Ionization Chamber Array MatriXX Research Articles

Patient-specific quality assurance of dynamically-collimated proton therapy treatment plans.

The dynamic collimation system (DCS) provides energy layer-specific collimation for pencil beam scanning (PBS) proton therapy using two pairs of orthogonal nickel trimmer blades. While excellent measurement-to-calculation agreement has been demonstrated for simple cube-shaped DCS-trimmed dose distributions, no comparison of measurement and dose calculation has been made for patient-specific treatment plans. To validate a patient-specific quality assurance (PSQA) process for DCS-trimmed PBS treatment plans and evaluate the agreement between measured and calculated dose distributions. Three intracranial patient cases were considered. Standard uncollimated PBS and DCS-collimated treatment plans were generated for each patient using the Astroid treatment planning system (TPS). Plans were recalculated in a water phantom and delivered at the Miami Cancer Institute (MCI) using an Ion Beam Applications (IBA) dedicated nozzle system and prototype DCS. Planar dose measurements were acquired at two depths within low-gradient regions of the target volume using an IBA MatriXX ion chamber array. Measured and calculated dose distributions were compared using 2D gamma analysis with 3%/3mm criteria and low dose threshold of 10% of the maximum dose. Median gamma pass rates across all plans and measurement depths were 99.0% (PBS) and 98.3% (DCS), with a minimum gamma pass rate of 88.5% (PBS) and 91.2% (DCS). The PSQA process has been validated and experimentally verified for DCS-collimated PBS. Dosimetric agreement between the measured and calculated doses was demonstrated to be similar for DCS-collimated PBS to that achievable with noncollimated PBS.

Single pencil beam benchmark of a module for Monte Carlo simulation of proton transport in the PENELOPE code.

PENH is a recently coded module for simulation of proton transport in conjunction with the Monte Carlo code PENELOPE. PENELOPE applies class II simulation to all type of interactions, in particular, to elastic collisions. PENH uses calculated differential cross sections for proton elastic collisions that include electron screening effects as well as nuclear structure effects. Proton-induced nuclear reactions are simulated from information in the ENDF-6 database or from alternative nuclear databases in ENDF format. The purpose of this work is to benchmark this module by simulating absorbed dose distributions from a single finite spot size proton pencil beam in water. Monte Carlo simulations with PENH are compared with simulation results from TOPAS Monte Carlo (v3.1p2) and RayStation Monte Carlo (v6). Different beam models are examined in terms of mean energy and energy spread to match the measured profiles. The phase-space file is derived from experimental measurements. Simulated absorbed dose distributions are compared to experimental data obtained with the ionization chamber array MatriXX 2D detector (IBA Dosimetry) in a water tank. The experiments were conducted with a clinical IBA pencil beam scanning dedicated nozzle. In all simulations a Fermi-Eyges phase-space representation of a single finite spot size proton pencil beam is used. In general, there is a good agreement between simulated results and experimental data up to a distance of 3cm from the central axis. In the core region (region where the dose is more than 10% of the maximum dose) PENH shows, overall, the smallest deviations from experimental data, with the largest radial rms (root mean square) smaller than 0.2. The results achieved by TOPAS and RayStation in that region are very close to those of PENH. For the halo region, that is the area of the dose distribution outside the core region reaching down to 0.01% of the maximum intensity, the largest rms achieved by TOPAS is always smaller than 0.5, yielding better results than the rest of the codes. The physics modeling of the PENELOPE/PENH code yields results consistent with measurements in the dose range relevant for proton therapy. The discrepancies between PENH appearing at distances larger than 3cm from the central-beam axis are accountable to the lack of neutron simulation in this code. In contradistinction, TOPAS has a better agreement with experimental data at large distances from the central-beam axis because of the simulation of neutrons.

Effect of angle correction factors of MatriXX on VMAT dose verification

Objective To investigate the effect of the angle correction factors of ionization chamber array Matrixx on the dosimetric verification of volumetric modulated arc therapy plan. Methods The Matrixx (IBA) was put in the middle of the MultiCube Phantom. Computed tomography scan was performed and the results were sent to the Raystation treatment planning system (TPS). All data calculated from TPS and actually measured using MatriXX were gained under the following conditions: 6 MV, 100 MU, 28 cm×28 cm radiation field. And the directions of beam were from 0°to 180°with an interval of 5°(except an interval of 1° from 85° to 95°). The angle correction factors (CF)(θ) cor of MatriXX were calculated by the calculated dose values from TPS and the measured values using MatriXX and then mirrored to the opposite side. CF (θ) cor values were compared with CF (θ) def values that were given by manufacturer. The comparison was made by paired t-test. Results CF (θ) cor and CF (θ) def were different, and the difference was significant between 85° and 95°, with a maximum of 7.5%(P<0.05). The CF (θ) cor correction method had a higher gamma pass rate than the CF (θ) def correction method, with a maximum of 17%(P<0.05). Conclusions The ionization chamber array MatriXX has an angular dependence and the angle correction should be performed. The angle correction factors have the individual characteristics and are different in every unit ionization chamber of the MatriXX. Therefore, it is necessary to do angle correction for every unit ionization chamber of the MatriXX and the possible errors caused by various factors should be considered in order to improve the accuracy rate and gamma pass rate of the plan verification. Key words: Ionization chamber matrix; Angle correction factor; Volumetric modulated arc radiotherapy; Plan verification

A comparative analysis of Matrixx and EPID for dosimetric verification of intensity-modulated radiotherapy

Objective To compare the dosimetric verification results of Varian Portal Dosimetry and Matrixx, and to assess the reliability of the clinical application of electronic portal imaging device (EPID) verification. Methods Varian TrueBeam linear accelerator, which was equipped with a 120-leaf multileaf collimator and an amorphous silicon EPID, as well as portal dose prediction software. IBA I′mRT Matrixx ion chamber array was used. EPID algorithm configuration, dose calibration, and testing before use were performed. The sliding-window protocol was used. There were 77 patients with tumors involving the head and neck (mainly nasopharyngeal carcinoma), mediastinum, abdomen, and pelvic cavity were selected. The verification plan of the portal dose was created with a source-detector distance of 100 cm, and the gantry angle was kept the same as the treatment plan. The verification plan was carried out in the TrueBeam machine, and the data were collected at the same time by EPID. Comparison between the measured and calculated dose images was performed, and the evaluation standard was gamma index (3%/3 mm). The paired t-test was used for difference analysis. Results For the 77 patients, the Gamma passing rates of both methods were above 97%. Except for head and neck carcinoma were a significant difference between the results of dosimetric results using EPID and Matrixx in intensity-modulated radiotherapy (P=0.018) other remaining all P> 0.05. Conclusions The dosimetric verification results of EPID are consistent with those of Matrixx. EPID can be used for dosimetric verification, and Matrixx ion chamber array can be used only in case of a low Gamma passing rate. Key words: Dosimetric verification; Portal dosimetry; Matrixx

SU-F-P-62: The Sensitivity of Routine IMRT QA Metrics to Monitor Unit Errors

Purpose: To assess the sensitivity of routine IMRT QA metrics in relative or absolute dose analysis modes to monitor unit (MU) errors. Methods: Monitor units for two IMRT plans were altered by scaling them uniformly as well as by introducing random errors. The uniformly scaled plans were created by increasing or decreasing the total MUs by 5, 10, or 20% and the random ones were created by modifying two fields’ MUs to varying degrees. The composite plans were delivered to a MatriXX ion chamber array and evaluated using OmniPro I'mRT software. The analysis was done both in relative and absolute dose modes. Isodose overlay, Gamma Index, percent difference, and X and Y profiles were evaluated. The Gamma Index pass criterion was 3%/3mm for 95% of the points and that of percent difference was ±3% for 90 or 95% of the points depending on plan type. Results: Analysis of the QA of the plans without and with induced errors indicates using one single metric, like Gamma, may not point to MU errors and often more than one analysis tool is needed to detect errors. In addition, analyzing the QA results in relative mode may not detect errors as well. Overall, the random MU errors were detected by analyzing X and Y profiles and the uniformly scaled ones were detected by a combination of metrics. Conclusion: Using a single IMRT QA metric such as Gamma analysis to analyze IMRT QA may not detect errors such as MU discrepancies due to data entry or transfer in cross vender treatment planning and record and verify system environments. Hence, a combination of analysis matrices should be used, preferably in absolute dose mode.

Dose distributions verification for High dose rate brachytherapy plans by using ionization chambers 2D array

The purpose of this paper is to perform dosimetric verification (in-phantom) of dose distributions calculated with treatment planning system (TPS) by using 2D chamber array device in HDR brachytherapy. HDR brachytherapy treatment plans’dose distribution verification is performed using the two-dimensional (2D) ionization chamber array MatriXX Evolution developed by IBA Dosimetry (IBA Dosimetry, Germany) whose detector area is covered with the Nucletron Freiburg Flap Applicator Set (Nucletron BV,Veenendaal, the Netherlands) with catheters such that the detector plane was set to 0.86 cm from the catheter plane. Fixed slabs of RW3 (Perspex) were added below the 2D-ARRAY to provide full scattering conditions. The phantom was scanned on computed tomography (CT) for treatment planning with 2.5-mm slice thickness. Based on the CT data of the phantom, three different plans were calculated by the planning system (Oncentrabrachy version 4.3, Nucletron BV) and then are exported to the VERISOFT software for comparison with measured data. For comparison of dose distributions, both dose planes – measured &amp; calculated – were normalized  to the global maximum dose of the reference matrix (measured data set) and compared using the Gamma index method. Gamma indexes were evaluated using a dose-difference criterion of 3% and a distance criterion of 3 mm (γ≤1).The total number of evaluated dose points for the vault test case is 9755, 98.6 % of them (9623 point) passed the criteria of acceptability (3% delta dose and 3-mm distance criteria) and 1.4% of them (132 point) failed it. The total number of evaluated dose points for the full test case is 19964, 93.6 % of them (18683 point) passed  the criteria of acceptability and 6.4% of them (1281 point) failed it. And the total number of evaluated dose points for the cylinder test case is 19871, 96.9 % of them (19258 point) passed the criteria of acceptability and 3.1% of them (613 point) failed it. By thisThe use of the two-dimensional (2D) ionization chamber array MatriXX Evolution for brachytherapy applications has been successfully demonstrated. The array measurements in these planes have shown acceptable agreement with the TPS, generally within 3% delta dose and 3-mm distance agreement criteria within each plane. The comparisons made led to make a relation between the passing percentage values to the number of the evaluated dose points for each test case and it was found that as the number of these pixels increases the possibility of having more failing points increases.

SU‐E‐T‐413: Examining Acquisition Rate for Using MatriXX Ion Chamber Array to Measure HDR Brachytherapy Treatments

Purpose:There are unique obstacles to implementing the MatriXX ionchamber array as a QA tool in Brachytherapy given that the device is designed for use in the MV energy range. One of the challenges we investigate is the affect of acquisition rates on dose measurement accuracy for HDR treatment plans.Methods:A treatment plan was optimized in Oncentra Brachy TPS to deliver a planar dose to a 5×5cm region at 10mm depth. The applicator was affixed to the surface of the MatriXX array. The plan was delivered multiple times using a Nucleatron HDR afterloader with a 2.9Ci Ir192 source. For each measurement the sampling rate of the MatriXX movie mode was varied (30ms and 500ms). This experiment was repeated with identical parameters, following a source exchange, with an 11.2Ci Ir192 source. Finally, a single snap measurement was acquired. Analysis was preformed to evaluate the fidelity of the dose delivery for each iteration of the experiment. Evaluation was based on the comparison between the measured and TPS predicted dose.Results:Higher sample rates induce a greater discrepancy between the predicted and measured dose. Delivering the plan using a lower activity source also produced greater discrepancy in the measurement due to the increased delivery time. Analyzing the single snap measurement showed little difference from the 500ms integral dose measurement.Conclusion:The advantage of using movie mode for HDR treatment delivery QA is the ability for real time source tracking in addition to dose measurement. Our analysis indicates that 500ms is an optimal frame rate.

MO‐D‐213‐05: Sensitivity of Routine IMRT QA Metrics to Couch and Collimator Rotations

Purpose:To assess the sensitivity of gamma index and other IMRT QA metrics to couch and collimator rotations.Methods:Two brain IMRT plans with couch and/or collimator rotations in one or more of the fields were evaluated using the IBA MatriXX ion chamber array and its associated software (OmniPro‐I'mRT). The plans were subjected to routine QA by 1) Creating a composite planar dose in the treatment planning system (TPS) with the couch/collimator rotations and 2) Creating the planar dose after “zeroing” the rotations. Plan deliveries to MatriXX were performed with all rotations set to zero on a Varian 21ex linear accelerator. This in effect created TPS‐created planar doses with an induced rotation error. Point dose measurements for the delivered plans were also performed in a solid water phantom.Results:The IMRT QA of the plans with couch and collimator rotations showed clear discrepancies in the planar dose and 2D dose profile overlays. The gamma analysis, however, did pass with the criteria of 3%/3mm (for 95% of the points), albeit with a lower percentage pass rate, when one or two of the fields had a rotation. Similar results were obtained with tighter criteria of 2%/2mm. Other QA metrics such as percentage difference or distance‐to‐agreement (DTA) histograms produced similar results. The point dose measurements did not obviously indicate the error due to location of dose measurement (on the central axis) and the size of the ion chamber used (0.6 cc).Conclusion:Relying on Gamma analysis, percentage difference, or DTA to determine the passing of an IMRT QA may miss critical errors in the plan delivery due to couch/collimator rotations. A combination of analyses for composite QA plans, or per‐beam analysis, would detect these errors.

SU-E-J-157: Evaluation of Surface-Tracking and Respiratory Gating Capacities of a Novel Surface-Mapping System Using Deformable Image Registration

Purpose: We have tested the capabilities of an innovative surface-mapping system that utilizes deformable image registration tools (C-RAD Catalyst). Its 3D positioning stability has been verified as well as its respiratory capabilities. Methods: The Catalyst daily QA phantom was used to test the stability of the system. A MatriXX ion-chamber array in a solid water phantom was mounted on a motion platform which can provide longitudinal 1D motion.The vertical-motion plate (respiratory plate) of the platform was used to mimic the chest movement in respiratory cycles. The respiratory plate was tracked by the Catalyst surface-mapping system. A static 10×10 field was first delivered to establish the baseline. Gated deliveries using different gating windows (from 10% to 90% duty cycle) were also performed and the results were recorded. A comparison between the Catalyst surface tracking signal and ABC data was performed for a breast cancer patient on-treatment. Results: Analysis of the experiment data indicates that the Catalyst is stable and reliable in detecting surface position and movement. With 50 numbers of phantom setups, the mean deviation of the shifts and rotations along all three directions was 0.6mm and 0.02 degrees, respectively. The gating capacity study suggested that respiratory gating using Catalyst system on an Elekta linac can be delivered safely and accurately. In fact, in gated beam delivery with the Catalyst system, dose distribution, dose profiles, gamma passing rates all show improvements as the gating window gets smaller. Gamma passing rates increase from 84.7% at non-gating to 95.6% at 10% gating window. The Catalyst system also correctly recognizes the respiratory pattern of a patient under our ABC gated treatment delivery. Conclusion: Based on a phantom study and clinical observations, we have found that Catalyst system is robust in surface tracking and positioning. It also can serve as a respiratory gating solution. Funded by Elekta

SU-E-T-597: Influence of Smoothing Parameters on Dynamic IMRT Plan Quality and Deliverability.

To study the impact of different smoothing parameters on IMRT plan quality and deliverabilityMethods: Five previously treated patients of carcinoma cervix were chosen. Planning target volume (PTV) and organ at risk (OAR) i.e. bladder and rectum were contoured. In each case, five different dynamic IMRT plans with 6MV photon beam were created in eclipse TPS for Varian 2300C/D linear accelerator. During optimization, dose volume constraints and priorities were kept constant and smoothing parameters were varied as follows: 10/5, 40/30 (TPS default value), 80/60, 100/80 and 200/150 in x/y direction. Total dose was 5040cGy in 28 fractions and prescribed at 95% isodose. Plan quality was analyzed by means of coverage index (CI=PTV covered by prescription dose/PTV), OAR mean doses and total monitor units (MUs) required to deliver a plan. In each case, deliverability of treatment plans were verified with I'matriXX ion-chamber array and compared with TPS dose-plane using gamma index of 3% dose difference and 3mm distance to agreement criteria. The CI values were 0.9435±0.032, 0.9418±0.034, 0.9380±0.041, 0.9330±0.047 and 0.8681±0.072 for 10/5, 40/30, 80/60, 100/80 and 200/150 in x/y direction. PTV dose maximum decreases with the increase of smoothing parameters and values were 5724.38±106.08 5723.30±131.60, 5708.44±1 16.74, 5697.92±116.82 and 5587.50±189.50cGy. The bladder mean doses were 4027.46±630.40, 3821.62±420.62, 3819.58±427.08, 3813.42±435.02 and 3814.78±438.0cGy. Rectum mean doses were 3839.88±466.02, 3835.52±473.18, 3837.52±472.88, 3839.10±471.20 and 3918.94±469.76cGy. Similarly, Total MUs were 1588±205, 1573±214, 1513±274, 1456±335 and 1219±68. Gamma pass rate increases with the increase of smoothing parameters and values were 99.16±0.21%, 99.07±0.19%, 99.24±0.28%, 99.29±0.29% and 99.75±0.15%. When smoothing parameters decreased below TPS default value, plan quality increases, but deliverability decreases. If smoothing parameters increased above TPS default value, deliverability increases but plan quality decreases. Total MU decreases with the increase of smoothing parameters. Therefore, it's a trade-off between plan quality and deliverability which needs to be justified clinically.

Characterization and use of a 2D-array of ion chambers for brachytherapy dosimetric quality assurance

The two-dimensional (2D) ionization chamber array MatriXX Evolution is one of the 2D ionization chamber arrays developed by IBA Dosimetry (IBA Dosimetry, Germany) for megavoltage real-time absolute 2D dosimetry and verification of intensity-modulated radiation therapy (IMRT). The purpose of this study was to (1) evaluate the performance of ion chamber array for submegavoltage range brachytherapy beam dose verification and quality assurance (QA) and (2) use the end-to-end dosimetric evaluation that mimics a patient treatment procedure and confirm the primary source strength calibration agrees in both the treatment planning system (TPS) and treatment delivery console computers. The dose linearity and energy dependence of the 2D ion chamber array was studied using kilovoltage X-ray beams (100, 180 and 300 kVp). The detector calibration factor was determined using 300 kVp X-ray beams so that we can use the same calibration factor for dosimetric verification of high-dose-rate (HDR) brachytherapy. The phantom used for this measurement consists of multiple catheters, the IBA MatriXX detector, and water-equivalent slab of RW3 to provide full scattering conditions. The treatment planning system (TPS) (Oncentra brachy version 3.3, Nucletron BV, Veenendaal, the Netherlands) dose distribution was calculated on the computed tomography (CT) scan of this phantom. The measured and TPS calculated distributions were compared in IBA Dosimetry OmniPro-I‘mRT software. The quality of agreement was quantified by the gamma (γ) index (with 3% delta dose and distance criterion of 2 mm) for 9 sets of plans. Using a dedicated phantom capable of receiving 5 brachytherapy intralumenal catheters a QA procedure was developed for end-to-end dosimetric evaluation for routine QA checks. The 2D ion chamber array dose dependence was found to be linear for 100–300 kVp and the detector response (kuser) showed strong energy dependence for 100–300 kVp energy range. For the Ir-192 brachytherapy HDR source, dosimetric evaluation kuser factor determined by photon beam of energy of 300 kVp was used. The maximum mean difference between ion chamber array measured and TPS calculated was 3.7%. Comparisons of dose distribution for different test plans have shown agreement with >94.5% for γ ≤1. Dosimetric QA can be performed with the 2D ion chamber array to confirm primary source strength calibration is properly updated in both the TPS and treatment delivery console computers. The MatriXX Evolution ionization chamber array has been found to be reliable for measurement of both absolute dose and relative dose distributions for the Ir-192 brachytherapy HDR source.

WE‐A‐BRB‐07: Characterization and Use of a 2D‐Array of Ion Chambers for Brachytherapy Dosimetric Quality Assurance

Purpose: The 2D ionization chamber array MatriXX Evolution is one of the 2D ionization chamber arrays developed for megavoltage real‐time absolute 2D dosimetry and verification of IMRT. The purpose of this study is to evaluate the performance of ion chamber array for sub‐megavoltage range brachytherapy beam dose verification and quality assurance. Methods: The dose linearity and energy dependence of the 2D ion chamber array was studied using kilovoltage X‐ray beams (100 to 300 kVp). For the Ir‐192 brachytherapy HDR source dosimetric evaluation the detector response (kuser) factor was determined using 300 kVp beam. The phantom used for this measurement consists of multiple catheters, the IBA MatriXX detector and a slab of RW3 to provide full scattering conditions. The treatment planning system dose distribution was calculated on the CT scan of this phantom. The measured and TPS calculated distributions were compared in IBA Dosimetry OmniPro‐IˈmRT software. The quality of agreement was quantified by the gamma index for nine sets of plans. Using a dedicated phantom capable of receiving 5 brachytherapy intralumenal catheters a QA procedure was developed for end‐to‐end dosimetric evaluation for routine quality assurance checks. Results: The 2D ion chamber array dose dependence was found to be linear for 100 to 300 kVp and kuser showed strong energy dependence for 100 to 300 kVp energy ranges. The maximum mean difference between ion chamber array measured and TPS calculated was 3.7%. Comparisons of dose distribution for different test patterns have shown agreement with greater than 94.5 % for &amp;#947; &amp;#8804; 1 (with 3% delta dose and distance criterion of 2 mm). Dosimetric QA can be performed with the 2D ion chamber array. Conclusions: The MatriXX Evolution ionization chamber array has been found to be reliable for measurement of both absolute dose and relative dose distributions for the Ir‐192 brachytherapy HDR source.

The inter‐ and intrafraction reproducibilities of three common IMRT delivery techniques

Intensity modulated radiation therapy (IMRT) treatment delivery requires higher precision than conventional 3D treatment delivery because of the sensitivity of the resulting dose to small geometric misalignment of the modulated beamlets. The chosen treatment delivery technique will affect the treatment precision in different ways, based on the characteristics of the delivery method. Delivery using a multileaf collimator (MLC) can reduce treatment time and therapist workload, but typically requires a greater number of monitor units and the fields are prone to both systematic and random leaf positioning errors. An alternative to MLC-based fields, patient specific brass compensators, do not suffer from these leaf positioning errors. In our study, we set out to investigate which delivery method will provide the highest levels of dosimetric reproducibility and the minimum amount of interfraction variability. In our study, a seven field IMRT plan for a head and neck treatment was created using the Pinnacle3 treatment planning system and the intensity maps for each field were obtained. The intensity maps of the fields were delivered with a Varian 2100C/D linear accelerator, using solid compensators and sliding window (SW) and step-and-shoot (SS) MLC segments. Three fields were selected from the seven-beam IMRT plan for comparison. Analysis was carried out using the MatriXX ion chamber array, radiochromic film, and Varian dynalog files. Our results show that the error in MLC leaf positioning has no gantry angle dependence. The compensator and SW deliveries showed excellent agreement, even when stricter than usual gamma criteria were applied. However, we noted that under these strict conditions, the SS field had at least ten times more pixels out of range than did the compensators. When using step-and-shoot MLC fields, it was observed that the increase in dose rate or the increase of MU/segment degrades the quality of the plan. Analysis of the dynalog files showed that while each individual field had its own propensity for error, all fields showed the same trend: a greater percentage of time the leaves are out of position as dose rate increases, MUs decrease, or both. The compensator-based field and both types of MLC-based fields have MatriXX results that are within the clinically acceptable tolerance of 3% dose difference and 2 mm DTA. However, when the criteria are tightened, it becomes evident that the compensators have a definite advantage over their comparable MLC-based competitors in terms of interfraction reproducibility. Fewer monitor units are required to deliver each portal, potentially improving patient outcomes and reducing unwanted side effects to both patients and therapists. In centers without MLC, compensators represent a simple and cost effective way to offer patients state of the art treatment. Based on the results of this study, compensator-based IMRT is a reliable, viable option for use in clinics both with and without MLC-equipped linacs.

Dose distribution features of periodic motion target based on MatriXX system

Objective To investigate the dose distribution features of periodic motion target based on MatriXX system. Methods The static and periodic motion targets were separately irradiated by step and shoot and sliding window intensity modulated radiation therapy (IMRT) beams. The dose distribution was measured by MatfiXX system combined with domestic made periodic motion platform. Then the dose distribu-tions were compared by using the OmniPro I'mRT software system. Results When the target was irradiated by one type of IMRT beams (step and shoot or sliding window), the differences of dose distributions between periodic motion target and static were mainly located at the edge of the target, about 15%. When the target (static or motion) was irradiated separately by two types of IMRT beams (step and shoot and sliding window), the differences of dose distributions between step and shoot and sliding window irradiation were very small. Con-clusions The differences of dose distributions between periodic motion and static target were mainly located at the edge of the fields, and we should pay more attention. In the central area of the target, the dose distribution differences were very small. Key words: Radiotherapy; Step and shoot IMRT; Two-dimensional ionization chamber array MatriXX system; Sliding window IMRT

SU-GG-T-225: Gantry-Angle Dependence of a 2D Ion Chamber Array for IMAT QA

Purpose: Gantry‐angle dependence of dose measurement for intensity‐modulated arc therapy (IMAT) quality assurance (QA) using a two‐dimensional ion chamber array may be significant. A new device, the gantry angle sensor, has been provided to compensate for this dependence. However, the default correction factors (CF) assume that the ion chamber angular response is symmetrical, uniform across all chambers, and independent of field size. Our goal was to test these three assumptions, and verify the use of the default CF. Method and Materials: The Matrixx ion chamber array (IBA Dosimetry, Inc.) was irradiated every 5° gantry rotation with a 27×27cm2 static field. The central ion chamber CF = calculation/measurement. To test the asymmetry, dose was delivered over 360°, and compared for symmetrical angles. To test for chamber response uniformity, CFs for off‐axis chambers were calculated and compared to the central ion chamber. To test for field size dependence, the dose delivery was repeated with 10×10cm2 and 2×2cm2 fields, and the central ion chamber CF recalculated. Results: The measured chamber response is asymmetrical with gantry angle. The median difference between the default and measured central ion chamber CFs is 1.2% (range: −0.3%–8.5%) from Gantry 0–180°, and 2.9% (range: 0.7%–7.6%) from Gantry 180–360°. The asymmetry appears to arise from both the device itself (median 1.4%, range 0.1%–3.4%) and from the device holder (median 1.7%, range 0.5%–4.8%). The central‐axis and peripheral CFs varied by up to 10%. There was no observed field size dependence at 10×10cm2, but differences of up to 4.7% were seen with the 2×2cm2 field.Conclusion: The CF table for gantry angle dependence of an ion chamber array should account for asymmetry, nonuniformity of chamber response, and field size effect for small fields. Clinics are recommended to verify their own CFs.

MO‐D‐BRB‐01: Study of Systemic and Random Errors On VMAT and IMRT Plan Quality and Deliver Accuracy

Purpose: During the delivery of volumetric modulated arc therapy (VMAT), random errors exist in both the MLC leaf positions and the gantry angle. In this work, we investigated the impact of such random errors on VMAT plan quality and delivery accuracy. The impact of system calibration errors was also examined. For comparison purposes, we performed a similar study on step‐and‐shoot IMRT plans. Material and Method: VMAT plans for three treatment sites (prostate, pancreas and head‐&amp;‐neck) were created using a home‐grown arc sequencer. Next, random and systematic leaf position errors were introduced into these plans with the random errors sampled from Gaussian distributions of varying widths ranging from 1 to 3mm. Two types of systematic errors, including MLC bank shifts in the same direction (Type I: leaf gap unchanged) and MLC bank shifts in opposite directions (Type II: leaf gap increase/decrease). The plan quality variations were compared in the Pinnacle3 planning system. Plans with systematic errors were verified using the MatriXX ion chamber array with gamma evaluation criteria of 3%/3mm. Results: The plan degradation observed for VMAT plans was slightly less as compared to that for fixed‐field IMRT plans when random errors up to 3mm to the leaf positions were introduced. With type I systematic errors of 3mm on leaf positions, the average standard deviation of PTV dose increased by 10.2%. This value increased to 18.4% for the corresponding fixed‐field IMRT plans. A larger impact on the IMRT plans was also observed when type II systematic errors were introduced. The above results were confirmed by plan verification measurements with higher gamma passing rates for VMAT plans when systematic errors were applied. Conclusion: The VMAT delivery technique has better tolerance to random and systematic errors in gantry angle and MLC leaf position errors as compared with step‐and‐shoot IMRT.Research supported by Elekta.

SU-FF-T-209: Implementation of 2D Arrays for TomoTherapy Patient-Specific QA

Purpose: 2D arrays were not designed for rotational delivery modalities such as TomoTherapy. This study explores their clinical use and develops a methodology for minimizing their limitations. Method and Materials: The MatriXX ion chamber array (MULTICube phantom) and the MapCHECK diode array (MapPHAN phantom) were calibrated using a TomoTherapy plan with a contour enclosing the center diode/ion chamber and an adjacent ion chamber in solid water for absolute dose calibration. Eight prototype TomoTherapy treatment plans were created with various directional blocks to restrict beam delivery angles. Delivery verification plans for these prototype plans and for multiple clinical patient plans were created using MVCT images of each array in its respective phantom and then delivered. For comparison, film and ion chamber measurements were made with the cylindrical TomoTherapy cheese phantom. Results: Clinically, less than 5% of more than 100 patient treatment plans verified with these 2D arrays had pass rates less than 95% for a 3%/3-mm DTA criterion. Relative dose comparisons for the failing plans all had pass rates above 95%, indicating a calibration/scaling error. DQAs for a prototype plan that restricted beams to a 45 degree lateral arc had pass rates of ∼40% (absolute error > 5%) for both devices, indicating a problem with lateral beams. Similar arc restrictions in the AP/PA plane produced plans with pass rates > 95% for both devices. To minimize these effects, two approaches were explored. Histograms of MLC leaf opening times as a function of projection angle were used to either determine the optimal device orientation or to determine a calibration procedure that more closely matched the angular distribution of delivery beams. Conclusion: Both devices can be used with TomoTherapy satisfactorily, but care must be taken to orient and calibrate the 2D array under delivery conditions that more closely match the treatment delivery conditions.

MatriXX system in the study of the influence of respiratory motion on target dose distribution

Objective To investigate the influence of respiratory motion on target dose distribution in three-dimensional conformal radiation therapy(3DCRT) and intensity modulated radiation therapy(IM-RT). Methods A 2D air vented ionization chamber array MatriXX system,a platform which can mimic the clip motion of lung tumor, and a home-made solid water(RW3) phantom were used to measure the 2D dose distribution of static and moving targets irradiated by 3DCRT and IMRT beams using multiple gantry an-gles. The period of the moving platform was set to 3.5 s and the amplitude was ± 1 cm. Then the differences of dose distribution between the static phantom and moving phantom were compared and analyzed with the MatriXX system software. Results Compared with the static phantom in 3DCRT, the penumbra of dose distribution of the periodic moving phantom along the moving directions was increased by 6-9 mm, the high dose area was shrinked by about 5 ram,and the low dose area was extended by 5 mm,but the area of 50% i-sodose and the dose center area changed very little. When a single segment beam of IMRT was delivered and measured and the maximum dose of measuring plane was normalized to 100% ,the average difference of dose distribution between the static and dynamic phantoms was ±27% (from -56.4% to 56.1%) ;When all of the segment beams of IMRT were dehvered and the integrated dose distribution was measured ,the dose differ-ences were less than ±3% ,and the maximum difference of dose distribution was about ±15% which mainly appeared at the field margin and were similar to 3DCRT. Conclusions The dose distribution of most cen-ter areas of the periodic moving target using multi-fraction 3DCRT/IMRT beams is similar to that of the static target, while the high dose area of the former is shrinked and the low dose area is extended. Key words: Phantom/radiotherapy; Three-dimensional conformal; Intensity modulated; Ioni-zation chambers array; Dose verification