QA Measurements Research Articles

Suitability of Self-Fabricated Solid Tissue Phantom for Quality Assurance Examination of Shortwave Diathermy (SWD) Unit

Diathermy is commonly used treatment method for pain relief in musculoskeletal tissue, rheumatisms, inflammation etc., by which the increased temperature in tissue induces better blood perfusion and quickens the healing process. But improper maintenance and incorrect handling might lead to degrade the output power or may cause serious burns, respectively. So correct regular QA measurements for the output power would be necessary, in order to give the correct and safe treatment plan. A suitable tissue phantom is then needed to accomplish this task. For this purpose, a glycerol-based solid tissue phantom has been self-fabricated with insertion of four precission temperature sensors (LM 35) at locations of of 1.0 cm, 1.5 cm, 2.0 cm, and 2.5 cm from the surface (each of which are 2 cm laterally separated). Prior calibration of these four sensors for temperature range of 20 - 50°C (using a calibrated temperature calibrator) show very good accuracy and precission properties. The phantom is then tested by exposing it with RF waves output of SWD unit with time variation of 2-14 min (step 2 min), and output power variation of 20% - 80% of its maximum power (incremental step of 20%).

TU‐G‐BRD‐05: Results From Multi‐Institutional Measurements with An Anthropomorphic Spine Phantom

Purpose:To analyze the results from an anthropomorphic spine phantom used for credentialing institutions for National Cancer Institute (NCI) sponsored clinical trial.Methods:An anthropomorphic phantom that contains left and right lungs, a heart, an esophagus, spinal cord, bony material and a PTV was sent to institutions wishing to be credentialed for NCI trials. The PTV holds 4 TLD and radiochromic film in the axial and sagittal planes. The heart holds one TLD. Institutions created IMRT plans to cover ≥90% of the PTV with 6 Gy and limit the cord dose to <0.35cc receiving 3.75 Gy and <1.2cc receiving 2.63 Gy. They were instructed to treat the phantom as they would a patient, including making plan specific IMRT/SBRT QA measurements before treatment. The TLD results in the PTV were required to be within ±7% of the plan dose. A gamma calculation was performed using the film results and the submitted DICOM plan. ≥85% of the analyzed region was required to pass a 5%/3 mm criteria.Results:176 institutions irradiated the spine phantom for a total of 255 results. The pass rate was 73% (187 irradiations) overall. 44 irradiations failed only the gamma criteria, 2 failed only the dose criteria and 22 failed both. The most used planning systems were Eclipse (116) and Pinnacle (52) and they had pass rates of 76% and 71%, respectively. The AAA algorithm had a pass rate of 77% while superposition type algorithms had a 71% pass rate. The average TLD measurement to institution calculation ratio was 0.99 (0.04 std dev.). The average percent pixels passing the gamma criteria for films was 89% (12% std dev.)Conclusion:Results show that this phantom is an important part of credentialing and that we have room for improvement in IMRT/SBRT spine treatments.This work was supported by PHS CA180803 and CA037422 awarded by NCI, DHHS.

SU‐E‐T‐88: Acceptance Testing and Commissioning Measurements of a Newly Released InCiseâ„¢ Multileaf Collimator for CyberKnife M6â„¢ System

Purpose:Accuray recently released a new collimator, the InCise™ Multileaf Collimator (MLC), for clinical use with the CyberKnife M6™ System. This work reports the results of acceptance testing and commissioning measurements for this collimator.Methods:The MLC consists of 41 pairs of 2.5 mm wide leaves projecting a clinical maximum field size of 110 mm × 97.5 mm at 800 mm SAD. The leaves are made of tungsten, 90 mm in height and tilted by 0.5 degree. The manufacturer stated leaf positioning accuracy and reproducibility are 0.5 mm and 0.4 mm respectively at 800 mm SAD. The leaf over‐travel is 100% with full interdigitation capability. Acceptance testing included, but are not limited to, the verification of the specifications of various parameters described above, leakage measurements and end‐to‐end tests. Dosimetric measurements included, but not limited to, measurements of output factors, open beam profiles, tissue‐phantom ratios, beam flatness and symmetry, and patient specific QA.Results:All measurements were well within the manufacturer specifications. The values of output factors ranged from 0.804 (smallest field size of 7.6 mm × 7.5 mm) to 1.018 (largest field size of 110.0 mm × 97.5 mm). End‐to‐end test results for the various tracking modes are: Skull (0.27mm), fiducial (0.16mm), Xsight Spine (0.4mm), Xsight Lung (0.93 mm) and Synchrony (0.43mm). Measured maximum and average leakage was 0.37% and 0.3%, respectively. Patient‐specific QA measurements with chamber were all within 5% absolute dose agreement, and film measurements all passed 2%/2mm gamma evaluation for more than 95% of measurement points.Conclusion:The presented results are the first set of data reported on the InCise™ MLC. The MLC proved to be very reliable and is currently in clinical use.

SU‐E‐T‐374: Evaluation and Verification of Dose Calculation Accuracy with Different Dose Grid Sizes for Intracranial Stereotactic Radiosurgery

Purpose:In this study, we aim to evaluate the effect of dose grid size on the accuracy of calculated dose for small lesions in intracranial stereotactic radiosurgery (SRS), and to verify dose calculation accuracy with radiochromic film dosimetry.Methods:15 intracranial lesions from previous SRS patients were retrospectively selected for this study. The planning target volume (PTV) ranged from 0.17 to 2.3 cm3. A commercial treatment planning system was used to generate SRS plans using the volumetric modulated arc therapy (VMAT) technique using two arc fields. Two convolution‐superposition‐based dose calculation algorithms (Anisotropic Analytical Algorithm and Acuros XB algorithm) were used to calculate volume dose distribution with dose grid size ranging from 1 mm to 3 mm with 0.5 mm step size. First, while the plan monitor units (MU) were kept constant, PTV dose variations were analyzed. Second, with 95% of the PTV covered by the prescription dose, variations of the plan MUs as a function of dose grid size were analyzed. Radiochomic films were used to compare the delivered dose and profile with the calculated dose distribution with different dose grid sizes.Results:The dose to the PTV, in terms of the mean dose, maximum, and minimum dose, showed steady decrease with increasing dose grid size using both algorithms. With 95% of the PTV covered by the prescription dose, the total MU increased with increasing dose grid size in most of the plans. Radiochromic film measurements showed better agreement with dose distributions calculated with 1‐mm dose grid size.Conclusion:Dose grid size has significant impact on calculated dose distribution in intracranial SRS treatment planning with small target volumes. Using the default dose grid size could lead to under‐estimation of delivered dose. A small dose grid size should be used to ensure calculation accuracy and agreement with QA measurements.

TH-CD-304-10: A Method for Correcting Ion Recombination Effects in OCTAVIUS 1000 SRS Measurements

Purpose: PTW's Octavius 1000 SRS array performs IMRT QA measurements with liquid filled ionization chambers to allow closer detector spacing and higher resolution. Increased density of liquid relative to air, leads to increased ion recombination effects, which are proportional to dose/pulse experienced at the detectors. During IMRT QA, dose/per pulse will vary across the array, leading to varying collection efficiencies. Magnitudes of recombination effects were investigated and a method was developed to correct them in QA measurements. Methods: The OCTAVIUS 1000 SRS was used to measure different IMRT plans using varying dose/pulse and results were normalized to air filled chamber measurements to obtain collection efficiencies. Regression analysis was performed to relate collection efficiency to dose/pulse. Matlab software was developed to correct IMRT QA measurements using these correlations. Measurements were performed on a Truebeam equipped with high-definition MLC, using 6MV and 10FFF RapidArc plans. Plans were measured in the OCTAVIUS 4D system, which measures dose in time intervals. Dose/time was converted to dose/pulse using pulse repetition information. Collection efficiency to dose/pulse correlations were used to correct measured dose. 3D gamma analysis was performed on plans with and without corrections. Results: Liquid filled ionization chamber collection efficiency correlated linearly to dose/pulse with slopes of 2.15%/(mGy/pulse) for 6MV and 5.74%/(mGy/pulse) for 10FFF. After corrections, average gamma pass rates improved by [0.02%,0.06%,0.24%] for 6MV and [0.19%,0.54%,2.01%] for 10FFF using [3%/3mm,2%/2mm,1%/1mm] criteria. Maximum changes in gamma pass rates were [0.1%,0.3%,0.5%] for 6MV and [0.4%,0.8%,5.1%] for 10FFF using [3%/3mm,2%/2mm,1%/1mm] criteria. Conclusion: OCTAVIUS 1000 SRS ion recombination effects have little effect on 6MV measurements. However, the effect is larger for higher dose/pulse unflattened beams when using tighter gamma tolerances, as is common for SRS/SBRT. This researched was supported by PTW.

SU‐E‐T‐51: A National QA Audit of QA Systems Used for IMRT and VMAT Patient QA

Purpose:To independently validate patient‐specific quality assurance for IMRT and VMAT plans using the same set of treatment plans for all institutes.Methods:In February 2014 we devised a set of treatment plans: simple IMRT/VMAT plans; more complex IMRT/VMAT plans and a stereotactic VMAT plan, all 6MV for both Varian and Elekta linacs. In total we used 5 Varian and 8 Elekta plans. The plans were imported in the institute's treatment planning system for dose computation on the phantom of the audit team and the institute's phantom. Additionally, 10x10 cm2 fields were made and computed on both phantoms. Next, the audit team performed measurements using the audit equipment. So far, 18 audits have been performed and we expect to have concluded the audits by June 2015. The measurements were performed using an ionization chamber (PinPoint, PTW), Gafchromic film and a 2D ionization chamber array, all in an octagonal phantom (Octavius, PTW, see Figure). Differences between the measured and computed 2D dose distributions were investigated using a gamma analysis with a 5%/1mm criterion for the stereotactic treatment plan and 3%/3mm for the other plans. Additionally, the participating centres performed QA measurements of the same treatment plans according to their local protocol and equipment.Results:For the 10x10 field on the phantom, the first 18 audits showed differences with respect to the planning of −0.21 (range: −1.8; 2.2)%. See Table 1 for more results. The findings compared well with the QA measurement results reported by the institutions according to their local protocols.Conclusion:These preliminary results demonstrate that such a national QA audit is feasible. Importing and computing the prepared treatment plans in the planning systems in use in the country is achievable. The local QA systems provided similar results as found with the audit.

TH‐CD‐201‐01: Radiation Therapy Databases

Radiation Oncology is a data intensive discipline. Data moves among various systems (imaging, treatment planning, image guidance and delivery) and the development of troubleshooting methods and appropriate QA and QC measures strongly depends on the medical physicists’ understanding of fundamental information technology concepts. This session will cover the concepts involved in networking, data transfer, DICOM and HL7 connectivity, database operations and QA for IT. Databases are at the heart of all modern radiotherapy clinics and are used for planning as well as treatment delivery, and they also drive the observed state of the data at various clients. Querying and reporting from the database is an important skill that medical physicists need to learn. Interactions between clients and databases need to be well understood to ensure that correct, synchronized, and up‐to‐date information is properly communicated amongst all stakeholders involved in the treatment management process. Entrance into the database via DICOM, HL7 and other proprietary transfer mechanisms must also be tested for accuracy of information storage and presentation. Issues with inter‐operability between systems must also be well understood during acceptance testing and commissioning activities. The ongoing efforts of IHE‐RO will be explained in the context of the need for increased engagement on the part of medical physicists.Learning Objectives: Understand the basic IT infrastructure required to run a radiotherapy department Become familiar with database concepts to improve troubleshooting skills Learn the principles of designing a QA program tailored to one's specific clinical IT environment. Curan: Funding from AAPM for travel and expenses DICOM and Funding from ASTRO for travel and expenses IHE RO

TH‐CD‐201‐03: QA for Radiotherapy IT

Radiation Oncology is a data intensive discipline. Data moves among various systems (imaging, treatment planning, image guidance and delivery) and the development of troubleshooting methods and appropriate QA and QC measures strongly depends on the medical physicists’ understanding of fundamental information technology concepts. This session will cover the concepts involved in networking, data transfer, DICOM and HL7 connectivity, database operations and QA for IT. Databases are at the heart of all modern radiotherapy clinics and are used for planning as well as treatment delivery, and they also drive the observed state of the data at various clients. Querying and reporting from the database is an important skill that medical physicists need to learn. Interactions between clients and databases need to be well understood to ensure that correct, synchronized, and up‐to‐date information is properly communicated amongst all stakeholders involved in the treatment management process. Entrance into the database via DICOM, HL7 and other proprietary transfer mechanisms must also be tested for accuracy of information storage and presentation. Issues with inter‐operability between systems must also be well understood during acceptance testing and commissioning activities. The ongoing efforts of IHE‐RO will be explained in the context of the need for increased engagement on the part of medical physicists.Learning Objectives: Understand the basic IT infrastructure required to run a radiotherapy department Become familiar with database concepts to improve troubleshooting skills Learn the principles of designing a QA program tailored to one's specific clinical IT environment. Curan: Funding from AAPM for travel and expenses DICOM and Funding from ASTRO for travel and expenses IHE RO

TH‐CD‐201‐00: Clinical Networks: IT for Radiation Oncology

Radiation Oncology is a data intensive discipline. Data moves among various systems (imaging, treatment planning, image guidance and delivery) and the development of troubleshooting methods and appropriate QA and QC measures strongly depends on the medical physicists’ understanding of fundamental information technology concepts. This session will cover the concepts involved in networking, data transfer, DICOM and HL7 connectivity, database operations and QA for IT. Databases are at the heart of all modern radiotherapy clinics and are used for planning as well as treatment delivery, and they also drive the observed state of the data at various clients. Querying and reporting from the database is an important skill that medical physicists need to learn. Interactions between clients and databases need to be well understood to ensure that correct, synchronized, and up‐to‐date information is properly communicated amongst all stakeholders involved in the treatment management process. Entrance into the database via DICOM, HL7 and other proprietary transfer mechanisms must also be tested for accuracy of information storage and presentation. Issues with inter‐operability between systems must also be well understood during acceptance testing and commissioning activities. The ongoing efforts of IHE‐RO will be explained in the context of the need for increased engagement on the part of medical physicists.Learning Objectives: Understand the basic IT infrastructure required to run a radiotherapy department Become familiar with database concepts to improve troubleshooting skills Learn the principles of designing a QA program tailored to one's specific clinical IT environment. Curan: Funding from AAPM for travel and expenses DICOM and Funding from ASTRO for travel and expenses IHE RO

TH‐CD‐201‐02: Networks, Connectivity, and Interoperability

Radiation Oncology is a data intensive discipline. Data moves among various systems (imaging, treatment planning, image guidance and delivery) and the development of troubleshooting methods and appropriate QA and QC measures strongly depends on the medical physicists’ understanding of fundamental information technology concepts. This session will cover the concepts involved in networking, data transfer, DICOM and HL7 connectivity, database operations and QA for IT. Databases are at the heart of all modern radiotherapy clinics and are used for planning as well as treatment delivery, and they also drive the observed state of the data at various clients. Querying and reporting from the database is an important skill that medical physicists need to learn. Interactions between clients and databases need to be well understood to ensure that correct, synchronized, and up‐to‐date information is properly communicated amongst all stakeholders involved in the treatment management process. Entrance into the database via DICOM, HL7 and other proprietary transfer mechanisms must also be tested for accuracy of information storage and presentation. Issues with inter‐operability between systems must also be well understood during acceptance testing and commissioning activities. The ongoing efforts of IHE‐RO will be explained in the context of the need for increased engagement on the part of medical physicists.Learning Objectives: Understand the basic IT infrastructure required to run a radiotherapy department Become familiar with database concepts to improve troubleshooting skills Learn the principles of designing a QA program tailored to one's specific clinical IT environment. Curan: Funding from AAPM for travel and expenses DICOM and Funding from ASTRO for travel and expenses IHE RO

WE-D-BRA-01: FEATURED PRESENTATION and BEST IN PHYSICS (THERAPY): Predicting Potentially Problematic VMAT Treatment Plans Before Patient Specific QA Measurements

Purpose: Several promising IMRT QA tools have been developed in recent years to combat problems found in the lack of sensitivity in planar dose measurements analyzed using consensus gamma analysis criteria. The increased complexity and added information with such devices adds not only increased time, but new challenges in determining endpoints for pass/fail criteria. Using a large cohort of previously measured planar IMRT QA data, it may be possible to correlate potentially problematic plans with calculated plan metrics that can be done a priori, such that these tools can be used only in clinically relevant situations. Methods: 90 previously measured, clinically delivered VMAT plans were exported in DICOM RT format. Using a Matlab program, plan metrics were computed based on a previously developed set of equations (Du et al. 2014). These metrics included MU-weighted beam irregularity, which quantifies an MLC shape’s deviation from that of a circle. Machine delivery parameters such as MU delivered per degree and leaf movement in millimeters per MU were also calculated. Based on a previous analysis of 394 IMRT QA measurements, a “failing” plan was defined as one with less than 85% gamma pass rate when computed at 1% dose difference and 2 mm distance to agreement; 16 of the 90 plans were identified as failing Results: A ROC curve was generated with an AUC of 0.9409. All 16 outliers were detected with a specificity of 85.1% when using a threshold value based on a linear combination of the MU-weighted plan irregularity and the leaf speed in mm/degree (average MU delivered per degree multiplied by average leaf movement in mm per MU). Conclusion: This data supports that potentially problematic VMAT plans can possibly be predicted before measurement by assessing the MU-weighted irregularity of the MLC shapes combined with the averaged leaf speed of the MLC.

SU‐E‐J‐53: A Phantom Design to Assist Patient Position Verification System in Daily Image‐Guided RT and Comprehensive QA Measurements

PurposeThis study is to implement a homemade novel device with surface locking couch index to check daily radiograph (DR) function of adaPTInsight™, stereoscopic image guided system (SIGS), for proton therapy. The comprehensive daily QA checks of proton pencil beam output, field size, flatness and symmetry of spots and energy layers will be followed by using MatriXX dosimetry device.MethodsThe iBa MatriXX device was used to perform daily dosimetry which is also used to perform SIGS checks. A set of markers were attached to surface of MatriXX device in alignment of DRR of reconstructed CT images and daily DR. The novel device allows MatriXX to be fit into the cradle which was locked by couch index bars on couch surface. This will keep the MatriXX at same XY plane daily with exact coordinates. Couch height Z will be adjusted according to imaging to check isocenter‐laser coincidence accuracy.ResultsadaPTInsight™ provides robotic couch to move in 6‐degree coordinate system to align the dosimetry device to be within 1.0 mm / 1.0°. The daily constancy was tightened to be ± 0.5 mm / 0.3° compared to 1.0 mm / 1.0° before. For gantry at 0° and couch all 0° angles (@ Rt ARM 0 setting), offsets measured of the couch systems were ≤ 0.5° in roll, yaw and pitch dimensions.ConclusionSimplicity of novel device made daily image guided QA consistent with accuracy. The offset of the MatriXX isocenter‐laser coincident was reproducible. Such easy task not only speeds up the setup, but it increases confidence level in detailed daily comprehensive measurements. The total SIGS alignment time has been shortened with less setup error. This device will enhance our experiences for the future QA when cone beam CT imaging modality becomes available at proton therapy center.

SU‐E‐T‐727: The Application of Regularization‐Based Multi‐Channel Radiochromic Film Dosimetry for Patient‐Specific QA of Stereotactic Body Radiation Therapy (SBRT) Treatment

Purpose:To examine the feasibility of using radiochromic film with advanced calibration techniques for absolute and relative dose measurement in patient‐specific SBRT QA.Methods:Ten SBRT cases were randomly selected. The treatment plans were mapped to a cylindrical Perspex phantom for QA measurement. A radiochromic film of 8”x10” was cut into a large piece of 20×21cm2 and 2 small pieces of 4×4cm2 each. The large piece was inserted in the mid‐plane of the phantom which was irradiated with the treatment beam. One of the small pieces was irradiated with known dose. All irradiated films were allowed to stabilize for 24 hours before scanning. The protocol of single‐scan with recalibration was followed and the 2 small films were used to adjust the calibration curve. The treatment film was calibrated with the dose gradient regularization method by an in‐house developed MATLab program. The calculated dose planes were then exported to RIT113V4.4. Both treatment films and planned dose planes were normalized to the corresponding prescribed fractional dose for analysis.Results:The analysis was performed by normalizing both the treatment film and the planned dose planes to the prescribed fractional dose. Isodose line comparison was performed to visually check the alignment of the isodose level of steep dose gradient regions. Good agreements were observed in all 10 cases. Gamma index analysis with tolerance of 3% and 2mm was also carried out for quantitative analysis. All 10 cases were reported with a gamma index passing percentage of at least 98.9%, which were higher than the institute's passing criteria of over 95%.Conclusion:By employing recalibration and regularization‐based calibration techniques, radiochromic film can measure both the absolute and relative dose of SBRT treatment plans. This study suggested that radiochromic film, with proper application of advanced calibration methods, can be a promising tool for patient‐specific SBRT QA.

SU‐E‐T‐616: Plan Quality Assessment of Both Treatment Planning System Dose and Measurement‐Based 3D Reconstructed Dose in the Patient

Purpose:Systematic radiotherapy plan quality assessment promotes quality improvement. Software tools can perform this analysis by applying site‐specific structure dose metrics. The next step is to similarly evaluate the quality of the dose delivery. This study defines metrics for acceptable doses to targets and normal organs for a particular treatment site and scores each plan accordingly. The input can be the TPS or the measurement‐based 3D patient dose. From this analysis, one can determine whether the delivered dose distribution to the patient receives a score which is comparable to the TPS plan score, otherwise replanning may be indicated.Methods:Eleven neuroblastoma patient plans were exported from Eclipse to the Quality Reports program. A scoring algorithm defined a score for each normal and target structure based on dose‐volume parameters. Each plan was scored by this algorithm and the percentage of total possible points was obtained. Each plan also underwent IMRT QA measurements with a Mapcheck2 or ArcCheck. These measurements were input into the 3DVH program to compute the patient 3D dose distribution which was analyzed using the same scoring algorithm as the TPS plan.Results:The mean quality score for the TPS plans was 75.37% (std dev=14.15%) compared to 71.95% (std dev=13.45%) for the 3DVH dose distribution. For 3/11 plans, the 3DVH‐based quality score was higher than the TPS score, by between 0.5 to 8.4 percentage points. Eight/11 plans scores decreased based on IMRT QA measurements by 1.2 to 18.6 points.Conclusion:Software was used to determine the degree to which the plan quality score differed between the TPS and measurement‐based dose. Although the delivery score was generally in good agreement with the planned dose score, there were some that improved while there was one plan whose delivered dose quality was significantly less than planned. This methodology helps evaluate both planned and delivered dose quality.Sun Nuclear Corporation has provded a license for the software described.

SU‐E‐T‐679: Retrospective Analysis of the Sensitivity of Planar Dose Measurements To Gamma Analysis Criteria

Purpose:IMRT QA using planar dose measurements is still a widely used method for checking the accuracy of treatment plans. A pass/fail judgment is made using gamma analysis based on a single endpoint. Using more stringent criteria is a way to increase the sensitivity to planning and delivery errors. Before such implementation, it is necessary to understand how the sensitivity to different gamma criteria settings affects gamma passing rates (GPR).Methods:752 IMRT QA measurements were re‐analyzed with varying distance to agreement (DTA) and dose difference (DD) percentages using a Matlab program. Other quantifying information such as the mean dose difference in the treatment target (defined as points that are greater than 80% of maximal dose) were stored in a relational database for retrospective analysis.Results:The average and standard deviation of GPR (%) fell from 99.84 ± (0.43) to 89.61 ± (6.08) when restricting DD from 5 − 1% respectively, as compared to a drop from 99.15 ± (1.19) to 95.00 ± (4.43), when restricting the DTA from 5 − 1 mm respectively. The mean dose difference (%) in the treatment target between measured and calculated dose was −1.96 ± (0. 83), −0.09 ± (0.98), and 1.44 ± (0. 86) for each of our institution's three matched linear accelerators (LINAC 1, 2, and 3 respectively). For plans that are approximately 2.7 sigma below the mean GPR, an average of 78.4% of those plans were measured on LINAC 1 or 3, while only 48% of the total plans were run on those machines.Conclusion:The data demonstrates that when restricting gamma criterion, such as the DD, the greatest indicator of reduced GPR in our institution is which matched LINAC the plan was measured on. While small, these differences manifest themselves to levels comparable to other treatment related differences and possibly confound the gamma analysis.

Decadal 10Be, 26Al and 36Cl QA measurements on the SUERC 5 MV accelerator mass spectrometer

We quantify the routine performance and uncertainties of cosmogenic 10Be, 26Al and 36Cl QA measurements made on the SUERC 5MV accelerator mass spectrometer since 2004. Our analysis compiles data from primary (NIST SRM4325 for 10Be, Purdue Z92-0222 for 26Al and Purdue Z93-0005 for 36Cl) and secondary (Nishiizumi’s series for 10Be, 26Al and 36Cl) reference samples with 10Be/9Be, 26Al/27Al and 36Cl/Cl ratios ranging between 10−11 and 10−13. Our decadal datasets indicate that the 10Be, 26Al and 36Cl secondary standard samples have average standard deviations 1.1%–2.4%, 1.1%–2.8% and 3.0%–3.1%, respectively. The average statistical uncertainties based on counting statistics are 1.0%–1.8%, 0.9%–2.8% and 2.5%–2.7% for 10Be, 26Al and 36Cl, respectively. These indicate additional uncertainties (0.6%–1.6% for 10Be, 0.5%–2.4% for 26Al and 1.4%–1.7% for 36Cl) above those calculated from counting statistics alone. The average differences between the measured and the nominal values are within ±1% in 13 of 14 secondary samples.

Quasi 3D dosimetry (EPID, conventional 2D/3D detector matrices)

Patient specific pretreatment measurement for IMRT and VMAT QA should preferably give information with a high resolution in 3D. The ability to distinguish complex treatment plans, i.e. treatment plans with a difference between measured and calculated dose distributions that exceeds a specified tolerance, puts high demands on the dosimetry system used for the pretreatment measurements and the results of the measurement evaluation needs a clinical interpretation. There are a number of commercial dosimetry systems designed for pretreatment IMRT QA measurements. 2D arrays such as MapCHECK® (Sun Nuclear), MatriXXEvolution (IBA Dosimetry) and OCTAVIOUS® 1500 (PTW), 3D phantoms such as OCTAVIUS® 4D (PTW), ArcCHECK® (Sun Nuclear) and Delta4 (ScandiDos) and software for EPID dosimetry and 3D reconstruction of the dose in the patient geometry such as EPIDoseTM (Sun Nuclear) and Dosimetry CheckTM (Math Resolutions) are available. None of those dosimetry systems can measure the 3D dose distribution with a high resolution (full 3D dose distribution). Those systems can be called quasi 3D dosimetry systems. To be able to estimate the delivered dose in full 3D the user is dependent on a calculation algorithm in the software of the dosimetry system. All the vendors of the dosimetry systems mentioned above provide calculation algorithms to reconstruct a full 3D dose in the patient geometry. This enables analyzes of the difference between measured and calculated dose distributions in DVHs of the structures of clinical interest which facilitates the clinical interpretation and is a promising tool to be used for pretreatment IMRT QA measurements. However, independent validation studies on the accuracy of those algorithms are scarce. Pretreatment IMRT QA using the quasi 3D dosimetry systems mentioned above rely on both measurement uncertainty and accuracy of calculation algorithms. In this article, these quasi 3D dosimetry systems and their use in patient specific pretreatment IMRT/VMAT QA will be discussed.

Dosimetric comparison of tools for intensity modulated radiation therapy with gamma analysis: a phantom study

Dosimetry of the Intensity Modulated Radiation Therapy (IMRT) is very important because of the complex dose distributions. Diode arrays are the most common and practical measurement tools for clinical usage for IMRT. Phantom selection is critical for QA process. IMRT treatment plans are recalculated for the phantom irradiation in QA. Phantoms are made in different geometrical shapes to measure the doses of different types of irradiation techniques. Comparison of measured and calculated dose distributions for IMRT can be made by using gamma analysis. In this study, 10 head-and-neck IMRT QA plans were created with Varian Eclipse 8.9 treatment planning system. Water equivalent RW3-slab phantoms, Octavius-2 phantom and PTW Seven29 2D-array were used for QA measurements. Gantry, collimator and couch positions set to 00 and QA plans were delivered to RW3 and Octavius phantoms. Then the positions set to original angles and QA plans irradiated again. Measured and calculated fluence maps were evaluated with gamma analysis for different DD and DTA criteria. The effect of different set-up conditions for RW3 and Octavius phantoms in QA plan delivery evaluated by gamma analysis. Results of gamma analysis show that using RW3-slab phantoms with setting parameters to 00 is more appropriate for IMRT QA.

Does the extracorporeal blood flow affect survival of the arteriovenous vascular access?

Hemodiafiltration with high-convective volumes is associated with improved patient survival, whereby practical realization is contingent on high extracorporeal blood flow (Qb) and dialysis treatment time. However, Qb is restricted by vascular access (VA) quality and/or concerns that high Qb could damage the VA. Taking VA quality into consideration, one can investigate the relationship between Qb and VA survival. We analyzed data from 1039 patients treated by hemodiafiltration over a 21-month period where access blood flow (Qa) measurements were also available at baseline. VA failure was defined as a surgical intervention resulting in the generation of a new VA. Qa was included as a stratification variable within a Cox regression model. A second Cox proportional hazard model with a penalized spline was used to describe the association between Qb and VA survival. Compared with Qb in the 350-357 mL/min range, a significantly higher hazard ratio (HR) for VA failure was detected for fistula only, and then only for Qb < 312 mL/min (HR: 2.361, 95% confidence interval [CI]: 1.251-4.453), Qb = 387-397 mL/min (HR: 1.920, 95% CI: 1.007-3.660) and Qb >414 mL/min (HR: 2.207, 95% CI: 1.101-4.424). Age, gender, diabetes, VA vintage, position of the VA, and arterial pressure were not significantly associated with outcome. The form of the penalized spline confirmed higher risk for VA failure for the lowest and the highest values of Qb. Taking Qa into consideration, no association was found between VA failure and Qb up to flows as high as approximately 390 mL/min.

Poster — Thur Eve — 19: Performance assessment of a 160‐leaf beam collimation system

In this study, the performance of the new beam collimation system with 160 leaves, each with a 5 mm leaf width projected at isocenter, is evaluated in terms of positional accuracy and plan/delivery quality. Positional accuracy was evaluated using a set of static and dynamic MLC/jaw delivery patterns at different gantry angles, dose rates, and MLC/jaw speeds. The impact on IMRT plan quality was assessed by comparing against a previous generation collimation system using the same optimization parameters, while delivery quality was quantified using a combination of patient‐specific QA measurements with ion chambers, film, and a bi‐planar diode array. Positional accuracy for four separate units was comparable. The field size accuracy, junction width, and total displacement over 16 cm leaf travel are 0.3 ± 0.2 mm, 0.4 ± 0.3 mm, and 0.5 ± 0.2 mm, respectively. The typical leaf minor offset is 0.05 ± 0.04 mm, and MLC hysteresis effects are 0.2 ± 0.1 mm over 16 cm travel. The dynamic output is linear with MU and MLC/jaw speed, and is within 0.7 ± 0.3 % of the planning system value. Plan quality is significantly improved both in terms of target coverage and OAR sparing due, in part, to the larger allowable MLC and jaw speeds. γ‐index pass rates for the patient‐specific QA measurements exceeded 97% using criteria of 2%/2 mm. In conclusion, the performance of the Agility system is consistent among four separate installations, and is superior to its previous generations of collimation systems.