2D Ion Chamber Array Research Articles

An algorithm to calculate a collapsed arc dose matrix in volumetric modulated arc therapy

The delivery of volumetric modulated arc therapy (VMAT) is more complex than other conformal radiotherapy techniques. In this work, the authors present the feasibility of performing routine verification of VMAT delivery using a dose matrix measured by a gantry mounted 2D ion chamber array and corresponding dose matrix calculated by an inhouse developed algorithm. Pinnacle, v9.0, treatment planning system (TPS) was used in this study to generate VMAT plans for a 6 MV photon beam from an Elekta-Synergy linear accelerator. An algorithm was developed and implemented with inhouse computer code to calculate the dose matrix resulting from a VMAT arc in a plane perpendicular to the beam at isocenter. The algorithm was validated using measurement of standard patterns and clinical VMAT plans with a 2D ion chamber array. The clinical VMAT plans were also validated using ArcCHECK measurements. The measured and calculated dose matrices were compared using gamma (γ) analysis with 3%/3 mm criteria and γ tolerance of 1. The dose matrix comparison of standard patterns has shown excellent agreement with the mean γ pass rate 97.7 (σ = 0.4)%. The validation of clinical VMAT plans using the dose matrix predicted by the algorithm and the corresponding measured dose matrices also showed good agreement with the mean γ pass rate of 97.6 (σ = 1.6)%. The validation of clinical VMAT plans using ArcCHECK measurements showed a mean pass rate of 95.6 (σ = 1.8)%. The developed algorithm was shown to accurately predict the dose matrix, in a plane perpendicular to the beam, by considering all possible leaf trajectories in a VMAT delivery. This enables the verification of VMAT delivery using a 2D array detector mounted on a treatment head.

A methodology for dosimetry audit of rotational radiotherapy using a commercial detector array

PurposeTo develop a methodology for the use of a commercial detector array in dosimetry audits of rotational radiotherapy. Materials and methodsThe methodology was developed as part of the development of a national audit of rotational radiotherapy. Ten cancer centres were asked to create a rotational radiotherapy treatment plan for a three-dimensional treatment-planning-system (3DTPS) test and audited. Phantom measurements using a commercial 2D ionisation chamber (IC) array were compared with measurements using 0.125cm3 IC, Gafchromic film and alanine pellets in the same plane. Relative and absolute gamma index (γ) comparisons were made for Gafchromic film and 2D-Array planes, respectively. ResultsComparisons between individual detectors within the 2D-Array against the corresponding IC and alanine measurement showed a statistically significant concordance correlation coefficient (both ρc>0.998, p<0.001) with mean difference of −1.1±1.1% and −0.8±1.1%, respectively, in a high dose PTV. In the γ comparison between the 2D-Array and film it was that the 2D-Array was more likely to fail planes where there was a dose discrepancy due to the absolute analysis performed. ConclusionsIt has been found that using a commercial detector array for a dosimetry audit of rotational radiotherapy is suitable in place of standard systems of dosimetry.

SU‐E‐T‐211: Development of Correction Factors for a 2D Ion Chamber Array Inserted in a Cubic and Homogeneous Phantom for VMAT Patient‐Specific QA

Purpose: Volumetric modulated arc therapy (VMAT) requires, as another kind of intensity‐modulated radiation therapy (IMRT), patient‐specific QA procedures. 2D ion chamber arrays are used to analyze and compare dose distributions. When these detectors are used in a stationary phantom, the measured dose is dependent on the angle of the beam. This work shows the development of correction factors (CFs) needed to obtain good agreement between calculated and measured dose distribution. Methods: VMAT treatments are delivered in our institution with an Elekta Synergy linac. 2D ion chamber array (MatriXX, IBA Dosimetry) is used to perform dose distribution comparisons for patient‐specific QA, inserted in a cubic and homogeneous special phantom (MultiCube, IBA Dosimetry). CFs are determined for each individual chamber in MatriXX detector by comparing measurements taken with MatriXX/MultiCube set to dose reference values calculated by our TPS (Monaco 3.1, Elekta) with the same setup. The detector has been set up to avoid irradiation through the treatment couch. Measurements consist on a set of open fields of 27×27 cm2 with gantry angles covering the range from 0° to 180° (interval of 1° between 85° y 95° and 5° for any other angle). Assuming a symmetric response, values from 180° to 360° are the same. An external gantry angle sensor has been used to match the measurements with their gantry angle. Results: CFs have been evaluated for each chamber in all gantry positions. Deviations of up to 18% and 7% are observed for gantry angles at 90° and 180°. These values must be used to correct dose measurements performed with MatriXX/MultiCube set. Conclusion: Angular dose dependency of MatriXX detector inserted on MultiCube phantom has been studied and calibrated. This calibration should be used to evaluate actual treatments as a part of patient‐specific QA procedures.

SU‐E‐T‐220: Comparison of Two Different Setups for VMAT Patient‐Specific QA

Purpose: Patient‐specific QA methods are needed for verification of intensisty‐modulated radiation therapy (IMRT). VMAT treatment is delivered through dynamic arcs with a continuous variation of gantry speed, dose rate and MLC position. 2D ion chamber arrays can be used to compare dose distributions as a part of patient‐specific QA procedure in VMAT technique. This work shows the gamma tests results for two different setups using a 2D array in the verification of VMAT plans. Methods: VMAT treatments are delivered with an Elekta Synergy linac. A 2D ion chamber array (MatriXX Evolution, IBA Dosimetry) is used to measure and evaluate dose distributions calculated by the TPS (Monaco 3.1, Elekta). Detector is calibrated to perform absolute dose measurements. Two different setups are presented as a part of QA verification procedure. First setup mounts the MatriXX detector inserted in a dedicated gantry holder with SSD of 100 cm. Dose measurements are compared with calculations collapsed to 0° because the detector is attached to the gantry during the gantry arcing. Second setup consists on MatriXX inserted in a cubic and homogeneous phantom (MultiCube, IBA Dosimetry). Coronal and sagital dose planes are measured. A set of correction factors are needed in order to correct dose in this arrangement. The sampling time is 150 ms. 111 patients have been verified with previous setups with a gamma index criteria of 3%/3mm. Results: Mean value for % points meeting gamma test criteria is (99.02±1.14)% in the first setup. The worst value is 92.9%. Results for second setup are (99.05±0.67)% for the coronal planes and (98.19±1.10)% for the sagital planes. The worst value is 95.22% for a sagital plane. Conclusion: The suitability of MatriXX detector for patient‐specific QA in VMAT techniques has been tested for two different setups with excellent agreement between calculation and measurement.

SU-E-T-10: Streamlining and Automating Dosimetric Analysis for Treatment Planning System and Linac QA

Purpose: To streamline and automate analysis of dosimetry checks for treatment planning system (TPS) commissioning, and TPS and linac QA, through scripts. Methods: A dosimetric analysis tool was developed using scripts written in Python language. A “standard” dosimetry dataset, ideally data measured for commissioning and/or verification of linac and TPS using a water scanning system, is generated and structured into an Excel spreadsheet. Annual dosimetry measurements, made with water a scanning system, are converted to the same format; measurements with a 2D ion chamber array (Matrixx) are kept in their original format in a separate folder. Plans for the same setup geometries are generated, or existing plans recalculated, and associated 3D dose files are exported in DICOM format into proper folders. Python scripts cycle through the planned DICOM dose files, match the information (beam energy, field size, SSD, depth) in the DICOM header with the data in the Excel files, or in the Matrixx data files, extract relevant dosimetric data from the 3D dose files (PDDs, profiles, output factors), compare to measurement using an in-house developed gamma analysis routine and display the results in a pdf file (profiles, PDDs, planar dose distributions) or tabulate in an Excel sheet (output factors, absolute doses for reference geometries, wedge factors). For points failing the gamma analysis default passing criteria, an optimization is done to iteratively find at which tolerance levels these points pass for further evaluation. Results: Using our dosimetric analysis software, a test analysis of planned and measured dosimetric data for TPS commissioning of photon and electron beams was completed in about half hour, once data were converted into the proper format. Conclusion: Scripts written in Python facilitate more comprehensive analysis of dosimetric data for TPS and linac commissioning/QA. Streamlining the data analysis through scripts increases efficiency by reducing repetitive manual tasks.

SU‐E‐T‐418: Dynamic Arc Technique Improves Delivery Accuracy of Stereotactic Body Radiation Therapy (SBRT) for Spine

Purpose: To investigate whether delivery accuracy can be improved with volumetric intensity modulated arc (VMAT), a hybrid dynamic arc therapy (HA) with fixed beam IMRT when compared to fixed beam IMRT alone for spine SBRT. Methods: Ten spine patients who underwent SBRT treatment using fixed beam IMRT were randomly chosen from an IRB approved registry. All cases were replanned with VMAT and HA techniques. The VMAT plans were created using the Pinnacle 9.0 with four 120 degree coplanar arcs and 120 segments. HA plans and clinical IMRT plans were created from Brainlab iplan. All HA plans used one 360 dynamic arc and 5–7 IMRT fixed beams, consisting 150–180 segments. All clinically IMRT plans used 7–9 fixed IMRT beams with 200–250 segments. All VMAT and HA plans were similar or better than clinical IMRT plans, meeting clinical objectives. All thirty plans were delivered with the same linear accelerator and measured with a 2D ion chamber array. The gamma index (GI) (3mm/3% and 2mm/2%) was used to test the delivery accuracy for each plan. Results: For the three delivery techniques, VMAT, HA, and IMRT, the average GI of 3mm/3% were 98.9±0.6%, 96.5±2.7%, and 92.6±1.1%, respectively. The average GI of 2mm/2% were 92.3±2.4%, 90.4±2.1%, and 82.0±2.2%, respectively. There is no significant difference on delivery accuracy between the VMAT plans and HA plans(P =0.09 for GI (3mm/3%) and P=0.229 for GI(2mm/2%)). There is a significant difference (P&lt;0.05) on delivery accuracy between VMAT (or HA) and IMRT plans based on GI of 3mm/3% and 2mm/2% criteria. Conclusions Dynamic arc techniques improve delivery accuracy of stereotactic body radiation therapy for the spinal lesions. VMAT or HA is a better choice than fixed beam IMRT with improved delivery accuracy.

TH‐C‐144‐11: Radiological Physics Center (RPC) Approval of Proton Centers for NCI‐Sponsored Clinical Trials

Purpose: To describe the approval process for enrolling patients on NCI‐funded cooperative group clinical trials with proton therapy and the impact of data obtained from the process. Methods: Institutions interested in participating in clinical trials with proton therapy must first complete five RPC‐coordinated approval steps for each proton delivery Method: a) completion of the proton facility questionnaire; b) annual monitoring of beam output; c) electronic transfer of treatment plans; d) successful completion of baseline anthropomorphic proton phantoms; and e) successful completion of a dosimetry review visit and corresponding site visit report. The RPC implemented a new fee structure for proton therapy site visits on 1/1/2013, establishing a cost‐share between the NCI/MGH Federal Share Funds and the institutions visited. These visits consist of QA review as well as dosimetric measurements with an ion chamber, 2D ion chamber array, and multi‐layer ion chamber to evaluate treatment delivery. Once an institution is approved for clinical trials with protons, there may be additional credentialing for specific trials, such as irradiations of the RPC's protocol‐specific anthropomorphic proton phantoms. Results: To date, the RPC has approved eight proton centers for use of proton therapy in clinical trials. These approvals cover scattered, uniform scanning, and spot scanning proton therapy delivery techniques. The RPC has performed 12 proton therapy site visits and has consensus data to establish acceptance criteria for site visit measurements that will ensure clinical trial consistency. With some institutions going through the approval process for a second or third delivery modality, the RPC anticipates eight proton centers will have 12 new beam delivery systems approved for use of proton therapy in clinical trials by the end of 2013. Conclusion: The RPC's proton therapy approval process and credentialing requirements help ensure consistency across clinical trial participants. Work supported by grants CA10953, CA059267, and CA81647 (NCI, DHHS).

SU-E-T-90: Sensitivity of MatriXX 2D Array in Detection of MLC Leaf Positioning Errors

Purpose: MLC leaf positioning errors significantly impact the accuracy of IMRT delivery. This study was to investigate the detection sensitivity of a commercial MatriXX 2D ion chamber array for MLC leaf positioning errors. Methods: Systematic MLC leaf positioning errors (±1, ±2, ±3 mm) were purposely introduced on one side of the leaf bank in a square field (10 × 10 cm2) and a clinical prostate IMRT plan with 5 coplanar fields (0°, 50°, 100°, 260°, and 310°) within a clinical treatment planning system (TPS). To test the effect of the ion chamber resolution (7.62 mm), the center of the square field was shifted to different locations at 1 mm intervals in relationship to the chamber positions. Both treatment plans, with and without leaf positioning errors, were delivered and measured with a commercial MatriXX device. To eliminate potential beam modeling errors from the TPS, the Gamma index of each field was directly calculated between measured planar doses with and without errors. Results: For the square field with 1 mm and 2 mm leaf positioning errors, respectively, the Gamma indices of 3%/3mm were 99.8% and 98.8%; the Gamma indices of 2%/2mm were 98.8% and 97.5%. The Gamma index was independent of the error positions relative to the chamber positions. For the clinical prostate IMRT plan with 1 mm, 2 mm, and 3 mm leaf positioning errors, respectively, the average Gamma indices of 3%/3mm for 5 fields were (100 ± 0.0)%, (99.8 ± 0.1)%, and (98.6 ± 1.2)%; the average Gamma indices of 2%/2mm were (99.8 ± 0.2)%, (98.3 ± 0.8)%, and (96.6 ± 1.8)%. Conclusion: With the Gamma index of 3%/3mm having a passing rate of 99%, the MatriXX 2D array can detect 2 mm leaf positioning errors for a single field, and 3 mm errors for IMRT plans with multiple fields.

SU-E-T-91: The Effect of Phantom Setup Uncertainties On Gantry Angle Correction Factors for MatriXX Evolution

Purpose: Prior research has shown that the MatriXX Evolution™ 2D ion chamber array has an angular-dependent dose response. This can be corrected using user-defined gantry angle correction factors (GACFs). The GACF for a given angle is defined as the ratio of the user's TPS-calculated dose to the phantom-measured dose at that angle. This work evaluates the effect of phantom setup uncertainties on GACF values. Methods: Measurements were made with the MatriXX in a MultiCube phantom (IBA Dosimetry). The GACFs were created using the vendor's recommended procedure. The phantom was irradiated with a 6 MV, 27×27 cm2 field at gantry angles 0° to 355° (IEC convention) in 5-degree increments. Additional measurements were made in 1-degree increments from 85° to 95° and 265° to 275°. The reference planar doses for each angle were computed using Philips Pinnacle3 TPS. Translation errors of 1 mm and 2 mm along each axis were simulated, along with rotational errors about the longitudinal axis of 0.5° and 1°. GACFs were computed for each shift and compared to the values for the unshifted phantom. Results: Vertical shifts produced changes in GACFs of up to 1.5% at a gantry angles near 180°. Vertical shifts produced changes of up to 8% near 90° and 270°, where the GACFs vary rapidly with angle. Lateral shifts produced changes of up to 1.7%. Longitudinal shifts, which occur parallel to the gantry rotation axis, caused negligible changes to GACFs. Rotational shifts of 1° produced up to 6% changes in GACFs near 90° and 270°. Conclusion: Shifts in phantom position and orientation of less than 2 mm or 1 degree generally produce changes in computed GACFs of less than 2%. Larger changes in GACFs at certain angles may be important for IMRT plan verification for plans with beam angles near the more sensitive angles.

WE-G-108-06: Evaluating Pre-Treatment IMRT &amp; VMAT QA Techniques Quantitatively Using Receiver Operating Characteristic (ROC) Analysis

Purpose: Pre‐treatment IMRT and VMAT QA techniques are often commissioned without knowledge of their sensitivity to clinically relevant delivery errors. The purpose of this work is to develop a method to quantify the sensitivity and specificity of pre‐treatment IMRT and VMAT QA techniques to treatment delivery errors. Methods: To evaluate a QA technique, a population of treatment plans and a population of clinically relevant delivery errors are defined. For each delivery error, a threshold magnitude is determined that induces a substantial change in clinically relevant dosimetric indices. Errors at the threshold magnitude are introduced into the plans and QA is performed with and without intentionally introduced errors. The QA technique is treated as a binary classifier to predict error plans using Receiver Operator Characteristic (ROC) analysis. We applied this technique to evaluate portal imager and 2D ion chamber array based QA for VMAT treatment of brain lesions. Delivery errors included discrepancies in MLC positioning (single leaf and leaf bank); lag of MLC trajectory; and discrepancy in dose rate per control point or gantry angle. The threshold magnitude was determined by achieving a 5% change in target conformity index. Results: The area under the curve (AUC) for the ROC analysis was 0.592 and 0.509 for the ion chamber array and portal imager, respectively, using a gamma index of 3%, 3mm. The AUC increased to 0.632 and 0.777 when 2%, 2mm was used for the ion chamber array and portal imager, respectively. Comparison based on 3% dose agreement resulted in an AUC of 0.557 and 0.693, respectively. Conclusion: For both portal imager and ion chamber array based QA, stricter tolerance than 3%, 3mm is needed to detect clinically relevant delivery errors. This method can be used to quantitatively compare the sensitivity of various QA techniques to clinically relevant dosimetric errors.

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.

SU‐E‐T‐375: Evaluation of the Ability of Three Commercially Available Dosimeters to Detect Systematic Delivery Errors in IMRT Plans

Purpose: To evaluate the sensitivity of three patient‐specific IMRT QA devices to systematic delivery errors. Methods: Four clinical step‐and‐shoot IMRT plans were selected, two simple prostate, one medium complexity head and neck (H&amp;N) and one complex H&amp;N. Twelve plan variations with errors introduced were created: Multi‐Leaf Collimator (MLC) positional errors (all leaf pairs shifted in the same direction (1, 2, 3 mm) and opposite direction (1, 2, 3 mm)); collimator rotation offsets (1, 2, 3 degrees) and gantry rotation offsets (0.5, 1, 2 degrees). The total dose for each plan was measured using an ArcCHECK helical diode array. Each field for each plan, excluding those with gantry offsets, was measured using an Electronic Portal Imager and a MatriXX Evolution 2D ionisation chamber array. The measured dose was compared to the calculated dose for the no error plan using Gamma analysis with 3%/3mm, 3%/2mm, and 2%/2mm criteria. Results: The ability of the different devices to detect errors was similar. In most cases pass rates reduced with increasing errors and/or stricter gamma criteria as expected. The ArcCHECK demonstrated sensitivity to 2 degree gantry errors, with a reduction in pass rate of 6–14% between the 1 and 2 degree errors in all plans for all gamma criteria. Even for the 2%/2mm criteria, no device detected a 3 degree collimator error in either prostate plan, however collimator errors were detected for the H&amp;N plans which had elongated fields. For all criteria, there were different scenarios for each detector where MLC position errors of 3 mm were not detected. Conclusion: None of the devices could reliably detect MLC position or collimator rotation errors that were not within recommended linear accelerator monthly QA tolerances. There were no consistent differences between the ability of the devices to detect systematic delivery errors. Financial support has been provided by the Saudi Arabian Cultural Mission towards this work.

SU‐E‐T‐204: Preliminary Study of the Comparison of Patient‐Specific QAs Using the Diode and Ionization Chamber Based Dosimetry Systems

Purpose: Patient QA comparison by using the ArcCHECK diode cylindrical and PTW 2D ionization chamber array based dosimetry systems for in‐homogenous and small field size tumor. Methods: Four head and neck (HN), 4 lung and 3 brain tumor patients were selected to study the behavior of in‐homogeneity on QA and, 3 SRS and SRT patients were selected to investigate small field size effect. The detectors used for this study are the PTW Seven29 2D ion‐chamber planar array (PTW, Freiburg, Germany) and the ArcCHECK cylindrical array (Sun‐Nuclear, FL). All the patient QA plans were generated using the Eclipse (version‐11) TPS; all the treatment plans were delivered on the TrueBeam‐STX Accelerator and measured by both the systems. A gamma‐index criterion of minimum 3% (3 mm) is used to evaluate the QA results. The Local‐Dose and the Van‐Dyk criteria are used to calculate the gamma, respectively. Results: The QA results show that gamma‐index scores are higher than 94% (3mm, 3%) for both the systems for the brain and lung patients; whereas for the HN patients, the gamma‐index score is 86% and 98% for the PTW and ArcCHECK, respectively. For the small field size, the gamma‐index scores (3%, 3 mm) are higher than 98% for the ArcCHECK; whereas the gamma‐index score is lower for the PTW. Conclusion: The tumor inhomgeneity does not affect the QA results since both systems give similar QA results for the brain and lung patients. However, ArcCHECK is better as compared to the PTW for the HN patients; this may be due to the relatively higher sensitivity of the diode and could pick up the lower signal for low dose area. For the SRS and SRT QAs, there will be volume averaging effect because of the large sensitive volume; hence, the PTW system may not be a good choice of detectors for these QAs.

Performance evaluation of respiratory motion‐synchronized dynamic IMRT delivery

The purpose of this study was to evaluate the capabilities of DMLC to deliver the respiratory motion‐synchronized dynamic IMRT (MS‐IMRT) treatments under various dose rates. In order to create MS‐IMRT plans, the DMLC leaf motions in dynamic IMRT plans of eight lung patients were synchronized with the respiratory motion of breathing period 4 sec and amplitude 2 cm (peak to peak) using an in‐house developed leaf position modification program. The MS‐IMRT plans were generated for the dose rates of 100 MU/min, 400 MU/min, and 600 MU/min. All the MS‐IMRT plans were delivered in a medical linear accelerator, and the fluences were measured using a 2D ion chamber array, placed over a moving platform. The accuracy of MS‐IMRT deliveries was evaluated with respect to static deliveries (no compensation for target motion) using gamma test. In addition, the fluences of gated delivery of 30% duty cycle and non‐MS‐IMRT deliveries were also measured and compared with static deliveries. The MS‐IMRT was better in terms of dosimetric accuracy, compared to gated and non‐MS‐IMRT deliveries. The dosimetric accuracy was observed to be significantly better for 100 MU/min MS‐IMRT. However, the use of high‐dose rate in a MS‐IMRT delivery introduced dose‐rate modulation/beam hold‐offs that affected the synchronization between the DMLC leaf motion and target motion. This resulted in more dose deviations in MS‐IMRT deliveries at the dose rate of 600 MU/min.PACS numbers: 87.53.kn, 87.56.N‐

VMAT monthly QA using two techniques: 2D ion chamber array with an isocentric gantry mount and an in vivo dosimetric device attached to gantry

AbstractPurposeVarian RapidArc is a volumetric modulated arc therapy (VMAT) that obtains a conformal dose around the desired structure by employing variable gantry speed, dose rate and dynamic multileaf collimator (DMLC) speed as the gantry rotates about machine isocenter. This study is meant to build upon previous research by Ling et al. by completing the tests with an in vivo dosimetric device attached to the linac gantry and a 2D ionisation chamber array with an isocentric gantry mount.Materials and methodsTwo PTW detectors, seven29 array with gantry mount and DAVID, were attached to the linear accelerator gantry, allowing each device to remain perpendicular to the beam at all gantry angles. Three tests for RapidArc evaluation were performed on these devices including: dose rate and gantry speed variation, DMLC speed and dose rate variation and DMLC position accuracy. The reproducibility of the arc data was also reported.ResultsA picket fence plan varying dose rates (111 to 600 MU/minute) and gantry speeds (5·5 to 4·3°/second) was delivered consisting of seven sections of different combinations. These measurements were compared with static gantry, open field measurements and found to be within 2·39% for the DAVID device and 0·84% for the seven29. A four-section picket fence of varying DMLC speeds (0·46, 0·92, 1·84 and 2·76 cm/second) was similarly evaluated and found to be within 1·99% and 3·66% for the DAVID and seven29, respectively. For DMLC position accuracy, a picket fence arc plan was compared with a static picket fence and found to agree within 0.38% and 2.91%. Reproducibility for these three RapidArc plans was found to be within 0·30% and 2·70% for the DAVID and seven29.ConclusionThe DAVID and seven29 detectors were able to perform the RapidArc quality assurance tests efficiently and accurately and the results were reproducible. Periodic verification of DMLC movement, dose rate variation and gantry speed variation relating to RapidArc delivery can be completed in a timelier manner using this equipment.

The Influence of Volume Effect in 2D-array Ion Chamber on the Measurement of IMRT Dose Distribution

‡‡ † † , † ‡ We evaluated the influence of volume effect on the measurement of IMRT dose distribution by comparing a 2D-array ion chamber and other dosimeters. Matrix phantom which is a 2D-array ion chamber having volume effect was compared with beam image system and film for the mea surement of dose distribution. Five intensity-modulated radiation therapy plans were created using five fields in thevirtual phantom. The measured dose distribution was compared with the calculated one by radiation treatment planning system and analysis program. We evaluated the conformity of dose distribution by calculating correlation coefficients and gamma values. The highest error rate of 1.3% was associated with matrix phantom in which volume effect in small field sizes was substantial.

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

Impact of dose rates on the position accuracy of multi-leaf collimator

PurposeTo study the impact of dose rates on the position accuracy of a Varian multi-leaf collimator (MLC) in dynamic IMRT. MethodsFour nasopharyngeal carcinoma (NPC) patients who received dynamic IMRT were chosen. The dosimetric verification at different dose rates (400–600MU/min) was performed using a PTW 2D ion chamber Array in a Varian Clinac iX linear accelerator. Meanwhile, the actual MLC leaf position data was taken by the MLC controller. The passing ratios of dosimetric verification were compared at different dose rates, and the MLC leaf position deviations were evaluated through the parameters of Error Histogram and Error RMS. ResultsThe passing ratios of dosimetric verification both of the criteria of 3%/3mm and 2%/2mm did not show statistical differences between two dose rates (P>0.05). But using the criteria of 2%/1mm, significant differences were observed between two dose rates (P<0.05). Moreover, according to the analysis of the Error Histogram and Error RMS, the MLC leaf position deviations were higher with the increase of dose rate, and the differences between 400 and 600MU/min were statistically significant (P<0.05). ConclusionsThe higher dose rate will affect the accuracy of MLC leaf position in dynamic IMRT.

SU-E-T-100: How to Improve the Dose Accuracy for Gantry Angle Dependent Patient Specific IMRT QA Using 2D Ion Chamber Array with Octavius Phantom.

To determine the cross calibration factors which can predict more accurate dose distribution for fixed beam IMRT QA using Octavius phantom. The ion chamber based Octavius 2D-array detector (PTW, Freiburg, Germany) is a step in the right direction to measure the absolute dose and dose distribution for patient specific IMRT QA. However, the directional dependency of this detector made it less than desirable for angle dependent IMRT QA. We evaluated the new Octavius system (PTW, Freiburg, Germany) for angle dependent IMRT QA which compensates the response due to directional dependency. The system is designed for full arc VMAT QA, but does not always work for the discrete angle IMRT QA due to non-averaging of errors caused by directional dependence of detectors. The proposed method uses correction factors for each gantry angle. The dose for a 10cm × 10cm open field for each gantry angle was calculated by treatment planning system and measured using the Octavius phantom. The correction factors were determined at each gantry angle and the dose distribution was renormalized at each angle using correction factors. The discrepancy between measured and planned dose per monitor unit depended on the gantry angle and were in the range of +-4% using the PTW method. Using our method, uncertainty due to the detector angle dependency was eliminated. The new method removes the angle dependency of ion chamber based 2D array detector for the fixed beam IMRT QA. It provides fast, accurate and more realistic results for angle dependent IMRT QA.

SU-E-T-114: Characterization of the Spatial Response Functions of Ionization Chambers for Photon Beam Dosimetry.

The finite extension of an ionization chamber gives rise to a spatial averaging effect, known as the "volume effect". In order to provide the appropriate corrections, the response functions along its lateral and longitudinal directions are characterized using Gaussian distributions, whose standard deviations slat and slong have been determined for a large set of clinical dosimeters. Nine cylindrical ionization chambers, two parallel-plate chambers and two 2D ionization chamber arrays have been examined by scanning rectangular photon fields along their short axes. The true profiles D(x) were known from scans with a small Si diode. The ionization chambers were aligned with their symmetry axes either perpendicular or parallel to the scan direction in order to obtain slat and slong separately. In a search process, D(x) was numerically convolved with normalized one-dimensional Gaussian kernels K(x) of varying s. The best fit between the convolution product D(x) * K(x) and the measured profile M(x) of the ionization chamber was used to determine parameters slat and slong of the Gaussian kernels. For both the lateral and longitudinal directions, very good agreement was found between M(x) and the convolution products of D(x) with Gaussian kernels K(x). For all chambers, their 2s values are similar to the cavity dimensions, which means that the "tails" of the Gaussian response functions reach into the exterior of the chambers, - an effect of the ranges of the secondary electrons. At higher photon energies response functions K(x) are slightly wider, but no detectable depth dependence has been observed. We have shown that the response functions of ionization chambers can be described by Gaussian distributions, confirming earlier observations, and we determined their standard deviations in both the lateral and longitudinal directions. Using these response functions, appropriate correction methods determined to eliminate the volume effect can be applied.