bluuebunny/crossref_metadata_2025 huggingface dataset:
SU‐E‐T‐127: Two Dosimetric Methods of Measuring Linac Beam Spot Position
Authors: Nyiri B , Smale J , Gerig L
Published: November 2011

Purpose: Two dosimetric methods of measuring the electron beam spot position of photon beams w.r.t. the Collimator Axis Of Rotation (CAOR) are described. Methods: Method 1 (SNG test) uses a cylindrical ion chamber (IC) (A1SL) in a phantom mounted on a jig co‐rotational with the collimator making the relationship among the chamber, phantom, jaws and CAOR fixed and independent of collimator angle. A jaw parallel to the IC axis is set to zero and the phantom adjusted so the IC signal is 65% of the open field value. cGy/mu is measured every 30 degrees of collimator rotation. The phantom is then translated normal to the IC axis until the signal is 35%. The shift is recorded to calculate sensitivity (%dose/mm). For a radially symmetric spot centred on CAOR, the IC signal is angularly constant. Deviation from constant indicates a displacement from the CAOR and a shape distortion. The displacement and spot shape are calculated using Fourier analysis and penumbra shape (sensitivity). The 2nd method uses a hard wedge instead of the jaw and sensitivity is obtained from wedge profiles. Test measurements were performed on an Elekta 6 MV: the In Plane beam spot position was measured before and after it was moved by changing bending magnet current (Bending Fine). Results: Independent beam spot measurement (Flexmap Image Registration) showed the initial beam spot, projected back to source, 0.29 mm from CAOR, moving to 0.91 mm (delta=0.62 mm) with steering. The SNG test reported the beam spot moving from 0.13mm to 0.92 mm from CAOR (delta=0.79 mm). The hard wedge test gave an initial position of 0.09 mm off axis and a final position of 0.79 mm (delta=0.7 mm) Conclusions: The methods described provide a quantitative measure of beam spot position, not requiring the user to independently define the CAOR.

Jason Smale is an Employee of Elekta Canada
SU‐E‐T‐133: A Method to Improve the Accuracy of Optical‐CT 3D Dosimetry by Correcting for Stray Light
Authors: Thomas A , Newton J , Oldham M
Published: November 2011

Purpose: Radiochromic plastics have recently emerged which can yield 3D dose information over clinical volumes in high resolution. These dosimeters require a fast, accurate read‐out, and broad beam optical‐Computed‐Tomography (optical‐CT) systems have shown promise, but can be sensitive to stray light artifacts. Here we present and evaluate a new method to correct for stray light artifacts in optical‐CT by deconvolving a measured, spatially invariant, point spread function (PSF). Methods: The correction was developed for the DLOS telecentric scanner (Duke Large field of view Optical‐CT Scanner). The PSF was constructed from a series of varying sized aperture images. The spatial invariance and robustness of the PSF is key to implementation, and was thoroughly investigated, including with/without the presence of an irradiated/unirradiated dosimeter, and across the field of view. The efficacy of the correction was evaluated both on individual projection images, containing known OD variations, and onknown 3D dose distributions. Results: The dominant source of stray light in this telecentric system was determined to originate in the imaging lens. Corrections of up to 3% required for heavily dosed dosimeters. Slight variations in PSF were observed under wide conditions, but this corresponded to small uncertainty in the correction (<1%). Accounting for stray light extended the dynamic range of the system from ∼30dB to ∼60dB giving significantly better potential for CNR. Conclusions: The method is shown to accurately account for stray light artifacts. The telecentric nature of the DLOS system brings two major advantages over non‐telecentric systems. First, the general amount of stray light in the projections is much less, due to removal through the telecentric imaging lens. Second, the remaining stray light appears to behave such that the PSF is spatially invariant and robust under a variety of conditions. The latter fact enables elegant accurate stray light correction using deconvolution.
SU‐E‐T‐135: 3D Dose Distribution Measurement with a New TLD Sheet
Authors: Kurokawa C , Urushiyama A , Ozawa S , Toramatsu C , Sugimoto S , Fujita T , Tsutsumi Y , Ito K , Karasawa K
Published: November 2011

Purpose: To evaluate the feature of a TLD sheet for dose distribution measurements of photon and electron beams Method and Materials:3D dose distributions from 10×10 cm2 to 20 × 20 cm2 field sizes for photon and electron beams were measured using a TLD sheet, which has 40 × 40 cm2 dimension and 0.3 mm thickness. Its density and effective atomic number are 1.0 g/cm3 and 7.4, respectively. Before the measurements of dose distributions, the irradiation was done to calibrate the response of each TLD sheet. The read out temperature was 180 °C and generated light was detected with a CCD camera which has 1024 × 1024 pixels (FLIML 1001E‐2, Finger Lakes Instrument). Results: Dose profiles were obtained as smooth curve after the calibration. Measured PDD with the TLD sheet well reproduced the results with ionization chamber, except for the surface region. The measured dose with TLD at surface is around 20%, which is lower more than 20% comparing to the dose with ionization chamber. The 3D dose distribution with TLD sheet shows that the low dose region has isotropic form at surface. We demonstrated the prostate IMRT dose distribution also. The result reproduces the calculated result of treatment planning system qualitatively. Conclusion: It is shown that the TLD sheet has the potential to measure the 3D dose distribution including low dose at surface. For the IMRT dose distribution, the measured result with TLD qualitatively reproduces the calculated dose distribution. For a quantitative comparison with TPS dose calculation, we need to improve the TLD feature.
SU‐E‐T‐137: Evaluating How the Sample‐Volume Dependence of Well‐Type Chambers Affects the Current Clinical Determination of Y‐90 Microsphere Activity
Authors: Paxton A , Micka J , DeWerd L
Published: November 2011

Purpose: To evaluate how the sample‐volume dependence of well‐type chambers affects the current clinical determination of Y‐90 microsphere activity.Methods: A Y‐90 standard activity solution from the NIST SRM program was used for all measurements. Ionization current measurements were made in a Standard Imaging IVB1000 well chamber and a Capintec 12‐atm dose calibrator with Sirtex Medicalˈs glass shipping vial and plastic v‐vial for volume levels from 1mL to 5mL in 1mL increments. A calibration coefficient was calculated for each volume level and the percent difference from the 5mL calibration coefficient was calculated. To evaluate how the differences in calibration coefficients between the volume levels affect the delivered activity, a theoretical prescription level of 2.4GBq was assumed. Using the appropriate calibration coefficients and the source measurement methods suggested by Sirtex Medical, the actual activity that would be delivered and percent difference from 2.4GBq were calculated.Results: The largest variation in calibration coefficient for each chamber and vialconfiguration was between the 5mL and 1mL volume levels. The largest difference of 45% was seen in the IVB1000 well chamber with the shipping vial and the smallest difference of 1.3% was seen in the 12‐atm dose calibrator with the v‐vial. These differences translated into 11.3% and 0.3% differences, respectively, from the theoretical 2.4GBq prescription. A 2.6% difference from the 2.4GBq prescription was found for the Capintec 12‐atm with the shipping vial held in the dipper holder, which represents the current clinical measurement geometry Conclusions: Based on the results, the IVB1000 would not be recommended for the current activity determination methods as proposed by Sirtex Medical for microspheres. The 2.6% difference determined for the currently used measurement geometry may be resolved with calibration and measurement with the v‐vial.
SU‐E‐T‐138: Evaluation of CAFCHROMIC® EBT2 Films as An Absolute and Relative Dosimeters in Small Field Dosimetry
Authors: Huang Y , Kim J , Li H , Kim S , Wen N , Nurushev T , Chetty I , Jin J
Published: November 2011

Purpose: The accuracy of second generation EBT film (EBT2) in commissioning of small fields for stereotactic radiosurgery (SRS) has yet to be demonstrated. We evaluate EBT2 films against a diode in a set of small circular fields collimated by cones. We investigate the challenges of EBT2 films for small field dosimetry and evaluate its accuracy as an absolute or relative dosimeter against diode measurements. Methods: Output factors (OFs), central‐axis percentage depth dose curves (PDDs) and cross‐beam profiles from a set of 9 Brainlab® SRS cones, with diameters ranging from 5 to 30 mm, were measured with a Scanditronix stereostatic diode(SFD) on a Varian Trilogy Linac. Measurements were repeated using EBT2 films in a Solid Water phantom. Blue‐color channel correction, as recommended by the vendor, was also investigated for reducing the effect of film inhomogeneity. Results: The OFs with EBT2 and diode agree within 1.1%, on average, for cone sizes 10 mm and above. SFD measures higher OFs than EBT2 for cones smaller than 10mm, likely due to the water nonequivalence of the silicon material in the diode detector which effectively reduces lateral electronic disequilibrium in small fields. Profiles and PDDs between EBT2 and diode match closely with each other. Film measurements were sometimes irregular due to non‐homogeneous film response. Blue‐color channel correction did not mitigate the film inhomogeneity. Conclusions: This study demonstrates that EBT2 films can be applied as absolute and relative dosimeters for measuring dose in small fields. Because EBT2 are more water‐equivalent than diode, it is likely that OFs measured by EBT2 in cones smaller than 10 mm are closer to truth. Inhomogeneity of the EBT2 films needs to be carefully evaluated before clinical use and the blue‐color channel correction is not recommended.
SU‐E‐T‐142: Impact of Quality of Tri‐Axial Cable on Radiation Dose Measurement in Proton Beam
Authors: Gautam A , Srivastava S , Fanelli L , Das I , Cheng C
Published: November 2011

Purpose: Ionization chambers (IC) and electrometers are calibrated periodically at least every 2 years. However, the triaxial cables providing measuring system are neither tested nor calibrated periodically. Radiation treatment with small fields requires dose measurements with high degree of precision. In this study, we investigated the impact of different cables from various manufacturers on signals in a proton beam. Methods: Nine different commercially available cables from various vendors with different diameters and flexibility were studied. These cables were characterized by connecting to four different IC (volume 0.65–0.016cm3). The response of the cable‐connector‐IC combinationˈs were analyzed in a proton beam under the reference irradiation condition. Signals from cables were evaluated without and with connecting to an IC to investigate the effect of the quality of cable and connectors on the measured signals. Results: A significant signal is detected without an IC when irradiating the cable connector alone. Connecting to an IC, the signal is linearly related to the chamber volume with a slope of 13.23nC/cm3 for all nine cables. In a proton beam, there is no significant change in the magnitude of signals for various cables for large volume chambers. There is a 2% variation in signal for the parallel plate chamber for the different cable combination s. The signal is within ±0.02 % for large volume IC (0.65 cm3). For small volume (0.016 and 0.02cm3) chambers, the response is dominated by the quality of cable with variations up to ±0.3 % and ± 0.5 % respectively. Conclusions: The quality of cable has a larger effect on small volume chambers especially for the parallel plate chamber. Uncertainty due to cable should be considered during calibration for small volume chambers in proton beam.
SU‐E‐T‐145: Patient Specific Quality Assurance ‐ Not Just Stamp Collecting
Authors: Barnett E , Moseley D , Comsa D , Abbas A , van Prooijen M , Yeung I
Published: November 2011

Purpose: To assess the feasibility of using MapCheck 2D dose maps to improve quality in radiation therapy. Methods: A MapCheck2 device is used to validate each inverse planned step and shoot IMRT field at our centre. To date over 2000 fields have been measured. It is of interest to use this wealth of data effectively to establish appropriate control limits by applying principals of statistical process control, change planning practice if required as well as evaluate beam model performance. An application has been developed that generates a report comparing the measured 2D dose maps with the planar dose maps exported from the Pinnacle treatment planning system (Philips Radiation Oncology Systems, Madison, WI). This application reports mean error and standard deviation for each field within the following three distinct regions of the distribution consistent with AAPMˈs TG53: inner (80 – 100%), outer (0 – 20 %) and penumbra (20 – 80 %) where the percentages are of the maximum dose in the 2D dose map. Results: Results of the N=2193 measured fields are: inner field mean of the mean error −0.3%, SD 1.0%, outer field mean of mean error −0.1%, SD 1.0%, penumbra mean of mean error 0.6%, SD 2.3%. Control limits have been established. Mean error as a function of the number of control points has been reviewed and more appropriate control point limits established for treatment plan generation. A test suite of patients has been created for validation of new beam models or software releases. Conclusions: This quantity of measured data allows for a thorough evaluation of beam model performance and TPS software version in terms of IMRT delivery. By effectively analyzing the large quantity of measured clinical data, continual quality improvement becomes a less labour intensive task.
SU‐E‐T‐148: The IMRT and VMAT QA Experience Using PTW 2D‐ARRAY
Authors: Hung C , Liu K , Chen W , Lai C , Chen H , Liu C , Huang Y , Chen M
Published: November 2011

Purpose: Accurate dose delivery is a key component in the success of radiation therapy. The dose verification is especially important in the high complexity treatments, such as Intensity Modulated Radiation Therapy (IMRT) and Volumetric Modulated Arc Therapy (VMAT). Our purpose was to evaluate the dosimetric performance of our newly installed Elekta Synergy linear accelerator and Philips Pinnacle3 Planning system. Methods: The PTW Seven29 2D‐ARRAY consists of 729 vented ionization chambers with a dedicated phantom called Octavius were used to acquire the 2D dose distributions for the IMRT and VMAT plans. One limitation of this 2D‐ARRAY is the significant angular dependency, so we had measured thecorrection factors for VMAT plans. Another dosimetry system consists of a farmer type ion chamber (Wellhofer FC‐65P) and solid water phantom was used to measure the selected point dose. Total of 6 IMRT and 3 VMAT plans were included in this study. IMRT plans were verified with perpendicular deliveries at 0 degree gantry angle. VMAT plans were measured with actual treatment arcs, and then the corrections were applied after the measurements. We performed gamma comparison with 3% and 3mm criteria to evaluate the results of all 2D dose maps. Results: Our results showed good agreements in chamber dose validations. The differences between calculated and measured doses ranged from 0.7% to 3.1% (Mean 1.7%, SD 0.9%). The mean gamma index of 2D comparison from IMRT plans was 96.4% +/− 2.7% (SD). The mean gamma index of 2D comparison from VMAT plans was 95.1% +/− 2.8% (SD). Conclusions: Our preliminary evaluation has given us a satisfactory result, but the number of plans we have verified is just too small to give us enough confidence for all the situations we may encounter. We shall continue to validate the dosimetric performance of our planning and delivery system.
SU‐E‐T‐150: Filmless Quality Assurance for Linear Accelerator with Electronic Portal Image Device
Authors: Chen H , Jacob D , Simpson L , Chen W
Published: November 2011

Purpose: Our intention was to perform some machine quality assurance (QA) tests that require the use of films with electronic portal imaging device (EPID). A software tool was designed for these tasks which include Light vs Radiation Field, Isocenter Star Shot, Winston‐Lutz and MLC Picket Fence images. Methods: For Light vs Radiation Field, BB dots were placed on the tabletop to indicate the light field and double‐exposure technique was used to enhance the display of BB dots. For Table Isocenter Star Shot, a square field was used, and a steel ruler was placed on the table to show one of the major axis on the images. For Winston‐Lutz test, we placed a small steel ball at isocenter and irradiate it with a square field from various gantry angles. For MLC Picket Fence test, we used a multi‐segment MLC field to deliver six rectangular fields adjacent to each other. In addition to the composite image, the EPID also saved the image from each segment. Our software analyzed the images of all segments. Results: The ability to save multiple images without entering the treatment room greatly shortened the overall time spent. The test results of Light vs Radiation Field, Isocenter Star Shot and Winston‐Lutz from the EPID are similar to the film, except the image quality is much better on EPID images. A significant advantage of EPID is that individual image of every segment can be saved and analyzed; where as one cannot determine accurately the position errors on a composite image of film. This analysis of EPID based images facilitates easy calibration of MLC leaves Conclusions: Compared to film method, the EPID images are of higher quality and better consistency. The software we developed provides us with fast, convenient, and accurate analysis for this application.
SU‐E‐T‐151: Application of a 2D Ionization Chamber Array for Monthly and Annual Linear Accelerator QA
Authors: Buckey C , Myers P , Mavroidis P , Esquivel C , Mihailidis D , Gutiérrez A , Papanikolaou N , Stathakis S
Published: November 2011

Purpose: An evaluation of a commercially available 2D array for monthly and annual linear accelerator QA. Methods: The 2D‐ARRAY seven29 (PTW, Freiburg, Germany) is a two‐dimensional ion chamber array, consisting of 729 (27 × 27) vented ion chambers, with separately housed evaluation electronics. The parallel‐plate chambers are 5 × 5 × 5 mm in size, and have a center to center spacing of 10 mm. To increase resolution, the array can be shifted three times in increments of 5 mm, and the resulting 2916 measurement points can be summed into one image. The array was placed under 10 cm of water‐equivalent material, and irradiated to evaluate: flatness and symmetry, static MLC positioning, dynamic MLC positioning, and output. It was also irradiated without any buildup for the light field and radiation coincidence test. All deliveries were completed using a Varian 2100 C/D linear accelerator with a Millennium‐80 MLC. For improved accuracy, the static and dynamic MLC fields were delivered 4 times each, and were then summed using the “merge” function in the PTW VeriSoft software. Results: Over one month of data acquisition, output fluctuated less than 2%. For light field and radiation coincidence, reproducibility is excellent, with a maximum of 1% variation in asymmetric jaw settings. When using the “merge” function on dynamic MLC measurements, results were less than 4% for transmission and 0.5 mm for positioning errors. Measurements for flatness and symmetry and static MLC positioning also had good reproducibility. Conclusions: The PTW 2D‐Array seven29 can be used to evaluate many areas of monthly linear accelerator QA. Particularly in light of the trend for departments to go filmless, it is a viable alternative for obtaining flatness and symmetry, static and dynamic MLC positioning, light field and radiation coincidence, and for measuring output.

This work partially supported by PTW (Freiburg, Germany).
SU‐E‐T‐157: A Statistical Technique to Unmask Accelerator Performance Trends
Authors: Saito A , Rangaraj D , Yaddanapudi S , Palaniswaamy G , Ozawa S , Karasawa K , Mutic S , Klein E E
Published: November 2011

Purpose: To improve the performance monitoring capability of linear accelerators by applying a statistical process control (SPC) technique to daily quality assurance (QA). Extracted data from daily QA can yield information related to delivery performance. Method and Materials:Daily QA data of 3 linear accelerators (Varian Trilogy, Varian EX, Elekta Precise) had been acquired by a Daily QA device (Sun Nuclear Co.) from July 2007 to present. The exported data from the Daily QA 3 was analyzed by an inhouse SPC developed using MatLab software. Acquired data from the Daily QA 3 were; absolute dose, flatness, symmetry, beam energy, and field size. In addition to this standard data, dose rate was deduced from the Beam On Time (BOT), recorded with 0.5 second resolution. The moving average (MA) and the exponentially weighted moving average (EWMA) techniques were applied to extract performance trends. Results: The MA and the EWMA techniques strongly increased visibility of measured quantities. Specifically the variability of dose rate, which initially calculated with limited resolution of BOT, was drastically improved by these techniques. A trend of declining dose rate was evidenced by EWMA filtering, clearly identified a performance problem with one particular accelerator. Two dimension plots indicate dependencies (i.e. flatness vs. dose rate) not otherwise obvious. Conclusions: This work demonstrates that the SPC is a powerful tool for efficient drift detection of QA data, especially compared with the low fluctuation indicated by low digital resolution. The SPC enables us to understand variability of QA data for performing real time feedback.
SU‐E‐T‐158: An End‐To‐End Quality Assurance Procedure for Advanced Radiation Therapy
Authors: Liu H , Xue J , Chen X , Lin M , Koren S , Dong Z , Chen L , Ma C
Published: November 2011

Purpose: This work investigates a simple end‐to‐end QA procedure for the entire radiotherapy process from treatment simulation, treatment planning to treatment delivery. Methods: This procedure utilizes a commercial QA phantom, the IBA MatriXX Evolution with the MULTICube. The initial establishment of this QA procedure requires some baseline data according to the following steps: (1) CT scan the phantom, (2) transfer the CT data to all TPS, (3) generate typical IMRT/VMAT plans, (4) transfer all treatment plans to the R&V system, (5) set up the phantom using lasers and other target localization devices, (6) deliver the plans to the phantom, and (7) analyze/record the planned/delivered doses and Gamma analysis passing rates. This procedure is performed at monthly and annual QA to repeat all steps above except for (3) where original plans are used to calculate dose distributions on new CT scans. The new measured doses will be compared with the baseline data. Results: This procedure checks the entire radiotherapy process including CT scanning, CT#‐to‐ED conversion, dose calculation, TPS‐to‐R&V data transfer, patient setup/target localization with the lasers, OBI, EPID, and CBCT, and linac delivery. The baseline Gamma passing rates were 99%(3%/3mm) and 96%(2%/2mm), respectively. In the subsequent QA measurements, any >3% reduction in the Gamma passing rates will trigger a step‐by‐step examination of the entire process to find the cause. We have tested our procedure by mimicking setup/localization misalignment by 2mm, 5mm and 10mm and found up to 4%, 10% and 14% passing rate reduction, respectively. Conclusions: The end‐to‐end QA procedure is practical and efficient for checking the entire radiotherapy process. Baseline data can be established after the initial calibration and commissioning of all the software/hardware components and the end‐to‐end QA procedure can check the accuracy/consistency of the entire radiotherapy process monthly/annually and after major software/hardware upgrade or component replacement.
SU‐E‐T‐160: Absolute MLC Positions Derived from Epid Measurements on a Siemens Artiste
Authors: Haering P , Lang C , Schwahofer A
Published: November 2011

Purpose: To present a method to derive absolute leaf positions from Epid measured MLC QA fields on a Siemens Artiste. Methods: The MLC 160 on a Siemens Artiste has a unique leaf end design. This results in a varying attenuation of the beam through the leaf edge, depending on the field position. As flat panel based Epids show a strong spectral signal dependence, the measured image does not represent the real dosimetric leaf positions. Leaf positions derived from data show a characteristic nonlinear mismatch. As films taken simultaneously, referenced to Epid images by markers seen on both systems, did not show this behavior, the effect is caused by the combination of MLC and Epid. Comparisons between film and Epid gave rise to a calibration method Epid to film. As a result there is a polynomional fit of the fourth grade for both MLC sides which is used to correct the Epid measured leaf positions. To derive the raw positions from the images the MLCSoftepid, PTW, Germany was used. Data then undergo the correction in an Excel file. The results can be displayed using color coded error limits. Results: So far, positions were derived from measurements taken over a 5 month period using nineteen 2cm stripes fields. Evaluations with and without the film based Epid calibration show clearly a much improved measurement accuracy. Corresponding error histograms show a reduction in half width of more than 20% and a systematic shift could also be reduced. Conclusion: This correction method is an attempt to eliminate the position errors seen in Epid based MLC QA fields. Although it is not easy to establish the film based calibration measurements, this method might help to get a better understanding of this effect and to advance MLC QA with Epids.
SU‐E‐T‐161: Small Field Surface Doses for 6X and 10X Photon Beams ‐ FFF versus FF
Authors: Mynampati D , Yaparpalvi R , Clift C , Tsai S , Mah D
Published: November 2011

Purpose: To measure and compare small field surface doses for Flattening filter free (FFF) and flattened (FF) photon beams. Methods: Measurements were performed for photon beams of two nominal energies (6 and 10 MV) of a Varian Linac (TrueBeamSTX). TrueBeam delivers 6 MV and 10 MV photon beams with both ‐ a flattening filter in place (FF) or using an open port where a thin foil is used instead of a flattening filter (FFF). We refer to the four photon beams investigated to as 6XFF, 6XFFF, 10XFF, and 10XFFF. All measurements were performed in IBA Blue Water Phantom (IBA‐Dosimetry) and a SFD diode (IBA, thickness of measuring volume 60 m) was employed to measure surface doses. Surface doses were expressed relative to the dose at dose maximum for the respective energy. Field sizes were varied from 1×1 cm2 to 10×10 cm2. Results: Measurements in the 1×1 cm2 to 10×10 cm2 field size range resulted in higher surface doses for FFF beams compared to FF beams. Percentage surface dose increases for FFF beams compared to FF beams were — for a 1× 1 cm2 increases were 36% and 60% for 6X and 10X beams respectively compared to corresponding 31.5 and 50.0% increases for a 5×5 cm2 field size. Overall, surface doses relative to dmax ranged from 0.37 – 0.44 and 0.21 – 0.28 for field sizes of 1×1 cm2 to 10×10 cm2 for the 6XFF and 10XFF beams respectively. The corresponding range of values for the 6XFFF and 10XFFF beams were 0.50 – 0.54 and 0.34 – 0.37 respectively. Conclusions: FFF beam change the dosimetric characteristics of photon beams by softening the energy spectra compared to beam hardened FF beams, thus changing surface doses. Our study evaluates the magnitude of these changes for small field sizes. The results confirm literature observations.
SU‐E‐T‐165: Systematic Evaluation of Uncertainties Associated with GAFCHROMIC EBT2 Film Dosimetry for 6MV Photon Beams
Authors: Kim J , Kim S , Shaikh M , Li H , Huang Y , Wen N , Glide‐Hurst C , Jin J , Nurushev T , Chetty I J
Published: November 2011

Method and Materials:Eight packets of films were exposed to 13.5cm ×13.5cm, 6MV radiation fields in a solid water phantom. Dose levels of 1.1, 3.2, 5.3, 7.4, and 9.5 Gy were delivered to five films in each packet. Films were scanned both before and after irradiation using an Epson flat‐bed scanner (24hr wait‐time for post‐irradiation coloration). Corresponding 2D dose distributions were measured with a detector‐array (MatriXX). Point dose comparisons were performed with an ion chamber. Digitized film images were registered to the 2D dose distribution to generate a correction map that compensated the scanner non‐uniform response as a function of dose. Optical density (OD) and net optical density (NetOD) values were calculated for all images. Dose response curves were established using mean values of a central 0.5cm × 0.5cm region‐of‐interest (ROI). Images were converted to dose, and error uncertainties (1SD) were measured in the central 8cm × 8cm ROI. Results: The overall dosimetric uncertainties (1SD) of the NetOD approach were 2.2%, 1.9%, and 3.5% for red, green, and blue channels, respectively. The corresponding uncertainties of OD were 2.7%, 3.1%, and 8.3%, respectively. For low dose range (<3 Gy), the green channel revealed higher uncertainty (SDgreen= 3.3%) than the red channel (SDred=2.6%). However, for high doses (3∼9 Gy), the green channel showed less variability (SDgreen=1.6%, SDred=2.9%). Minimum SDred and SDgreen were 1.6% at 5.3Gy and 1.3% at 7.4 Gy, respectively. Scanner non‐uniformity correction mitigated the irregular response of scanner detector elements observed initially. Conclusion: NetOD may be a more useful metric for benchmarking EBT2 than OD. We demonstrated that the lowest dose uncertainties were achieved using the red channel for low dose range, while the green channel was preferred for higher doses. Scanner non‐uniformity correction is necessary for higher precision dosimetry.
SU‐E‐T‐166: Dosimetric Considerations for Applying a Commercial 2D Ion Chamber Array in Evaluation of IMRT Plans on Account of Big Gradient Areas
Authors: Cordero‐Ramirez A
Published: November 2011

Purpose: To evaluate a commercial system based on ion chamber detectors by several dosimetric measurements, in order to use it as part of an IMRT QA program. Methods: The device 2D Array Seven29TM was evaluated for clinical assessment of IMRT plans. This is a two dimensional detectors array with of 729 air‐filled ionization chambers. A Clinac iX was used in different fields, the first irradiation open and all of them with the same quadrant blocked by the MLC afterwards, using the MLC Shaper of Varian. The SSD is set to 95 cm, with 5 cm of RW3 in the back of the device for backscatter production and 4.5 cm in from of it for build up. In order to evaluate the 2D array measurements, a 3D PinPoint chamber was irradiated for the same configurations. Results: There exist a good agreement between measurements from 2D Array and the chamber, even when both methods had a different distance to effective point of detectors (in the array the 100 cm distance is in the entrance plane of the air‐filled chamber volumes, while in the PinPoint chamber is in its central point). Reducing the field size, the PinPoint chamber keeps its response but the array takes not expected values especially in the points out of the centre. The fact that the PinPoint has a very small volume gives it a better resolution, and the bigger volume size of the chambers in the array restricts its detection capabilities in big gradient areas. Conclusions: The evaluated system shows a good performance and similar values than novel methods. Consequently, this system might be a good tool to be incorporated in a patient dependent QA program even for absolute dose, taking into account the issues regarding to vertical position of detectors and incidence of the leaves.
SU‐E‐T‐171: Quantitative Evaluation and Correction of the Gantry Angular Dependence of MapCheck2 for Patient QA Measurement
Authors: Zhou H , Wu C
Published: November 2011

Purpose: To quantitatively evaluate and correct the gantry angular dependence of MapCheck2 for patient QA measurements Methods: A MapCheck2 device (SunNuclear Corp.) is used for the study. The phantom setup uses 5cm solid water slabs both above and below the device. A set of 10 × 10 cm2 fields were measured at different gantry angles from 0 to 360 degree at 20 degree interval on a Varian 21iX LINAC. To the first order approximation, the center diode detector measurement values are extracted and compared against the corresponding TPS calculated values. The ratio of the two is assumed to represent the angular correction factors for the whole 2‐D diode array. MapCheck2 phantom QA for a typical prostate 7‐fields 6 MV IMRT case was conducted with the beams delivered at actual planned gantry angles. Measurement data corrected with the above factors were compared with TPS computed values to evaluate the effectiveness of the correction. Results: Significant diode angular dependence is found for both 6 MV and 15 MV photons. The worst angular dependence (about 13% over‐response for 6 MV and 8% for 15 MV) is found at gantry angles of 90 (or 270 degree), where the beam irradiates parallel to the 2‐D diode array plane. On the representative 7‐fields IMRT plan QA, when absolute dose comparison is made with a Gamma criteria of 3%/3mm DTA and 10% low dose threshold, the QA pass rate is significantly improved when the derived correction factors for the 2‐D diode array are applied. The corrected MapCheck2 measurement gives a 98.7% pass rate, compared to the un‐ corrected raw measurementˈs pass rate of 73.1%. Conclusions: MapCheck2 gantry angular dependence is quantitatively measured and a set of approximate correction factors derived. When applied, MapCheck2 measurement gives very good agreement with the TPS computed planar dose.
SU‐E‐T‐176: A Web Based Tool for the Implementation of Routine Linac QA Following TG1 42
Authors: Papanikolaou N , Beck G , Peyton J , Gutierrez A , Esquivel C , Mavroidis P , Stathakis S
Published: November 2011

Purpose: To develop a web environment where the TG142 proposed test actions and results can be entered and retrieved for review and electronic approval. Methods and Materials: AAPM Task Group 142 published recently a report with recommendations and tests for a comprehensive quality assurance program for external beam radiation therapy for linear accelerators. The recommendations address frequency and tolerances for the proposed tests. In addition, there are qualifiers for tests that are specific to linacs that are used for IMRT and stereotactic procedures. We have developed a web based environment using the asp.net tools where the TG142 tests have been implemented for the recommended frequencies for seven different linear accelerators in our institution. The end user (therapist or medical physicist) has a unique login and password and follows the prompts of the web page to complete each test. Baselines and tolerances are built into the program and data analysis is performed automatically, reporting a pass or fail result. Data trend analysis can be plotted and the medical physicist can review and approve the qa results electronically. The pdf report of the qa analysis can be printed or uploaded to the RV system as part of an EMR QA environment. Results: We have implemented this web based QA tool in our department with great success. The sequence ofmorning qa tasks has also been optimized so that the entire suite of tests per TG142 takes no more than 20min to perform. Electronic approval of the QA is available only to the faculty physicist. Once the document is approved, it is automatically uploaded to Mosaiq under the patient name corresponding to the machine name. Conclusion: A web based TG142 qa tool was developed and implemented in our clinic, allowing for data entry, analysis, trend plotting and electronic approval.
SU‐E‐T‐179: Quality Assurance of Enhanced Dynamic Wedge Profiles Using IˈˈmRT MatriXX
Authors: Gao S , Balter P , Ohrt J , Pollard J , Chung H
Published: November 2011

Purpose: To evaluate the performance of the IˈmRT MatriXX for periodical quality assurance (QA) of enhanced dynamic wedge (EDW) profile. To verify the treatment planning system (TPS) EDW models. Methods: The ionization chamber based MatriXX (IBA, Schwarzenbruck, Germany) was used to measure the EDW profiles of the Varian 21EX Linac with various wedge angles (15, 30, 45, and 60 degrees) and field sizes. The source to detector distance was 100 cm and 4 or 5 cm of buildup was used for 6 or 18 MV respectively. Planar dose fluences were generated by TPS (Philips Pinnacle version 9.0), then we compared the calculated TPS data to the data measured with MatriXX using the OmniPro‐IˈmRT software. Profiles were then extracted and exported to a spreadsheet for point‐to‐point comparisons between the TPS calculated and the measurements. We also measured selected profiles using a 0.6 cc ionization chamber in solid slab phantom. Results: Point‐to‐point differences between planned and measured EDW profiles for wedge angles of 15°, 30°, 45°, and 60° in both Y1‐in and Y2‐out directions were analyzed for field sizes of 5×5, 10×10 and 20×20 cm̂2 on three Varian Linacs. For 10×10 cm̂2 field, all four wedge angles the point‐to‐point differences within 80% of the field width were less than 2% for both 6 MV and 18 MV beams. Agreements were not as good for smaller field sizes. Comparisons with a 0.6 cc chamber were only done for the 60° wedge 6 MV beam with field of 20×20 cm̂2, the point‐to‐point comparison both Y1‐in and Y2‐out wedge orientations shows that the ion chamberˈs results agreed with the MatriXX within 1% in 80% field width. Conclusions: This study demonstrates that MatriXX is a reliable tool and is an efficient approach for both TPS commissioning and periodical QA for EDWs.
SU‐E‐T‐180: Using the Gafchormic EBT2 Film and the NACP Parallel Plate Ion Chamber to Commission the Total Skin Electron Treatment
Authors: Wu X , Wang Y , Jones G , MacPherson M
Published: November 2011

Purpose: Total skin electron treatment with the six‐field, dual‐angle technique is commissioned. Two types of dosimeters, the Gafchromic EBT2 films and the NACP parallel plate ion chamber, are employed to allow cross‐comparison of the measurement results. Methods: The commissioning followed AAPM report 23 guidelines and was performed on a Varian 2300EX under the HDTse‐mode. The SSD was 4.2m. Dual gantry angles were determined by sampling the vertical profile using the chamber. Both EBT2 films and the chamber were used to determine PDDs and perform the output calibration in solid water under a single‐dual field setting. The beam output was verified by a 3rd party dosimetry service using TLDs. Dual‐angle six‐field measurements, including the x‐ray contamination measurement, were performed using the EBT2 films in an anthropomorphic phantom. Results: +/−16deg was selected for the dual angles to achieve a dose uniformity of 80% along the vertical profile on the central axis. The single dual field R_50s determined using the EBT2 films and NACP chamber matched within 0.5mm, and the dmax determined using the two dosimeters matched within 1 mm. The output calibration results between the EBT2 films and NACP matched with 2.1%. The 3rd party measured beam output matched ours to within 3%. The x‐ray contamination was 0.8%. PDD results of the‐six dual field demonstrated that the R_50 was smaller than those of a single‐dual field and the doses at the clinical depth of interest of 4mm varied up to 35%. B factor, defined as the ratio of the dose at the Rando surface from the six‐dual field over the dose at dmax of a single‐dual field was within the range suggested in AAPM report 23. Conclusions: Commissioning results obtained using the two different dosimeters are comparable and satisfactory. Future work is to establish a QA program for this treatment modality.
SU‐E‐T‐182: Comparison of Techniques in Irradiating RPC Credentialing Phantoms
Authors: Larson L , Burmeister J , Chen Z , Liang L , Bossenberger T , Liu Q , Nalichowski A
Published: November 2011

Purpose: In recent years, the pass rate for clinics irradiating Radiological Physics Center (RPC) phantoms for protocol credentialing is 76–80%. This study looks at two clinics with different philosophies towards the irradiation process and evaluates the results from each. Methods: 12 Head and Neck and 5 Lung phantoms for credentialing IMRT protocols have been irradiated at two separate institutions. At Institution 1 (6 irradiations), multiple members of the physics team were involved with each step in the process (including simulation, planning, and delivery). At Institution 2, each step was performed by the respective staff clinically responsible for the procedure for 9/11 irradiations, and a single medical physicist performed all procedures for 2/11 irradiations. Three separate treatment planning and delivery systems were employed and four different dose calculation algorithms were used. Results: 100% of the phantoms irradiated by Institution 1 met the RPC credentialing criteria. At Institution 2, 73% (8/11) of the phantoms met the RPC criteria. Of the 3 that failed, 2 were performed solely by one physicist and one included a positioning error not discovered until the irradiation was finished. Comparing the Head and Neck phantom results, the average ratio of measured to expected dose was (0.996 SD=0.01) for Institution 1 and (.988 SD=0.04) for Institution 2 Conclusions: Whether an RPC phantom credentialing is completed only by physicists or by multiple members of the treatment team executing their respective duties, acceptable results can be achieved. The data from this study suggest that with appropriate care, better results can be achieved by a team approach than by a solo individual. They also suggest that multiple physicists can achieve better results than multiple members of the treatment team. However, the meaning of this is unclear since physicists do not perform all duties clinically.
SU‐E‐T‐189: Determination of Output Factors with Different Types of Active Detectors and Passive Dosemeters for Stereotactic Systems Equipped with MicroMLC and Circular Cones
Authors: Bassinet C , Huet C , Derreumaux S , Brunet G , Chéa M , Baumann M , Trompier F , Roch P , Boisserie G , Clairand I
Published: November 2011

Purpose: Radiotherapy techniques involving the use of small fields, especially for stereotactic treatments, require accurate determination of output factors (OF). Moreover OF reference values are not available. The aim of this work is to determine small fields OF using several active detectors and passive dosemeters for a dedicated medical linear accelerator (Novalis) equipped with m3 microMLC (BrainLAB) and circular cones and for an adaptative one (Varian Clinac 2100) equipped with an additional microMLC. Methods: Small field sizes were defined either by microMLC down to 6×6 mm2 or by circular cones down to 4 mm in diameter. Output available active detectors dedicated to measurements in small fields (high resolution diodes: IBA SFD, Sun Nuclear EDGE, PTW 60016, PTW 60017; ionizing chambers: PTW 31014 PinPoint chamber, PTW 31018 microLion liquid chamber and PTW 60003 natural diamond) and in a solid water phantom with passive dosemeters (TLD micro‐cubes, EBT2 Gafchromic films). Results: Significant differences between the results obtained by the different detectors are observed, particularly for the smallest field size for which the difference of the measured OF can reach 20%. Results obtained with both passive dosemeters present a good agreement (better than 2%). Some of these differences can be explained with the detector characteristics (detector size, tissue equivalence …) Conclusions: This study provides data on the behavior of different detectors in terms of dose measurements for small beams. Results obtained with the passive dosemeters in this study as well as in another study performed on a Cyberknife system (Huet et al., 2011) are promising. The perspective of this study is to propose a methodology for the determination of correction factors for stereotactic systems.
SU‐E‐T‐194: Accounting for Support Arm Backscatter in EPID‐Based IMRT QA
Authors: Nichita E , Ostapiak O , Das M
Published: November 2011

Purpose: To devise a method for IMRT QA using a Pinnacle model for the EPID response which accounts for non‐uniform backscatter from the support arm. Method: The method is based on a Pinnacle treatment planning system model. It relies on determining a correction matrix (CM) that multiplies the (flood‐field calibrated) EPID image to restore pixel response to backscatter and subsequently constructing a backscatter region within the Pinnacle model that reproduces the effect of backscatter from the EPID support arm. The CM is constructed from a series of profiles calculated as the ratio of modelled to acquired slit‐beam fields. Each slit beam is offset so that the series covers the entire EPID area. Slit beams are used to minimize the dependence of response to backscatter. The CM represents the flood‐field response of the EPID mounted on the support arm as if it had been calibrated in the absence of backscatter. An iterative fitting procedure is used to adjust the backscatter region in the Pinnacle model so that the modelled flood‐field response matches the CM. Results: As expected, arbitrary‐field EPID images multiplied by the CM agree well with Pinnacle model results incorporating the backscatter region. The method was tested for 10, 20, and 30 cm square fields and shown to reduce discrepancies between the Pinnacle calculation and EPID images from 3.3, 8.3 and 9.4 %, to 0.8, 2.2 and 1.1 % respectively for the three fields. Conclusions: A novel method for IMRT QA was developed whereby acquired EPID images are multiplied by a pre‐computed correction matrix and then compared to the response from a Pinnacle model which incorporates a non‐uniform backscatter region. The method is easy to use in clinical practice and does not require measurements involving the removal of the EPID from the support arm.
SU‐E‐T‐196: A Quality Assurance Method for Stereotactic Radiosurgical Cones Output Factor Check Using Kodak EDR2® Film
Authors: Permanente Kaiser
Published: November 2011

Purpose: In the wake of the recent negative publicity for Radiation Oncology, there has been an increase in interest in quality assurance methodology and the frequency of which the methods are performed. Stereotactic radiosurgery procedures are performed in clinics using small cone sizes that introduce complex dosimetry calculations. Measuring small cones output factors are challenging due to the small field size, presence of high dose gradient, as well as lack of spatial resolution of the ion chambers, and diode detectors. The purpose of this study was to compare current clinical cone output factors to the measurement of these same cones using a method of EDR2 film and the RIT Technology high resolution film scanning system.Methods: Kodak EDR2 film was exposed to 200cGy 6MV x‐rays at a depth of 1.5 cm in solid water phantom. The following cones were measured for the BrainLab Novalis linear accelerator: 4, 6, 7.5, 10, 12.5, 15, 17.5, and 20 mm. The cones measured for the Varian Trilogy linear accelerator: 4, 7.5, 10, 12.5, 15, 17.5, 20, 25, and 30 mm. A 10×10 cm2 field size reference field was exposed at the same depth, on the same film, to negate film and developer discrepancies. Using the RIT Technology high resolution film scanning system the maximum film optical density for each cone was measured and relative output factors were calculated. Results: Cone output factor values relative to the 10×10 cm2 reference field for the Novalis linear accelerator differed from the commissioned values by an average 4.14%±2.78% (range of −0.5% to 8.2%). The values for the Trilogy linear accelerator cones differed by 2.77%±2.53% (ranged from −0.50% to 7.00%) Conclusions: EDR2 film dosimetry is a useful tool for checking output factors for SRS small cones on a more frequent basis as an implemented quality assurance method.