MatriXX Dose Research Articles

Dosimetric verification of stereotactic body radiation therapy for lung cancer treatment plans using flattening filter-free beams.

This study aimed to analyze the dose distribution using different dosimetric tools in conducting pretreatment quality assurance for lung cancer stereotactic body radiation therapy (SBRT) plans with flattening filter free (FFF) beams. Nine patients with lung cancer treated via SBRT were randomly selected, and their treatment plans were generated using the Eclipse treatment planning system (TPS) with FFF beams. For each patient, the same plan was applied to the Delta4 phantom and MatriXX by the TPS, which calculated the dose distribution. The Delta4 and MatriXX phantoms were then used to measure the actual dose distribution at the linear accelerator, and these measured doses were compared to with the calculated doses. Gamma analysis was employed in verifying the correspondence between the dose distributions. The absolute point doses were measured by a 0.016 mL Microchamber with the RW3 phantom and Thorax phantom. The absolute point doses measured by the 0.016 mL Microchamber were within ±3% of the calculated results for the central point of the RW3 and Thorax phantoms. The Delta4 and MatriXX dose distributions agreed well with the measured and calculated doses, over 98%, based on the 3% maximum dose and 3 mm gamma criteria. Both measured and calculated doses for the Delta4 and MatriXX phantoms agreed well for each patient with lung cancer. The absolute point dose measurements using the 0.016 mL Microchamber exhibited excellent agreement with the TPS calculated between the RW3 and Thorax phantoms.

Transit dosimetry in dynamic IMRT with an a-Si EPID.

Using an amorphous silicon (a-Si) EPID for transit dosimetry requires detailed characterization of its dosimetric response in a variety of conditions. In this study, a measurement-based model was developed to calibrate an a-Si EPID response to dose for transit dosimetry by comparison with a reference ionization chamber. The ionization chamber reference depth and the required additional buildup thickness for electronic portal imaging devices (EPID) transit dosimetry were determined. The combined effects of changes in radiation field size, phantom thickness, and the off-axis distance on EPID transit dosimetry were characterized. The effect of scattered radiation on out-of-field response was investigated for different field sizes and phantom thicknesses by evaluation of the differences in image profiles and in-water measured profiles. An algorithm was developed to automatically apply these corrections to EPID images based on the user-specified field size and phantom thickness. The average phantom thickness and an effective field size were used for IMRT fields, and images were acquired in cine mode in the presence of an anthropomorphic phantom. The effective field size was defined as the percentage of the jaw-defined field that was involved during the delivery. Nine head and neck dynamic IMRT fields were tested by comparison with a MatriXX two-dimensional array dosimeter using the Gamma (3%, 3 mm) evaluation. A depth of 1.5 cm was selected as the ionization chamber reference depth. An additional 2.2 mm of copper buildup was added to the EPID. Comparison of EPID and MatriXX dose images for the tested fields showed that using a 10% threshold, the average number of points with Gamma index <1 was 96.5%. The agreement in the out-of field area was shown by selection of a 2% threshold which on average resulted in 94.8% of points with a Gamma index <1. The suggested method is less complicated than previously reported techniques and can be used for all a-Si EPIDs regardless of the manufacturer.

SU-E-T-608: Performance Comparison of Four Commercial Treatment Planning Systems Applied to Intensity-Modulated Radiotherapy

Purpose: To compare the performances of four commercial treatment planning systems (TPS) used for the intensity-modulated radiotherapy (IMRT). Methods: Ten patients of nasopharyngeal (4 cases), esophageal (3 cases) and cervical (3 cases) cancer were randomly selected from a 3-month IMRT plan pool at one radiotherapy center. For each patient, four IMRT plans were newly generated by using four commercial TPS (Corvus, Monaco, Pinnacle and Xio), and then verified with Matrixx (two-dimensional array/IBA Company) on Varian23EX accelerator. A pass rate (PR) calculated from the Gamma index by OminiPro IMRT 1.5 software was evaluated at four plan verification standards (1%/1mm, 2%/2mm, 3%/3mm, 4%/4mm and 5%/5mm) for each treatment plan. Overall and multiple pairwise comparisons of PRs were statistically conducted by analysis of covariance (ANOVA) F and LSD tests among four TPSs. Results: Overall significant (p>0.05) differences of PRs were found among four TPSs with F test values of 3.8 (p=0.02), 21.1(>0.01), 14.0 (>0.01), 8.3(>0.01) at standards of 1%/1mm to 4%/4mm respectively, except at 5%/5mm standard with 2.6 (p=0.06). All means (standard deviation) of PRs at 3%/3mm of 94.3 ± 3.3 (Corvus), 98.8 ± 0.8 (Monaco), 97.5± 1.7 (Pinnacle), 98.4 ± 1.0 (Xio) were above 90% and met clinical requirement. Multiple pairwise comparisons had not demonstrated a consistent low or high pattern on either TPS. Conclusion: Matrixx dose verification results show that the validation pass rates of Monaco and Xio plans are relatively higher than those of the other two; Pinnacle plan shows slight higher pass rate than Corvus plan; lowest pass rate was achieved by the Corvus plan among these four kinds of TPS.

SU‐E‐T‐181: Commissioning of COMPASS System for Flattening Filter Free Beams Patient Specific Quality Assurance

Purpose: To validate and implementate the COMPASS and MatriXX Evolution systems (Iba dosimetry) for specific patient quality assurance using flattening filter free beams from a TruebeamSTx (Varian Medical Systems) linear accelerator. Methods: We use MatriXX Evolution 2D array mounted on isocenter using a gantry holder combined with angle sensor. 6MV FFF beams are modeled and verified into Pinnacle v9.2 (Philips) and Compass software. Matrixx dose rate response was assesed for dose rates from 4 Gy/min to 14 Gy/min. We begin the commissioning process comparing measurements and calculation of simple fields, and then we choose several SBRT cases comparing TPS doses and Compass calculated and reconstructed doses using as quality index the number of points with gamma index &lt;1 inside 50% isodose. Verification include comparison of MatriXX measured fluences with Compass predicted fluences. Results: Compass modelling of FFF beams is accurate within 0.5%, same accuracy than conventional beams. Matrixx shows dose rate dependence below 0.2%. Comparison between Compass and TPS shows an agreement of 96% (points with gamma index &lt;1), with no essential different between reconstructed and calculated measurements. Compass predicted fluences agree with matrixx measurements, with differences between +−2% inside beam and with higher diferences in penumbra region (some chambers with 10% differences) Conclusion: Compass was commissioned and validated for use with flattening filter free beams. Low dose rate dependence of Matrixx detector allows for measurements of IMRT or VMAT plans with high dose rate modulation. Compass calculated dose is as accurate as reconstructed dose, but last one allows for verification of machine delivery together with TPS verification.

SU‐E‐T‐199: A Software Tool for the Efficient Correction and Analysis of IMRT QA Experimental Data Using MatriXX

Purpose: The rationale for developing this software was to address shortcomings found while using OmniPro‐I' mRT software to evaluated patient specific IMRT QA measurements taken with MatriXXEvolution 2D ion chamber array. Our software addresses three major issues. It calculates average doses within each ion chamber of the array using 3D TPS DICOM field‐by‐field doses exported as input, corrects the resulting doses to account for gantry angle dependence of Matrixx, and automates the analysis of the measured data resulting in a concise report document ideal for patient electronic medical records (EMR).Materials/Methods: The software was developed in MATLAB to improve and streamline analysis of patient‐specific IMRT QA. It compares measured Matrixx doses to average doses within each ion chamber of the array, which are computed from 3D DICOM doses exported from TPS for each field. A correction factor is applied to the TPS doses to account for angular dependence. The Data is automatically evaluated using the 2D gamma analysis, evaluating a range of passing criteria (3%,4%,5%). This software allows for user‐defined passing criteria and automated reporting of quantitative results, eliminating potential user errors. A single‐sheet report is generated displaying patient information, planar dose distributions, x‐y‐profiles, and gamma plot noting the pixel based passing criteria. The effect of misalignment in direction perpendicular to detector plane is simulated by allowing a non‐zero shift along that direction. Results: This home‐built IMRT QA analysis tool gives a clear quantitative answer based on the user defined criteria. The implementation of an angular dependence correction and the averaging over the volume of the ion chambers improves the accuracy of the analysis accounting for ion chamber size (∼5mm x 5mm x5mm) and spacing (∼7mm × 7mm) Conclusion: Though this tool does not replace the commercial software completely, it improves the accuracy of the analysis and streamlines the process.

Angular dependence correction of MatriXX and its application to composite dose verification

We measured the angular dependence of central and off‐axis detectors in a 2D ionization chamber array, MatriXX, and applied correction factors (CFs) to improve the accuracy of composite dose verification of IMRT and VMAT. The MatriXX doses were measured with a 10° step for gantry angles (θ) of 0°–180°, and a 1° step for lateral angles of 90°–110° in a phantom, with a 30×10 cm2 field for 6 MV and 10 MV photons. The MatriXX doses were also calculated under the same conditions by the Monte Carlo (MC) algorithm. The CFs for the angular dependence of MatriXX were obtained as a function of θ from the ratios of MatriXX‐measured doses to MC‐calculated doses, and normalized at θ=0°. The corrected MatriXX were validated with different fields, various simple plans, and clinical treatment plans. The dose distributions were compared with those of MC calculations and film. The absolute doses were also compared with ionization chamber and MC‐calculated doses. The angular dependence of MatriXX showed over‐responses of up to 6% and 4% at θ=90° and under‐responses of up to 15% and 11% at 92°, and 8% and 5% at 180° for 6 MV and 10 MV photons, respectively. At 92°, the CFs for the off‐axis detectors were larger by up to 7% and 6% than those for the central detectors for 6 MV and 10 MV photons, respectively, and were within 2.5% at other gantry angles. For simple plans, MatriXX doses with angular correction were within 2% of those measured with the ionization chamber at the central axis and off‐axis. For clinical treatment plans, MatriXX with angular correction agreed well with dose distributions calculated by the treatment planning system (TPS) for gamma evaluation at 3% and 3 mm. The angular dependence corrections of MatriXX were useful in improving the measurement accuracy of composite dose verification of IMRT and VMAT.PACS number: 87.55.Qr, 87.56.Fc

SU-E-T-129: Angular Dependence and Its Correction of the MatriXXEvolution System for IMRT and VMAT Dose Verification

Purpose: The purpose of this study was to evaluate the angular dependence of the MatriXX Evolution and establish its correction method to improve the dose verification accuracy of IMRT and VAMT. Methods: The CT study was performed by inserting the MatriXX into the MULTICube, which was exported to iPlan. The regions of interest that correspond to the size and locations of the MatriXX detectors were configured in CTimages. The angular dependence for the MatriXX detectors was measured with gantry angles 0°–180° at a field size of 30cm×10cm for 6 and 10MV photons. The correction factor (CF) was calculated as the ratio of the MatriXX dose to the Monte Carlo(MC) calculated dose for central detectors and detectors of every line, and was normalized at the gantry angle 0°.To verify two correction methods (central and entire correction), the MatriXX detectors was irradiated with gantry angles 0°–180° at different field sizes for 6 and 10MV photons. The results were compared with MC calculations by gamma index with 2% and 2mm using the OmniPro I'mRT. Absolute doses for central and off‐axis locations were measured with a PTW31010 chamber inserted into the MULTICube, and were compared with the MatriXX doses. Results: The angular dependence of the MatriXX detectors reached up to14% and 11% for 6 and 10MV photons, respectively. The difference in CF between central and off‐axis detectors was up to 7% and 5% for 6 and 10MV photons, respectively. The entire corrected MatriXX doses agreed within 1.5% with those of a PTW31010 chamber for central and off‐axis regions. The passing rate for gantry angle 90° shows a significant deviation between central and entire correction methods. Conclusions: The dose verification accuracy of the MatriXX Evolution improved gamma index up to 10% by the use of the entire correction method that includes off‐axis detectors

SU-GG-T-220: Comparison of RapidArc and IMRT Peripheral Doses for Pediatric Patients Based on Measured and TPS Doses

Purpose: To evaluate RapidArc and IMRT pediatric plans in peripheral regions based on measured and TPS doses. Method and Materials: IMRT and RapidArc plans for six pediatric patients were created and delivered to a solid water phantom containing a 2D ion chamber array - MatriXX. A correction technique was developed to correct for MatriXX dose errors in the peripheral regions. Thus corrected MatriXX doses were evaluated against TPS plans. Results: (1) MatriXX, with corrections, shows good agreement with a reference Exradin A12 ion chamber and EBT film, allowing for accurate measurements of low doses; (2) In the peripheral regions without direct beams, TPS was found to underestimate the dose by as much as 7cGy (prescription 200cGy); (3) The mean TPS dose error was smaller for RapidArc in five of the six cases and comparable in the other; (4) The error shows clear dependence on the equivalent square jaw size due to MLC scatter; (5) The distance to agreement (DTA) was smaller for RapidArc in all six cases (1 – 100cGy); (6) The impact of TPS dose error on VxGy was evaluated along y-axis in low-dose regions. Disagreement between MatriXX and TPS was found to be insignificant (<1cm) for x>5Gy, independent of RapidArc or IMRT. For jaw size >15cm, as large as 6 and 4.2cm were observed in V1Gy and V2Gy, respectively, for IMRT. The corresponding values were 2.6 and 0.6 cm for RapidArc. The disagreement in VxGy was smaller for RapidArc in all six cases. Conclusion: In terms of TPS error in peripheral dose, its impact on VxGy or DTA, RapidArc was found to be better than IMRT from our study of six pediatric patients. Our method can be useful in determining the best plan based on accurate assessment of the TPS doses in the peripheral regions through measurements.

WE‐D‐BRB‐04: Evaluation of MatriXX in Peripheral Dose Regions for IMRT and VMAT Dose Verifications

Purpose: To evaluate MatriXX and to develop a MatriXX correction technique for IMRT and VMAT dose verifications in peripheral (low) dose regions. Method and Materials: We compare a 2D ion chamber array — MatriXX with a reference Exradin A12 ion chamber in solid water phantoms. Comparisons were performed for a number of cases that involve different gantry and MLC dynamics including open beam, sweeping MLC gap mimicking IMRT, oscillating sweeping MLC gap mimicking VMAT, as well as clinical IMRT and RapidArc plans. Results: We have identified four types of error associated with MatriXX dose in peripheral regions: (1) Pixel‐dependent positive bias which is independent of dose level, cumulative over time, and dependent on operation parameters; (2) Over‐response to scattered beams by 2.0% on average for regions 2 – 15 cm from the field edge where patient and collimator scatters dominate; (3) Angular dependence up to 11% and 8% for primary and scattered (peripheral) radiations, respectively; (4) Round‐off error due to insufficient precision in conversion of raw signals to MatriXX software data for very small doses. In order to extend the application of MatriXX to peripheral regions, we have developed a method to correct for these errors. With corrections, MatriXX shows good agreement with A12 in all cases involving different gantry and/or MLC dynamics as well as the clinical plans. For both primary and peripheral doses, MatriXX shows good dose linearity down to about 1 – 2 cGy with an accuracy of within 1% of the local doses. Conclusion: Inherent dose errors of MatriXX were identified and thoroughly studied. A correction method was developed and successfully tested by a series of experimental and clinical plans. Therefore, we have demonstrated that, with proper corrections, MatriXX can be used for accurate 2D peripheral (low) dose measurements.

SU‐GG‐T‐359: The Applications of MatriXX to Electron Beam Dosimetry

Purpose: The objective of this study is to investigate the properties of I'mRT Matrixx device in electron beams, and to develop methods of electron dosimetry using Matrixx. Method and Materials: The measurements were conducted using Matrixx in electron beams from Siemens linacs. The Matrixx was placed horizontally on the linac tabletop, supported with solid water blocks. Solid water layers were used for dose buildup on top of the Matrixx. For all the measurements, the linac gantry angle was 0°, and the SSD was 100 cm from the surface of solid water buildup. The depth ionization was measured for different energies and field sizes, thus the depth of maximum ionization and the effective water equivalent depth of Matrixx could be determined. The electron cone factors and beam profiles were measured at the maximum ionization depths. The 2D dose maps and cutout factors of custom blocks were studied using Matrixx. Results: It was estimated that the intrinsic effective water equivalent depth of Matrixx is about 4.5 mm. When measuring at the respective depth of maximum electron intensity (peak of ionization), the Matrixx has slightly different dose responses for different electron energies. The cone factors measured by Matrixx are nearly identical to those derived by ionization chambers. Beam profiles (thus the flatness and symmetry) can be easily determined using Matrixx, and they are comparable to water tank results. The planar dose map of cutout blocks can be visually observed, and the cutout factors can be conveniently measured. Conclusion: The MatriXX dose response to electron beams is different from photon beams, and separate dose calibration factors should be established for electron dosimetry. The ionization chamber array in Matrixx has the ability to display planar dose map in a single measurement. Matrixx has the potentials in electron beam dosimetry and routine QA.

SU‐FF‐T‐360: Correction for Angular Dependence of the MatriXXEvolution Ionization Chamber Array

Purpose: The MatriXXEvolution ionchamber array (IBA Dosimetry) is an attractive tool for plan verification of both conventional IMRT and rotational therapies (RapidArc, VMAT, TomoTherapy, etc). However, the response of this array has a significant angular dependence. We have quantified this dependence, and developed a correction method suitable for plan verification. Method and Materials: A MatriXX phantom (MatriXXEvolution ionchamber array + solid water) and a reference phantom (A12 ionchamber + solid water) were irradiated from different gantry angles (0°–360°, steps of 10°, with finer steps of 3° for lateral angles). Reference measurements were taken using a phantom of the equivalent dimensions. The calibration factor was calculated as the ratio of the MatriXX dose to the A12 dose for a given gantry angle. The water equivalent thickness of the MatriXXEvolution reported by the manufacturer was also examined in the TPS and by measuring 6MV attenuation due to MatriXXEvolution vs. solid water for variable total thickness of the phantom. Three patient‐specific IMRT and two VMAT cumulative plan verifications were delivered to the MatriXX phantom. Dose measurements collected by MatriXXEvolution were then corrected with in‐house software. Measurements were then repeated to the reference phantom. Corrected MatriXX, uncorrected MatriXX and A12 measurements were compared to the treatment planning system. Results: The angular response of the MatriXX array was found to vary as much as 11%, depending on the irradiation angle. The most variation was found to occur around 91–110° and 260–269°, and also close to 180°. The corrected MatriXX doses were found to agree within 1.0% of the A12 ion chamber dose, vs. 2.5% for uncorrected. Conclusion: The MatriXXEvolution ionchamber array has an angular dependent response which is clinically significant, and must be corrected. We have developed and tested the use of an angular‐dependent correction factor which corrects for these response variations.

SU-FF-T-215: Evaluation of a Planar Ion Chamber Array for Patient Specific IMRT QA

Purpose: To evaluate the IBA MatriXX planar ion chamber array for patient specific IMRT QA by comparing measurements to ion chamber measurement and isodose distribution. Method and Materials: Ten IMRT plans were delivered to the MatriXX which was placed into the IBA MultiCube virtual water phantom. The dose distribution was compared to the Brainscan treatment planning system (TPS) by using the Omnipro I'mRT software. Ion chamber measurements were also compared to TPS as well as coronal isodose distributions taken with film. A PTW TN31010 cylindrical ion chamber and Kodak EDR were used. Each dosimeter was placed at similar depth in phantom and the composite dose of all treatment fields was used for comparison. The deviation between measured and planned dose was determined for each dosimetry system. Isodose distributions were analyzed with gamma routines meeting a 3%/3mm criteria. The average gamma as well as percentage pixel failure rate were compared. Results: CAX dose measured on MatriXX differs from plan by an average of 2.8% with a standard deviation of 1.5%. CAX dose measured with the ion chamber differed from plan by an average of 1.8% with a standard deviation of 1.1% The average gamma for MatriXX dose distribution was 0.59 with standard deviation 0.17. The average gamma for film distribution was 0.64 with a standard deviation of 0.21. The average percentage of pixels that fail gamma was 3.6% for MatriXX and 7.5% for film. Conclusion: The MatriXX performed as well as or better than ion chamber or film measurements, and the MatriXX provided better agreement to plan than film in nine out of ten cases measured. The results show that the MatriXX is a reliable and consistent device for patient specific IMRT QA.