H Liu

SU-F-J-42: Comparison of Varian TrueBeam Cone-Beam CT and BrainLab ExacTrac X-Ray for Cranial Radiotherapy

PURPOSEnTo compare online image registrations of TrueBeam cone-beam CT (CBCT) and BrainLab ExacTrac x-ray imaging systems for cranial radiotherapy.nnnMETHODnPhantom and patient studies were performed on a Varian TrueBeam STx linear accelerator (Version 2.5), which is integrated with a BrainLab ExacTrac imaging system (Version 6.1.1). The phantom study was based on a Rando head phantom, which was designed to evaluate isocenter-location dependence of the image registrations. Ten isocenters were selected at various locations in the phantom, which represented clinical treatment sites. CBCT and ExacTrac x-ray images were taken when the phantom was located at each isocenter. The patient study included thirteen patients. CBCT and ExacTrac x-ray images were taken at each patients treatment position. Six-dimensional image registrations were performed on CBCT and ExacTrac, and residual errors calculated from CBCT and ExacTrac were compared.nnnRESULTSnIn the phantom study, the average residual-error differences between CBCT and ExacTrac image registrations were: 0.16±0.10 mm, 0.35±0.20 mm, and 0.21±0.15 mm, in the vertical, longitudinal, and lateral directions, respectively. The average residual-error differences in the rotation, roll, and pitch were: 0.36±0.11 degree, 0.14±0.10 degree, and 0.12±0.10 degree, respectively. In the patient study, the average residual-error differences in the vertical, longitudinal, and lateral directions were: 0.13±0.13 mm, 0.37±0.21 mm, 0.22±0.17 mm, respectively. The average residual-error differences in the rotation, roll, and pitch were: 0.30±0.10 degree, 0.18±0.11 degree, and 0.22±0.13 degree, respectively. Larger residual-error differences (up to 0.79 mm) were observed in the longitudinal direction in the phantom and patient studies where isocenters were located in or close to frontal lobes, i.e., located superficially.nnnCONCLUSIONnOverall, the average residual-error differences were within 0.4 mm in the translational directions and were within 0.4 degree in the rotational directions.

SU-E-J-47: Comparison of Online Image Registrations of Varian TrueBeam Cone-Beam CT and BrainLab ExacTrac Imaging Systems

Purpose To compare online image registrations of TrueBeam cone-beam CT (CBCT) and BrainLab ExacTrac imaging systems. Methods Tests were performed on a Varian TrueBeam STx linear accelerator (Version 2.0), which is integrated with a BrainLab ExacTrac imaging system (Version 6.0.5). The study was focused on comparing the online image registrations for translational shifts. A Rando head phantom was placed on treatment couch and immobilized with a BrainLab mask. The phantom was shifted by moving the couch translationally for 8 mm with a step size of 1 mm, in vertical, longitudinal, and lateral directions, respectively. At each location, the phantom was imaged with CBCT and ExacTrac x-ray. CBCT images were registered with TrueBeam and ExacTrac online registration algorithms, respectively. And ExacTrac x-ray image registrations were performed. Shifts calculated from different registrations were compared with nominal couch shifts. Results The averages and ranges of absolute differences between couch shifts and calculated phantom shifts obtained from ExacTrac x-ray registration, ExacTrac CBCT registration with default window, ExaxTrac CBCT registration with adjusted window (bone), Truebeam CBCT registration with bone window, and Truebeam CBCT registration with soft tissue window, were: 0.07 (0.02–0.14), 0.14 (0.01–0.35), 0.12 (0.02–0.28), 0.09 (0–0.20), and 0.06 (0–0.10) mm, in vertical direction; 0.06 (0.01–0.12), 0.27 (0.07–0.57), 0.23 (0.02–0.48), 0.04 (0–0.10), and 0.08 (0– 0.20) mm, in longitudinal direction; 0.05 (0.01–0.21), 0.35 (0.14–0.80), 0.25 (0.01–0.56), 0.19 (0–0.40), and 0.20 (0–0.40) mm, in lateral direction. Conclusion The shifts calculated from ExacTrac x-ray and TrueBeam CBCT registrations were close to each other (the differences between were less than 0.40 mm in any direction), and had better agreements with couch shifts than those from ExacTrac CBCT registrations. There were no significant differences between TrueBeam CBCT registrations using different windows. In ExacTrac CBCT registrations, using bone window led to better agreements than using default window.

SU-E-T-678: Normal Tissue Dose-Volume Constrains for Inverse Planning of Acoustic Neuroma Stereotactic Radiosurgery (SRS)

PURPOSEnStereotacic radiosurgery (SRS) of acoustic neuroma was traditionally planned using forward planning techniques, either with Elekta GammaKnife ball-packing technique, or with multiple conformal dynamic arcs technique using micro-MLC and Linac (BrainLAB Novalis). As the recent development of Volumetric Modulated Arc Therapy (VMAT), inverse planning using multiple-arcs VMAT technique for SRS becomes commercially available, and it has the advantage of faster delivery with comparable dosimetry. However, planners need proper dose volume constrains to drive the optimizer to achieve the dosimetric goals, both clinically acceptable and technically achievable. This study is investigating how to set up the proper dose volume constraints for SRS plans of acoustic neuroma.nnnMETHODSnTwenty cases were selected with their volumes cover the general range of acoustic neuroma SRS (0.1 cc to 5cc). Each case was planned using Gammaknife Perfexion with sector optimization. Prescription is 12Gy to 50% isodose, with 100% target coverage. The total tissue volumes that receive greater than 12, 8, and 6 Gy (V12, V8, and V6) were recorded. The relationship between the target volumes (Vtarget) and V12, V8, and V6 were plotted and linear regressions were fitted for each parameter.nnnRESULTSnA set of linear relationships between the target volume and the volumes that receive greater than 12, 8, and 6Gy was constructed as the following: V12 = 1.476Vtarget (R2 = 0.995), V8 = 3.04Vtarget +0.16(R2 = 0.990), and V6 = 4.64 Vtarget + 0.35 (R2 = 0.982).nnnCONCLUSIONSnTraditional plan quality indices such as conformity index, selectivity, etc. are good for evaluating forward plans, but cannot be used to drive the inverse planning engine to optimize inverse plans. The normal tissue dose volume constraints obtained in this study, combined with clinical constraints for brainstem and cochlea, can provide inverse planner with objectives to obtain an inverse plan for acoustic neuroma SRS.

Template-Based Inverse Planning Simulated Annealing for CT-Based High-Dose-Rate Brachytherapy of Cervical Cancer: Feasibility Study

Abstract: Purpose : To investigate the feasibility of using an inverse planning technique for CT-based ring and tandem high-dose rate brachytherapy of cervical cancer. Methods and Materials : Two patients previously treated with high-dose-rate brachytherapy for cervical cancer were retrospectively identified for this study. Each patient had five intracavitary insertions using CT/MR-compatible tandem and ring applicators. The 6Gy isodose lines from the original clinical plans were converted into a structure set (S6) using MIMvista. Inverse plans were then generated in Oncentra using the inverse planning simulated annealing (IPSA) with S6 as the optimization target. The dose to 0.1cm 3 , 1cm 3 , 5cm 3 of bladder (D B0.1 , D B1 , and D B5 ) and rectum (D R0.1 , D R1 , D R5 ) were determined from the dose volume histogram (DVH). Percentage of physician drawn clinical target volume (CTV) and S6 coverage (V 100CTV , V 100S6 ) were also recorded. Results : The mean V 100%CTV of the original clinical plans and the inverse plans were 88.14% and 87.57%. The mean V 100%S6 of the original clinical plans and the inverse plans was 98.68% and 97.00%. The mean dose reduction for D B0.1 , D B1 and D B5 were 5.4%, 5.4%, and 4.7%, respectively. The mean dose reduction for D R0.1 , D R1 and D R5 were 6.4%, 5.5%, and 4.8%. Conclusions : This work demonstrated the feasibility of this structure-based inverse planning. It can achieve comparable CTV coverage while reducing dose to critical structures. Once template structure set is constructed, this procedure can not only reduce planning time, but improve quality assurance by standardizing the procedure. This approach can be directly extended to other applicator-based brachytherapy procedures.

SU‐GG‐T‐313: A Procedure for Standardizing MLC Quality Assurance for Elekta Linacs

Purpose: As specified in TG142, MLC position accuracy needs to be tested on weekly/monthly basis, with 1mm tolerance. This study focuses on developing techniques, hardware and software tools for implementation of MLC QA tests for Elekta Linacs.Material and Methods: This process was tested with an Elekta Synergy S, Beam Modulator™, which has 40 leaf pairs of 4mm width (maximum 16cm×21cm field size). Based on the machine characteristics, two picket‐fence IMRT plans were designed: one has 5 2cm×16cm strips separated by 2cm gap; the other has the same setup with individual leafs intentionally displaced by ±1mm, ±1.2mm, etc. Both plans used 6MV x‐rays and 50MU on each strip. We overcame the limitation of Xio planning system in generating picket‐fence IMRT plan by modifying leaf positions from a DICOM RT plan file. In‐house software was executed to validate the files before imported into Record and Verify system (Mosaiq) for delivery. Radiographic images were acquired using Kodak XV films. The borders of a 16cm×21cm light field were first traced on the film. These reference lines helped reduce the orientation errors during image registration. Two sets of films were exposed with full buildup. After development, each film was digitized with 0.06mm resolution using a high‐resolution scanner. The images were then imported into Matlab. In‐house code was used to detect leafs exceeding the 1mm threshold. Results: The plans were delivered smoothly. Leaf positions in the first image were used as baselines, instead of using reference leaf positions from the same exposure. This reduced systematic errors. After image registration, leafs displacing from the baseline by 16 pixels (1mm) or more were detected. Conclusion: This efficient procedure provides a sufficiently accurate test for MLC positioning reproducibility. It is a simple and straightforward procedure that can be used for routine MLC position checks.

WE-C-BRC-07: Image Quality QA for Three Radiotherapy Cone-Beam CT Systems

Purpose: With the increased presence of volumetric radiotherapy cone beam CT (RT‐CBCT) systems, the importance of high grade image quality is crucial to achieve daily image‐guided adaptive radiotherapy (IGART) capabilities. We subjected three commercially available RT‐CBCT systems to a battery of standard diagnostic image quality tests acquired under clinical conditions. This study reports on the evaluation of image qualities for RT‐CBCTs and a diagnostic CT.Methods and Materials: RT‐CBCT scans were performed on Elekta XVI, Nucletron Simulix, and Varian OBI Advanced Imaging using clinical pelvis scan settings on a CATPHAN Model 600. They were then compared to results from a GE Lightspeed CT scanner. The phantom contained modules allowing measurement of low contrast resolution, slice thickness, Hounsfield Unit (HU) sensitivity, spatial resolution, and image uniformity. Results: No CBCT system was able to detect any targets in the low contrast module. Six targets could be seen on diagnostic CTs. All RT‐CBCT systems had greater than 33% variations in slice thickness reconstruction. The HU sensitometry test showed absolute differences between accepted HU values and measured values for XVI and Simulix systems to be on average 6 times and 3 times the error seen in a diagnostic CT respectively. HU sensitivity for OBI is within 15% of a diagnostic CT. Both OBI and Simulix had spatial resolution twice that of XVI but were 3 lp/cm worse than a diagnostic CT. Measurements of image uniformity showed that all three RT‐CBCT systems have a standard deviation of HU values on the order of 10 times that of a diagnostic CT.Conclusions: The image qualities of three evaluated RT‐CBCT systems are relatively comparable, yet still inferior to those from helical CT. It is important to note that dosimetric settings can have a clear impact on image quality, and dose measurements were not done for this work.

SU-FF-T-42: A Novel Approach to Scar Boost in Mesothelioma Treatment

Purpose: To evaluate the potential advantages of high dose rate(HDR) brachytherapy for boosting mesothelioma incisions extending beyond the hemithoracic target volume. HDR treatment was compared with abutted, enface electron fields. Methods and Materials: One case describes catheter placement, dosimetric advantages, setup uncertainties, and treatment delivery. The right lateral decubitus position, in alpha cradle, was selected to permit consideration of HDR versus electrons. Comparative analyses of treatment plans were performed, and HDR setup and dosimetry were preferred. The HDR treatment utilized 3 catheters atop 5mm of bolus. The HDR setup required physician to confirm catheter placement. Tape, 5mm bolus, and wet towels secured the catheters and provided shielding. The prescription was 3.0Gy to 1.2cm from catheter including bolus (7mm depth in tissue). Dwell positions every 5mm were used to cover the entire length of the scars (22cm and 34cm). The corresponding electron plan required 5 thin strip cutouts and a physician present to approve field setup and feathering of the junctions. A phantom was used to simulate treatment of lesion with electrons and entire process was timed. Results: The total setup and treatment time for 6 HDR treatments was approximately 270minutes (time varies with activity, prescription dose, and volume) compared with 450minutes for 15 electron fractions (2Gy/fraction). Planning time for either modality is comparable. Electron treatments involve additional time for cutting blocks, measuring cutout factors, and physician approved junction changes. One additional concern is dose homogeneity caused by feathering electrons versus brachytherapy which provides a reliable dose distribution. Conclusions: HDR brachytherapy provided an optimal solution over the complicated and time‐consuming electron treatment and should be considered as a viable option for mesothelioma patients with a considerable scar length beyond hemithoracic target volume. Clinical data to support the use of brachytherapy over electrons for coverage of incisional scars is still needed.

SU-FF-T-48: Dosimetric Feasibility of Using Low Energy Sources in Multi-Lumen Device for Partial Breast Brachytherapy

Purpose: To evaluate the dosimetric feasibility of using low energy sources (Cs‐131, I‐125, Pd‐103) in the multi‐lumen device for partial breast brachytherapy. The sources are placed into the device for multiple days until the desired dose is achieved, then withdrawn with the device. This outpatient procedure only requires two visits, one for source placement and another for device withdrawal. Method: A 10‐1 SAVI device was fully expanded, CT‐scanned and imported into VariSeed. Cavity, PTV (cavity + 1cm margin) and PTV_eval (PTV − cavity) were created. The same type and activity of seeds were added into the multi‐lumen device to cover the PTV_eval and seed activity was adjusted to achieve a nominal dose (50Gy in one half‐life of source). Different seed types and distributions were tested. Oncentra Brachy and IPSA optimization was used to generate a HDR plan for comparison. Dosimetric parameters (D90, V100, V150, and V200 of PTV_eval) and skindose were compared. Results: D90, V100, V150, and V200 of PTV_eval of HDR plan are 107%, 97%, 52cc, and 24cc. The most comparable LDR plan is from a non‐uniform distributed 90 3.2U Cs‐131 source, which gives 102%, 92%, 52cc, and 26cc. Skindose is lower than the HDR plan by ∼10% of the prescription dose from 0.5 ∼ 5 cm away from the PTV edge. With the comparable coverage, I‐125 gives slightly higher V150 and V200 (56cc and 30cc, respectively), while Pd‐103 gives much higher V150 and V200 (66cc and 46cc respectively). Conclusion: Cs‐131 is the best choice among the 3 commercially available low energy sources. It can provide adequate target coverage and similar dose homogeneity compared to HDR dosimetry in reasonable days (10 days). The faster dose fall‐off of the low energy sources give it advantage of less dose to the surrounding healthy tissues including skin,heart, and lung.

SU‐FF‐T‐364: Skin Dose Measurement for Partial Breast Brachytherapy Using OneDose MOSFET Dosimeter

Purpose: To calibrate a new‐designed OneDose MOSFETdosimeter to Ir‐192 HDR radiation and use it to measure the skindose from partial breast HDR brachytherapy.Method and Materials: A set of cylindrical phantoms of different sizes (from 2cm to 8cm diameter) were made with a central hole to hold the HDR catheter, so that the treatment distance of 1cm to 4cm can be measured. Ten OneDose MOSFETdosimeters were placed outside of the phantom equal‐distance in order to get the average reading and thus remove the inter‐dosimeter variance for each phantom. The dosimeters build‐up side (1mm water‐equivalent thickness) was facing the HDR source. The dosimeters and phantom were immersed into water in a 30×30×30cm tank to get full scattered environment. Nucletron microSelectron HDR was used to deliver the radiation. Monte Carlo data published on CLRP website http://www.physics.carleton.ca/clrp/seed_database/Ir192_HDR/microSel ectron_v2/ was used to calculate the expected dose. The ratio of the measured dose and expected dose is the calibration factor for each distance. For patient measurement, a CT‐simulation process was performed to find a skin point and a fiducial marker was placed on that point, so that it is shown in the treatment planning CT and the expected dose from the treatment planning system (Nucletron Oncentra Brachy) can be obtained. A skin mark was made on that point so that the dosimeters placement is inter‐fraction reproducible. The measured dose is compared to the TPS calculated dose and the differences are recorded. Results: The calibration factors at different treatment distances were obtained and used for dose correction from the dosimeter reading. Two patients were measured so far and more are expected (work in progress). The expected results are that the measured dose is less than the TPS estimated dose since there are no backscatter as assumed in the TPS.

SU‐FF‐T‐76: Non‐Template Dosimetry for Prostate Seed Implant

Purpose: Template guidance has been used in modern prostate brachytherapy with 3D image‐based planning. In robotically assisted prostate seed implants, template is only a virtual concept for the planning procedure, because the robot can insert needles at any desired location. A non‐template dosimetry concept was presented and compared with traditional template dosimetry plan. Materials and Methods: Variseed (V7.1) planning software was used to create the non‐template plans. Based on 12 pretreated patients contours, both template and non‐template plans were created for comparison. Two different template plans were used: one used modified periphery needle pattern and the other used the same number of needles as non‐template. Novel planning rules were designed to create more periphery‐conformable non‐template needle patterns. For all methods, inverse planning was used to optimize the seed spacing along the needles after needle pattern was determined. Dosimetry parameters (prostate V100, V150, urethra D1 and rectum V100) and number of needles and seeds were compared. The correlation of dosimetry outcome between two methods was analyzed. Isodose distributions were compared. Results & Discussion: Compared to the template plan using modified periphery needle pattern, the non‐template plans decreases number of needles by 20% and increases average V100 by 1%. Compared to the limited number of needle template plans, the non‐template plans resulted in 2% increase in V100 and 4% decrease in maximum urethra dose, but the rectum V100 increased by 0.02 cc (mean). The correlation between urethra dose decrease and rectum dose increase has p‐value 0.00032. That was because the needles inserted below the urethra and above the rectum play an important role to affect both doses. Non‐template plan generally has greater conformal dose coverage than the template plan. Conclusions: Non‐template plan dosimetry has observable advantage over template one with similar number of needles.