QA Results Research Articles

WE‐C‐BRB‐12: Moving from Gamma Based to Patient DVH Based IMRT QA

Purpose: In this work we study 1) the correlation between patient DVH deviations and 3D gamma passing rate of the whole patient volume, 2) the correlation between patient DVH deviations and gamma passing rates of each corresponding ROI, and 3) the capability of an software/algorithm that predicts patient DVH deviations based on planar QA result. Methods: 96 unique “imperfect” step‐and‐shoot IMRT plans were generated by applying 4 different types of error—induced beam models on 24 clinical Head/Neck patients. The doses in each patient as well as QA phantom were then recalculated using an error‐free beam model to simulate the real delivery. The degree of induced errors was tuned to result in planar QA results that are commonly achieved in clinics. Clinically relevant patient DVH deviations, as well as 3D Gamma passing rates for both the whole patient volume and each corresponding ROI were derived by comparing the “delivered” and planed patient dose. Predicted patient DVH deviations were generated by perturbing planned dose based on simulated results of per beam planar QA in a phantom, using a commercial software/algorithm. Results: Weak to moderate correlation were found between clinically important patient DVH metrics (CTV‐D95, parotid Dmean, spinal cord D1cc, and larynx Dmean) and 3D patient gamma passing rate (3%/3mm, 2%/2mm, 1%/1mm) for whole patient volume as well as for each corresponding ROI. Predicted DVH deviations were in good agreement with actual DVH deviations. Conclusions: Gamma passing rate, even calculated in 3D for each specific ROI in real patient, has limited power in predicting clinical impact in terms of patient DVH deviations. The software/algorithm has potential to accurately predict these DVH deviations using the plan information and phantom planar QA results. Moving from Gamma based to patient DVH based IMRT QA appears to be both necessary and possible, and is therefore suggested.This is an unfunded work however one of the authors (Dr. Benjamin E. Nelms) serves as a consultant to the vendor (Sun Nuclear, Melbourne, FL) of the software that is used in this study.

Quality assurance of volumetric modulated arc therapy: Evaluation and comparison of different dosimetric systems

To compare and evaluate different dosimetric techniques and devices for the QA of VMAT plans created by two treatment planning systems (TPSs). A total of 50 VMAT plans were optimized for treatment of anatomical sites of various complexities by two TPSs which use rather different approaches to VMAT optimization. Dosimetric plan verifications were performed both as part of commissioning and as patient specific QA of clinical treatments. Absolute point doses were measured for all plans by a micro ion chamber inserted in a dedicated water-filled cylindrical phantom. Delivered dose distributions were verified by four techniques based on different detectors: radiographic and gafchromic films, two systems based on 2D diode arrays and an ion chamber array. Gamma index analysis with various tolerance levels (3%, 3 mm and 3%, 2 mm) was used to analyze differences between calculated and delivered doses. Sensitivity to possible delivery errors was also evaluated for three of the considered devices introducing +/-3 mm shifts along the three directions and a 3 degrees gantry offset. Ion chamber measured point doses were within 3% of calculated ones for 48 out of 50 values. For delivered dose distribution, the average fraction of passed gamma values using 3% and 3 mm criteria was above 95% for both TPSs and all detectors except gafchromic film which yielded on average of 91.4%. For 49 out of 50 plans, a pass-rate above 94% was obtained by at least one of the four techniques. Shrinking the tolerance to 3% and 2 mm, the average pass-rate by all detectors (except film) was still above 95% for one of the two TPSs, but lower for the other one. The detector sensitivity to 3 mm shifts and to gantry angle offset was strongly plan and partially detector dependent: the obtained pass-rate reduction ranged from 2% to 30%. The presented results for VMAT plans QA assess the reliability of the delivered doses for both TPSs. The slightly lower pass-rate obtained for one of the considered TPS can be attributed to a higher level of complexity of the optimized plans. The results by different dosimetric techniques are coherent, apart from a few measurements by gafchromic films. The detector sensitivity to delivery errors, being strongly plan dependent, is not easy to evaluate.

SU-GG-T-226: Reproducible QA of RapidArc Treatment Plans with MapCHECK2 and MapPHAN

Purpose: To report on an investigation of “best practices” for achieving precise and reproducible results with the MapCHECK2/MapPHAN (MC2/MP) dosimetry system (Sun Nuclear Corporation) for QA of RapidArc (Varian Medical Systems) volumetric modulated arc therapy (VMAT) treatments. Method and Materials: MapCHECK2 (MC2) is a 2D diode array developed for planar IMRTdosimetry; MapPHAN (MP) is a water equivalent phantom that adapts MC2 for rotational dosimetry. The MC2/MP system was checked to verify uniform and equivalent response to 6 MV photon beams incident from anterior and posterior directions. A CT scan of the system was ported to the Eclipse planning system, (v8.0) where it was assigned a generic mass density of 1.05 g/cm^3. RapidArc treatment plans (15 prostate, 2 SBRTlung, and lhead/neck) were projected onto this phantom and planar dose was computed at 5 cm depth. Parameters for specification of phantom and QA delivery were varied to deduce best practices for reproducible QA of RapidArc VMAT plans. Parameters included mass density assignment, collimator/MLC rotation, position of diode array relative to MLC leaves, and resolution of the dose plane exported from Eclipse. QA results were evaluated by absolute dose agreement and by the percentage of points passing a Gamma analysis with criteria 3% dose difference or 3 mm distance to agreement. Results: The most important steps for reproducible QA of RapidArc plans with MC2/MP were zero collimator rotation for QA beam delivery, and a positional offset of the diode array by half the width of an MLC leaf. These actions enhanced the ability of MC2 to measure dose modulation, and increased the percentage of points passing the Gamma analysis by approximately 5% and 3%, respectively. Conclusion: With simple setup interventions, the MapCHECK2/MapPHAN system is capable of measuring precise and reproducible QA of RapidArc‐delivered VMAT plans.

SU-GG-T-236: Quality Assurance of VMAT Treatment Delivery: Comparison of Four Different Dosimetric Equipments for the Verification of Plans Created by Two Treatment Planning Systems

Purpose: To compare four dosimetric techniques for the QA of VMAT plans created by two TPS. Method and Materials: 46 VMAT plans for treatment of anatomical sites of various complexity were created using ERGO++TPS (Elekta) (35 plans) and Oncentra VMAT module (Nucletron) (11 plans) for a 6 MV Elekta Synergy. Gamma analysis was used to compare calculated dose distributions with doses measured by 4 equipments: films (EDR2 and EBT2) in a solid water phantom, a 2D diode array (Mapcheck and Mapphan phantom), 2 crossing diode arrays in a cylindrical phantom (Delta4 system), an ion chamber array (PTW seven29 and Octavius). Mapcheck, delta4 and seven29 sensitivity to possible delivery errors was evaluated introducing +−3mm shifts along the 3 directions and a 3° gantry offset. Results: The average fraction of passed gamma values with a 3% and 3 mm criteria was above 95% for both TPS and all detectors except EBT2 which yielded an average of 91.0%. For 45 out of 46 plans a pass‐rate above 93% was obtained by at least one of the 4 equipments. Shrinking the tolerance to 3% and 2mm the average pass rate by Mapcheck, delta4 and seven29 was still above 95% for ERGO, but lower for Oncentra. The detectors sensitivity to 3 mm shifts and to gantry angle offset was strongly plan and partially detector dependent: the obtained pass‐rate reduction ranged from 2% to 30%. Conclusion: The presented results for VMAT plans QA assess the reliability of the delivered doses for both TPS. The slightly lower pass rate obtained for Oncentra can be attributed to a higher complexity of some of the created plans. The results by different dosimetric techniques are coherent, apart from a few measurements by EBT2. The detectors sensitivity to delivery errors, being strongly plan dependent, is not easy to evaluate.

SU‐GG‐J‐16: Impacting Parameter Analysis for IMRT Quality

Introduction: Intensity‐modulated radiation therapy (IMRT) accurately delivers radiation doses with high degree of conformity by modulating the intensity of the radiation beam in multiple small segments. Usually small fields have large variation in dose. For some TPS, there are no restrictions on plan parameters. Guideline for plan optimization is needed that allows the IMRT QA to pass satisfactorily. IMRT plan parameters are analyzed to correlate the success and failure of an IMRT QA plan. Materials and Methods: Based on IMRT QA results, 15 IMRT treatment plans, divided into 3 groups, are studies. Plans in group 1 passed IMRT QA with high gamma index passing rates and plan in group 2 passed with marginal passing rates. Plans in group 3 failed the IMRT QA. Statistical analysis has been performed on plan parameters, including beam number, segment number for each beam, MU in total or for each segment, the width variations of the leaf/jaw positions for each segment, the segment area sizes, and dose delivery for different segments of each beam, to discover the relationships between IMRT quality and these parameters. Results: The statistical results showed there is no correlation between plan quality and MU or beam/segment numbers. However, there are noticeable correlations between the IMRT quality and the segment sizes and widths. For each plan group, the IMRT quality decreased with the decreasing field sizes and segment widths. The histograms of these factors showed that failed IMRT plans have peak distributions with small field sizes (< 30cm2) and narrow widths (<20mm). Conclusion: Initial results showed that the passing rates of IMRT treatment plans have strong correlation with the segment field sizes and the opening widths of the leaf/jaw positions. Large number of segments with small fields produces unacceptable IMRT QA and should be avoided during IMRT planning.

SU-GG-T-252: The Comparison of Matrixx IMRT QA and MapCheck EMRT QA

Purpose: Compare Matrixx and MapCheck IMRT QA techniques and understand their similarity and difference on expressing IMRT QA results. This will help ones set up consistent criteria to evaluate IMRT QA results when both techniques are used (especially at the same clinic). Methods and Materials: Matrixx (IBA Dosimetry) and MapCheck (Sun Nuclear Corporation) IMRT QA techniques were used to perform IMRT QA for the same IMRT treatment plan. Ten IMRT treatment plans on different disease sites including head and neck, prostate, brain, esophagus, and pelvis were used in this study. In order to make a consistent comparison for these two QA methods, for each treatment plan we created the same IMRT QA verification plan with all beam's gantry angles set at zero degree and then exported the 2‐dimension dose plane at a certain water‐equivalent depth. Before doing QA dose measurements, both Matrixx and MapCheck devices were calibrated by using manufactures' manuals and verified by delivering open‐field beams at different sizes and depths. For the same IMRT QA plan, in respectively Matrixx and MapCheck measured the dose at the depth that the calculated dose plane was located. We compared the measured doses and the passing rates for the same DTA and Gamma criteria. Results: The doses measured by Matrixx were consistent with the ones measured by MapCheck, but the passing rate might be quite different depending on how the dose threshold was set for MapCheck and the dose region of interest was set for Matrixx although the same DTA and Gamma criteria for the comparison were used. Conclusion: A higher passing rate for Matrixx IMRT QA compared with the lower one by MapCheck doesn't mean that the treatment plan QA by Matrixx is better than the one QA by MapCheck, and vice versa. Consistent evaluation criteria need to be consent.

TU-E-BRA-02: A Novel Technique for Correcting Interleaf Edge Modeling Errors

Purpose: Evaluate MLC interleaf edge modeling errors using a dual interleaf edge correction to predict and correct calculated IMRT dose distributions, allowing more accurate IMRT QA and prediction of patient dose distributions. Materials and Methods: Interleaf edge profiles from a Varian Millenium‐120 MLC were measured using Kodak EDR2 film. Films were compared to interleaf dose profiles predicted using the Photon_AAA_8117 algorithm in Varian's Eclipse Treatment Planning System(VETPS). Differences between the measured and predicted edge profiles were modeled in user‐generated Matlab software MEDIC(Multifunctional Environment for Dual Interleaf Corrections) to create a mechanism for correcting VETPS dose distributions. 7 IMRT cases(47 fields) which failed departmental QA standards were chosen for correction. Dynamic MLC leaf positions were used to determine locations of exposed interleaf edges during delivery where corrections were applied. In addition, calculated dose distributions were corrected using a traditional T&G correction for comparison with our novel dual‐correction method. The original and modified distributions were compared to distributions measured with a diode matrix, using a 3mm/3% gamma index to test the correction accuracy. Results: The dual interleaf edge corrections combine to produce traditional T&G underdosing throughout the distributions, while single interleaf edge corrections appeared at high dose gradient locations. Modifying the VETPS‐calculated distributions resulted in average gamma index increases of 1.48% using traditional T&G corrections and 3.23% using dual interleaf edge corrections. Conclusion: Application of the MEDIC software quickly determines whether failing IMRT QA results are caused by interleaf edge modeling errors. Dual interleaf edge corrections were more than twice as effective at improving gamma indices compared to traditional T&G corrections and were better at predicting delivered dose in high gradient regions. MEDIC provides corrected dose distributions which are more representative of that delivered to the patient and can be used to asses clinical acceptability of the plan.

SU-FF-T-211: Influence of Anatomical and Physical Aspects of Treatment Planning On Prostate and H&N IMRT Plan QA Results

Purpose: To study the influence of combinations of Linear Accelerators and treatment planning‐systems, anatomical site and PTV volumes on MUs, and the magnitude and direction of deviation of prostate and H&N IMRT plan QA dose from plan‐dose. Method and Materials:IMRT plans were generated using Pinnacle3 or Eclipse treatment planning‐systems for treatment with Elekta or Varian Linacs, respectively. There were 29 H&N plans for Varian, and 59 and 33 prostate plans for Varian and Elekta, respectively. Dose at a point in the MED‐TEC phantom was measured with MT‐ERI‐A12 ion chamber. QA results were calculated as percentage deviation between measured and calculated doses in the phantom. A deviation of ±5% was acceptable. The magnitude and direction of deviation of QA results vs. PTV volumes, treatment site, Linac and planning system combinations were studied. PTV volumes vs. MUs was also analyzed. Linear regression analysis was used to study the relationship between paired variables. Results: The direction and magnitude of QA result deviation was Linac dependent. For Elekta prostate cases, QA measured dose deviated to negative direction (−2.49±0.93, −0.5 to −5%). For Varian cases, QA measured dose for prostate (1.67±0.91, −0.1 to 4.9%) and H&N cases (3.58±1.80, 0.2–6.4%) deviated to positive side. These results suggest that the delivered dose could vary between Linacs and might result in under‐ or over‐dosing. Prostate QA result deviation was independent of PTV volumes (P>0.1), but MUs increased with increase in PTV volumes (P<0.001). For H&N cases, MUs and the deviation between measured and planed‐dose increased with increase in PTV volumes (84–691 cm3, P<0.001). Conclusions: This type of analysis would help to evaluate the influence of Linac and/or planning systems, anatomical site and PTV volumes on the magnitude and direction of deviation between planned and delivered dose and to develop correction strategies to minimize radiation delivery variations.

SU-FF-T-254: Factors Study for MapCheck IMRT QA Evaluation

Purpose: To examine the factors that function in MapCheck IMRT QA result. Method and Materials:IMRT plan was calculated with Eclipse planning system and projected into a phantom at gantry angle zero position to create a verification plan. MapCheck QA result variation was evaluated with (1) a linac output fluctuation by calibrated MapCheck at previous 1.007 and current 0.995 linac outputs, (2) an effect of projected phantom slice interval at 0.5cm and 0.3cm, and (3) the number of point passing the criteria with MapCheck comparision models (AD/DTA, RD/DTA, AD/γ and RD/γ). A normalization point was selected as the value difference ⩽2% and fixed the point was used for all comparison models. All comparisons were done at 3% and 3mm criteria with a threshold 10. Results: MapCheck result varied with linac output showed an average improvement of 1.94% from 4 plans and 2.00% from 20 individual fields at CAX reading, and no signification improvement with the comparison models through MapCheck re‐calibration. Phantom slice interval difference related with a QA result showed no signification change in CAX reading, and the mean improvement from five field measurements was 0.76%, 1.30%, 5.80% and 4.10% in four comparison models from 0.5cm to 0.3cm interval. There was 100% point passing observed with a normalization point off 3.44%, and 91.8% point passing observed from same testing when the normalization point difference reduced to 0.32%. An average point passing the criteria from four comparison models was 91.81%, 95.71%, 85.77% and 95.30% from 50 fields, and 95.59%, 98.49%, 90.76% and 98.03% from 16 plans. Conclusions: A phantom slice interval 0.3cm is suggested for IMRT verification plan projection. MapCheck re‐calibration should be considered when a linac output changes. All comparison model results vary with normalization point selection. RD model is more sensitivity to the normalization point selection than AD model.

SU‐EE‐A1‐04: LINAC Quality Assurance and Calibration Using Electronic Portal Imaging Device

Purpose: Radiation treatment machine QA can be time consuming and oftentimes not accurate. The EPID on linacs can possibly be utilized for machine QA and calibration. In this study, we have developed methods to perform QA using EPID images. Method and Materials: A modified ball bearing phantom was constructed and set up according to the room laser center. Portal images were taken at different collimator, gantry, and table angles on an Elekta Synergy LINAC equiped with an a‐Si EPID. Portal images of several jaw and MLC control points were taken also. All images were analyzed using custom developed software in Matlab to calculate collimator/table angle, collimator/table/gantry runout, and jaw/MLC positions. Multiple CBCT images were also taken to verify KV‐MV coincidence and table correction accuracy. Results: Accurate quantitative results for mechanical, MLC, jaw and CBCT QA can be obtained using the developed method. Conclusions: We have developed automatic treatment machine QA methods using EPID images. This QA procedure is faster and more accurate than traditional QA methods.

SU‐FF‐T‐309: Quality Assurance for Imaging Guided Stereotactic RadioSurgery with Novalis Tx™ System

Purpose: To present QA measurements over extended periods at our institute and recommend a QA program for image‐guided radiosurgery on Novalis Tx system. Method and Materials: Novalis Tx system is developed based on a Trilogy linear accelerator featuring BrainLAB 6D‐ExacTrac, HD120‐MLC, MV‐EPID and KV‐OBI&amp;CBCT. Utilization of any new technology and equipment necessitates a comprehensive QA program to maintain and monitor system performance characteristics. In this study, we carried out a systematic QA program, including linear accelerator QA, treatment planning QA, planning MRI and CT QA, treatment imaging QA, patient‐specific QA, and system setup procedures for image‐guided radiosurgery. Methodology, frequencies and criteria of QA tests were based on AAPM TG40, TG42, TG51, TG66, and ACR MRI QC. Results: Comprehensive QA was performed from March to December 2008. 1) Daily Winston‐Lutz tests of 199 measurements showed that isocenter of gantry rotation was 0.44±0.21mm; isocenter of couch rotation was 0.34±0.20mm, within 1mm tolerance. Other linear accelerator QA results were within TG40 criteria. 2) Treatment planning QA was conducted with spot‐checks and a RPC SRS head phantom test. In spot‐checks, dosimetric discrepancies were within 3% (3.5% for the 4mm cone). RPC radiosurgery test was passed with less than 1% dosimetric deviation, and 100% geometric match. 3) Daily MRI QA showed that average geometric distortion was −0.2±0.4mm, −0.3±0.4mm, and −0.9±0.1mm along vertical, lateral, and longitudinal directions, within 1mm on phantom, 50% of ACR specifications. 4) OBI QA showed that imaging isocenter displacement was 0.46±0.30mm at AP and 0.76±0.19mm at RLAT. 5) Over ten months, 128 patients with intracranial lesions were treated with radiosurgery. All second MU checks were within 3%, and there were none treatment errors associated with the SRS treatments. Conclusion: Comprehensive QA program for image‐guided radiosurgery has been developed with proper methodology, frequency and criteria on Novalis Tx system, demonstrated with extensive measurements.

SU‐FF‐T‐111: Head‐And‐Neck IMRT Without Beam‐Splitting

Purpose: Varian MLC leaf travel is limited to 14.5 cm. For large PTV, an IMRT field will be split into two or three sub‐fields. Thus a 9‐beam head‐and‐neck plan may end up with 18 or more treatment fields. The purpose of this study is to develop a non‐split IMRT planning technique and compare the quality of non‐split plans with beam‐split counterparts. Method: Varian Eclipse user can choose fixed‐jaws and set field‐size less than 14.5 cm so that IMRT field will not split. A small part of the PTV may be blocked by x‐jaws in a particular beam, but the missing dose from one beam can be covered by other beams at different gantry angles. We compared our previously treated 9‐beam‐split‐18‐field head‐and‐neck IMRT plans with corresponding non‐split plans of 9, 11, and 15 gantry angles. Plan DVH and IMRT QA results were analyzed. Results: Non‐split plans produced the same dose coverage as beam‐split plans. DVH curves of PTV for split and non‐split plans almost overlapped with each other. DVH of organs‐at‐risk were slightly different. Total treatment MU were about the same for all the plans. All IMRT QA plans delivered on a Varian Trilogy passed physics QA criteria, with non‐split plans showing slightly better passing scores. Conclusions: Creating IMRT plans without beam‐splitting is encouraged. Non‐split IMRT plans can be as good as beam‐split plans, but use only half of the number of beams. Quality of non‐split plans improves with increasing number of beams. The amount of improvement was larger for the number of beams going from 9 to 11 than that from 13 to 15, demonstrating a balance of costs and benefits. Cutting the number of beams by half may result in a number of benefits: more accurate dose delivery, shorter treatment time, and increased machine throughput and productivity.

Moving from IMRT QA measurements toward independent computer calculations using control charts

Background and purpose In this study, we investigated IMRT QA using Statistical Process Control for the purpose of comparing the processes of patient-specific measurements and the corresponding independent computer calculations. Materials and methods Point dose data from the treatment planning system (TPS), independent computer calculations, and physical measurements for prostate and head and neck cases were studied. Control charts were used to analyze the IMRT QA processes from several institutions in the academic and community setting. Control charts are a method to describe the performance of a process. The width of the control chart limits (or action limits) describes the process’ ability to meet clinical specifications of ±5%. In all, 24 process comparisons were made (12 measurement QA and 12 independent computer calculation QA). Results For head and neck IMRT QA, the average process ability for the measurement QA was ±6.9% compared to ±7.2% for the independent computer calculation QA. For prostate IMRT QA, the average process ability was 4.4% for both measurement QA and independent computer calculation QA. It was found that 11 of the 24 processes were in control. At none of the institutions were the processes of measurements and independent computer calculations both in control and performing within clinical specifications. Conclusion There is room to improve the processes of IMRT QA measurements and independent computer calculations. In situations where the improvement of the processes is such that each is in control and well within clinical specifications, it may be appropriate to suspend patient-specific IMRT QA measurements for every patient in the place of independent computer calculations.

SU‐GG‐T‐157: Commission of 2.5 Mm ModuLeaf MLC for Stereotactic Radiation Therapy

Purpose: Improvements of MLC technology in radiotherapy, including smaller leaf sizes, allow greater accuracy in delivering radiation doses to the target with better conformality. For example, the high ablative doses used in hypo‐fractionated stereotactic body radiotherapy (SBRT) require high precision in dose delivery, thus high precision in commissioning the MLC system to verifiy MLC accuracy. The purpose of this study was to evaluate the procedure and challenges in the commissioning and application of 2.5‐mm small‐leaf MLC. Method and Materials: Siemens' newly‐developed Moduleaf MLC (MMLC) with 2.5mm leaf‐size was commissioned for SBRT. Step‐by‐step QA and application procedures for SBRT commission guidelines were followed. Error tolerance and QA results were analyzed through the entire commission process from beam data preparation, to treatment planning system commission, to treatment plan verification. The commission was divided into 3 phases: mechanical commission, software commission, and comprehensive commission. Isocenter accuracy was assured for mechanical precision. For software commission, the effect of small field factor was analyzed. In comprehensive phase, a single field and a head‐and‐neck case were tested with the QA standard. Results: The commission and clinical implementation of 2.5mm MMLC were described in this study, with provided guidelines for QA procedures from beam data collection and modeling to treatment planning methodology. The accuracy of the MLC mechanic isocenter was within 0.2 mm. At the same time, the treatment modalities with or without the flatten filter were compared, and they were consistent to each other except for the dose‐rate effect. Different MMLC clinical protocols, from SBRT to head‐and‐neck and intracranial treatments, have been suggested. Conclusion: After strictly following the physical QA requirements, we successfully commissioned the new mounted‐on 2.5mm MMLC for SRS/SBRT. In the follow‐up studies, clinical applications with MMLC will be further compared to other on‐site SRS systems, including Gamma‐Knife, and Tomotherapy system.

SU-GG-J-09: A DICOM Screen Dumper That Links Tomotherapy Units and R&amp;V Systems

Purpose: To present a DICOM screen dumper that captures 3D image registration results of MVCT vs. KVCT on Tomotherapy delivery console and sends them to R&V system to make remote image reviewing feasible. Method and Materials: Helical Tomotherapy is a true IGRT modality. Its on-line imaging feature allows us to produce patient's position accurately by means of 3D image registration of MVCT vs. KVCT prior to treatment so that inter-fraction motion is compensated. Unfortunately, this system lacks the feature to communicate with R&V systems due to its unique design. The image registration results are either approved by radiation oncologists at the console or on the printouts. To smooth the workflow, a software utility, ScreenBee, was developed for Tomotherapy system. Instead of acquiring entire 3D registration results and plan parameters that currently are not supported by any R&V system, it takes snapshots of either entire screen or a specific region, i.e., the graphical representations of 3D image registration and plan, encodes them in DICOM RT format and transfers them to R&V system. Results: It has been extensively tested and evaluated for about 8 months in our clinic. More than 2,000 images have been successfully sent to IMPAC. It also works on any computer with Microsoft Windows NT/2000/XP wherever the screen is desired to be seen and reviewed remotely in a timely fashion. So the treatment prescription and QA results can also be sent to R&V system if it is installed on the Tomotherapy planning workstation. Conclusion: The technical limitation on DICOM RT implementation of Tomotherapy system can be remedied by dedicated software without extra hardware capital investment. It helps radiation therapy clinics with cutting edge technologies move towards digital solutions so that the clinic workflows become more efficient and smooth with permanent records.

SU‐GG‐T‐331: A New Monte Carlo Code for Gamma Knife Simulations

Purpose: Gamma Knife is widely used in stereotactic radiosurgery (SRS) to treat brain tumors. In this study, a new Monte Carlo code was developed to simulate Gamma Knife system, which can be used to calculate Gamma Knife collimator factors, to verify Gamma Knife monthly and annual QA results and to evaluate treatment plan from Leksell GammaPlan system. Method and Materials: An EGSnrc based user code DOSIMETER was used for this study. The Cobalt‐60 source and collimators were simulated based on their material, size and location in the system. Because of the equal geometries and materials of the sources and collimators, one source system is generated for all 201 sources in corresponded angle. For each collimator (4, 8, 14 or 18 mm), a phase space file was produced at the final helmet collimator surface. A CT image of a round QA phantom with 160 mm in diameter was introduced in the simulation system. Source linearity and profiles at isocenter were calculated using Monte Carlo simulations and compared with pin chamber and film measurement results. The relative collimator factors for 4, 8 and 14 mm were calculated and compared with ultra‐thin TLD and OSL measurements. Finally, a plan from Leksell GammaPlan system was evaluated by the Monte Carlo simulations. Results: A Monte Carlo code was implemented for Gamma knife simulation. It showed good dose linearity. The profiles for each collimator were consistent with film measurements. The relative collimator factors from Monte Carlo simulations are 0.970, 0.938 and 0.873, which are within 3% against TLD and OSL results. The DVHs from Monte Carlo are in good agreement with GammaPlan system. The treatment plan dose difference at isocenter between Monte Carlo and the planning system is less than 1%.

SU‐GG‐T‐156: Patient Specific QA Results for Step and Shoot and Sliding Window Deliveries of IMRT Beams as Implemented by Two Treatment Planning Systems

Purpose: To present the patient specific QA results for step and shoot (SS) and sliding window (SW) deliveries of IMRT beams as implemented by two treatment planning systems (TPS). Method and Materials: From a sample of our clinical practice 648 (108 cases) Eclipse AAA_7_5_‐18‐SW and 464 (66 cases) Pinnacle 7.4f ‐SS IMRT beams were evaluated using the MapCheck diode array system. The planar dose measurements at 5 or 10 cm depth in water were compared with the predictions from both TPS. A 3 % −3mm distance to agreement (DTA) criteria was used to score the percent of passing points (PPP) for each individual field. The samples for both TPS were composed of 75 % of Head and Neck cases, 15% prostates and 10 % other sites. Results: The distribution of the PPP showed that 83% and 85% of the evaluated beams were above the 95 % PPP for Eclipse and Pinnacle respectively. Between 90 and 95 % the PPP were 15% and 14% and below 90% the PPP were 1.9 % and 1.7 % for Eclipse and Pinnacle respectively. The passing rates for Eclipse‐SW are significantly improved when the first 20 Eclipse‐SW cases (130 beams) are not considered for this comparison. The Eclipse‐SW PPP above 95% increases to 91.1 %. The patterns of failure observed for the two TPS were distinctively different with Pinnacle‐SS exhibiting clusters of hot points (measurements higher than calculations) while Eclipse‐SW showed cold points aligned in the direction of the MLC leaves movement. Conclusion: The results from our patient specific IMRT QA using the Eclipse‐SW delivery system exhibited a better agreement when compared with our previous experience using Pinnacle‐SS. A detail analysis is required in order to assess the clinical significance of the different failure patterns observed between these two TPS.

SU-FF-T-228: Factors Affecting IMRT QA

Purpose: We evaluate the impact of plan parameters and setup errors on IMRT QA results. Methods and Materials: Prostate (23MV, 5 field) and H& N (6 MV, 7 field) IMRT plans were exported to a cubic QA phantom. First, the magnitude of random errors in IMRT QA was estimated by repeating each measurement ten-times. Next, the following parameters were independently adjusted: intensity levels (5 to 20), minimum MLC segments (0.5 to 1.5cm), phantom setup error (0.5cm), and their impact on IMRT QA was evaluated via ionization chamber and EDR film. Coronal plane film results were analyzed using γ-index, dose difference, and distance to agreement. Results: Based on repeat QA measurements, the central axis ion chamber dose was found to be highly reproducible and in good agreement with planned dose (mean deviation, prostate: 1.79±0.10%; and H&N: 2.62±0.25%). Measured isodose distributions displayed similar consistency (mean γ-index, prostate: 0.72±0.04; H&N: 0.60±0.09). Varying the number of intensity levels and MLC segment size had a small effect on the central axis ion chamber dose (<1.2%). However, film results showed that mean γ-index changed by 10% when minimum MLC segment size was increased from 0.5 to1.5cm. The number of intensity levels had a smaller effect on γ-index. For a 0.5cm setup error in H&N plan, 3.8% deviation was noted in the measured dose, however this coincidentally improved QA results. No corresponding dose discrepancy was seen in a fiducial-less film. Conclusions: For an accurately modeled linac, IMRT QA results show remarkable consistency and agreement with planned dose. Plan complexity tended to worsen QA results for H&N patients. Sub-optimal IMRT plans with incorrect plan parameters and setup errors can have detrimental effect on patient treatment. However, IMRT QA does not always flag these errors, as their impact on QA results may not be large enough to be noticed.

WE‐D‐AUD‐08: Analysis of 1200 Delivery QA Measurements and Patient Plan Parameters in Helical Tomotherapy

Purpose: We have treated over 1200 patients on different anatomic sites using available beams widths. Every patient plan was verified with a delivery QA plan on a cylindrical water‐equivalent phantom. This study analyzes the variations of the delivery QA results and attempts to find some correlations with the patient plan parameters. Method and Materials: Following approval of an IMRT plan, a dose verification plan was generated for delivery on a Cheese phantom. An EDR2 film was placed in the central coronal plane of the phantom. An #a1SL ion chamber was placed 5 mm below the film for simultaneous point dose measurement. After appropriate setup and irradiation of the phantom with the delivery QA plan, the recorded charge was converted to dose and the film processed for dose distribution analysis. The films were scanned with a Vidar scanner and transferred to the Tomotherapy planning station for comparison of the delivered and planned dose distribution. Altogether 903 patient plans have been analyzed of which 11 are on 1 cm beam, 196 are on 2.5 cm beam and 696 are on 5 cm beam. Results: The difference between measured and calculated doses for all beam widths correlates (R2 = 0.87) with a normal distribution (μ = 0.6, σ = 3.152). The 2.5 cm and 5.0 cm beams independently are (μ,σ,R2) 2.15, 2.93, 0.85 and 0.338, 2.92, 0.87, respectively. The 1.0 cm beam measurements had μ = −0.9, σ = 3.8. No treatment planning parameters (pitch, treatment time, modulation factor) were found to correlate with the percent difference. Conclusions: The results are randomly distributed and planning parameters do not appear to affect the delivery QA results. The random nature indicates variation in machine output, phantom positioning, and position within the treatment field.

SU-FF-T-206: Effects of Static Dosimetric Leaf Gap On MLC-Based Small Beam Dose Distribution for Intensity Modulated Radiosurgery

Purpose: To evaluate the impact of static dosimetric leaf gap on MLC‐based small beam dose distribution and to compare the calculated results from planning system with actual measurement for intensity modulated radiosurgery.Methods and Materials: We determined the optimal dosimetric static leaf gap by comparing the profiles of MLC based small beam with those of the collimated fields. The applied leaf gaps were 0, 1, and 2 mm for comparison. The doughnut shaped PTV (6.1 cm3) and inner OAR (0.3 cm3) were delineated for delicate intensity modulated radiosurgery test plan. For the test, Millennium 120 leaf MLC and Eclipse Radiation Therapy Planning system (Varian) were used. For the measurement of dose, we used radiosurgery head phantom (model 605, CIRS, Norfolk, Virginia). Results: We found that 2 mm gap was optimal for the MLC based small beam. The maximum dose differences at the inside PTV, outside PTV, and inner OAR were 22.3%, 20.2%, and 35.2% for the 0 mm leaf gap, 17.8%, 22.8%, and 30.8% for the 1 mm leaf gap, and 5.5%, 8.5%, and 6.3% for the 2 mm leaf gap, respectively. In a humanoid head phantom study, the final dose distribution from the Eclipse planning system was significantly different from the measured values. The planned results were similar, while the measured showed large differences in dose according to the leaf gaps (range: 1.3 – 12.7%). Conclusion: An inadequate determination of the dosimetric static leaf gap during the RTP configuration can make errors from the final dose calculation, which can sometimes be confused with unwanted QA results of IMRS. An appropriate dosimetric leaf gap setting is critical during the commissioning of an inverse planning system and an incorrect setting can produce large dose delivery errors particularly in the delicate IMRS treatment.