MLC Tests Research Articles

The preparation of polyamide 11 composites with extremely long ignition time

AbstractThe wildly‐used charring agent, pentaerythritol (PER), was solely incorporated into polyamide 11 (PA 11) matrix by melt compounding. The addition of 15%PER improved the UL rating of PA 11‐PER composites to V‐0 and reduced the peak heat release rate by 22.1%. Most importantly, the time to ignition had been incredibly increased from 93 s of neat PA 11 to 193 s of PA 11‐15PER sample. The thermal behaviors were investigated by thermogravimetric analysis (TGA) and TGA‐Fourier transform infrared (FTIR). The morphology, chemical structure and physical properties (such as crystallinity and viscosity) of char residues that were collected at ignition point or after MLC tests of PA 11 and PA 11‐15PER composites were fully demonstrated. It was suggested that the interactions between PA 11 and PER led to the rapid formation of a compact char layer which exhibited sufficient viscosity before ignition in the condensed phase, while the early released H2O and CO2from PER took effects in the gas phase.

Characterization of radiotherapy component impact on MR imaging quality for an MRgRT system.

Radiotherapy components of an magnetic resonnace‐guided radiotherapy (MRgRT) system can alter the magnetic fields, causing spatial distortion and image deformation, altering imaging and radiation isocenter coincidence and the accuracy of dose calculations. This work presents a characterization of radiotherapy component impact on MR imaging quality in terms of imaging isocenter variation and spatial integrity changes on a 0.35T MRgRT system, pre‐ and postupgrade of the system. The impact of gantry position, MLC field size, and treatment table power state on imaging isocenter and spatial integrity were investigated. A spatial integrity phantom was used for all tests. Images were acquired for gantry angles 0–330° at 30° increments to assess the impact of gantry position. For MLC and table power state tests all images were acquired at the home gantry position (330°). MLC field sizes ranged from 1.66 to 27.4 cm edge length square fields. Imaging isocenter shift caused by gantry position was reduced from 1.7 mm at gantry 150° preupgrade to 0.9 mm at gantry 120° postupgrade. Maximum spatial integrity errors were 0.5 mm or less pre‐ and postupgrade for all gantry angles, MLC field sizes, and treatment table power states. However, when the treatment table was powered on, there was significant reduction in SNR. This study showed that gantry position can impact imaging isocenter, but spatial integrity errors were not dependent on gantry position, MLC field size, or treatment table power state. Significant isocenter variation, while reduced postupgrade, is cause for further investigation.

MLC parameters from static fields to VMAT plans: an evaluation in a RT-dedicated MC environment (PRIMO)

BackgroundPRIMO is a graphical environment based on PENELOPE Monte Carlo (MC) simulation of radiotherapy beams able to compute dose distribution in patients, from plans with different techniques. The dosimetric characteristics of an HD-120 MLC (Varian), simulated using PRIMO, were here compared with measurements, and also with Acuros calculations (in the Eclipse treatment planning system, Varian).Materials and methodsA 10 MV FFF beam from a Varian EDGE linac equipped with the HD-120 MLC was used for this work. Initially, the linac head was simulated inside PRIMO, and validated against measurements in a water phantom. Then, a series of different MLC patterns were established to assess the MLC dosimetric characteristics. Those tests included: i) static fields: output factors from MLC shaped fields (2 × 2 to 10 × 10 cm2), alternate open and closed leaf pattern, MLC transmitted dose; ii) dynamic fields: dosimetric leaf gap (DLG) evaluated with sweeping gaps, tongue and groove (TG) effect assessed with profiles across alternate open and closed leaves moving across the field. The doses in the different tests were simulated in PRIMO and then compared with EBT3 film measurements in solid water phantom, as well as with Acuros calculations. Finally, MC in PRIMO and Acuros were compared in some clinical cases, summarizing the clinical complexity in view of a possible use of PRIMO as an independent dose calculation check.ResultsStatic output factor MLC tests showed an agreement between MC calculated and measured OF of 0.5%. The dynamic tests presented DLG values of 0.033 ± 0.003 cm and 0.032 ± 0.006 cm for MC and measurements, respectively. Regarding the TG tests, a general agreement between the dose distributions of 1–2% was achieved, except for the extreme patterns (very small gaps/field sizes and high TG effect) were the agreement was about 4–5%. The analysis of the clinical cases, the Gamma agreement between MC in PRIMO and Acuros dose calculation in Eclipse was of 99.5 ± 0.2% for 3%/2 mm criteria of dose difference/distance to agreement.ConclusionsMC simulations in the PRIMO environment were in agreement with measurements for the HD-120 MLC in a 10 MV FFF beam from a Varian EDGE linac. This result allowed to consistently compare clinical cases, showing the possible use of PRIMO as an independent dose calculation check tool.

12 Dynamic Wave Arc with SBRT dedicated system VERO: Implementation and quality control

Introduction With SRS/ SBRT, non coplanar beams with fixed table positions are widely used. Since 2017, the dedicated accelerator SBRT VERO (Brainlab, Mitsubishi) has a new treatment approach allowing simultaneous gantry/ring rotations according to predefined trajectories: Dynamic Wave Arc (DWA), realizing in 1 arc a dynamic non coplanar VMAT treatment. This work presents the implementation and the quality control of the DWA. Methods The Vero system is equipped with a gimballed irradiation head with MLC in order to follow tumor movements (Dynamic Tacking DT), on board on a O’ ring. TPS Raystation (Raysearch) has recently developed for the VERO system, a specific optimization module for the DWA technique. Dosimetric data for the VERO beam modelling in the TPS and its validation, were measured with Edge diode (SunNuclear) and 0.125 cc ionization chamber (PTW) with the 3DS circular water tank (SNC) mounted on the O’ring (Jack screwed on the ring). The agreement measurements/calculations was evaluated with ScanDoseMatch software (qxrayconsulting.com). MLC characteristics (MLC position, effet Tongue and Groove, Leaf tip, Transmission..) were defined with MLC test fields, EBT3 films and analysis film software Doselab Pro. The validation of the beam modeling was performed according the ESTRO 2004 protocol. Clinical tests for IMRT, VMAT and DWA irradiations were performed with several detectors and phantoms: EBT3 films calibrated according to a well defined procedure allowing a dosimetric accuracy of 3%, the CIRS062 phantom, the 3D matrix Archeck (SunNuclear), and the StereoPhan system (SunNuclear) combined with the new diodes matrix SRSMapcheck (SunNuclear). This new matrix is composed of 1013 diodes in a 7.7x7.7 cm2 field and allows measurements with non coplanar beams (angular diode corrections). The agreement between calculated and measured dose distributions was calculated with gamma index method and different criteria according to the measurement system used. Results The beam modeling in Raysearch shows that 95% of the tests have γ 1 for 2%1 mm. The MLC characteristics MLC position, Transmission, Leaf Tip et Tongue and Groove allowing an agreement for the tested MLC are respectively 0.008 cm, 0.2%, 0.3 cm and 0.03 cm. The clinical results give an agreement between calculations and measurements with γ 1 for (2%2 mm) with Archeck, (3%1 mm) for EBT3 films et (2%1 mm) for SRSMapcheck. Conclusion DWA is a reliable technology for a fast treatment with 1 modulated arc according non coplanar trajectories. The next step would be to associate DWA to the Dynamic Traking(DT).

P22. Evaluation and implementation of the module myQA Machines for a standard Elekta Synergy accelerator, with setup of acceptance criteria according to the guidelines defined by the decision of the ANSM of July 27th, 2007, and setting in routine

Introduction myQA platform and devices is a computer program hosting applications developed by IBA. It provides different modules for dosimetry qality assurance (QA) including machine QA, patient QA, and radiotherapy QA. This project regards myQA Machines which provides a complete set of functions to plan, perform, analyze and document quality assurance of treatment units, imaging devices and their accessories. All QA activities are based on customizable protocols. The objective of this project is to implement a protocol according to ANSM decision of July 27th, 2007 in order to evaluate the application myQA Machines for a setting in routine in the service. Methods We use an Elekta Synergy MLC80 accelerator delivering X-ray beams of 6 and 18 MV and electron beams from 6 to 18 MeV. It has a system of Portal and CBCT imagery. In myQA Machines, there are two possible types of tests: Generic tests , when the measurements are carried out without the software and results are manually entered, and Plugin tests , when the complete QA procedure is performed with the software using the multi detector MatriXX for example. The results of dosimetry and MLC tests will be compared respectively with those obtained in reference conditions (water phantom standard MP3 PTW) and with the Aquilab solution. A study of influence of mechanical, physical and dosimetric parameters of the accelerator is carried out in the aim of evaluating the performances and the limits of the system myQA Machines as well as defined acceptability criteria. Results The template related to the QA according to ANSM decision is operational on myQA Machines software. Results of Plugin tests are conforms. The largest difference between constancy measurements and references for homogeneity and symmetry, of all the beams available on the machine, is lower than 1%. The study on the sensitivity of the system shows the importance to choose the tolerances well because a variation of more than 1 mm in the centering of MatriXX induces a non-conformity of at least one of the controlled parameters. Conclusion The application myQA Machines is operational for the quality controls on machines. The analysis of QA results and sensitivity study’s results will enable us to determine the feasibility, reliability and adaptability of myQA Machines software for a setting in routine in an external service of radiotherapy. This implementation would make it possible to have control of data by connecting all the applications of QA on a single platform.

Clinical commissioning of the INCISE™ multileaf collimator for cyberknife M6™ system

Introduction The National Center for Cancer Care and Research (NCCCR) at Hamad Medical Corporation in Qatar is amongst the first institutions in Asia and Europe to have the CyberKnife® M6 FIM system (Accuray, Inc., Sunnyvale, CA, USA) including Fixed Collimators, Iris Variable Aperture Collimator and InCise™ Multileaf Collimator. Purpose To report our commissioning experiences of the CyberKnife M6 InCise™ Multileaf Collimator system. Materials and methods Accuray guidelines were used to conduct measurements. A detailed description of the equipment used, the mechanical alignment, the verification procedures, and the alignment of the detector motion with the radiation beam is described. Incise MLC testing was conducted as per Accuray procedure. Further MLC tests were developed to verify its feasibility under certain clinical settings. Results Our results were within the manufacturer specifications. Results for both the Sun Nuclear Edge detector and PTW SRS diode 60018 were found to be comparable. Measurement conditions specified by Accuray were sufficient; however the use of a reference chamber introduced additional noise for the continuous profile acquisitions. As compared to fixed and IRIS collimators, the incise MLC produced acceptable results with an average of 20% decrease in treatment delivery time. Conclusions Our results are amongst the first set of data reported on the InCise™ MLC and were within the manufacturer specifications. These results will be used to develop a benchmark reference for periodic Quality Control (QC). Disclosure Authors have nothing to disclose.

SU‐F‐T‐313: Clinical Results of a New Customer Acceptance Test for Elekta VMAT

Purpose:To report the results of a customer acceptance test (CAT) for VMAT treatments for two matched Elekta linear accelerators.Methods:The CAT tests were performed on two clinically matched Elekta linear accelerators equipped with a 160‐leaf MLC. Functional tests included performance checks of the control system during dynamic movements of the diaphragms, MLC, and gantry. Dosimetric tests included MLC picket fence tests at static and variable dose rates and a diaphragm alignment test, all performed using the on‐board EPID. Additionally, beam symmetry during arc delivery was measured at the four cardinal angles for high and low dose rate modes using a 2D detector array. Results of the dosimetric tests were analyzed using the VMAT CAT analysis tool.Results:Linear accelerator 1 (LN1) met all stated CAT tolerances. Linear accelerator 2 (LN2) passed the geometric, beam symmetry, and MLC position error tests but failed the relative dose average test for the diaphragm abutment and all three picket fence fields. Though peak doses in the abutment regions were consistent, the average dose was below the stated tolerance corresponding to a leaf junction that was too narrow. Despite this, no significant differences in patient specific VMAT quality assurance measured were observed between the accelerators and both passed monthly MLC quality assurance performed with the Hancock test.Conclusion:Results from the CAT showed LN2 with relative dose averages in the abutment regions of the diaphragm and MLC tests outside the tolerances resulting from differences in leaf gap distances. Tolerances of the dose average tests from the CAT may be small enough to detect MLC errors which do not significantly affect patient QA or the routine MLC tests.

TU-FG-201-02: Is Weekly MLC QA Necessary? Two Year EPID-Based Weekly MLC QA Experience at the University of Maryland

Purpose: Appropriate MLC QA is important to assure treatment accuracy. In addition to the TG-142 recommended picket fence test, our institution performs four MLC tests on a weekly basis. The purpose of this study was to evaluate the clinical necessity of our weekly MLC QA program after a two-year run. Methods: Our Weekly MLC QA tests: 1) MLC positioning; 2) Interdigitation for testing hard MLC motion; 3) Picket fence at static gantry angle; 4) Leaf-end leakage and 5) dose match between a VMAT plan and the same plan with static-gantry-sliding window. For this study, a total of 794 Weekly MLC QA plans using EPID systems (3878 in total) for 10 machines from June 2014 to February 2016 were analyzed. Failure Mode Effect Analysis (FMEA) was performed with five failure modes. For each failure mode, a risk probability number (RPN) was calculated for a Weekly MLC QA (Monthly + Weekly) and for a Monthly MLC QA Only scheme. The probability of occurrence was evaluated from the Weekly QA experience. Results: Total number of failures for 794 cases were 24 (0.23%, for MLC Position test, 0.11% for Interdigitation test, 0.11% for Picket Fence Test, 0% for Leaf end Leakage test and 2.5% for VMAT test). The QA goal was to maintain the RPN score < 125. The RPN scores for the Monthly MLC QA Only ranged from 144–180, while those with Weekly (Monthly + Weekly) MLC QA decreased to 96–120. Conclusion: The Weekly MLC QA decreased the RPN numbers to the desired level (RPN<125), while those from the program of Monthly QA only were found to be higher or borderline. This objectively proves the clinical necessity for comprehensive weekly MLC QA.

SU‐F‐T‐469: A Clinically Observed Discrepancy Between Image‐Based and Log‐ Based MLC Position

Purpose:To present a clinical case which challenges the base assumption of log‐file based QA, by showing that the actual position of a MLC leaf can suddenly deviate from its programmed and logged position by &gt;1 mm as observed with real‐time imaging.Methods:An EPID‐based exit‐fluence dosimetry system designed to prevent gross delivery errors was used in cine mode to capture portal images during treatment. Visual monitoring identified an anomalous MLC leaf pair gap not otherwise detected by the automatic position verification. The position of the erred leaf was measured on EPID images and log files were analyzed for the treatment in question, the prior day's treatment, and for daily MLC test patterns acquired on those treatment days. Additional standard test patterns were used to quantify the leaf position.Results:Whereas the log file reported no difference between planned and recorded positions, image‐based measurements showed the leaf to be 1.3±0.1 mm medial from the planned position. This offset was confirmed with the test pattern irradiations.Conclusion:It has been clinically observed that log‐file derived leaf positions can differ from their actual positions by &gt;1 mm, and therefore cannot be considered to be the actual leaf positions. This cautions the use of log‐based methods for MLC or patient quality assurance without independent confirmation of log integrity. Frequent verification of MLC positions through independent means is a necessary precondition to trusting log file records. Intra‐treatment EPID imaging provides a method to capture departures from MLC planned positions.Work was supported in part by Varian Medical Systems.

SU‐E‐T‐624: Portal Dosimetry Commissioning of Multiple (6) Varian TrueBeam Linacs Equipped with PortalVision DMI MV Imager

Purpose:To show that a single model for Portal Domisetry (PD) can be established for beam‐matched TrueBeam™ linacs that are equipped with the DMI imager (43×43cm effective area).Methods:Our department acquired 6 new TrueBeam™s, 4 “Slim” and 2 “Edge” models. The Slims were equipped with 6 and 10MV photons, and the Edges with 6MV. MLCs differed between the Slims and Edges (Millennium 120 vs HD‐MLC respectively). PD model was created from data acquired using a single linac (Slim). This includes maximum field size profile, as well as output factors and acquired measured fluence using the DMI imager. All identical linacs were beam‐matched, profiles were within 1% at maximum field size at a variety of depths. The profile correction file was generated from 40×40 profile acquired at 5cm depth, 95cm SSD, and was adjusted for deviation at the field edges and corners. The PD model and profile correction was applied to all six TrueBeam™s and imagers. A variety of jaw only and sliding window (SW) MLC test fields, as well as TG‐119 and clinical SW and VMAT plans were run on each linac to validate the model.Results:For 6X and 10X, field by field comparison using 3mm/3% absolute gamma criteria passed 90% or better for all cases. This was also true for composite comparisons of TG‐199 and clinical plans, matching our current department criteria.Conclusion:Using a single model per photon energy for PD for the TrueBeam™ equipped with a DMI imager can produce clinically acceptable results across multiple identical and matched linacs. It is also possible to use the same PD model despite different MLCs. This can save time during commissioning and software updates.

Clique-Based Clustering of Correlated SNPs in a Gene Can Improve Performance of Gene-Based Multi-Bin Linear Combination Test

Gene-based analysis of multiple single nucleotide polymorphisms (SNPs) in a gene region is an alternative to single SNP analysis. The multi-bin linear combination test (MLC) proposed in previous studies utilizes the correlation among SNPs within a gene to construct a gene-based global test. SNPs are partitioned into clusters of highly correlated SNPs, and the MLC test statistic quadratically combines linear combination statistics constructed for each cluster. The test has degrees of freedom equal to the number of clusters and can be more powerful than a fully quadratic or fully linear test statistic. In this study, we develop a new SNP clustering algorithm designed to find cliques, which are complete subnetworks of SNPs with all pairwise correlations above a threshold. We evaluate the performance of the MLC test using the clique-based CLQ algorithm versus using the tag-SNP-based LDSelect algorithm. In our numerical power calculations we observed that the two clustering algorithms produce identical clusters about 40~60% of the time, yielding similar power on average. However, because the CLQ algorithm tends to produce smaller clusters with stronger positive correlation, the MLC test is less likely to be affected by the occurrence of opposing signs in the individual SNP effect coefficients.

SU-E-T-24: Development and Implementation of an Automated Algorithm to Determine Radiation Isocenter, Radiation vs. Light Field Coincidence, and Analyze Strip Tests.

The aim of this study was to develop a phantom and analysis software that could be used to quickly and accurately determine the location of radiation isocenter using the Electronic Portal Imaging Device (EPID). The phantom could then be used as a static reference point for performing other tests including: radiation vs. light field coincidence, MLC and Jaw strip tests, and Varian Optical Guidance Platform (OGP) calibration. The solution proposed uses a collimator setting of 10×10 cm to acquire EPID images of the new phantom constructed from LEGO® blocks. Images from a number of gantry and collimator angles are analyzed by the software to determine the position of the jaws and center of the phantom in each image. The distance between a chosen jaw and the phantom center is then compared to the same distance measured after a 180 degree collimator rotation to determine if the phantom is centered in the dimension being investigated. The accuracy of the algorithm's measurements were verified by independent measurement to be approximately equal to the detector's pitch. Light versus radiation field as well as MLC and Jaw strip tests are performed using measurements based on the phantom center once located at the radiation isocenter. Reproducibility tests show that the algorithm's results were objectively repeatable. Additionally, the phantom and software are completely independent of linac vendor and this study presents results from two major linac manufacturers. An OGP calibration array was also integrated into the phantom to allow calibration of the OGP while the phantom is positioned at radiation isocenter to reduce setup uncertainty contained in the calibration. This solution offers a quick, objective method to perform isocenter localization as well as laser alignment, OGP calibration, and other tests on a monthly basis.

SU-E-T-534: Beam and MLC Commissioning and Assessment of Three Commercial Treatment Planning Systems.

To assess and compare the open beam and multi-leaf collimator modeling of Pinnacle, Ecilpse (AAA and Acuros) and RayStation planning systems. The 6MV photon beam of a Varian TrueBeam with Millennium 120 MLC was used for this study. Measurements made with combinations of ion chamber, radiochromic film, and diodes in water and plastic phantoms. Depth and crossplane profiles of open square fields shaped by jaws or MLC ranged from 3×3 to 40×40cm2 and from 0 to 20 cm depth. Depth dose, flatness (80% of FWHM), and penumbra (20-80%) of calculated and measured profiles were compared. Various MLC test patterns were calculated and compared with measurements to assess the modeling of the round leaf edge, tongue-and-groove, and interleaf transmissions. Calculated depth doses are within 1.0% and flatness is within 2% for all field sizes and depths. Jaw penumbrae are within 2mm and 3mm for 20×20 and 30×30cm2 at 10cm depth respectively. MLC penumbrae (20-80%) of the three systems are within 0.3mm and 1.0mm for a 3×3cm2 and 10×10cm2 MLC apertures. Notably, to match the measured MLC round-edge transmission, the half thickness (10% transmission) leaf-tip width of the current RayStation MLC model has to be broadened to 10mm. All three systems appear to adequately model the tongue-and-groove. Pinnacle explicitly models the interleaf transmission while Eclipse and RayStation simply use average MLC transmission. All three systems are capable of generating clinically acceptable beam models for open fields. Based upon the round-edge profile, Eclipse and Pinnacle provide better MLC models than RayStation. Among the three systems, Eclipse took the least time and effort to commission these features.

SU‐E‐T‐352: Commissioning and Quality Assurance of the First Commercial Hybrid MRI‐IMRT System

Purpose: To describe the commissioning process for the first installed commercial MRI‐guided IMRT system (ViewRay, Village of Oakwood, OH) and outline quality assurance methods for this novel treatment modality. Methods: The ViewRay™ System (510(k) pending) consists of a 0.35‐T double‐doughnut MRI coupled with a gantry that houses three Co‐60 sources,each with an activity up to 15,000 Ci (120° apart). IMRT delivery is enabled by doubly‐focused MLCs that serve as the only beam‐shaping collimators for each head, allowing a maximum field size of 27.3 cm2 at the 105‐cm isocenter. MRI imaging is used prior to and during delivery for setup evaluation, adaptive radiotherapy, and gating. The challenges in commissioning as well as periodic and patient‐specific QA arise due to the presence of the magnetic field, unique geometry of this device which is not compatible with many of conventional RT QA devices and techniques, and the penumbra of a 2‐cm wide Co‐60 source. The following devices were used for commissioning and quality assurance tests: radiochromic and radiographic film, ionization chambers (Exradin A12 &amp; A16), Sun Nuclear's IC Profiler (Melbourne, FL), and a water tank. Tests were conducted to evaluate the beam profiles, penumbra, and PDDs. Also, tests to check MLC accuracy and reproducibility were evaluated. Results: Tests were developed to validate geometric performance of the device including a comprehensive set of MLC tests. Due to the low strength of the magnetic field, the mean free path of electrons in the ionization chamber volume is too long to have a noticeable curvature; therefore the magnetic field is not expected to have a noticeable effect on dose measurement. Conclusions: While the presence of the magnetic field limited the choice of QA devices, it was found that satisfactory methods for MRI‐IMRT machine QA exist and can be successfully employed. Drs. Green, Goddu, and Mutic served as scientific consultants for ViewRay, Inc. Dr. Mutic is on the clinical focus group for ViewRay, Inc., and his spouse holds shares in ViewRay, Inc.

Dose verification of IMRT by use of a COMPASS transmission detector

Our purpose in this study was to evaluate the fundamental accuracy of reconstructed dose distributions from the COMPASS system using specific MLC test patterns and complicated IMRT neck plans. The COMPASS-reconstructed dose distributions were compared with those measured with EPID, MapCHECK, and EDR2 film and as well as Monte Carlo-calculated dose profiles with use of square-wave chart patterns of 20-, 10-, and 5-mm gaps and step and pyramid patterns. Additionally, the COMPASS dose distributions for clinical IMRT neck plans were tested. The COMPASS dose profiles were almost in agreement with the Monte Carlo-calculated dose profiles and point doses measured with MapCHECK for 20- and 10-mm gap patterns. The dose profile for a 5-mm gap pattern showed a narrow width due to the detector size in the penumbra region. For step and pyramid patterns, COMPASS agreed with MapCHECK and Monte Carlo calculation, except for EDR2 film. The COMPASS and MapCHECK dose distributions agreed with that of a treatment planning system by gamma analysis (criteria; 3mm/3%). In comparisons of clinical IMRT neck dose distributions, COMPASS was measured with almost the same accuracy as MapCHECK, but slight deviations were found for large IMRT fields. These deviations could be minimized by improvement of the beam model of the COMPASS system. The COMPASS system can be expected to be used for traditional QA methods in clinical routine with the same accuracy as a MapCHECK diode detector.

SU‐GG‐T‐387: Matching of Flattening‐Filter‐Less (FFL) to Standard (STD) Linac Beams Based on Monte Carlo Simulations

Purpose: Flattening‐Filter‐Less (FFL) linacs increase the dose rate and therefore decrease treatment time, which may minimize the possibility of intra‐fraction motion. However, up till now only a few FFL linac studies have been reported and FFL beam characteristics are not completely known. Method&amp;Materials: This study tries to investigate the impact of the MLC‐modulation for the same prescribed dose using FFL instead of STD linac, by matching two linac penetration (PDD) and dose output (calibration dose per MU for given reference conditions) and by determining the field size and depth dependence of the relative beam intensities across the field and relative MLC properties for the two linacs. We commissioned Monte Carlo (MC) dose calculation for Varian21EX(6MV) standard machine by matching the golden beam dose data (GBD). The electron beam energy was determined to be ESTDEGBD=6.5MeV. Next, beam properties were determined for FFL(EFFL=ESTD). Linac matching procedure was then performed by first matching the PDD by the increasing the energy, and by calculating the MU‐correction factor FMU equal to dose ratio FFLmatch/STD. Next, relative beam intensities FOAR(x,d,c)=OARSTD(x,d,c)/OARFFLmatch(x,d,c) were determined for open beams and parameterized for different field sizes c and depths d. Similar properties were determined for sweeping gap dynamic MLC motion tests for various gap sizes (G=1,5,10,20mm) and other more complex MLC tests. Finally, relative MLC transmission properties were determined FMLC=TSTD/TFFLmatch. Additional beam data was generated for IMRT‐relevant symmetric and asymmetric rectangular fields for open and MLC‐modulated beams for both linacs. Results: Adjusted electron energy for FFL was 8.0MeV to 8.5MeV, depending of the field size. Monitor Unit correction factor was FMU≈1.8 and 3.3 for FFL and FFLmatch linacs correspondingly. The other factors were: FMLC=1.2, FOAR(x,40)=1.003+0.003⋅x2 − 0.000003⋅x4+0.1⋅Erf[0.2⋅x]2 for c=40cm and FOAR(x,c)≈FOAR(x⋅(1+40/c),40) for other field sizes c. Conclusion: Presented parameterization of beam data is essential in analysis of factors influencing IMRT on FFL linacs.

SU‐GG‐T‐203: Dosimetric Properties of a Transmission Detector in a 6 MV Photon Beam

Introduction: The objective of this study was to investigate dosimetric properties of a new transmission dosimeter system. The system, COMPASS (IBA Dosimetry, Germany), contains 1600 plane parallel ionization chambers with a detector spacing of 6.5 mm and an active volume of 0.02 cm3. The COMPASS detector is mounted on the gantry of the linear accelerator to perform on‐line dose vilification during treatment. We checked capability of the system for dose verification in clinical intensity modulated radiotherapy (IMRT) plans. Methods and Materials: Fist, the COMPASS system was modeled by dosimteric data (percent depth dose, off‐axis ratio, and output factor) for a 6 MV photon beam. The dosimetric properties for simple square fields and various MLC test patterns were checked by comparison with those of MapCHECK and EDR2 films. The Monte Carlo simulations were also used as needed. The system was also compared for dose verification in clinical IMRT plans of neck cases. Results and Discussion: The influence of a transmission detector for radiation fields was insignificant except for buildup regions. For MLC test patterns, the COMPASS dose profiles agreed within 3% with those of MapCHECK, EDR2 films, and Monte Carlo simulations, except for complex MLC test patterns. Monte Carlo simulations agreed within 2% with EDR2 in the region more than 50cGy. In clinical examples, COMPASS agreed well with EDR2. COMPASS is a detector having the same accuracy as ion chambers in output measurements and is similar to a film for the dose profiles. The reason is because COMPASS dose distributions are based on measured fluence maps. Conclusions: The COMPASS system is useful for dose verification for IMRT plans. The system can use with the similar accuracy to traditional film measurements and 2D detectors. The beam model needs to be improved for complex dose profiles.

Poster — Wed Eve—27: Feasibility of Using Amorphous Silicon EPIDs for QC of MLCs

The correct delivery of dynamic IMRT treatments is dependent on proper functioning of the MLCs. Recent publications suggest that an error of 0.3 mm in leaf position could lead to a 2% error in absolute dose to the target volume. QC tests that allow quick checks of the reproducibility of leaf positions and speeds would provide additional confidence in the correct functioning of the MLCs on a frequent basis. The feasibility of utilizing amorphous silicon EPIDs for QC of MLCs was evaluated with aS1000 EPIDs. Portal images of the MLC test patterns were acquired and analyzed in Matlab. Portal images of a picket fence pattern (1 cm steps separated by standard gaps between leaf ends) were acquired at different times and collimator and gantry angles (0° and 90°). The consistency in MLC leaf positions was evaluated by calculating the second derivative of image intensity profiles, allowing analysis of an image in 1–2 minutes. The measured standard gap was consistent within 0.5 mm, indicating reproducibility over time, and at gantry angles of 0° and 90°, indicating no measurable gravitational effects. Portal images of a wedge intensity pattern were acquired at various MUs to investigate effects of MLC acceleration/deceleration. Analysis of the wedge intensity pattern demonstrated a linear relationship between dose delivered and response of the EPID indicating consistency in leaf speed. QC tests developed will allow for daily robust evaluation of the performance of MLCs at different gantry and collimator angles without using films.

WE-D-AUD B-09: Fluence Variability and Errors in Dose Reconstructed From Exit Fluence Due to Patient Anatomical Variations

Purpose: To test the basic tenets of exit‐fluence‐based dose reconstruction by measuring incident fluence variability and by determining the error introduced by incorrectly attributing measured patient‐induced exit fluence deviations to incident fluence deviations. Method and Materials: Fluence reproducibility of a Varian linac's 6 and 18 MV beams is measured for open fields, MLC test fields, and a sample complex patient field using an aS1000 EPID. Both short‐ and long‐term reproducibility are measured, using a method designed to separate fluence and detector variability. To demonstrate potential pitfalls of attributing patient‐related fluence deviations to incident fluence variations, EPIDimages are calculated using a Monte Carlo‐based algorithm for both the original patient anatomy and for a variant anatomy in which the patient's rectum is filled with air. Deviations between the computed exit images are used to derive entrance fluence ‘corrections’, which are then used in the Monte Carlo‐based algorithm to reconstruct the patient dose. Results: For MLC test fields and the complex patient field, measured fluence variability is <2%. Comparison of the planned, actual and reconstructed doses finds that the difference between the planned and actual dose is substantially smaller than the difference between the reconstructed and actual dose. The uncorrected original planned dose is a better estimator of the actual dose than the reconstructed dose. Conclusion: Attributing measured exit fluence variations to incorrect incident fluence delivery appears to be both unjustified and incorrect. Assigning patient‐induced exit fluence deviations to incident‐fluence deviations increases the error in patient dose estimation. Dose reconstruction breaks down when its basic premise is violated, that is, when fluence deviations are caused by anatomical changes instead of fluence delivery variations. Conflict of Interest: This work was supported in part by Varian Medical Systems.

TU‐D‐M100F‐02: Automated Quality Assurance for Helical Tomotherapy Using Exit Detector Data

Purpose: The purpose of this work was to develop techniques for utilizing exit detector data in a helical tomotherapy system to automate the QA process. The clinical significance of this study is that an analysis of the exit detector data acquired during treatment delivery could be used to ensure that the correct delivery sequence is being administered to the patient. Method and Materials: Software applications were developed to automatically analyze uncompressed detector data. To test this software, a MLC test sequence was designed that allows each MLC leaf to be tested using the exit detector data. The goals of this test are 1.) To identify MLC problems (stuck leafs, bad valves, etc…) before failure, and 2.) Perform QA Tests on MLC parameters that affect the delivered dose. Additionally, a clinical test case was created using the treatment delivery sequences for a head &amp; neck patient. The original treatment delivery sequence was modified and 12 known MLC errors were inserted in the MLC controller file. The procedures were delivered and the software was used to analyze the exit detector data. Results: The MLC QA Test was delivered on four occasions with two MLCs. A software application developed by the investigators was then used to analyze the exit detector data from these deliveries. The software correctly performed Latency Tests, Projection Centering Tests, and MLC Transit Time Tests. For each delivery, the tests showed that the MLCs were properly functioning. For the Head &amp; Neck test case, the shape‐detection algorithm was able to identify 11 out of 12 known MLC errors &gt;10 msec. Conclusion: A technique was developed for performing automated QA of the MLC and individual patient deliveries using exit detector data in a helical tomotherapy system. With the tomotherapy detector array, errors in MLC position &gt; 10 msec have been detected.