J Peng

Calibration of the Gamma Knife Perfexion using TG‐21 and the solid water Leksell dosimetry phantom

PURPOSE To calibrate a Gamma Knife (GK) Perfexion using TG-21 with updated chamber-dependent values for modern microionization chambers in the new solid water Leksell dosimetry phantom. This work illustrates a calibration method using commercially available equipment, instruments, and an established dosimetry protocol that may be adopted at any GK center, thus reducing the interinstitutional variation in GK calibration. The calibration was verified by three third-party dosimetry checks. In addition, measurements of the relative output factors are presented and compared to available data and the new manufacturer-provided relative output factors yet to be released. METHODS An absolute dose calibration based on the TG-21 formalism, utilizing recently reported phantom material and chamber-dependent factors, was performed using a microionization chamber in a spherical solid water phantom. The result was compared to other calibration protocols based on TG-51. Independent verification of the machine output was conducted through M.D. Anderson Dosimetry Services (MDADS), using thermoluminescent dosimeters (TLDs) in an anthropomorphic head phantom; the Radiological Physics Center (RPC), using TLDs in the standard Elekta ABS plastic calibration phantom (gray phantom), included with the GK; and through a collaborative international calibration survey by the University of Pittsburgh Medical Center (UPMC) using alanine dosimeters, also in the gray phantom. The alanine dosimeters were read by the National Institute of Standards and Technology. Finally, Gafchromic EBT film was used to measure relative output factors and these factors were compared to values reported in the literature as well as new values announced for release by Elekta. The films were exposed in the solid water phantom using an included film insert accessory. RESULTS Compared to the TG-21 protocol in the solid water phantom, the modified and unmodified TG-51 calibrations resulted in dose rates which were 1.8% and 1.3% lower, respectively. Ratios of the doses measured by third parties to the dose reported showed excellent agreement. MDADS returned ratios of 1.00 and 0.98 for the two TLDs irradiated. The RPC returned a mean ratio of 0.98 of the dose reported and the UPMC alanine study returned a mean ratio of 1.008. Relative output factors were found to be 0.817 +/- 0.009 and 0.897 +/- 0.008 for the 4 and 8 mm collimators, respectively, which are in excellent agreement with revised Monte Carlo-derived relative output factors Elekta is expected to recommend with the next version of the GK treatment planning software (GAMMAPLAN version 10). CONCLUSIONS The TG-21 dosimetry protocol, performed in a solid water phantom in conjunction with modern dosimeters and phantom material and chamber-dependent factors, can yield an accurate dose measurement in the unique GK treatment geometry. The technique described here can be easily adopted by institutions worldwide since all equipment and instruments used are commercially available, thus reducing the existing interinstitutional variation in GK calibration techniques. Relative output factor measurements made in this same solid water phantom were used to verify the relative output factors provided by Elekta and agreed excellently with output factors expected to be released in conjunction with GAMMAPLAN version 10.

Comparison of radiation dose spillage from the Gamma Knife Perfexion with that from volumetric modulated arc radiosurgery during treatment of multiple brain metastases in a single fraction

OBJECT The objective of this study was to examine radiation dose distributions created by 2 competing radiosurgery modalities for treating multiple brain metastases: single-isocenter volumetric modulated arc radiosurgery (VMAS) and Gamma Knife Perfexion (GKP). In addition, the effectiveness of multiple radiosurgery quality metrics was evaluated and compared between these advanced treatment modalities. METHODS Seven anonymized MRI data sets, each showing 2-5 metastases, were used to create plans on each system. The GammaPlan (version 10.1) program was used for planning of GKP. A neurosurgeon contoured the volumes to be treated, and no planning target volume expansion was used. A prescription dose coverage of ≥ 99% was achieved for each tumor volume. The Philips Pinnacle (version 9.2) program was used for planning of VMAS, using the SmartArc optimization algorithm for delivery on a Varian iX linear accelerator. Contours were transferred from GammaPlan, and again no planning target volume expansion was used. Between 2 and 5 arcs with table angles of 90°-270° were used. Again, a V100% of ≥ 99% was achieved for each tumor volume. After planning, the MRI scans, tumor volumes, and dose information from each plan were exported according to the Digital Imaging and Communications in Medicine standard to the VelocityAI program for analysis. Brain dose-volume histograms (DVHs) for normal brain tissues were generated, and the volume of these tissues receiving 20%-90% of the prescription dose was tabulated. Finally, the prescription isodose to tumor volume ratio (PITV; Shaw et al., 1993), conformity index (CI; Paddick, 2000), gradient index (GI, Paddick and Lippitz, 2006), and conformity/gradient index (CGI, Wagner et al. 2003) were calculated for each plan. Both the PITV and CI have ideal values of 1, while the GI and CGI have ideal values of lowest and highest achievable, respectively. RESULTS The DVHs consistently showed that with VMAS a higher amount of normal brain tissues received each dose level than with GKP. These increases were largest for lower isodose levels, with the volumes of normal brain that received 20%-50% and 60%-90% of the prescription dose showing average increases of 403% and 227%, respectively. Prescription isodose conformality showed only minor differences between the 2 modalities. Radiosurgery quality metrics including measures of the dose gradient (GI and CGI) indicated that the GKP plan was superior in each case, with respective average GI and CGI values of 3.04 and 57.75 for GKP and of 10.22 and 10.85 for VMAS. Metrics evaluating prescription isodose conformality alone differed only slightly between the modalities. Average respective PITV and CI values were 2.13 and 0.53 for GKP and 2.27 and 0.51 for VMAS. CONCLUSIONS Stereotactic radiosurgery plans for the treatment of multiple metastases with VMAS delivered significantly more dose to the normal brain tissues than plans for GKP. Radiosurgery quality metrics including a measure of the dose gradient are better suited to providing contrast between modern radiosurgery treatment platforms.

SU-F-T-27: A Comparative Case Study Among Four Modalities for the Superficial Treatment of Squamous Cell Carcinoma

PURPOSE We performed a comparative planning study among High Dose Rate (HDR) brachytherapy, superficial electrons, Volume Modulated Arc Therapy (VMAT), and Helical IMRT (Tomotherapy) for squamous cell carcinoma of the abdominal wall with consideration for the underlining bowel. METHODS A 69-year old female presented with squamous cell carcinoma protruding 8mm beyond the anterior skin surface of the midabdomen was considered for treatment. The patient had a ventral hernia which resulted in the reduction of the abdominal wall thickness and the adjacent small bowel being the dose limiting structure. Four plans were generated using different treatment modalities: a) an enface electron field (eMC, Eclipse v. 11), b) Tomotherapy (HI-Art II v.5.0.5), c) VMAT (Acuros, Eclipse v. 11), and d) HDR using a Freiburg applicator (Oncentra v. 4.3). The following plan objectives were used for all four plans: for the CTV target, V90% ≥90% (61.8Gy2/2).For the small bowel, D0.1cc < 56.2 Gy2/2 was a hard constraint and expressed as a percentage of the prescription for comparison to demonstrate the dose fall-off achieved among the modalities.For HDR, V200% <0.1cc was an additional constraint. Multiple dosimetric parameters, including those listed above, were compared among the four modalities. RESULTS The HDR plan showed comparable target coverage compared to the Tomotherapy plan and better coverage compared to the electron plan. Small bowel doses (D0.1cc) were lower in HDR plan compared to Tomotherapy, electron, & VMAT plans (88.8%, 89.6%, 90.9%, & 96.6%). Integral dose to the whole body (V5%) was much higher for HDR, VMAT, and Tomotherapy when compared to electron plan by factors of seven, eight, and ten, respectively. After reviewing all treatment modalities, the physician selected HDR owing to better control of the small bowel dose while maintaining adequate target coverage. CONCLUSION This case study demonstrated HDR can successfully treat superficial lesions with superior sparing of underlying structures.

SU-F-T-20: Novel Catheter Lumen Recognition Algorithm for Rapid Digitization

PURPOSE Manual catheter recognition remains a time-consuming aspect of high-dose-rate brachytherapy (HDR) treatment planning. In this work, a novel catheter lumen recognition algorithm was created for accurate and rapid digitization. METHODS MatLab v8.5 was used to create the catheter recognition algorithm. Initially, the algorithm searches the patient CT dataset using an intensity based k-means filter designed to locate catheters. Once the catheters have been located, seed points are manually selected to initialize digitization of each catheter. From each seed point, the algorithm searches locally in order to automatically digitize the remaining catheter. This digitization is accomplished by finding pixels with similar image curvature and divergence parameters compared to the seed pixel. Newly digitized pixels are treated as new seed positions, and hessian image analysis is used to direct the algorithm toward neighboring catheter pixels, and to make the algorithm insensitive to adjacent catheters that are unresolvable on CT, air pockets, and high Z artifacts. The algorithm was tested using 11 HDR treatment plans, including the Syed template, tandem and ovoid applicator, and multi-catheter lung brachytherapy. Digitization error was calculated by comparing manually determined catheter positions to those determined by the algorithm. RESULTS he digitization error was 0.23 mm ± 0.14 mm axially and 0.62 mm ± 0.13 mm longitudinally at the tip. The time of digitization, following initial seed placement was less than 1 second per catheter. The maximum total time required to digitize all tested applicators was 4 minutes (Syed template with 15 needles). CONCLUSION This algorithm successfully digitizes HDR catheters for a variety of applicators with or without CT markers. The minimal axial error demonstrates the accuracy of the algorithm, and its insensitivity to image artifacts and challenging catheter positioning. Future work to automatically place initial seed positions would improve the algorithm speed.

SU-F-T-36: Dosimetric Comparison of Point Based Vs. Target Based Prescription for Intracavitary Brachytherapy in Cancer of the Cervix

PURPOSE Improved patient imaging used for planning the treatment of cervical cancer with Tandem and Ovoid (T&O) Intracavitary high-dose-rate brachytherapy (HDR) now allows for 3D delineation of target volumes and organs-at-risk. However, historical data relies on the conventional point A-based planning technique. A comparative dosimetric study was performed by generating both target-based (TBP) and point-based (PBP) plans for ten clinical patients. METHODS Treatment plans created using Elekta Oncentra v. 4.3 for ten consecutive cervical cancer patients were analyzed. All patients were treated with HDR using the Utrecht T&O applicator. Both CT and MRI imaging modalities were utilized to delineate clinical target volume (CTV) and organs-at-risk (rectum, sigmoid, bladder, and small bowel). Point A (left and right), vaginal mucosa, and ICRU rectum and bladder points were defined on CT. Two plans were generated for each patient using two prescription methods (PBP and TBP). 7Gy was prescribed to each point A for each PBP plan and to the target D90% for each TBP plan. Target V90%, V100%, and V200% were evaluated. In addition, D0.1cc and D2cc were analyzed for each organ-at-risk. Differences were assessed for statistical significance (p<0.05) by use of Students t-test. RESULTS Target coverage was comparable for both planning methods, with each method providing adequate target coverage. TBP showed lower absolute dose to the target volume than PBP (D90% = 7.0Gy vs. 7.4Gy, p=0.028), (V200% = 10.9cc vs. 12.8cc, p=0.014), (ALeft = 6.4Gy vs. 7Gy, p=0.009), and (ARight = 6.4Gy vs. 7Gy, p=0.013). TBP also showed a statistically significant reduction in bladder, rectum, small bowel, and sigmoid doses compared to PBP. There was no statistically significant difference in vaginal mucosa or ICRU-defined rectum and bladder dose. CONCLUSION Target based prescription resulted in substantially lower dose to delineated organs-at-risk compared to point based prescription, while maintaining similar target coverage.

SU-E-T-379: Evaluation of An EPID-Based System for Daily Dosimetry Check by Comparison with a Widely-Used Ionization Chamber-Based Device

Purpose: To examine the feasibility of using Varian’s EPID-based Machine Performance Check (MPC) system to track daily machine output through comparison with Sun Nuclear’s DailyQA3 (DQA) device. Methods: Daily machine outputs for two photon energies (6 and 16MV) and five electron energies (6, 9, 12, 16, 20MeV) were measured for one month using both MPC and DQA. Baselines measurements for MPC were taken at the start of the measurement series, while DQA baselines were set at an earlier date. In order to make absolute comparisons with MPC, all DQA readings were referenced to the average of the first three DQA readings in that series, minimizing systematic differences between the measurement techniques due to baseline differences. In addition to daily output measurements, weekly averages were also calculated and compared. Finally, the electron energy dependence of each measurement technique was examined by comparing energy-specific measurements to the average electron output of all energies each day. Results: For 6 and 16MV photons, the largest absolute percent differences between MPC and DQA were 0.60% and 0.73%, respectively. Weekly averages were within 0.17% and 0.23%, respectively. For all five electron energies, the greatest absolute percent differences between MPC and DQA for each energy ranged from 0.49%–0.83%. Weekly averages ranged from 0.07%–0.28%. DQA energy-specific electron readings matched the average electron output within 0.29% for all days and all energies. MPC energy-specific readings matched the average within 0.21% for 9–20MeV. However, 6MeV showed a larger distribution about the average with four days showing a difference greater than 0.30% and a maximum difference of 0.51%. Conclusion: MPC output measurements correlated well with the widely-used DQA3 for most beam energies, making it a reliable back up technique for daily output monitoring. However, MPC may display an energy dependence for lower electrons energies, requiring additional investigation.

MO-F-CAMPUS-J-01: A Novel and Efficient Daily QA Program for Both Modern Linear Accelerator and Optical Surface Motion Systems

Purpose: The camera-based optical surface monitoring system(OSMS) is a standard motion-tracking system for TrueBeam™ machine. Daily quality assurance(DQA) procedures for the TrueBeam and OSMS systems currently require multiple phantoms and test sequences. Machine Performance Check(MPC) uses the IsoCal™ phantom to test geometric and dosimetric aspects, and has been validated for routine TrueBeam DQA. This work examines the novel use of MPC to simultaneously conduct DQA of OSMS,utilizing one phantom and test sequence for QA of both the TrueBeam and OSMS systems. Methods: TrueBeam with 6 degree-of-freedom(6DOF) couch, MPC and OSMS were installed. The therapists routinely use MPC with the IsoCal phantom to verify geometric and dosimetric parameters with an automated sequence of varying gantry and collimator angles, MLC positions, and couch translations and rotations as the DQA for TrueBeam. A surface outline of the IsoCal phantom, generated from a CT dataset, was imported to OSMS and used to monitor the position of the phantom at isocenter and to track its movement up to 5cm/10°(yaw) and rotation in pitch and roll up to 3° in real time during the automated MPC sequence. Motion of the phantom throughout the MPC routine was compared to the couch position shown by the digital readout. Results: The coincidence of isocenter, as determined by OSMS, and TrueBeam radiation isocenter was within ±0.8mm/±0.8° in three axes. Between OSMS and TrueBeam 6DOF couch, the mean couch relative shifts/angles were within ±0.7mm/±0.8° for each axis. Reported pitch and roll values were within ±0.4mm/±0.4°. The total measurement time for each DQA session took approximately 5 minutes. Conclusion: Use of the IsoCal phantom and MPC for simultaneous DQA of TrueBeam and OSMS increases QA efficiency and reduces complexity. OSMS isocenter position and motion-tracking capability in 6DOF were verified in real-time and were found to be within 1mm of the expected value.

SU-E-T-417: The Impact of Normal Tissue Constraints On PTV Dose Homogeneity for Intensity Modulated Radiotherapy (IMRT), Volume Modulated Arc Therapy (VMAT) and Tomotherapy

PURPOSE Complex intensity modulated arc therapy tends to spread low dose to normal tissue(NT)regions to obtain improved target conformity and homogeneity and OAR sparing. This work evaluates the trade-offs between PTV homogeneity and reduction of the maximum dose(Dmax)spread to NT while planning of IMRT,VMAT and Tomotherapy. METHODS Ten prostate patients,previously planned with step-and-shoot IMRT,were selected. To fairly evaluate how PTV homogeneity was affected by NT Dmax constraints,original IMRT DVH objectives for PTV and OARs(femoral heads,and rectal and bladder wall)applied to 2 VMAT plans in Pinnacle(V9.0), and Tomotherapy(V4.2). The only constraint difference was the NT which was defined as body contours excluding targets,OARs and dose rings.NT Dmax constraint for 1st VMAT was set to the prescription dose(Dp).For 2nd VMAT(VMAT_NT)and Tomotherapy,it was set to the Dmax achieved in IMRT(~70-80% of Dp).All NT constraints were set to the lowest priority. Three common homogeneity indices(HI),RTOG_HI=Dmax/Dp,moderated_HI=D95%/D5% and complex_HI=(D2%-D98%)/Dp*100 were calculated. RESULTS All modalities with similar dosimetric endpoints for PTV and OARs. The complex_HI shows the most variability of indices,with average values of 5.9,4.9,9.3 and 6.1 for IMRT,VMAT,VMAT_NT and Tomotherapy,respectively.VMAT provided the best PTV homogeneity without compromising any OAR/NT sparing.Both VMAT_NT and Tomotherapy,planned with more restrictive NT constraints,showed reduced homogeneity,with VMAT_NT showing the worst homogeneity(P<0.0001)for all HI. Tomotherapy gave the lowest NT Dmax,with slightly decreased homogeneity compared to VMAT. Finally, there was no significant difference in NT Dmax or Dmean between VMAT and VMAT_NT. CONCLUSION PTV HI is highly dependent on permitted NT constraints. Results demonstrated that VMAT_NT with more restrictive NT constraints does not reduce Dmax NT,but significantly receives higher Dmax and worse target homogeneity. Therefore, it is critical that planners do not use too restrictive NT constraints during VMAT optimization. Tomotherapy plan was not as sensitive to NT constraints,however,care shall be taken to ensure NT is not pushed too hard. These results are relevant for clinical practice. The biological effect of higher Dmax and increased target heterogeneity needs further study.

SU‐E‐T‐57: Assessment of Systematic Uncertainties On Beam Data Collection Using Blue Phantom HelixTM Tomotherapy Scanning System

PURPOSE Tomotherapy users are not allowed to modify the beam-data or parameters for beam modeling in Tomotherapy treatment planning system(TPS).The gold beam-data in TPS for modeling and commissioning was measured with TomoScannerTM system(TS).This study investigates the use of the Blue Phantom HelixTM(BPH) as an alternative scanner to establish a benchmark dataset for commissioning and quantifies systematic differences between TS and BPH. METHODS Reproducibility of scanning with BPH was tested by 3 experienced physicists taking 5 measurements over 3 month period.Several enhancements of BPH over TS were included a 3D scanning arm which acquires beam-data with one tank setup,a universal chamber mount,and the OmniPro software,which allows provides online data collections and processing.Discrepancy was estimated by acquiring datasets with each tank under the most recent target conditions.The possible uncertainty due to TPS modeling was quantified by comparison to the golden beam-data.The total systematic uncertainty defined as the combination of scanning system and beam modeling uncertainties, was determined through numerical analysis,and tabulated according to scan type.OmniPro was used for all analysis to eliminate variances of data processing. RESULTS The setup reproducibility of BPH remained within 0.5mm/0.5%.For PDDs,the systematic uncertainties were within 1.4mm/2.1% except for the buildup region up to 7.0±2.5%,which was attributed to output instabilities at surfaces and differences of chamber alignments and dimensions.The differences in field width and penumbra of in-line profiles between BPH,TS and golden were within 0.7±0.9mm.The displacements of field width were increased in cross-line profiles at depth >=10cm up to 3.5%/3.5mm among three datasets.Use of BPH reduced measurement time by 1-2 hrs per session. CONCLUSION The uncertainty magnitudes of BPH have been quantified as an efficient,reproducible and accurate scanning system capable of providing a reliable benchmark beam data. Without the flexibility of replacing the TPS beam-data with BPH measurements,further investigation of dose uncertainties in beam modeling from different measurement datasets is suggested.

SU‐E‐J‐54: The Characterization of a 3D Real‐Time Surface System with a Single High‐Definition (HD) Camera

PURPOSE To investigate a single high-definition (HD) camera of 3D realtime surface system (AlignRT_1HD, VisionRT, Inc.) as the potential alternative to the standard configuration of the system with three charge-coupled device (CCD) cameras (AlignRT_3C) and to evaluate the localization accuracy of AlignRT_1HD compared to AlignRT_3C. METHODS Both AlignRT_3C and AlignRT_1HD were installed in the same treatment room with Varian LINAC. A commercial daily QA phantom (DailyQA3TM, Sun Nuclear) and 5 IMRT patients with intracranial and head and neck tumors receiving non-coplanar treatment were used to evaluate system characteristics and position-tracking accuracies of AlignRT_1HD through comparison with the AlignRT_3C and kV-CBCTs. Surface-image data sets were acquired simultaneously by both systems for the evaluation of inter-and intra-fractional motion at daily treatment of each patient. RESULTS The system origin displacements agreed to within 0.5 mm/0.5°. Compared to the CBCTs without couch rotations, the mean registration errors obtained using AlignRT_1HD were approximately 0.4 mm/0.8° (for phantom displacements up to ± 3cm in 3 axes), 1.1 mm/0.8° (for inter_frational motion in patients), and 0.3 mm/0.4° (for intra_frational motionin patients) less than those obtained using the AlignRT_3C. For non-coplanar treatments, without the surface images acquired by two side camera pods using AlignRT_1HD in this study, the accuracy of patient setup was only guaranteed up to ±10° couch angles. The average setup time using AlignRT_1HD was reduced by approximately 1 min per session. CONCLUSION As judged by CBCTs, AlignRT_1HD is capable of increasing in positioning accuracy and setup efficiency for high precision treatment, with the enhancements in image resolution, frame rate and field of view using HD cameras. The standard configuration with 3-camera-pods of AlignRT_3HD was essential for non-coplanar treatment and the localization accuracy with couch angle up to 90° needs to be further quantified.