EDR2 Film Research Articles

Independent quality assurance of a helical tomotherapy machine using the dose magnifying glass

Helical tomotherapy is a complex delivery technique, integrating CT image guidance and intensity modulated radiotherapy in a single system. The integration of the CT detector ring on the gantry not only allows patient position verification but is also often used to perform various QA procedures. This convenience lacks the rigor of a machine-independent QA process. In this article, a Si strip detector, known as the Dose Magnifying Glass (DMG), was used to perform machine-independent QA measurements of the multileaf collimator alignment, leaf open time threshold, and leaf fluence output factor (LFOF). The DMG measurements showed good agreements with EDR2 film for the MLC alignment test while the CT detector agrees well with DMG measurements for leaf open time threshold and LFOF measurements. The leaf open time threshold was found to be approximately 20 ms. The LFOF measured with the DMG agreed within error with the CT detector measured LFOF. The DMG with its 0.2 mm spatial resolution coupled to TERA ASIC allowed real-time high temporal resolution measurements of the tomotherapy leaf movement. In conclusion, DMG was shown to be a suitable tool for machine-independent QA of a tomotherapy unit.

Comparison of four commercial devices for RapidArc and sliding window IMRT QA

For intensity‐modulated radiation therapy, evaluation of the measured dose against the treatment planning calculated dose is essential in the context of patient‐specific quality assurance. The complexity of volumetric arc radiotherapy delivery attributed to its dynamic and synchronization nature require new methods and potentially new tools for the quality assurance of such techniques. In the present study, we evaluated and compared the dosimetric performance of EDR2 film and three other commercially available quality assurance devices: IBA I'MatriXX array, PTW Seven29 array and the Delta 4 array. The evaluation of these dosimetric systems was performed for RapidArc and IMRT deliveries using a Varian NovalisTX linear accelerator. The plans were generated using the Varian Eclipse treatment planning system. Our results showed that all four QA techniques yield equivalent results. All patient QAs passed our institutional clinical criteria of gamma index based on a 3% dose difference and 3 mm distance to agreement. In addition, the Bland‐Altman analysis was performed which showed that all the calculated gamma values of all three QA devices were within 5% from those of the film. The results showed that the four QA systems used in this patient‐specific IMRT QA analysis are equivalent. We concluded that the dosimetric systems under investigation can be used interchangeably for routine patient specific QA.PACS numbers: 87.55.Qr, 87.56.Fc

Dosimetric accuracy of tomotherapy dose calculation in thorax lesions

BackgroundTo analyse limits and capabilities in dose calculation of collapsed-cone-convolution (CCC) algorithm implemented in helical tomotherapy (HT) treatment planning system for thorax lesions.MethodsThe agreement between measured and calculated dose was verified both in homogeneous (Cheese Phantom) and in a custom-made inhomogeneous phantom. The inhomogeneous phantom was employed to mimic a patient's thorax region with lung density encountered in extreme cases and acrylic inserts of various dimensions and positions inside the lung cavity. For both phantoms, different lung treatment plans (single or multiple metastases and targets in the mediastinum) using HT technique were simulated and verified. Point and planar dose measurements, both with radiographic extended-dose-range (EDR2) and radiochromic external-beam-therapy (EBT2) films, were performed. Absolute point dose measurements, dose profile comparisons and quantitative analysis of gamma function distributions were analyzed.ResultsAn excellent agreement between measured and calculated dose distributions was found in homogeneous media, both for point and planar dose measurements. Absolute dose deviations <3% were found for all considered measurement points, both inside the PTV and in critical structures. Very good results were also found for planar dose distribution comparisons, where at least 96% of all points satisfied the gamma acceptance criteria (3%-3 mm), both for EDR2 and for EBT2 films. Acceptable results were also reported for the inhomogeneous phantom. Similar point dose deviations were found with slightly worse agreement for the planar dose distribution comparison: 96% of all points passed the gamma analysis test with acceptable levels of 4%-4 mm and 5%-4 mm, for EDR2 and EBT2 films respectively. Lower accuracy was observed in high dose/low density regions, where CCC seems to overestimate the measured dose around 4-5%.ConclusionsVery acceptable accuracy was found for complex lung treatment plans calculated with CCC algorithm implemented in the tomotherapy TPS even in the heterogeneous phantom with very low lung-density.

Analysis of changes in dose distribution due to respiration during IMRT

PurposeIntensity modulated radiation therapy (IMRT) is a high precision therapy technique that can achieve a conformal dose distribution on a given target. However, organ motion induced by respiration can result in significant dosimetric error. Therefore, this study explores the dosimetric error that result from various patterns of respiration.Materials and MethodsExperiments were designed to deliver a treatment plan made for a real patient to an in-house developed motion phantom. The motion pattern; the amplitude and period as well as inhale-exhale period, could be controlled by in-house developed software. Dose distribution was measured using EDR2 film and analysis was performed by RIT113 software. Three respiratory patterns were generated for the purpose of this study; first the 'even inhale-exhale pattern', second the slightly long exhale pattern (0.35 seconds longer than inhale period) named 'general signal pattern', and third a 'long exhale pattern' (0.7 seconds longer than inhale period). One dimensional dose profile comparisons and gamma index analysis on 2 dimensions were performedResultsIn one-dimensional dose profile comparisons, 5% in the target and 30% dose difference at the boundary were observed in the long exhale pattern. The center of high dose region in the profile was shifted 1 mm to inhale (caudal) direction for the 'even inhale-exhale pattern', 2 mm and 5 mm shifts to exhale (cranial) direction were observed for 'slightly long exhale pattern' and 'long exhale pattern', respectively. The areas of gamma index >1 were 11.88%, 15.11%, and 24.33% for 'even inhale-exhale pattern', 'general pattern', and 'long exhale pattern', respectively. The long exhale pattern showed largest errors.ConclusionTo reduce the dosimetric error due to respiratory motions, controlling patient's breathing to be closer to even inhaleexhale period is helpful with minimizing the motion amplitude.

Dosimetric analysis of beam-matching procedure of two similar linear accelerators

Linear accelerators from the same vendor in a radiation therapy center are usually beam-matched following Vendor's acceptance criteria. This protocol is limited to check the difference at particular points on the ionization curve for depth dose or beam profiles. This article describes different tests done after commissioning to evaluate the level of agreement between matched beams of two ONCOR Impression plus linear accelerators from Siemens. Total scatter factors, collimator scatter factors, wedge transmission factors were measured in water for 6-MV photon. All these factors for ONCOR2 were within ±1% of those values for ONCOR1. Along with these point dose measurements we have essentially used γ-index to compare the planar dose distribution from two beam-matched accelerators. For this study a set of ready packed EDR2 films was exposed on both accelerators. The set consisted of films for percentage depth dose, beam profiles, a pyramid shape, multileaf collimator's positional and dose delivery accuracy, and a film to compare head scatter at tray level. To include treatment planning system calculations, a film kept in axial plane was exposed to 3DCRT and IMRT plans with actual gantry angles and monitor units. These films were analyzed for γ in OmniPro IMRT software using different combinations of Δdose and - Δdistances. All these films have shown good agreement for - Δdistance of 3 mm and Δdose of 3 %.

Ionization chamber array for patient specific VMAT, Tomotherapy and IMRT QA

The evaluation between measured and calculated dose is essential in the patient specific quality assurance procedures for intensity modulated radiation therapy. The high complexity of volumetric arc radiotherapy, Tomotherpay and intensity modulated radiation therapy deliveries attributed to the dynamic and synchronization requirements of such techniques require new methods and potentially new tools for the quality assurance of such techniques. Studies evaluating the dosimetric performance of EDR2 film and a 2D ionization chamber array quality assurance device have been performed in our institution. Our results showed that differences between the detector systems are small. The respective gamma index histograms showed that when 3% dose difference and 3mm distance to agreement are used, more than 90% of the evaluated points were within the tolerance criteria

Dosimetric characterization of a multileaf collimator for a new four‐dimensional image‐guided radiotherapy system with a gimbaled x‐ray head, MHI‐TM2000a)

To present the dosimetric characterization of a multileaf collimator (MLC) for a new four-dimensional image-guided radiotherapy system with a gimbaled x-ray head, MHI-TM2000. MHI-TM2000 has an x-ray head composed of an ultrasmall linear accelerator guide and a system-specific MLC. The x-ray head can rotate along the two orthogonal gimbals (pan and tilt rotations) up to +/- 2.5 degrees, which swings the beam up to +/- 41.9 mm in each direction from the isocenter on the isocenter plane perpendicular to the beam. The MLC design is a single-focus type, has 30 pairs of 5 mm thick leaves at the isocenter, and produces a maximum field size of 150 x 150 mm2. Leaf height and length are 110 and 260 mm, respectively. Each leaf end is circular, with a radius of curvature of 370 mm. The distance that each leaf passes over the isocenter is 77.5 mm. Radiation leakage between adjacent leaves is minimized by an interlocking tongue-and-groove (T&G) arrangement with the height of the groove part 55 mm. The dosimetric characterizations including field characteristics, leaf position accuracy, leakage, and T&G effect were evaluated using a well-commissioned 6 MV photon beam, EDR2 films (Kodak, Rochester, NY), and water-equivalent phantoms. Furthermore, the field characteristics and leaf position accuracy were evaluated under conditions of pan or tilt rotation. The differences between nominal and measured field sizes were within +/- 0.5 mm. Although the penumbra widths were greater with wider field size, the maximum width was < 5.5 mm even for the fully opened field. Compared to the results of field characteristics without pan or tilt rotation, the variation in field size, penumbra width, flatness, and symmetry was within +/- 1 mm/l% at the maximum pan or tilt rotational angle. The leaf position accuracy was 0.0 +/- 0.1 mm, ranging from -0.3 to 0.2 mm at four gantry angles of 0 degrees, 90 degrees, 180 degrees, and 270 degrees with and without pan or tilt rotation. The interleaf leakage was up to 0.21%, whereas the intraleaf leakage was < 0.12%. T&G decreased the doses by 10.7%, on average. This study demonstrated that MHI-TM2000 has the capability for high leaf position accuracy and low leakage, leading to highly accurate intensity-modulated radiotherapy delivery. Furthermore, substantial changes in the dosimetric data on field characteristics and leaf position accuracy were not observed even at the maximum pan or tilt rotation.

Validation of a treatment plan‐based calibration method for 2D detectors used for treatment delivery quality assurance

Dosimetry using film, CR, electronic portal imaging, or other 2D detectors requires calibration of the raw image data to obtain dose. Typically, a series of known doses are given to the detector, the raw signal for each dose is obtained, and a calibration curve is created. This calibration curve is then applied to the measured raw signals to convert them to dose. With the advent of IMRT, film dosimetry for quality assurance has become a routine and labor intensive part of the physicist's day. The process of calibrating the film or other 2D detector takes time and additional film or images for performing the calibration, and comes with its own source of errors. This article studies a new methodology for the relative dose calibration of 2D imaging detectors especially useful for IMRT QA, which relies on the treatment plan dose image to provide the dose information which is paired with the raw QA image data after registration of the two images (plan-based calibration). Validation of the accuracy and robustness of the method is performed on ten IMRT cases performed using EDR2 film with conventional and plan-based calibration. Also, for each of the ten cases, a 5 mm registration error was introduced and the Gamma analysis was reevaluated. In addition, synthetic image tests were performed to test the limits of the method. The Gamma analysis is used as a measure of dosimetric agreement between plan and film for the clinical cases and a dose difference metric for the synthetic cases. The QA image calibrated by the plan-based method was found to more accurately match the treatment plan doses than the conventionally calibrated films and also to reveal dose errors more effectively when a registration error was introduced. When synthetic acquired images were systematically studied, localized and randomly placed dose errors were correctly identified without excessive falsely passing or falsely failing pixels, unless the errors were concentrated in a majority of pixels in a contiguous narrow dose band. Irregularities seen in the calibration curve expose these errors. The plan-based calibration method was found to be an accurate, efficient procedure, capable of detecting IMRT QA relative dosimetry errors as well as, or better than conventional calibration methods.

Verification measurements and clinical evaluation of the iPlan RT Monte Carlo dose algorithm for 6 MV photon energy

This study presents data for verification of the iPlan RT Monte Carlo (MC) dose algorithm (BrainLAB, Feldkirchen, Germany). MC calculations were compared with pencil beam (PB) calculations and verification measurements in phantoms with lung-equivalent material, air cavities or bone-equivalent material to mimic head and neck and thorax and in an Alderson anthropomorphic phantom. Dosimetric accuracy of MC for the micro-multileaf collimator (MLC) simulation was tested in a homogeneous phantom. All measurements were performed using an ionization chamber and Kodak EDR2 films with Novalis 6 MV photon beams. Dose distributions measured with film and calculated with MC in the homogeneous phantom are in excellent agreement for oval, C and squiggle-shaped fields and for a clinical IMRT plan. For a field with completely closed MLC, MC is much closer to the experimental result than the PB calculations. For fields larger than the dimensions of the inhomogeneities the MC calculations show excellent agreement (within 3%/1 mm) with the experimental data. MC calculations in the anthropomorphic phantom show good agreement with measurements for conformal beam plans and reasonable agreement for dynamic conformal arc and IMRT plans. For 6 head and neck and 15 lung patients a comparison of the MC plan with the PB plan was performed. Our results demonstrate that MC is able to accurately predict the dose in the presence of inhomogeneities typical for head and neck and thorax regions with reasonable calculation times (5–20 min). Lateral electron transport was well reproduced in MC calculations. We are planning to implement MC calculations for head and neck and lung cancer patients.

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.

SU-GG-T-243: Quality Assurance Measurements for Varian/ RapidArc

Purpose: As Varian revealed “RapidArc radiotherapy technology is a major advance from Varian Medical Systems that improves dose conformity while significantly shortening treatment times.” It delivers IMRTtreatment plan with 360 degree rotation of linear accelerator gantry. The purpose of this analysis is to perform Quality Assurancemeasurements by using regular IMRT QA tools to verify the doses delivered with the new RapidArc radiotherapy technology. Method and Materials: 20 treatment plans had been validated which generated from VARIAN/ Eclipse (RapidArc Progressive Resolution Optimizer V.8.6.15). All plans were measured with two different dosimetric systems. The first system included Plastic‐Water phantom, EDR2 films (Kodak), Ion Chamber (Wellhofer) and Electrometer (FLUKE F35040). The other system contained PTW “Seven‐29 Ion Chamber Array” with two 5‐cm thick Plastic‐Water phantoms. Film Dosimetry was performed by VIDAR's DosimetryPRO Advantage film scanner. The dose distribution analysis and comparison were completed by PTW/ VeriSoft tool to verify dose distributions measured in phantom with dose distributions computed by a RTPS using RapidArc radiotherapy technology. Results: The results show both Dosemeasurements are within acceptable criteria (the ±5% tolerance limit). The data from ion chambermeasurements shows %Diff are within 2%. It is more accurate if comparing it to the results from PTW's IMRT plan verification tool. After normalized the Dose at measured QA point, both results show the ISO Dose line are well comparable to the Dose distribution generated from Eclipse/RapidArc treatment plans. Data from PTW shows 94%–100% of the measured points passing criteria Gamma ≤ 1.0 (acceptance criteria 3%/3mmDTA). Conclusion: For all two dosimetric systems, fine agreement was experimental between measured and calculated doses distribution. Results show that 2 independent QA tools (PTW & Ion‐Chamber with EDR2 films) could capably perform Quality Assurance Test for MU & Dose distrubutionVerification in RapidArc Treatment Planning.

SU-GG-T-105: Dosimetric Analysis of Matching Photon and Electron Fields for the Treatment of Head and Neck Cancers

Purpose: To evaluate the current method of treatingcancers of the head and neck at our institution. The method consists of using anterior off‐cord photon fields to treat the site of the primary tumor, matched with an overlapping posterior electron field. Methods and Materials: Nine patient specific treatment plans, along with their electron cone cutouts, were applied to a 15cm solid water phantom and recalculated to deliver the prescription dose. Each plan consisted of laterally opposed 6MV photon field's incident on the anterior portion of the neck, treating 180 or 200cGy prescribed to the isocenter, and a matching field of 9MeV electrons treating 180 or 200cGy to the 90% isodose line. EDR2 film was placed at depths of 2.0cm, 2.2cm, and 2.5cm from the phantom's left surface. A 6MV photoncalibration curve was generated and applied to the films. The plan is delivered with a 3mm overlap of the photon and electron fields. The dose was evaluated using three profiles evenly distributed through the junction. Each profile was normalized to the uniform portion of the photon field, and the maximum dose and the length of tissue receiving 125% of the prescription dose were measured. Results: The average maximum dose was found to be 148 ± 6% of the prescribed dose at 2.0cm depth and slightly less at other depths. Tissue irradiated to 125% dose ranged from 0mm to 31mm, with an average of 11 ± 6mm. Three plans had profiles exceeding 150%, with an average of 3 ± 1 mm receiving this high dose. Conclusion: At our institution we intentionally overlap photon and electron fields by 3mm at the match‐line to ensure there are no cold regions. The maximum dose can exceed 150% of the prescribed dose, with nearly the length of the match‐line experiencing dose exceeding the prescription.

SU-GG-T-427: Development of Monte Carlo Dose Verification System for MHI-TM2000

Purpose: MHI‐TM2000 is an innovative image‐guidedradiotherapy system with a C‐band X‐ray head on an O‐ring shaped gantry. We are developing an integrated Monte Carlo (MC)dose calculation system for four‐dimensional radiotherapy using MHI‐TM2000. The purposes of this study were to propose specific MC models of the X‐ray head and the multi‐leaf collimator(MLC) for MHI‐TM2000 and to validate their accuracy. Methods and Materials: 6 MV photon beam delivered by the MHI‐TM2000 unit was implemented by EGSnrc/BEAMnrc and EGSnrc/DOSXYZnrc. Subsequently, the X‐ray head composed of a target, a primary collimator, a flattening filter, a monitor chamber, a fixed secondary collimator, and a MLC was simulated based on the specification. Next, the central axis depth doses and the lateral doses at 15, 100, and 200 mm depth were simulated under the source to surface distance (SSD) of 900 mm. Then, Each of them was compared with the corresponding measurements using a CC06 ionization chamber and a water phantom. For the MLC model, Tongue‐and‐Groove (100 mm depth, SSD 900 mm), leaf leakage (50 mm depth, SSD 950 mm), and round leaf effects (100 mm depth, SSD 900 mm) were simulated. Meanwhile film measurements were performed using EDR2 film and a solid water phantom under similar conditions. These doses were normalized to the corresponding doses for the open field at the isocenter. The differences between simulated and measured doses were calculated. Results: For the X‐ray head, depth doses beyond the buildup region and lateral doses within the region of flatness showed agreement of within 1.3%. For each MLC test, the simulated and measured doses agreed less than 3.0%, respectively. Conclusions: We have demonstrated that the proposed MC models of the X‐ray head and the MLC for MHI‐TM2000 have reasonable accuracy. Conflicts of Interest: Research sponsored in part by Mitsubishi Heavy Industries, Ltd.

SU‐GG‐T‐312: Intrabeam‐Intraoperative Radiotherapy Unit Quality Assurarance Measurements with EDR2 Films and Radiological Imaging Technology

Purpose: EDR2‐QA films were developed for patient dosimetry in Intraoperative radiotherapy. In this study a method to evaluate the isotropy, PDD, Isodose and beam deflection using EDR2‐QA films and a vidar scanner with RIT Software was developed and tested. Methods and Materials: Intrabeam System is (50KeV, 40μA) intraoperative radiotherapy unit. X‐ray source (XRS) equipped with a 10 cm long (o/ 3.2 mm) probe. Five Films were irradiated for one minute each at a distance of 5cm from the source in all 5 directions (0°, 90°, 180°, 270° and perpendicular to the source) with flat surface facing towards the source. Percentage depth doses were derived by exposing the film vertically down from the source. Results: Horizontal and vertical profiles from films in all directions were taken and compared with the help of RIT software. A difference of 0.7% was found between the profiles at 12cm from the central axis. PDD's were obtained with the film (which was kept vertically down from the source) by normalizing the values at 2 cm depth (known dose) from the surface. These PDD's proved l/r3 and can be implemented on CT. Dose distributions were also derived by RIT software. Films were exposed at different distances from the source and found that these doses are comparable with the doses obtained from Ionization chamber (&lt;1%). Conclusion: Some of these QA procedures can be incorporated as periodic and some can be incorporated as regular QA procedures before taking any patient. These film measurements proved and shown the simple way of checking the Isotropy, PDD, Isodose and beam deflection. These tests also useful to take beam data for planning systems to view the dose on CT images in 2D and 3D, which is not available on the present system.

SU-GG-T-249: Delivery Quality Assurance for Stereotactic Body Radiotherapy (SBRT) Using a Teflon Cylindrical Phantom

Purpose: At our institution the delivery quality assurance (DQA) of Tomotherapy plans is accomplished with a cylindrical water equivalent phantom, EDR2 film, and two ion chambers for point dose measurements. Most plans deliver at most 5Gy to the dose plane, however, recently a stereotactic body radiotherapy(SBRT) protocol has produced plans delivering upwards of 12Gy to the film plane. EDR2 film saturates at a dose of ∼7Gy, requiring a modification of our DQA procedure for SBRT plans. To reduce the dose to the film plane and accommodate a possible move to SBRT using Varian RapidArc a new phantom of teflon has been constructed and tested. Method and Materials: Our teflon phantom is cylindrical in shape and of a similar design to the water equivalent phantom currently used for DQA. The cylinder is 4 inches in height, 12 inches in diameter, and bisected across the diameter. The phantom has two chamber positions located centrally, 5mm and 10mm above and below the film plane. The phantom was scanned into the Tomotherapy and Varian Eclipse planning systems using an MVCT scan on the Tomotherapy system. SBRT plans were delivered and film distributions and point dose measurements compared. Results: The phantom sufficiently reduced dose to the film plane for the DQA of SBRT plans. Both treatment planning systems appear capable of calculating an accurate absolute point dose in the phantom. The measured dose distribution in the phantom correlated very well with both planning system calculations (γ 95%). Conclusion: The teflon phantom appears to be a potential option for SBRT DQA. Preliminary data show that the planning systems are capable of calculating point doses in the teflon, and the dose to the film plane is reduced sufficiently to allow for a direct measured DQA without the need for dose rescaling.

SU-GG-T-85: Anisotropy Function Verification of I-125 Brachytherapy Source with Film Dosimetry

Purpose: For eye plaque implant with fixed source locations and orientations on the plaque, the variation of anisotropy function may significantly affect the brachy dose distribution following the TG43‐U1 algorithm. TLD‐based dosimetry for the brachy source anisotropy functions generally involves significant measurement time. In this study, we explore a convenient film‐based dosimetry verification of the anisotropy function for eye plaque I‐125 source, on a timely manner prior to the brachytherapytreatment planning. Method and Materials: EDR2 films (Kodak) were used to measure the dose distribution of I‐125 seeds (Amersham, model 6711). The center and alignment of the cylindrical I‐125 seed on the film dose plane were determined based on the anisotropic and symmetrical dose distribution fact. Then along the circles with radius R = 1cm, 2cm, 3cm from the seed center, point dose reading were taken to calculate the anisotropy function table and compared to published data (TG43‐U1). Results: For the comparisons between our film measurement data and the published data, the relative differences between the two data were small at radius R = 1cm i.e. mean ± STD as 0.9% ± 1.2%, while increasing with larger R — mean ±STD as 7.3% ± 6.5% at R = 2cm, and 11.3% ± 9.7% at R = 3cm. This increasing deviation trend with larger radius was anticipated considering the fact that more angle range will fall below the film lower dose response threshold at larger radius. Conclusion: The preliminary results showed that film dosimetry can give fairly precise estimation of the seed Anisotropy Function Table, as compared to the published data. For dosemeasurement at different radius from the seed, the film exposure time needs to be adjusted, avoiding the measureddose region exceeding the film dose responsive range.

TU-E-BRB-04: Confidence Limit Variation for a Single System Following the TG119 Protocol

Purpose: To evaluate the consistency of TG119-based quality assurance metrics for an IMRT system. Methods and Materials: Four planners constructed treatment plans for the five IMRT test cases described in TG119 using iPlan RT Dose 4.1 software. All plans were delivered on a Varian 6EX to a 30×30×15cm solid water phantom in one treatment session in order to minimize session-dependent variation from phantom setup, film quality, machine performance, etc. Composite measurements utilized EDR2 film and an Exradin A1SL ionization chamber (sensitive volume of 0.056 cm3). Each film was developed and digitized immediately after exposure. Per-field measurements were collected using a MapCHECK device at an effective depth of 5cm. All data collected were analyzed using the TG119 specifications to determine quality assurance metrics for each planner separately and then compared. Results: The mean variance of ion chamber measurements for each planner was within 1.7% of the planned dose. Resulting confidence limits were 3.77%, 1.98%, 4.50%, and 5.99%. Confidence limit values determined by composite film analysis were 26.43%, 25.02%, 21.50%, and 56.58%. Using normalization to a film point rather than to ionization chamber, these confidence limits were reduced to 12.27%, 12.73%, 6.96%, and 15.75%. Confidence limits from per-field measurements were 1.55%, 0.00%, 0.00%, and 2.89%. Conclusions: For a single IMRT system, the accuracy assessment provided by TG119-based quality assurance metrics showed significant variations across all composite and per-field evaluations. Performing the TG119 evaluation a single time may not provide an adequate estimation of IMRT system accuracy.

SU‐GG‐T‐283: Tool for Automated Monthly Quality Assurance on Tomotherapy Using Film

Purpose: To efficiently and accurately perform routine monthly quality assurance (QA) tests on a Helical Tomotherapy unit using an in‐house developed phantom and software Method and Materials: Helical Tomotherapy is an image guided intensity modulated radiation therapy delivery system. The system uses a dynamic delivery in which the gantry, treatment couch and multileaf collimator leaves are all in motion during treatment. The dynamic nature of helical tomotherapy requires QA that is different from a conventional linear accelerator. According to the AAPM Task Group report 148 (QA for Helical Tomotherapy) and previous guidelines, one needs to check longitudinal and transverse profiles along with field widths as part of the monthly QA. To perform this task we had to set up the tomotherapy water tank and use the tomotherapy electrometer measurement system (TEMS) for data collection and analysis. To simplify and expedite this process a film phantom was designed and MATLAB developed software was used for analyzing the irradiated film. One could simultaneously obtain several parameters like the longitudinal and transverse profiles, field widths, couch linearity and relative output factors for all the field widths from the film. A Kodak EDR2 film is placed in the phantom and exposed on the tomotherapy system to obtain all the field widths along with a step wedge for calibration on the same film. The irradiated film is then analyzed using the in‐house developed software and a report is printed out. Results and Conclusion: Using the phantom we could check the longitudinal and transverse profiles, field widths, couch linearity and relative output factors for all the field widths on the system. When compared to the couple of hours it would take for beam scanning, the film phantom and software have enabled us to cut down and quicken part of our monthly QA process to &lt;30 min.

SU-GG-T-99: Re-Evaluation of a Tungsten Made Commercial Eye Shield in Electron Beams

Purpose: To re‐evaluate the transmission factor for a tungsten made commercial eye shield in electron beams based on the dose to the periphery of lens. Method and Materials: Kodak EDR2 film and MOSFETdetector was used to measure the transmission factors of the tungsten internal eye shields in 6 and 9 MeV electron beams. The thickness of the tungsten is 3 mm with a 0.5 mm and a 1 mm thick aluminum caps provided to reduce backscatter to the eyelid. The doses to the periphery of lens were measured at 5 mm depth below the eye shields and 5 mm away from the central axis. The doses were then normalized to the dose to dmax under the same setup without eye shields to come up with the transmission factors. Comparatively the transmission factors referred by the manufacturer were determined with the lens dose measured along central axis. Results: The transmission factors for 3 mm tungsten, and 3 mm tungsten with 0.5 mm and 1 mm aluminum caps in a 6 MeV electron beam are 3.8%, 5.2% and 6.5%, respectively, based on film measurement, and 3.2%, 5.1% and 5.9%, respectively, based on MOSFETmeasurement. Those in a 9 MeV electron an d 4.5%, 4.9% and 5.3%, respectively, based on film measurement, and 4.5%, 4.9% and 5.3%, respectively, based on MOSFETmeasurement. The transmission factors referred by the manufacturer are 2.5%, 2.4%, 2.5%, respectively, for a 6MeV beam, and 3.3%, 2.9%, 2.8%, respectively, for a 9 MeV beam. Conclusion: The re‐evaluated transmission factors are significantly larger than those referred by the manufacturer. Aluminum caps can increase significantly the dose to the peripheral region of shielded area due to their scattering contribution.

SU‐GG‐T‐212: Verification of IMRT Plans for SRT with 2D Detector Array

Purpose: To evaluate the use of 2D detector array for verification of intensity modulated radiotherapy (IMRT) fields for stereotactic radiotherapy (SRT) treatments. Method and Materials: We evaluated ten SRT plans with field sizes ranging from 3 cm to 6 cm with MapCHECK device that is routinely used for IMRT plan verification. We calculated the percent of passing points (PPP) for the distance to agreement (DTA) for absolute dose between measured and planned isodose lines above 10% using the MapCHECK software. We selected a value of 3 mm for DTA and 3% dose difference criteria for comparison. Dose sampling by MapCHECK detector array is limited for small field sizes due to detector geometry with 0.7 cm spatial resolution diagonally. To increase the number of points for more accurate evaluation, we repeated the measurement four times by shifting the array 0.5 cm in orthogonal directions. We then compare the combined measurement for each plan. We also measured the relative dose fluence using Kodak EDR2 films. We performed the γ analysis both for 3%, 3 mm and 2%, 2 mm criteria. The OmniPro‐I'mRT software was used for film comparison with treatment plans. There was no direct comparison between film results and MapCHECK measurements. Results: The average PPP for merged shifted fields is 98.7%, while for single plan delivery is 98.5%. Slightly lower PPP was shown for a 3 cm field size plan with high dose gradient. For each individual plan, the variation between shifted measurements could rise as high as 8% for smaller fields. The average PPPs for film and MapCHECK compared to planned dose are 98.7% vs. 100% for γ analysis with 3%, 3 mm criteria and 92.2% vs. 97.7% for γ with 2%, 2 mm, correspondingly. Conclusion: MapCHECK is an appropriate clinical tool for verification of IMRT plans for SRT.