Sun Nuclear Corporation Research Articles

WE‐D‐BRB‐07: Wide Field Array Calibration Dependence on Beam Shape Stability

Purpose: To simulate the effects of beam symmetry instabilities on detector array calibrations and explore compensation methods during actual calibration with a linear accelerator (LINAC). Methods: The array calibration method that was investigated is known as wide field (WF) calibration. With this method, a linear array [y‐axis (65 detectors) of the IC PROFILER™; Sun Nuclear Corporation, Melbourne, FL USA] requires three calibration measurements (α, θ, and λ) using a radiation field that is larger than the array. Measurement θ is a 180° rotation from α, and λ is a physical shift by one detector from θ. The relative detector sensitivities are then determined through ratios of detector readings at the same field locations. This method results in error propagation that is proportional to the array size. The WF calibration postulates that the dose distribution is constant for each calibration measurement. Postulate violations were quantified by applying a sine shaped perturbation (of up to 0.1%) to a calibration measurement and determining the error relative to a baseline calibration. Actual postulate violations were limited by using a continuously on beam and mechanized array movement during θ and λ. Symmetry is more stable for a continuously on beam. Results: Simulated perturbations to θ or λ resulted in calibration errors of up to 2%, while α was relatively immune (<0.1% error). Wide field calibration error on a LINAC with similar symmetry instabilities was (±) 1.6%. Using a continuous beam during θ and λ reduced the calibration error to (±) 0.68%. Conclusion: This work increased the reproducibility of WF calibrations by limiting the effect of measurement perturbations due to LINAC symmetry instabilities. The same technique would work for any array using WF calibration. Conflict of Interest: Research sponsored by Sun Nuclear Corporation.

SU‐GG‐T‐296: Experience on Using Modified CHECKMATE for Hi‐Art Tomotherapy Daily QA

Purpose: To design a method utilizing a commercially available daily QA device to check the performance of the Hi‐Art Tomotherapy unit. This method is simple for therapists to finish in a short time and also can check the output and energy of the machine as well as the laser and imaging precisions as recommended by Tomotherapy. The initial experience of using this method is also reported. Method and Materials: A CHECKMATE (Sun Nuclear Corp. Melbourne, FL) with one vented parallel‐pate chamber at the center was used. BBs were added to serve as the markers to check the imaging precision and couch position accuracy. The 10 cm square markings and the BBs were also used to check the red laser movement accuracy. A procedure was designed to deliver 20s beam on time (∼287 cGy) to the chamber with SSD = 85cm. This procedure was delivered first to check the output of the machine, then a Cerrobend plate of ∼1.6 cm thick was put on top of the device and the procedure was delivered again. The ratio between the two readings was used to represent the energy of the machine. Results were compared with the monthly QA measurements and CT detector data measured by field service engineer (FSE) when available. Results Daily couch position variation was well within 1mm limit. The output measurements were in agreement with the monthly IC measurement as well as the CT detector data used by FSE. The energy measurements were consistent with the monthly QA data. When target was deteriorating, the FSE reported data had a different trend than the morning QA data. Conclusions The modified CHECKMATE provides a reliable and fast way to do daily check for Tomotherapy machine and can be easily done by a therapist

SU‐GG‐T‐210: An Examination of the Usefulness of Films in Addition to Electronic Methods in Pre‐Treatment IMRT QA

Purpose: Using radiographs for IMRT QA carries many disadvantages. Electronic QA methods, e.g. diode arrays, have become standard for efficiently and accurately verifying dose distributions. Because diode arrays have relatively low resolution, radiographs might be used in addition to array measurements to qualitatively verify geometric accuracy of fluence delivery. However, the finite ability of the human eye to compare films and TPS printouts is unreliable: diode arrays may detect geometric inaccuracies before the errors are recognizable on film. Does qualitative use of films contribute significantly to the pretreatment verification process? Methods and Materials: Four IMRT fluences (varying modulation) were delivered both on film and MapCHECK1 (Sun Nuclear Corporation). Errors were systematically introduced by omitting portions of the fluence, simulating a communication error between planning and delivery systems. Diode array measurements were compared to the TPS verification plan using DTA (3%, 3mm). Films were compared to TPS printouts by light‐box overlay. Increasingly larger portions of the fluence were omitted until the QA failed, either by errors observed on the film or MapCHECK DTA with <85% passing points. Results: For every modified fluence that failed QA, errors were apparent in MapCHECK before being observed on film. With up to 20% control points omitted, all radiographs appear virtually unchanged to the naked eye. On average, MapCHECK analysis of the same fluences failed due to omission of approximately 7% control points from the center of the fluences and 17% from the outer portions of the fluences. Conclusions: If an IMRT fluence is changed due to data transfer errors, qualitative analysis of radiographs does not improve upon the effectiveness of MapCHECK at detecting the error. Because such errors may not be detected if ≤20% control points are lost/corrupted, DTA analysis should always be accompanied by examination of isodose distributions and dose line profiles.

SU‐DD‐A1‐03: On Rotational Measurements with the ArcCHECK 3D Diode Array

Purpose: The ArcCHECK 3D diode array dosimeter (Sun Nuclear Corporation) represents a novel approach to detector placement. We investigate the effects this design can have on the rotational measurements' accuracy and suggest correction factors that could sometimes improve it. Method and Materials: The ArcCHECK design places 1386 0.8×0.8mm2 diodes arranged on a 1×1cm helical grid on a cylindrical surface 21cm in diameter inside a doughnut‐shaped phantom.. Beam arrangements tested in this work include single static beams, simple open arcs, and VMAT plans based on the AAPM TG1 19 test suite. Field size dependence, angular dependence, phantom inhomogeneity effects, and calculation angular‐discretization effects were investigated separately.Results: Unlike an existing device using the same diodes (MapCHECK), ArcCHECK exhibits non‐negligible field size dependence in detector response, likely due to the increased phantom volume. Angular‐dependent detector response is also shown, evidenced by measured beam profiles deviating from the calculations and ion chamber measurements by more than 2% for field widths exceeding approximately 15 cm. The issue is being addressed by the manufacturer. There is an additional small disagreement between measured and calculated exit doses (1.5%) due to the inhomogeneous phantom structure. Simple arc measurements demonstrate strong influence of the small changes in field width (e.g. 9%/mm at 2cm), while the isocenter dose is essentially unchanged. This is inherent and expected given the device design. The average gamma analysis passing rates for a well‐commissioned VMAT system were 98.4 and 92.9% at the 3%/3mm and 2%/2mm error thresholds, respectively. They improved to 98.7 and 94.2%, respectively, after manual application of the field size correction factor. Conclusion: Despite hypersensitivity to field width in the simple arc geometry, the VMAT gamma analysis results are in line with expectations based on independent dosimetry measurements. Introduction of correction factors for angular and field‐size dependence would further improve measurement accuracy.

SU-GG-T-228: A New Technique to Measure Orthogonal Dose Distribution in Phantom for IMRT QA with Gafchromic EBT2 Film

Purpose: To demonstrate the feasibility of a new technique for patient specific IMRT QA. Method and Materials: Two prostate plans with the prescription of 76 Gy (23 initial and 15 boost fractions) were used for this study. The dynamic MLC fields planned by Eclipse 6.5 were delivered to QA phantom by Varian Clinac 21EX. To measure the dose distribution on the coronal plane, a film (Gafchromic EBT2, ISP) placed on coronal plane of isocenter in the phantom was exposed by all fields with the planned gantry angles. For sagittal plane measurement, film was still placed on the coronal plane in the phantom and irradiated by all fields with adding 90 degrees to planned gantry angle. It is because the phantom has a 90 degree rotational symmetry shape. The coronal and sagittal dose distributions in the phantom were calculated by Eclipse (AAA 7.5.18) without any modification. The exposed films were scanned by an EPSON V700 scanner. The films were analyzed using the MapCHECK software (Sun Nuclear Corporation), which uses the red channel for film analysis. Dose distributions were analyzed using 3%/3 mm criteria and 10% threshold. The planar dose distributions obtained with the film was normalized to the center of axis which is Farmer chamber dosimetry measurement was made. MapCHECK were used for each filed QA as a reference. Results: The average pass rate of initial and boost plan of case #1 were 98.1 % and 99.7 % by film, 94.2 % and 94.0 % by MapCHECK (MU weighted), for case #2, 95.8 % and 98.7 % by film, 92.0 % and 96.1 % by MapCHECK. Conclusion: These results attest to the usefulness of the new technique for patient specific IMRT QA. We are applying this QA technique to VMAT QA.

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

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

SU‐GG‐T‐184: Per‐Patient Dose QA Based on Clinically Relevant Metrics: Validation of the Planned Dose Perturbation Method to Estimate Patient Dose/DVH Using Conventional QA Data

Purpose: In patient‐specific IMRT QA, there is a need for patient dose/DVH‐based metrics. A method called “Planned Dose Perturbation” (PDP) uses conventional IMRT QA to estimate patient dose by perturbing the 3D TPS planned dose based on conventional QA comparisons. The accuracy of the PDP method is assessed here. Materials and Methods: 3D IMRT doses were generated using an accurate beam model. Then, various errors were induced in each beam of each plan to produce an incorrect 3D patient dose and corresponding incorrect planar QA calculations. The error‐free beams were used to generate “virtual measurements” per beam. Then, conventional planar IMRT QA methods were used to generate per‐beam 2D dose error maps which became inputs to the PDP system. PDP perturbs the error‐induced 3D Patient Dose to generate a revised 3D Patient Dose. This revised dose was compared voxel‐by‐voxel to the error‐free 3D Patient Dose to generate matching rate scores at 2%/2mm DTA over the 3D grid. Anatomical Region‐of‐Interest dose metrics (mean and max) were also assessed per ROI. This process was repeated for many cases, from simple IMRT plans (single beams on phantom) to very complex clinical IMRT plans (head/neck, prostate). Multiple beam energies were studied along with a range of severity of induced errors. Results: All the PDP Patient Dose estimates achieved passing rates of 99.1% vs. error‐free Patient Dose (2%/2mm DTA, absolute, volumetric, global percent difference, 10% lower threshold). When analyzing absolute dose metrics per ROI, the mean dose and max dose errors of PDP Patient Dose vs. Gold Standard averaged less than 1.00% error for all ROI (max error was 0.58%). Conclusions: The PDP method is extremely accurate at estimating patient dose/DVH impact based on conventional IMRT QA phantom methods. Research sponsored by Canis Lupus LLC and Sun Nuclear Corporation.

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

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

SU‐DD‐A1‐02: The X's and O's of 3D Dosimetry Phantoms

Purpose: As rotational radiation therapy becomes more prevalent, the need grows for new methodologies of per‐patient dose QA. An approach commonly used for static‐field IMRT QA (per‐beam planar dose normal to the beam axis) is less relevant for rotating beams; thus, more attention has turned to composite dose in a 3D phantom. In this work, two commercial 3D dosimetry phantoms are studied under identical conditions (TPS, delivery) for rotational plans. Materials and Methods: The Delta4 (ScandiDos) and ArcCHECK (Sun Nuclear Corporation) devices were each entered into the TPS as QA phantoms for the transference of VMAT plans produced by the Pinnacle TPS (Philips). The sensitivity to the angular discretizing of a rotational plan into 2, 4, and 6 degree sub‐arcs was studied. In addition, small errors were introduced into the TPS calculation (MLC shifts, transmission changes) to assay each device's sensitivity to those errors. Results: The ArcCHECK's peripheral detector geometry was far more sensitive to TPS angular discretization of the dose calculation; however, the impact of the difference in 2‐degree and 4‐degree sub‐arcs was very small in the high dose central/target regions, as evidenced by the high passing rates of the Delta4. Both devices showed insensitivity to small errors induced in MLC position and transmission, but both were sensitive (D4 at 3%/3mm and 2%/2mm, AC at 2%/2mm) to MLC shifts of 0.9 mm. Comparison results between Delta4 and ArcCHECK trended similarly, but passing rates were not the same. Conclusions: The design of the detectors' locations in 3D space is critical to understanding the behavior of a 3D dosimetry phantom. Dose QA results measured by one system are not interchangeable with another's if the detector geometries are not identical. Ideally, it would be useful to correlate these phantom dose QA results to their impact to patient dose/DVH on a per‐patient basis.

An MLC calibration method using a detector array

The authors have developed a quantitative calibration method for a multileaf collimator (MLC) which measures individual leaf positions relative to the MLC backup jaw on an Elekta Synergy linear accelerator. The method utilizes a commercially available two-axis detector array (Profiler 2; Sun Nuclear Corporation, Melbourne, FL). To calibrate the MLC bank, its backup jaw is positioned at the central axis and the opposing jaw is retracted to create a half-beam configuration. The position of the backup jaws field edge is then measured with the array to obtain what is termed the radiation defined reference line. The positions of the individual leaf ends relative to this reference line are then inferred by the detector response in the leaf end penumbra. Iteratively adjusting and remeasuring the leaf end positions to within specifications completes the calibration. Using the backup jaw as a reference for the leaf end positions is based on three assumptions: (1) The leading edge of an MLC leaf bank is parallel to its backup jaw's leading edge, (2) the backup jaw position is reproducible, and (3) the measured radiation field edge created by each leaf end is representative of that leaf's position. Data from an electronic portal imaging device (EPID) were used in a similar analysis to check the results obtained with the array. The relative leaf end positions measured with the array differed from those measured with the EPID by an average of 0.11+/-0.09 mm per leaf. The maximum leaf positional change measured with the Profiler 2 over a 3 month period was 0.51 mm. A leaf positional accuracy of +/-0.4 mm is easily attainable through the iterative calibration process. The method requires an average of 40 min to measure both leaf banks. This work demonstrates that the Profiler 2 is an effective tool for efficient and quantitative MLC quality assurance and calibration.

Poster — Wed Eve—30: Routine QA and Re‐Commissioning of Linacs, TomoTherapy, and Treatment Planning Systems with MapCHECK™

Recent developments in radiation delivery and treatment planning systems require more complicated quality assurance (QA) procedures. There are various QA test tools and specialized equipment for performing linear accelerator (Linac) and treatment delivery QA tests. However in order to have an effective QA program and ensure execution of the program, QA procedures should be simple and yet tailored so that each procedure checks as many facets as possible, while keeping the operating cost reasonable. At our center we use a 2D detector array, MapCHECK™ from SUN NUCLEAR Corporation, not only for its intended filmless IMRT QA but also for routine Linac and TomoTherapy machine QA as well as QA and commissioning of radiation treatment planning system. Using MapCHECK™ has improved our QA program feasibility and accuracy and has brought us one step closer to a paperless QA program.

Poster — Wed Eve—29: Validation of VMAT Delivery Using a Commercial 2D Diode Array

Volumetric modulated arc therapy (VMAT) is a new modality that combines IMRT with rotational delivery. RapidArc (Varian Medical Systems) incorporates the capability to vary dose rate, gantry speed and dynamic multileaf collimators to optimize dose and delivery efficiency. While new systems are being developed for VMAT verification, diode arrays are efficient and convenient tools for beam‐wise verification of IMRT delivery, and also have been employed in composite rotational delivery verification. Mapcheck2 (Sun Nuclear Corporation) is a new diode array with 1527 detectors. In this study, we investigate the use of Mapchekc2 in the MapPhan solid water phantom to validate VMAT delivery. 5 VMAT plans were generated in Eclipse using the RapidArc optimizer. All plans were delivered several times to verify consistency of delivery and measurement. Gamma analysis was used to verify the results compared to calculated dose distributions. The system was found to be sensitive to small changes in position or orientation. RapidArc beam delivery has been verified to correspond well with calculated dose distributions. Gamma values were below 1 in greater than 90% of measured points for all treatment plans. Delivery was found to be insensitive to initial dose rate. The MapPhan, with Mapcheck2 2D array in solidwater is a convenient way to verify VMAT plans. MapPhan results showed that RapidArc delivery is stable and reproducible.

SU‐FF‐T‐266: Characterizing a Multi‐Axis Ion Chamber Array

Purpose: To characterize a commercially available multi‐axis ion chamber array for use as a scanning water tank alternative. Method and Materials: The ion chamber array used in this study was the IC Profiler (Sun Nuclear Corporation: Melbourne, FL). We characterized four items of the array: reproducibility, dose linearity, backscatter response, and water tank agreement. Short and long term reproducibility's were established on a 60Co teletherapy unit (Eldorado 6; Atomic Energy of Canada Limited: Mississauga, Canada). The remaining tests were conducted with a Synergy (Elekta: Crawley, UK) linear accelerator (LINAC) operated at a nominal photon energy of 6MV. Results: Over a short time period the array displayed a maximum standard deviation of 0.55% and a mean standard deviation of 0.15%; over a long time period the array displayed a maximum standard deviation of 1.80% and a mean standard deviation of 0.76%. The array was sensitive to startup characteristics of the LINAC when operating in pulsed mode; this affected the dose linearity relative to a Farmer chamber operating under the same geometry. This effect was not observed when the array was operated in continuous mode. Both the array's central axis detector and a Farmer chamber displayed a similar increase in measured signal with increasing backscatter. However, with increasing backscatter (up to 16.6 cm) the arrays in‐beam‐profile shape changed by less than 0.7% relative to a setup with no additional backscatter. The agreement between the array and a scanning water tank differed by less than 1% in the beam. Conclusion: The IC Profiler is a viable option for water tank ‘like’ measurements. The device provides a stable platform with good dose linearity, minimal backscatter response, and uniform profile measurements. Conflict of Interest: This work was supported in part by SBIR Contract No. HHSN261200522014C, the University of Florida, and Sun Nuclear Corporation

SU-FF-T-204: The Determination of Optimal Hounsfield Unit Assignment of Materials in a 2 Dimensional Diode Array Used in the Calculation of Rotational Therapy Verification Plans

Purpose: To determine the optimal Hounsfield unit (HU) assignments for the materials in a two dimensional diode detector array used for the quality assurance of rotational radiotherapy.Method and Materials: A 2 dimensional diode array (MapCheck, Sun Nuclear Corporation) with a surrounding uniform density phantom (MapPhan) was imaged with 2.5 mm CT slices. The array contains a plane of 445 solid state detectors in contact on both the top and bottom with a fiberglass pc board and polycarbonate plate. The attenuating properties of this entire detector assembly were modeled in the eclipse planning system by assigning different HU to the materials and comparing calculated (AAA 8.2.23 algorithm, Varian Medical Systems) and measured dose. The diodes were assigned HU of −50, 0, 50 or 3000, and the polycarbonate plates were assigned HU of −50, 0, 50, 150, 250 or 350. Rectangular fields of length 25cm and widths of 5cm, 10cm and 30cm, were planned with different HU assignments and measured in a 358° arc. Results: Comparisons were made using the agreement rate between calculated and measured results within 2% dose. The phantom defined by uniform density had 99.1%, 95.7% and 92.4% agreement for HU values of −50, 0, and 50 respectively. Definitions of the polycarbonate plates of HU of 50, 150, 250, and 350 yielded calculation agreements with measurement of 96.4%. 93.3%, 90.3% and 84.5%. When each of the 445 diodes is assigned 3000 HU, the 2% dose agreement rate is 94.2% and pixel density correction yielded an agreement of 60.4%. Conclusion: The best results were obtain with uniform HU assignments of 0 or −50 HU or by defining the polycarbonate plates with a HU of 50 and the remainder of the assembly 0 HU. Unacceptable results are obtained when using pixel based density corrections.

MO‐FF‐A1‐02: Calibration of a Novel 4D Diode Array for IMRT and VMAT QA

Purpose: To develop calibration procedures for a novel 4D diode array for IMRT as well as arc therapy quality assurance (QA). Method and Materials: A novel 4D diode array (ArcCHECK) was designed for rotational therapy QA. Its cylindrical and isotropic design presents a consistent detector image in beam's eye view for all gantry angles. Signals were acquired every 50 ms, allowing for real‐time per‐beam QA as well as measuring composite dose distributions. An efficient calibration procedure was developed to obtain diode sensitivity and directional response dependence with one full gantry rotation. In this process, each diode was in turn irradiated under the same beam condition on beam central axis. The directional response was obtained using TPS‐calculated beam profiles, which were validated in rectangular geometry. A real time algorithm to derive beam angle based on beam edge back projections was developed to apply the corresponding directional response factors. Its clinical application for IMRT QA was demonstrated with 17 IMRT head and neck beams delivered with planned beam angles. Measured dose distribution was compared to TPS calculation in three dimensions with %diff and distance‐to‐agreement analysis. Its feasibility for VMAT QA was investigated with a prostate conformal arc plan. Results: For the IMRT beams, average passing rate was 94.3%±2.1% and 99.6%±0.8% with 1%/2mm and 1%/3mm, respectively. For the conformal arc plan, the passing rate was 97.7% with 1%/2mm and 100% with 1%/3mm. Conclusion: An efficient calibration procedure was developed to obtain both the diode sensitivity and directional response dependence. Real‐time gantry angles were derived accurately based on beam edge back projections, which were used to apply directional response factors. Excellent agreement with TPS calculation was achieved for both IMRT and VMAT plans. ArcCHECK is an efficient and valuable tool for both IMRT and VMAT QA.Research sponsored by Sun Nuclear Corporation.

SU-FF-T-202: Dosimetric Evaluation of a New Two-Dimensional Diode Matrix System for IMRT Planning Validation

Purpose: To present a dosimetric study of a large‐area 2D diode matrix QA device recently introduced for the validation of planar dose distributions resulting from patient IMRT plans used for treating prostate and head and neck (H&N) cancers.Method and Materials: MapCHECK 2™ Model 1177 (Sun Nuclear Corporation, Melbourne, FL) is a new 2D diode array consisting of 1527 n‐diodes distributed over a 26×32 cm2 octagonal space with a uniform diode spacing of 7.07 mm. Its predecessor (MapCHECK model 1175) contained 445 n‐diodes arranged over a 22×22 cm2 area. Spatial resolution over the 10×10 cm2 central portion was 7.07mm and increased to 14.14 mm outside this area. TG‐51 dose calibrations and diode sensitivity measurements were performed on both diode arrays before their clinical application. From a pool of IMRT patients, 5 prostate and 5 H&N treatment plans were selected for plan validation using both devices. Dose plan verification was accomplished by superimposing measured isodose distribution, at 10‐cm depth, for each IMRT field on the companion distribution calculated using the Pinnacle3treatment planning system (8.0m). Results: (1) As expected, both diode arrays faithfully reproduce the calculated isodose distributions for all prostate treatment plans, because the same inner high‐resolution (7 mm) area is used. (2) For H&N plans, the isodose distribution measured with MapCHECK 2, owing to its uniform 7 mm diode spacing, gives superior agreement with the Pinnacle isodose distributions. (3) Similar results are obtained with the older MapCHECK by utilizing the inner high‐resolution area at the expense of making multiple exposures for each field following translational and rotational shifts. Conclusions: We have successfully implemented the new MapCHECK 2 and demonstrated that it is more efficient for H&N IMRT QA than its predecessor. It is better suited for measuring planar dose distributions of larger IMRT fields encountered in H&N IMRT treatments.

SU‐FF‐T‐359: MapPhan with MapCHECK and Im'RT MatriXX for Real Gantry Angle IMRT QA

Purpose: A study was performed using two commercially available 2D detector array systems for IMRT patient specific QA by resetting all beam angels to gantry angle at zero degree and leaving all beam angles as planned to investigate and verify the dose delivery accuracy, limitation of the devices in the QA procedures. Method and Materials: Calibration was established at 0 degree beam angle, with the beam normal to the array plane. This study utilized one complicated head and neck IMRT plan with 9 fields (18 spitted fields). The Varian Eclipse treatment planning system (TPS) with Anisotropic Analytical Algorithm and Varian 21Ex LINAC were used for this study. The 2D array systems tested were MapCHECK with MapPhan from Sun Nuclear Corporation, and Im'RT MatriXX from IBA. Two QA phantoms were constructed by (1) MapPhan with MapCHECK and (2) solid water with MatriXX. The summed dose measured was compared to calculated one using two phantoms. Results: At gantry zero degree, 99.7% pass rate for MapPhan with MapCHECK was achieved using 3% and 3 mm DTA criteria in absolute mode and 99.2% for MatriXX. At true planned angle, 93.4% pass rate for MapPhan with MapCHECK and 93.0% for MatriXX. Conclusions: Although all arrays performed reasonably well in the composite dose for real gantry angles, it may indicate the loss of QA information in the composite. Since with the beam normal to the detector plane of 2D array, all measurement points carry the same information weight. Whereas delivery angles between normal and parallel, measurement carries varying amounts of weight for each point samples or not sampled depending on field size and beam angles. The application of a 2D array to real gantry angle IMRT QA requires careful considerations.

SU‐FF‐T‐391: IC Profiler Study: The End of Water Tank Measurements for Monthly QA?

Purpose: To investigate the possibility of using the IC Profiler (Sun Nuclear Corporation) for monthly QA of linear accelerators. Method and Materials: Photon and electron profiles measured with the IC Profiler, consisting of 251 ionization chambers located along the X, Y and diagonal axes, were compared to the profiles measured in water with an IC10 ionisation chamber. Three different methods were used for energy measurement of the photon and electron beams: 1) different thicknesses of solid water slab on the IC Profiler, 2) an acrylic continuous energy wedge and 3) a step wedge made of solid water. For the last two methods, the measured PDD equivalent curve was corrected for the change in profiles in order to remove the profile effect. All photon measurements were performed at SSD=75 cm with a 30×30 cm2 field size in order to be equivalent to a 40×40 cm2 at the isocenter. The electron measurements were done at SSD=100 cm with a 25×25 cm2 field size. Results: The agreement between symmetry and flatness measurements with the IC Profiler and the water tank was very good: maximum relative difference of −0.7% for 6 MV and 25 MV photon beams and −1.3% for electron beams (from 4 MeV to 22 MeV). Energy measurement using methods 2 and 3 was very fast and seemed to be as sensitive to an energy change as method 1. Method 2 gives a linear interpolation of the relative dose as function of the depth whereas method 3 gives the relative dose averaged over 10 detectors at 4 different depths. Conclusion: The IC Profiler is well adapted to the measurement of photon and electron profiles. Three different methods were tested for energy determination and more measurements are needed to establish the most accurate one.

SU‐FF‐T‐365: Dosimetry QA of Scanned Proton Beam with An Ion Chamber Array

Purpose: To investigate an IC Profiler for clinical proton dosimetry QA. Methods and Materials: An IC Profiler (model 1122) (Sun Nuclear Corporation, Melbourne, FL) consisting of four linear arrays (X, Y, and two diagonals) with 251 vented ion chambers was investigated. The vented ion chamber has 2.6 mm effective collection width and 0.9 g/cm2 inherent buildup. It was irradiated with a proton beam with 16 cm penetration depth at a depth of 11 cm corresponding to the middle of a 10 cm spread‐out Bragg peak (SOBP) for a 10‐cm diameter circular field using uniform scanning proton beam delivery system. Lateral dose profiles measured with ICP was compared with results obtained from a Markus chamber (MC) positioned in water tank, a large multi‐pad ionization chamber (MPIC) transverse detector, and a MatriXX detector (Scanditronix / Wellhofer). Field width (50%–50%), lateral penumbra (20%–80%), field flatness and symmetry were analyzed. Results: The measured field widths were 10.92 cm, 10.70 cm, 10.21 cm and 10.51 cm from ICP, MC, MPIC and, Matrixx detector, respectively. The measured lateral penumbras were 5.6 mm, 6.0 mm, 8.2 mm and 8.1 mm for ICP, MC, MPIC, and Matrixx, respectively. The flatness values obtained with ICP, MC, MPIC and Matrixx were 107.40%, 104.03%, 108.05%, and 104.55%. The symmetries obtained with ICP, MC, MPIC and Matrixx are 0.62%, 0.16%, 0.15%, and 0.12%, respectively. Conclusions: Results obtained from the IC Profiler are similar to those obtained from the other three detectors. From these investigations it can be concluded that the newly available IC PROFILER is a suitable device for quality assurance and dose verifications in proton therapy either in double scattering or scanned beam.

A simple approach to using an amorphous silicon EPID to verify IMRT planar dose maps

A simplified method of verifying intensity modulated radiation therapy (IMRT) fields using a Varian aS500 amorphous silicon electronic portal imaging device (EPID) is demonstrated. Unlike previous approaches, it does not involve time consuming or complicated analytical processing of the data. The central axis pixel response of the EPID, as well as the profile characteristics obtained from images acquired with a 6 MV photon beam, was examined as a function of field size. Ion chamber measurements at various depths in a water phantom were then collected and it was found that at a specific depth d(ref), the dose response and profile characteristics closely matched the results of the EPID analysis. The only manipulation required to be performed on the EPID images was the multiplication of a matrix of off axis ratio values to remove the effect of the flood field calibration. Similarly, d(ref) was found for 18 MV. Planar dose maps at d(ref) in a water phantom for a bar pattern, a strip pattern, and 14 clinical IMRT fields from two patient cases each being from a separate anatomical region, i.e., head and neck as well as the pelvis, for both energies were generated by the Pinnacle planning system (V7.4). EPID images of these fields were acquired and converted to planar dose maps and compared directly with the Pinnacle planar dose maps. Radiographic film dosimetry and a MapCHECK dosimetry device (Sun Nuclear Corporation, Melbourne, FL) were used as an independent verification of the dose distribution. Gamma analysis of the EPID, film, and Pinnacle planar dose maps generated for the clinical IMRT fields showed that approximately 97% of all points passed using a 3% dose/3 mm DTA tolerance test. Based on the range of fields studied, the author's results appear to justify using this approach as a method to verify dose distributions calculated on a treatment planning system, including complex intensity modulated fields.