced margins as ten different plans were developed based on target location per image set. The 4D dose and DVH were derived from all plans in a similar manner.Radiobiological assessment was based on the linear quadratic-Poisson and relative seriality models. The radiobiological parameters for each tissue were the D50, which is the dose that cause response to 50% of the patients, γ, which is the steepness of the dose-response curve, and s, which is the relative seriality. These parameters are specific for each organ and injury type and have been calculated from clinical data. The radiobiological effectiveness of each plan was quantified by the complication free tumor control probability P+ = PB PI where PB and PI are the total target response (benefit) and normal tissue complication (injury) probabilities, respectively. Four different fractionation schemes were considered; 10Gy, 6Gy, 3Gy and 2Gy per fraction for 6, 10, 20 and 30 fractions, respectively. Results: The advantages of tracking over conventional delivery were best observed using radiobiological analysis (fig 1). Higher P+ values for tracking, which decreased towards hyperfractionated schedules, were observed across all patients. For patients with tumor motion extent greater than 5mm, the relative increase in P+ were 25%±10%; 15%±7%; 8%±4%; 6%±3%, respectively for the plans with 6, 10, 20 and 30 fractions. The corresponding values for two patients with tumor motion extent less than 5mm were 10%±5%; 7%±3%; 3%±1%; 2%±2%. Conclusions: Radiobiological analysis offers a more sensitive assessment of clinical outcome and show that tracking is advantageous over conventional method with significant improvements for hypofractionated schedules. 653 poster (Physics Track)

BEUD/SECONDARY MALIGNANCY ANALYSIS: COMPARISON OF HT, MLC-BASED IMRT, AND CRT IN PROSTATE TREATMENT PLANNING

Purpose: To show that radiobiological analysis offers a better clinical assessment of treatment outcome compared to physical dosimetric analysis and to apply this radiobiological analysis to characterize possible benefits of tumor tracking over conventional radiotherapy. Materials: 4DCT data from six (n=6) patients with tumor motion in the range of 2-10mmwere used. Ten image sets corresponding to various phases of the respiratory cycle were derived from each 4DCT image set. A single clinician contoured the target and organs at risk on all ten phase-dependent image sets. Two plans were developed per patient based on tracking and conventional radiotherapy. In the latter, larger margins were used to account for organ motion. First, a reference image set was chosen and a plan developed based on a composite target. The plan parameters were then exported to the remaining phases and 9 other plans were created. By applying a validated deformable image registration algorithm, a deformation field was derived between each phase and the reference image which was then used to deform the dose from each of the phases to the reference phase where a weighted sum of the dose was computed to constitute the 4D dose from which a 4D DVH was developed.Tracking was characterized by reduced margins as ten different plans were developed based on target location per image set. The 4D dose and DVH were derived from all plans in a similar manner.Radiobiological assessment was based on the linear quadratic-Poisson and relative seriality models. The radiobiological parameters for each tissue were the D50, which is the dose that cause response to 50% of the patients, γ, which is the steepness of the dose-response curve, and s, which is the relative seriality. These parameters are specific for each organ and injury type and have been calculated from clinical data. The radiobiological effectiveness of each plan was quantified by the complication free tumor control probability P+ = PB PI where PB and PI are the total target response (benefit) and normal tissue complication (injury) probabilities, respectively. Four different fractionation schemes were considered; 10Gy, 6Gy, 3Gy and 2Gy per fraction for 6, 10, 20 and 30 fractions, respectively. Results: The advantages of tracking over conventional delivery were best observed using radiobiological analysis (fig 1). Higher P+ values for tracking, which decreased towards hyperfractionated schedules, were observed across all patients. For patients with tumor motion extent greater than 5mm, the relative increase in P+ were 25%±10%; 15%±7%; 8%±4%; 6%±3%, respectively for the plans with 6, 10, 20 and 30 fractions. The corresponding values for two patients with tumor motion extent less than 5mm were 10%±5%; 7%±3%; 3%±1%; 2%±2%. Conclusions: Radiobiological analysis offers a more sensitive assessment of clinical outcome and show that tracking is advantageous over conventional method with significant improvements for hypofractionated schedules. 653 poster (Physics Track)

How many sets of 4DCT images are sufficient to determine internal target volume for liver radiotherapy?

Background and purpose To determine the feasibility of using limited four-dimensional computed tomography (4DCT) images for treatment planning. Materials and methods The 4DCT scans of 16 patients with hepatocellular carcinoma (HCC) were analyzed. Gross tumor volumes (GTVs) were manually contoured on all 10 respiratory phases, and different internal clinical target volumes (ICTVs) were derived by encompassing volumes of the respective CTVs. Volume, position, and shape of ICTVs were calculated and compared. Results The ICTV 2 phases, ICTV 3 phases, ICTV 4 phases, and ICTV 6 phases all showed excellent agreement with ICTV 10 phases, and the ICTV 2 phases encompassed ICTV 10 phases by 94.1 ± 1.8% on average. The 3D shift between the centers of mass of the ICTVs was only 0.6 mm. The surface distance between ICTV 10 phases and ICTV 2 phases was 1.7 ± 0.8 mm in the left-right (LR) and anteroposterior (AP) directions. Conclusions Contouring two extreme phases at end-inhalation and end-exhalation is a reasonably safe and labor-saving method of deriving ITV for liver radiotherapy with low and medium tumor motion amplitude (⩽1.6 cm). Whether the larger tumor movement affects the results is the subject of ongoing research.

WE‐C‐BRC‐08: A Method to Evaluate Region‐Specific Pulmonary Function Using 4D CT Images for Lung Cancer Patients Undergoing Radiation Therapy

Purpose: Collateral radiation exposure to healthy lung tissue during radiation therapy can result in changes in structural and biomechanical properties of the lung. These changes may cause various clinical symptoms. The purpose of this study was to develop a functional imaging technique to assess the lung's region‐specific ventilation and pressure during or after radiation treatment.Method and Materials: With an in‐house developed finite element framework, a heterogeneous elastic model was developed for a lung patient and its Young's moduli were derived from a set of 4D CT images, acquired during radiation treatment. Each phase of the 4D dataset was registered, using deformable image registration (ITK demons algorithm) with the end‐inhale reference dataset. The resultant deformation matrix was used first to calculate the volumetric variation of each image voxel to generate a 3D ventilation image, and then to compute its corresponding transpulmonary pressure with the mechanical model. Results: Lung volumes on each phase of the acquired 4D dataset were compared with those derived from the deformed model, and were found to be within 1% of each other. The maximum ventilation occurs from phase 1 to phase 2, the earliest expiration phase. The average ventilation increased from 20.2% in phase 2 to 30.8% in phase 5 and their correspondent pressures increased from 1.57 Kpa to 2.25 Kpa. This result is generally consistent with published measurements. Conclusion: This study describes a theoretical approach to calculate the region‐specific ventilation and mechanical functions using deformable image registration. The method may be applied toward understanding how the mechanical properties of damaged lung differ from that of healthy lung tissue, and therefore it has potential applicability as a diagnostic indicator, as well as a tool for predicting radiation‐induced lung damages. Work is underway to correlate this approach with other traditional functional‐imaging modalities used to assess lung function.

SU‐FF‐J‐53: Experience and Comparison of Two Image Guidance Methods for SBRT Treatment: 4D CT and ExacTrac Respiratory‐Triggered Imaging

Purpose: For Stereotactic Body Radiation Therapy (SBRT) treatment of lung and liver, we quantified the differences between two image guidance methods: 4D CT and ExacTrac respiratory triggered images. Method and Materials: 5 liver lesions and 1 lung lesion for a total of 21 fractions were studied. For the 4D CT method, a manual registration method was used between the 4D CT image sets from initial simulation and treatment day. 4D CT derived image guidance shifts were ascertained from comparison to the same breathing phase at simulation. We also used ExacTrac respiratory triggered imaging to verify the target positioning and calculated the differences in image guidance shifts for the two methods. Results: The mean of the observed differences in image guided shifts between 4D CT image guidance and ExacTrac respiratory triggered image guidance was x = 1.6 mm, y = 1.9 mm, z = 2.8 mm, with no difference larger than 5.0 mm noted in any given direction for any individual case. As expected, the largest error occurred in S/I direction, with a mean of 2.8 mm for the six patients. This is expected because respiratory motion and the resulting uncertainties are most pronounced in this S/I direction. The ExacTrac images triggered at two extreme breathing phase, e.g., full exhale vs. inhale, agreed well (&lt; 2.0mm) with each other. Conclusion: The two very promising image guidance methods of 4D CT and ExacTrac respiratory triggered imaging have been presented and compared for SBRT of lung and liver. The mean differences in image guided shifts between the two methods were less than 3 mm, with no individual direction difference greater than 5 mm for the six SBRT patients, 21 delivered fractions evaluated in this study. Potential contributing factors to the disagreement were presented and will be studied further in future work.

SU-FF-J-88: A Novel Workflow for SBRT Image Guidance Using 4D CT Method

Purpose: We presented a novel clinical workflow to determine interfractional image guidance shifts for SBRT treatment of lung using 4D CT scans, and validated this workflow in phantom studies. Method and Materials: An ITV was first generated on the planning 4D CT image set and later used for treatment planning. The ITV and the 4D CT image sets on both the planning and treatment days were loaded into GE AW software. A manual registration was then performed. Using the measurement tool on AW, image guidance shifts were measured based on the same phase of the two CT image sets. Phantom studies were performed to simulate the entire SBRT process. The target was programmed to be shift for three scenarios: a) L/R and A/P shift; b) Longitudinal shift; and c) combination of L/R, A/P and longitudinal shift. All these phantom studies were repeated for two breathing traces: a sinusoidal and a real patient breathing trace. Results: The phantom studies showed that the differences were within a CT voxel size for all three shifting scenarios between the programmed and the measured shifts using the 4D CT method, for both the sinusoidal and patient breathing traces. We also performed two other validation studies retrospectively for one patient. An average was calculated for the tumor centroid position on 10 phases using a segmentation program. The difference between the simulation day and the treatment day gave the image guidance shifts. The second method used the average intensity projection image on the AW software. The image guidance shifts determined by these two retrospective methods agreed within 2mm with our clinically-used method. Conclusion: The workflow to determine image guidance shifts for SBRT treatment was validated by phantom studies, and it was further verified retrospectively by two independent methods for one patient with a lung tumor.

SU-FF-T-558: Respiratory Motion and Delivered Dose Accuracy for Gated Intensity-Modulated SBRT for Lung Cancer

Purpose: Accuracy of delivered dose can be affected by the interplay between DMLC delivery and target motion in gated IMRTtreatment. For gated intensity‐modulated lungSBRT, this concern is even larger due to smaller target margins, fewer treatment fractions and higher dose rate (1000MU/minute). The purpose of this work is to investigate the dosimetric accuracy in the latter situation. Method and Materials: Data from 5 lung patients treated with gated SBRT were retrospectively reviewed. For each patient the gating window was individually determined to keep residual motion <5 mm through analyzing the target motion based on 4DCT. Treatment plans were generated with 50% phase CT. PTVs were generated from CTVs with a 5mm margin. Based on the respiratory cycle, MU and dose rate, the DMLC leaf sequence for each beam was segmented into multiple sections, which corresponded to different respiratory phases within the gating window. The segmented DMLC leaf sections were then used to calculate dose distributions with corresponding 4DCT image set. The 4D composite dose distribution was obtained by accumulating the contributions from all phases within gating window using a deformable registration. The 4D dose distributions and DVHs for targets and critical organs were compared with the planned. The influences of the breathing period and dose rate were investigated. Results: The results indicated, although PTV dose coverage can be significantly influenced by the interplay of the target motion and DMLC delivery, the difference in CTV dose coverage between the 4D composite and the planned results is not significant, typically less than 3%. Conclusions: Interplay between target motion and DMLC‐delivery has a clinically acceptable impact in delivered dose accuracy even for the high dose rate gated SBRTtreatment. Other factors, such as target position shift due to the irregularity of patient's breathing, may be the main reasons affecting dose accuracy.

TU-D-304A-09: Evaluation of Localization Accuracy of Motion in 4DCT

Purpose: To develop a method and corresponding analysis tool to accurately evaluate the performance of 4DCT in localizing moving objects and to verify the correlation between motion characterization and the respiratory curve. Methods: Several radio opaque markers were affixed to the surface of a Varian Real‐time Positioning Management (RPM) infrared reflective block which was mounted on a moving platform. One of the markers was located right between the two infrared reflective markers of the RPM block. The CTimages of the moving system were acquired with a GE Discovery STEscanner in cine mode. GE Advantage4D was applied to create phased images by dividing the motion cycle into 10 bins. A software tool was developed to process the phased image sets and an algorithm was developed to find the number of markers and to analyze the most probable location of each marker. The motion amplitude was compared with the respiratory curve as recorded by the RPM system during imaging.Results: The multiple markers were identified from the 4DCT image sets simultaneously by the model algorithm. Their phase motion matches the recording of the RPM infrared sensor in both pattern and magnitude. The positioning error of the procedure is <0.5 mm along lateral directions, and 2.5mm along sup‐inf direction, which is within the scope of the image acquisition setup. Conclusion: Tracking multiple point‐like fiducials provides an accurate and efficient means of evaluating the localization performance in 4DCT images. Phantom studies have validated the 4DCT performance in imaging moving object. The method and tool developed in this work can be useful for radiation therapy target margin study and imaging QA alike.

SU-FF-J-111: On the Estimation of Error in 4D Imaging in Regard to Radiotherapy Tumor Motion Management

Purpose: To quantify the accuracy of 4D‐CT as it relates to radiotherapy motion management techniques, specifically determination of tumor motion trajectory (x,y,z)(t), tumor motion envelope, and delineation of the GTV throughout 4D datasets. Methods: An ideal motion tracking configuration was created with a Standard Imaging Respiratory Gating Platform and four glass marbles of various diameter suspended in polyurethane foam. 4D‐CT images were acquired for motion periods and displacements ranging from 3 – 6 s and 1 – 3 cm, respectively. Phase binned 4D images were acquired with a 4‐slice GE LightSpeed CT scanner and RPM tracking system. Scans were performed in cine mode with a 2.5 mm slice thickness and a 0.4 s gantry rotation. Cine mode duration for each scanning position was performed for at least one gantry rotation greater than the phantom motion period. Marbles were autocontoured at −500, 0, 500, 1000, and 1500 HU in all ten phases of the nine total 4DCT image sets. Centroid positions were determined for each auto‐contour threshold. Actual motion trajectories were recorded to compare to the observed. Results: Maximum errors in tumor position throughout a given motion trajectory ranged from 1 – 4 mm. The observed errors showed an inverse relationship with the auto‐contour HU threshold used and a direct relationship with the phantom velocity. Motion envelope underestimations ranged from 0 – 7 mm with the highest CT threshold auto‐contour resulting in the largest error. Conformality indices were also determined for each contoured marble throughout all 4D image sets. Conclusion: Considerable errors in the definition of tumor trajectory and the motion envelope were observed in an idealized motion phantom using clinical imaging acquisition parameters and realistic tumor motion settings. Methods defined are useful in quantifying errors associated with 4D imaging modalities as they relate to radiotherapy motion management. Research supported by Philips Medical Systems.

A simplified method of four‐dimensional dose accumulation using the mean patient density representation

The purpose of this work was to demonstrate, both in phantom and patient, the feasibility of using an average 4DCT image set (AVG-CT) for 4D cumulative dose estimation. A series of 4DCT numerical phantoms and corresponding AVG-CTs were generated. For full 4D dose summation, static dose was calculated on each phase and cumulative dose was determined by combining each phase's static dose distribution with known tumor displacement. The AVG-CT cumulative dose was calculated similarly, although the same AVG-CT static dose distribution was used for all phases (i.e., tumor displacements). Four lung cancer cases were also evaluated for stereotactic body radiotherapy and conformal treatments; however, deformable image registration of the 4DCTs was used to generate the displacement vector fields (DVFs) describing patient-specific motion. Dose discrepancy between full 4D summation and AVG-CT approach was calculated and compared. For all phantoms, AVG-CT approximation yielded slightly higher cumulative doses compared to full 4D summation, with dose discrepancy increasing with increased tumor excursion. In vivo, using the AVG-CT coupled with deformable registration yielded clinically insignificant differences for all GTV parameters including the minimum, mean, maximum, dose to 99% of target, and dose to 1% of target. Furthermore, analysis of the spinal cord, esophagus, and heart revealed negligible differences in major dosimetric indices and dose coverage between the two dose calculation techniques. Simplifying 4D dose accumulation via the AVG-CT, while fully accounting for tumor deformation due to respiratory motion, has been validated, thereby, introducing the potential to streamline the use of 4D dose calculations in clinical practice, particularly for adaptive planning purposes.