T Li

SU‐FF‐J‐02: A Comparison of Amplitude‐ and Phase‐Based 4D CT

Purpose: Four‐dimensional (4D) CT depends on accurate correlation between temporally acquired CT slices and the patients respiratory cycle. One approach is to record the position of an external marker placed on the abdomen or chest during the scan, and retrospectively match the CT data with the phase of the marker motion. While very effective for regular breathing patterns, the phase‐based approach can lead to significant mismatch between adjacent image segments when the respiratory motion exhibits irregularities. We propose a method of extracting amplitude‐based 4D CT from cine‐acquired CTdata sets, and compare the amplitude‐based 4D CT with the phase‐based 4D CT for both phantom and patient data. Method and Materials:CTdata sets were acquired in cine mode on the GE Discovery ST, and motion of an infrared reflecting block was recorded using Varians Real‐time Position Management (RPM) camera. Rather than use the phase‐based calculations of the RPM system, we replaced the phase field with pseudo‐amplitude values spanning the full respiratory cycle (i.e., differentiating inspiration from expiration). The modified respiratory trace file was then sent, along with the cine CT data, to the GE Advantage Workstation for processing. The method was applied to a thoracic phantom moving irregularly in the longitudinal direction, and to an abdominal 4D scan of a lungcancer patient. Results: For both phantom and patient data, the phase‐based 4D CTimages showed boundary mismatches of up to 1 cm between couch positions. The mismatch on the amplitude‐based sets, however, was less than 2 mm throughout the field of view. Conclusion: Phase‐based 4D CT can lead to mismatched slices when the respiratory cycle involves irregularities. In such situations, by replacing phase with a modified definition of amplitude that distinguishes inspiration from expiration, a substantially improved 4D CTimage can be generated.

SU‐E‐T‐407: Dosimetric Influence of Setup Errors On RapidArc‐Based SRS for Simultaneous Irradiation of Multiple Intracranial Targets

PURPOSE To evaluate the dosimetric influence of setup errors on RapidArc-based SRS for simultaneous irradiation of multiple intracranial targets. METHODS Eight patients previously treated with RapidArc™ technique for multiple intracranial lesions were included in this study. A RapidArc plan was designed to irradiate multiple targets simultaneously with one isocenter and 4 non-coplanar arcs. 1 mm margin was added to generate PTVs from GTVs. BrainLAB Novalis system was used to position the targets with 6D corrections and monitor patient position during treatment. CBCT was acquired for verification before irradiation. Velocity AI was used for image registration and dose mapping for CBCT, planning CT, contours and dose matrix. The DVHs of targets and critical structures and dose distributions were compared with the planned results and dosimetric influence from setup errors was analyzed. RESULTS We found that the translational errors were less than 1 mm in the three directions and rotational errors were less than 0.6 degree for all the patients. The overall PTV coverage decreases of ∼10 percent on average while the overall GTV coverage slightly influenced. Although prescribed dosed still covered most of GTVs, there were still some targets with a GTV coverage drop greater than 5 percent even with a 1 mm margin. Most of influenced targets were small targets and relatively far from isocenter. Our study also showed that the dosimetric influence on critical structures was negligible. CONCLUSIONS Although RapidArc technique can generate good plans for effectively treating multiple intracranial lesions simultaneously using stereotactic radiosurgery, this technique is more susceptible to the setup errors, especially rotational errors. 0.5 degree rotational error may Result in non-ignorable drop in target coverage. Our study show that a 1 mm margin for PTV and 6D positioning are necessary for a successful treatment with this technique.

SU‐E‐T‐556: Interplay Effect Between Dynamic MLC and Moving Target for Lung SBRT with IMAT Technique Delivered by Flattening Filter Free Beam of True Beam Machine

Purpose: To investigate the dosimetric impact of the interplay effect between dynamic MLC and moving target on lungSBRT with intensity‐ modulated arc therapy (IMAT) technique delivered by flattening filter free (FFF) beam of True‐Beam machine. Methods: 6 lungcancer patients with 0.5–1.0 cm tumor motions were investigated in this study. All patients underwent 4DCT scan using audio coaching. For each patient, a 2‐arc IMAT plan was retrospectively generated using Varian RapidArc planning system. Based on the total MU, dose rate and respiratory cycle, the planned MLC control points of IMAT plans were assigned into different respiratory phases. Afterward, 10 new IMAT plans related to different respiratory phases were generated and imported back into Eclipse planning system to calculate the radiationdose on the CTimages of different respiratory phases. In‐house 4D dose calculation program with deformable registration capacity was used to calculate the cumulative doses from all respiratory phases. Following parameters were used to evaluate the dosimetric impacts: dose error (DE), i.e., the difference between the 3D dose of original IMAT plan and the calculated 4‐D dose, and the percentage of volume receiving 100% of the prescribed dose (V100).Results: For all patients, the average maximum DE of PTV and CTV are 32.6% and 4.8%. The DE is larger than 5% for 22% of PTV and 0.3% of CTV volume, and the DE is less than 3% for over 88% of the CTV volume. Compared to the original IMAT plans, the V100 of PTV are lower by 15%. However, the changes on V100 of CTV are less than 1%. Conclusions: For lung IMAT treatment delivered by FFF beam of True‐Beam machine, the interplay effect between dynamic MLC and moving target could change the absolute doses within the target, but its impact on CTV dose coverage is insignificant.

WE‐AB‐204‐09: Respiratory Motion Correction in 4D‐PET by Simultaneous Motion Estimation and Image Reconstruction (SMEIR)

Purpose: In conventional 4D-PET, images from different frames are reconstructed individually and aligned by registration methods. Two issues with these approaches are: 1) Reconstruction algorithms do not make full use of all projections statistics; and 2) Image registration between noisy images can Result in poor alignment. In this study we investigated the use of simultaneous motion estimation and image reconstruction (SMEIR) method for cone beam CT for motion estimation/correction in 4D-PET. Methods: Modified ordered-subset expectation maximization algorithm coupled with total variation minimization (OSEM- TV) is used to obtain a primary motion-compensated PET (pmc-PET) from all projection data using Demons derived deformation vector fields (DVFs) as initial. Motion model update is done to obtain an optimal set of DVFs between the pmc-PET and other phases by matching the forward projection of the deformed pmc-PET and measured projections of other phases. Using updated DVFs, OSEM- TV image reconstruction is repeated and new DVFs are estimated based on updated images. 4D XCAT phantom with typical FDG biodistribution and a 10mm diameter tumor was used to evaluate the performance of the SMEIR algorithm. Results: Image quality of 4D-PET is greatly improved by the SMEIR algorithm. When all projections are used to reconstruct a 3D-PET, motion blurring artifacts are present, leading to a more than 5 times overestimation of the tumor size and 54% tumor to lung contrast ratio underestimation. This error reduced to 37% and 20% for post reconstruction registration methods and SMEIR respectively. Conclusion: SMEIR method can be used for motion estimation/correction in 4D-PET. The statistics is greatly improved since all projection data are combined together to update the image. The performance of the SMEIR algorithm for 4D-PET is sensitive to smoothness control parameters in the DVF estimation step.

SU-E-T-40: A Method for Improving Dose Gradient for Robotic Radiosurgery

Purpose: For targets with substantial volume, collimators of relatively large sizes are usually selected to minimize the treatment time in robotic radiosurgery. Their large penumbrae may adversely affect the dose gradient around the target. In this study, we implement and evaluate an inner-shell planning method to increase the dose gradient. Methods: Ten patients previously treated with CyberKnife M6 system were randomly selected with the only criteria that PTV be larger than 2cm3. A new plan was generated for each patient, in which the PTV was split into two regions: an inner shell and a core, where the core was created by shrinking the PTV by 5mm using 3D erosion, and the shell was obtained by subtracting the core from the PTV; then a 7.5mm Iris collimator was exclusively applied to the shell, with other appropriate collimators applied to the core depending on its size. The optimization objective functions and constraints were kept the same as the corresponding clinical plans. The results were analyzed for V12Gy, V9Gy, V5Gy, and gradient index (GI). Results: Volume reduction is found for the inner-shell method at all studied dose levels as compared to the clinical plans. The absolute dose volume reduction ranged from 0.05cm3 to 18.5cm3 with a mean of 5.6cm3 for 12Gy, from 0.2cm3 to 38.1cm3 with a mean of 9.8cm3 for 9Gy, and from 1.5cm3 to 115.7cm3 with a mean of 24.8cm3 for 5Gy, respectively. The relative GI reduction ranged from 3.2% to 23.6%, with a mean of 12.6%. Paired t-test for GI has a p-value of 0.0014. The range for treatment time increase is from −3 min to 20 min, with a mean of 7.0 min. Conclusion: The inner-shell planning method can significantly increase the dose gradient outside the PTV, while maintaining good coverage, conformity, and reasonable treatment time.

SU-E-T-225: Correction Matrix for PinPoint Ionization Chamber for Dosimetric Measurements in the Newly Released Incise™ Multileaf Collimator Shaped Small Field for CyberKnife M6™ Machine

Purpose: For small field dosimetry, such as measurements of output factors for cones or MLC-shaped irregular small fields, ion chambers often Result in an underestimation of the dose, due to both the volume averaging effect and the lack of lateral charged particle equilibrium. This work presents a mathematical model for correction matrix for a PTW PinPoint ionization chamber for dosimetric measurements made in the newly released Incise™ Multileaf collimator fields of the CyberKnife M6™ machine. Methods: A correction matrix for a PTW 0.015cc PinPoint ionization chamber was developed by modeling its 3D dose response in twelve cone-shaped circular fields created using the 5mm, 7.5mm, 10mm, 12.5mm, 15mm, 20mm, 25mm, 30mm, 35mm, 40mm, 50mm, 60mm cones in a CyberKnife M6™ machine. For each field size, hundreds of readings were recorded for every 2mm chamber shift in the horizontal plane. The contribution of each dose pixel to a measurement point depended on the radial distance and the angle to the chamber axis. These readings were then compared with the theoretical dose as obtained with Monte Carlo calculation. A penalized least-square optimization algorithm was developed to generate the correction matrix. After the parameter fitting, the mathematical model was validated for MLC-shaped irregular fields. Results: The optimization algorithm used for parameter fitting was stable and the resulted response factors were smooth in spatial domain. After correction with the mathematical model, the chamber reading matched with the calculation for all the tested fields to within 2%. Conclusion: A novel mathematical model has been developed for PinPoint chamber for dosimetric measurements in small MLC-shaped irregular fields. The correction matrix is dependent on detector, treatment unit and the geometry of setup. The model can be applied to non-standard composite fields and provides an access to IMRT point dose validation.

SU-E-T-420: Impact of Different Prescription Isodose Lines On Plan Quality for Brain Metastases Using Multiplan System

PURPOSE With the sequential optimization algorithm in MultiPlan system, clinical objectives (homogeneity, PTV coverage, conformity, normal tissue protection) can be optimized in sequence. However, the prescription isodose line (RxIDL) varies widely among institutions, which can influence the optimized dose distribution. The aim of this study is to investigate the impact of different prescription isodose lines on plan quality for the treatment of brain metastases using CyberKnife Multiplan system. METHODS Ten patients with multiple metastases were selected for this study. Four plans were generated for each patient such that 100% of the target volume receives the prescribed dose of 18 Gy, which was 50%, 60%, 70%, and 80% prescription Isodose line, separately. The prescription isodose was calculated as the ratio of the prescription dose and the maximum dose in target volume. The dosimetric parameters, including PTV coverage, conformity index (CI), gradient index (GI) and the volume covered by 12 Gy (V12Gy) were analyzed. The plan Monitor Units (MU) and treatment time were also compared. RESULTS All plans can provide the same target coverage (100%) and similar conformity index (1.26, 1.30, 1.32, and 1.29 on average for 80%, 70%, 60%, and 50% RxIDL plans, separately); there was no difference in critical structure dose. The 50% RxIDL plans have much lower GI (4.21±1.79 for 50% and 5.56±2.92 for 80% RxIDL plans) and V12Gy (13.36±10.31cc for 50% and 15.87±11.85cc for 80% RxIDL plans). The variation in estimated treatment delivery time was insignificant. CONCLUSION The dose falloff is much faster for the lower RxIDL plans in terms of GI and V12Gy. For 50% RxIDL plans, the average V12Gy decreases by 16% compared to 80% RxIDL plans, which indicates that the normal tissue can be better protected by decreasing the prescription Isodose line.

SU-E-J-185: Gated CBCT Imaging for Positioning Moving Lung Tumor in Lung SBRT Treatment.

PURPOSE Lung stereo-tactic body radiotherapy(SBRT) treatment requires high accuracy of lung tumor positioning during treatment, which is usually accomplished by free breathing Cone-Beam computerized tomography (CBCT) scan. However, respiratory motion induced image artifacts in free breathing CBCT may degrade such positioning accuracy. The purpose of this study is to investigate the feasibility of gated CBCT imaging for lung SBRT treatment. METHODS Six Lung SBRT patients were selected for this study. The respiratory motion of the tumors ranged from 1.2cm to 3.5cm, and the gating windows for all patients were set between 35% and 65% of the respiratory phases. Each Lung SBRT patient underwent free-breathing CBCT scan using half-fan scan technique. The acquired projection images were transferred out for off-line analyses. An In-house semi-automatic algorithm was developed to trace the diaphragm movement from those projection images to acquire a patients specific respiratory motion curve, which was used to correlate respiratory phases with each projection image. Afterwards, a filtered back-projection algorithm was utilized to reconstruct the gated CBCT images based on the projection images only within the gating window. RESULTS Target volumes determined by free breathing CBCT images were 71.9%±72% bigger than the volume shown in gated CBCT image. On the contrary, the target volume differences between gated CBCT and planning CT images at exhale stage were 5.8%±2.4%. The center to center distance of the targets shown in free breathing CBCT and gated CBCT images were 9.2±8.1mm. For one particular case, the superior boundary of the target was shifted 15mm between free breathing CBCT and gated CBCT. CONCLUSION Gated CBCT imaging provides better representation of the moving lung tumor with less motion artifacts, and has the potential to improve the positioning accuracy in lung SBRT treatment.

SU-E-T-17: A Mathematical Model for PinPoint Chamber Correction in Measuring Small Fields

PURPOSE For small field dosimetry, such as measuring the cone output factor for stereotactic radiosurgery, ion chambers often result in underestimation of the dose, due to both the volume averaging effect and the lack of electron equilibrium. The purpose of this work is to develop a mathematical model, specifically for the pinpoint chamber, to calculate the correction factors corresponding to different type of small fields, including single cone-based circular field and non-standard composite fields. METHODS A PTW 0.015cc PinPoint chamber was used in the study. Its response in a certain field was modeled as the total contribution of many small beamlets, each with different response factor depending on the relative strength, radial distance to the chamber axis, and the beam angle. To get these factors, 12 cone-shaped circular fields (5mm,7.5mm, 10mm, 12.5mm, 15mm, 20mm, 25mm, 30mm, 35mm, 40mm, 50mm, 60mm) were irradiated and measured with the PinPoint chamber. For each field size, hundreds of readings were recorded for every 2mm chamber shift in the horizontal plane. These readings were then compared with the theoretical doses as obtained with Monte Carlo calculation. A penalized-least-square optimization algorithm was developed to find out the beamlet response factors. After the parameter fitting, the established mathematical model was validated with the same MC code for other non-circular fields. RESULTS The optimization algorithm used for parameter fitting was stable and the resulted response factors were smooth in spatial domain. After correction with the mathematical model, the chamber reading matched with the Monte Carlo calculation for all the tested fields to within 2%. CONCLUSION A novel mathematical model has been developed for the PinPoint chamber for dosimetric measurement of small fields. The current model is applicable only when the beam axis is perpendicular to the chamber axis. It can be applied to non-standard composite fields. Further validation with other type of detectors is being conducted.

SU-E-J-124: Prediction of the Characteristics of Moving Lung Tumor in CBCT Imaging Using Virtual CBCT Image Simulated From 4-D CT Dataset

Purpose: Due to the slow scanning speed of on‐board CBCT imaging, the characteristics of moving lung tumor acquired by such system could be complicated. Recent phantom studies have shown that the maximum intensity projection (MIP) image, which was used for target delineation in treatment planning, might overestimate the target volume determined by CBCT imaging. The purpose of this study is to propose a new strategy to predict the characteristics of moving lung tumor in CBCT imaging. Methods: Six non‐small‐cell‐lung cancer (NSCLC) patients were retrospectively selected in this study. All patients underwent 4D CT scan using GE LightSpeed scanner under audio coaching. The respiratory motion of all patients ranged from 7.5mm to 12.5mm. For each patient, ten sets of phase‐sorted CT images and a MIP image including all respiratory phases were acquired. To simulate the respiratory motion effect in CBCT imaging, 4‐D projection images were generated using ray‐tracing algorithm based on the 3‐D CT images of the corresponding phase at each gantry angle. Filtered‐back‐projection algorithm was used to reconstruct this simulated virtual CBCT (SVCBCT) image. The targets were then contoured at the MIP image, SVCBCT image and the clinic CBCT image acquired at first treatment, respectively. Results: For the three cases volume differences between SVCBCT and MIP image were less than 5%. For the other three cases the target volume determined by SVCBCT image were 30%∼62% less than that of MIP image, and the distance of target center between those two images could be up to 6mm. In comparison, for all patients, the volume differences between SVCBCT and CBCT were all less than 4%. Conclusion: The characteristics of a moving lung tumor determined by CBCT imaging are highly dependent on the specific respiratory motion pattern of each patient, and could be well predicted by SVCBCT image.