IC Profiler Research Articles

Out of field scatter from electron applicator in modern linear accelerators.

Electron out-of-field scatter is generally not given importance mainly in electron fields. However, this is important when applicator down and boost treatments are given usually at an angle from the central axis. The electron scatter dose is found to be far away from the central axis which could be easily ignored. This study aims to investigate the out-of-field radiation doses from electron applicators and their effects on clinical treatment. By identifying the parameters that contribute to out-of-field doses and to explore potential strategies for reducing these doses in order to improve patient outcomes from modern machines. Measurements were performed in water phantom using electron diode for modern Elekta and Varian machines. Dose profiles were acquired at surface and dmax with 0° and 90° collimation angle. Various gantry angles were also studied for some data with IC Profiler. The profiles were normalized with respect to the central axis dose. The scatter dose peaks were found at a distance between 11 and 28cm from the central axis on all machines. However, the peak shifts to 15cm at 90° collimator when beam is tilted. The position and intensity of the dose varies with depth, collimator, and gantry angles for both Elekta and Varian machines. Due to clearance issues more gantry angles were studied for Elekta applicator compared to Varian. In general, Varian TrueBeam has a lower scatter that Elekta Infinity. The 90° collimator angle has a higher scatter compared to zero degree for both machines. There are clinically significant peripheral doses around 3% of the central axis dose from the electron applicator. Elekta has a slightly higher scatter (3%) than Varian (2%) that peaks at 25cm which is clinically important but often overlooked.

Validation of an EGSnrc-based Monte Carlo model for a complex 2D-array for technical QA measurements of a linear accelerator.

Multi-axis ionization chamber arrays can be used for quality assurance (QA) and measurement of linear accelerator (linac) specific data. In this work, the ability of the IC Profiler (Sun Nuclear Corp., Melbourne, Florida) detector array to measure the photon beam quality specifier %dd(10) and TPR20, 10 was investigated. To investigate the method for beam energy QA using a two-dimensional detector array, a Monte Carlo-based model of the detector array was developed and validated. A Monte Carlo-based model of the IC Profiler detector array with Quad Wedge accessories was developed in detail from drawings provided by the manufacturer using the egs++ class library from the EGSnrc code system. Monte Carlo simulations were used to calculate the absorbed dose in the 251 ionization chambers of the IC profiler in the 6 MV Elekta Precise radiation field. To validate the results from the Monte Caro simulations, measurements were performed on clinical 6 MV linacs. To vary the photon beam quality of the Elekta 6 MV linac, the current of the bending magnet was varied. Furthermore, the area ratio was calculated from IC Profiler measurements with the Quad Wedgeaccessories. Measurements as well as Monte Carlo simulations confirmed the linear relationship between the area ratio and the investigated photon beam quality specifier %dd(10) and TPR20, 10 for the investigated radiation source. Furthermore, the Monte Carlo-simulated data were within the 95% confidence interval of the linear fit to the measured data. This enabled the Monte Carlo-based IC Profiler model to be used for further investigations. The values were calculated for various electron beam sizes and the angle of incidence on the target of thelinac. A Monte-Carlo-based model of the detector array was developed, which could successfully reproduce measurements, demonstrating that even very complex geometries can be modeled in EGSnrc. Moreover, the study showed that the validated Monte Carlo model has the potential to investigate variations in beam parameters and their effects on AR ratios and %dd(10) that may not be investigated experimentally. While these findings may help users gain a deeper understanding of the QA method, the Monte Carlo model enables other complex investigations, such as the simulation of measurements in the presence of magnetic fields, or the simulation of measurements on novel treatment delivery techniques anddevices.

Decomposing a Beam Profile Constancy into Energy and Symmetry Components and Its Evaluation Using an Ionization Chamber Array

We have proposed a new formulation that can decompose the profile constancy defined in the AAPM TG 142 into energy and symmetry constancies by measuring beam profiles using an IC profiler (ICP). Measured profiles were laterally inverted to calculate averaged profiles in the lateral direction, thereby cancelling asymmetric components. Validation tests were performed by comparing the proposed calculation and measured results under various experimental conditions. Calculated profile constancies were further compared to decomposed energy and symmetry constancies. The energy constancy calculated from the averaged beam profile by lateral inversion and the measured PDD(10) constancy agreed within 0.1% when only symmetries were varied. The calculated energy and symmetry constancies, and the measured results agreed within 0.2% when both energies and symmetries were varied. The linac beam profile constancy has been decomposed into energy and symmetry terms. The proposed formulation has been validated by comparing the calculations and the direct measurements using the ICP. We have shown that QA/QC for profile constancy tests can be efficiently performed using the proposed formulation.

Clinical LINAC-Based Electron FLASH: Pathway for Practical Translation to Trials of FLASH Radiotherapy

<h3>Purpose/Objective(s)</h3> Studies show ultra-high dose rate (UHDR) radiotherapy (RT), at rates of ≥40 Gy per second, can produce the FLASH effect: reduced toxicity with equal tumor control when compared to conventional RT. Leveraging these benefits for clinical translation can profoundly impact RT. Early clinical trials exhibited safe delivery of UHDR-RT using specialized RT machines. Our prior study showed that a reversible configuration of a standard linear accelerator (LINAC) can be optimized for UHDR output. Here, we explored the components necessary for clinically practical delivery of LINAC-based electron FLASH RT. <h3>Materials/Methods</h3> Using a standard, clinically operational system for image-guided radiation, we configured the program board for a decommissioned electron beam energy for UHDR delivery. No LINAC hardware manipulation or exchange was performed. Key changes included adjustment of the dose per pulse and pulse delivery control using the gating interface. Of note, the scattering foil remained in place within the beam so that field sizes and depth doses could approximate those of standard clinical operations. A commercial electron arc applicator was installed at the face of the gantry using the standard applicator mount to allow for short SSD electron collimation. Copper electron blocks were fabricated with apertures ranging from 3.0 to 20.0 cm. Dose rates were measured at dmax for 100 to 59 cm source-to-surface distances (SSD) (level of machine head) and beam profile measurements were captured at 80 cm SSD using an IC profiler and radiographic film. <h3>Results</h3> UHDR rates ranging from 36.8 to 112.8 Gy/s were measured at 100 to 59 cm SSD, respectively. An 80 cm SSD was selected for treatment arrangement measurements to maintain UHDR and clinical utility, allowing for 13.5 cm of clearance. UHDR ≥60 Gy/s was observed at 80 cm SSD with all field sizes, using a standard, scattered electron beam, with an R80 depth of 6.1 cm, approximating to a 16-18 MeV energy. Average dose per pulse was measured at 0.33 Gy/pulse. Circular block apertures of 3, 10, and 20 cm exhibited beam flatness <5.0 % variability and symmetry of 2.2 to 6.2%. Penumbra width between the 80 and 20% isodose lines (IDL) measured at 0.7, 1.1, and 1.4 cm, respectively. <h3>Conclusion</h3> Optimizing a reversible setup for UHDR irradiation on a standard LINAC, electron delivery at dose rates expected to produce the FLASH effect was achieved. Flat, homogenous field sizes, ranging from 3 to 20 cm of coverage, were obtained at a clinically practical SSD of 80 cm with a treatment depth of 6.1 cm to the 80% IDL. This setup would enable UHDR irradiation at useful depths for a number of different clinical indications, bringing us a step closer to practical UHDR RT using a standard LINAC in future clinical trials.

Determination of the electronic portal imaging device pixel-sensitivity-map for quality assurance applications. Part 2: Photon beam dependence.

PurposeThe EPID PSM is a useful EPID calibration method for QA applications. The dependence of the EPID PSM on the photon beam used to acquire it has been investigated in this study for the four available PSM methods. The aim is to inform upon the viability of applying a single PSM for all available photon beams to simplify PSM implementation and maintenance.MethodsFour methods of PSM determination were each measured once in a single session on a single TrueBeam ® STx linac using 6 MV, 10 MV, 6 MV Flattening‐Filter‐Free (FFF), and 10 MV FFF photon beams. The resultant PSM was assessed for both intra‐ and inter‐method beam dependence via comparison between PSM of the same method compared to the 6 MV PSM and via comparison between PSM of the same beam with the corresponding Monte Carlo PSM. Comparisons were performed via 2D percentage deviation plots with associated histograms, 1D crossplane profiles, and via mean, median, and standard deviation percentage deviation statistics. Generated beam‐response was compared qualitatively via 1D crossplane profile comparison and quantitatively via symmetry assessment with comparison to the IC profiler device.ResultsThe Varian method provided the most consistent PSM with varying photon beam, with median percent deviation from the 6 MV PSM within 0.14% for all other beams. Qualitatively, each method provided similar beam‐response profiles. The measured beam‐response symmetry agreed to within 0.2% between the Calvary Mater Newcastle (CMN) method and IC profiler, but agreement reduced to within 0.9% and 2.2% for the Varian and WashU methods. PSM percent deviation with Monte Carlo PSM was within 0.75% for all methods and beams.ConclusionResults suggest that the PSM may be independent of photon beam to clinically relevant levels. The Varian method of PSM determination introduces the least beam dependence into the measured PSM.

Validation of a Novel Cone-Less Set-up for Electron FLASH Radiation Delivery on a Clinical-Use Linear Accelerator

<h3>Purpose/Objective(s)</h3> Ultra-high dose rate (FLASH) radiotherapy (RT) at ≥40 Gray per second has been shown preclinically to achieve improved normal tissue sparing while maintaining tumor control similar to that of conventional dose-rate RT. Many published FLASH-RT delivery models are inaccessible for widespread use and/or not applicable for clinical translation. We previously described a model for reversible configuration of a clinical linear accelerator (LINAC) that can deliver ∼16-18 MeV electron FLASH-RT at source-to-surface distances (SSDs) < 95 cm. However, it precludes the use of a standard electron cone. This proof-of-concept study will characterize the beam profiles from a novel cone-less set-up, to verify its feasibility as a widely accessible method of electron FLASH-RT delivery for the intention of clinical treatment. <h3>Materials/Methods</h3> This novel set-up employs a flat 25 × 25 cm applicator with custom cut-outs that lies within the accessory mount at 66.6 cm SSD of a standard treatment delivery system. We tested a 10 × 10 cm circular field size at a target distance of 80 cm SSD (selected to maintain FLASH dose rates). Radiographic film and an IC Profiler were used to evaluate penumbra width, beam flatness, symmetry, and percentage depth dose (PDD). FLASH-RT was delivered in 20 pulses with 3 independent measurements each using film and the profiler. This was compared to a conventional 16 MeV beam using a standard cone with a 10 × 10 cm field size at 100 cm SSD. <h3>Results</h3> Our novel cone-less FLASH set-up achieved a beam profile that was similar to the conventional electron set-up. Respectively, for the cone-less and conventional set-up, measured at a standardized depth of half R85, penumbra width was 1.13 cm and 1.15 cm, and uniformity index was 0.72 and 0.74 (defined as the ratio of the areas inside the 90% and 50% isodose lines); measured at a standardized depth of R95, flatness (defined as the maximum dose deviation from the central axis at a specific depth over an area confined within lines 2 cm inside the geometric edge of fields) was 1.5% and 4.1%, beam diameter containing dose > 80% of the central axis was 10.6 cm and 9.6 cm, and symmetry (defined as the maximum dose difference between any two symmetric points about the central axis in the flat portion of the beam, as a percentage of central dose) was 2.24% and 2.20%. PDD values for the cone-less set-up at Dmax, R95, R85, and R50 were, respectively, 3.1, 4.1, 5.3, and 6.8 cm. <h3>Conclusion</h3> We have developed a novel cone-less set-up that allows for the delivery of electron FLASH-RT utilizing a clinical LINAC at practical SSDs and verified that its beam profiles are clinically feasible. Further work will be done to characterize additional SSDs and field sizes, along with the addition of a custom skin collimator to further shape the beam. This will pave the way for practical treatment using electron FLASH-RT to a host of oncologic indications, including cutaneous malignancies and partial breast irradiation in the future.

Reconstruction of volume averaging effect-free continuous photon beam profiles from discrete ionization chamber array measurements using a machine learning technique.

PurposeThe use of the ionization chamber array ICProfiler (ICP) is limited by its relatively poor detector spatial resolution and the inherent volume averaging effect (VAE). The purpose of this work is to study the feasibility of reconstructing VAE‐free continuous photon beam profiles from ICP measurements with a machine learning technique.MethodsIn‐ and cross‐plane photon beam profiles of a 6 MV beam from an Elekta linear accelerator, ranging from 2 × 2 to 10 × 10 cm2 at 1.5 cm, 5 cm, and 10 cm depth, were measured with an ICP. The discrete measurements were interpolated with a Makima method to obtain continuous beam profiles. Artificial neural networks (ANNs) were trained to restore the penumbra of the beam profiles. Plane‐specific (in‐ and cr‐plane) ANNs and a combined ANN were separately trained. The performance of the ANNs was evaluated using the penumbra width difference (PWD, the difference between the penumbra widths of the reconstructed and the reference profile). The plane‐specific and the combined ANNs were compared to study the feasibility of using a single ANN for both in‐ and cross‐plane.ResultsThe profiles reconstructed with all the ANNs had excellent agreement with the reference. For in‐plane, the ANNs reduced the PWD from 1.6 ± 0.7 mm at 1.5 cm depth to 0.1 ± 0.1 mm, from 1.8 ± 0.6 mm at 5.0 cm depth to 0.1 ± 0.1 mm, and from 2.4 ± 0.1 mm at 10.0 cm depth to 0.0 ± 0.0 mm; for cross‐plane, the ANNs reduced the PWD from 1.2 ± 0.4 mm at 1.5 cm depth, 1.2 ± 0.3 mm at 5.0 cm depth, and 1.6 ± 0.1 mm at 10.0 cm depth, to 0.1 ± 0.1 mm.ConclusionsThis study demonstrated the feasibility of using simple ANNs to reconstruct VAE‐free continuous photon beam profiles from discrete ICP measurements. A combined ANN can restore the penumbra of in‐ and cross‐plane beam profiles of various fields at different depths.

Scintillation imaging as a high-resolution, remote, versatile 2D detection system for MR-linac quality assurance.

To demonstrate the potential benefits of remote camera-based scintillation imaging for routine quality assurance (QA) measurements for magnetic resonance guided radiotherapy (MRgRT) linear accelerators. A wall-mounted CMOS camera with a time-synchronized intensifier was used to image photons produced from a scintillation screen in response to dose deposition from a 6MV FFF x-ray beam produced by a 0.35T MR-linac. The oblique angle of the field of view was corrected using a projective transform from a checkerboard calibration target. Output sensitivity and constancy was measured using the scintillator and benchmarked against an A28 ion chamber. Field cross-plane and in-plane profiles were measured for field sizes ranging from 1.68×1.66cm2 to 20.02×19.92cm2 with both scintillation imaging and using an IC profiler. Multileaf collimator (MLC) shifts were introduced to test sensitivity of the scintillation imaging system to small spatial deviations. A picket fence test and star-shot were delivered to both the scintillator and EBT3 film to compare accuracy in measuring MLC positions and isocenter size. The scintillation imaging system showed comparable sensitivity and linearity to the ion chamber in response to changes in machine output down to 0.5 MU (R2 =0.99). Cross-plane profiles show strong agreement with defined field sizes using full width half maximum (FWHM) measurement of <2mm for field sizes below 15cm, but the oblique viewing angle was the limiting factor in accuracy of in-plane profile widths. However, the system provided high-resolution profiles in both directions for constancy measurements. Small shifts in the field position down to 0.5mm were detectable with <0.1mm accuracy. Multileaf collimator positions as measured with both scintillation imaging and EBT3 film were measured within ± 1mm tolerance and both detection systems produced similar isocenter sizes from the star-shot analysis (0.81 and 0.83mm radii). Remote scintillation imaging of a two-dimensional screen provided a rapid, versatile, MR-compatible solution to many routine quality assurance procedures including output constancy, profile flatness and symmetry constancy, MLC position verification and isocenter size. This method is high-resolution, does not require post-irradiation readout, and provides simple, instantaneous data acquisition. Full automation of the readout and processing could make this a very simple but effective QA tool, and is adaptable to all medical accelerators.

Technical Note: Dosimetric characterization of the dynamic beam flattening MLC sequence on a ring shaped, Jawless Linear Accelerator with double stacked MLC.

To characterize the dosimetric features and limitations of the dynamic beam flattening (DBF) on the Halcyon 2.0 linear accelerator (Varian Medical Systems). A predefined multi-leaf collimator (MLC) sequence was introduced and used to flatten the 6MV flattening filter free (FFF) beam on the Halcyon 2.0. Dosimetric characterizations of the flattened beams, including beam flatness, symmetry, percent depth dose (PDD), output factor and MU linearity, were investigated. Flatness and symmetry were obtained from profile measurements with both radiographic films (EDR2) and a two dimensional ion-chamber array (IC Profiler, Sun Nuclear Corporation). MU linearity, output factors, and PDDs were measured in a water tank with a CC13 ion chamber (Scanditronix Wellhöfer, Nuremburg, Germany). In addition, the effect of the DBF sequence on 3D plan quality was evaluated by creating DBF plans for a 4-field box rectum and an AP/PA spine plan. Patient specific QA was performed on these plans. At 100cm SSD and 10cm depth, a flatness of <3% was observed on both transversal and radial profiles for all square field sizes ≥10cm with DBF. For both larger and smaller field sizes the flatness showed a tendency to increase as the fields got bigger or smaller, respectively. Similar trends in flatness were observed at all depths measured. All measured output factors for square field sizes ≥5cm were within 1% of the TPS prediction. Linearity was ≤2.02% for all measurements. For both treatment sites, the MD judged the plans created for the Halcyon without the use of DBF not to be clinically acceptable, however considered both the TrueBeam plan and the Halcyon plan with the DBF sequence to be clinically acceptable. The DBF sequence on the Halcyon and its characteristics were investigated. The analysis indicates that the DBF sequence can be used on the Halcyon to generate clinically acceptable 3D treatment plans.

Use of a 2-Dimensional Ionization Chamber Array to Measure Head Leakage of a Varian Truebeam&amp;lt;sup&amp;gt;&amp;#174;&amp;lt;/sup&amp;gt; Linear Accelerator

The purpose of this work is to evaluate the use of a two-dimensional (2D) planar ion chamber array to characterize leakage radiation from the head of the linear accelerator. Ion chamber arrays provide a benefit over a singular ion chamber measurement as they allow for the measurement of a larger area in order to isolate the point of maximum leakage dose and the small size of each individual ion chamber minimizes volume-averaging effects. A Varian Truebeam® undergoing acceptance testing was used for all measurements. The gantry was wrapped in Portal Pack for Localization (PPL) radiographic film in order to isolate the location of maximum leakage. A calibration curve was developed and used to determine dose-to-film. An Ion Chamber Profiler (IC Profiler™) manufactured by Sun Nuclear Corporation was used to confirm measurements by the PPL film. All measurements were normalized to leakage at 100 cm from the target relative to the central axis. Three points were investigated with the IC Profiler, including the top of the gantry, the Varian logo, and the side of the gantry. For the three locations, respectively, the PPL film and the IC profiler were measured 0.142% and 0.131%, 0.036% and 0.030%, and 0.014% and 0.019%. The good agreement between the PPL film and the IC Profiler provides confidence in the use of a more efficient and accurate ion chamber array for head leakage measurements.

Tungsten filled 3D printed field shaping devices for electron beam radiation therapy

PurposeElectron radiotherapy is a labor-intensive treatment option that is complicated by the need for field shaping blocks. These blocks are typically made from casting Cerrobend alloys containing lead and cadmium. This is a highly toxic process with limited precision. This work aims to provide streamlined and more precise electron radiotherapy by 3D using printing techniques.MethodsThe 3D printed electron cutout consists of plastic shells filled with 2 mm diameter tungsten ball bearings. Five clinical Cerrobend defined field were compared to the planned fields by measuring the light field edge when mounted in the electron applicator on a linear accelerator. The dose transmitted through the 3D printed and Cerrobend cutouts was measured using an IC profiler ion chamber array with 6 MeV and 16 MeV beams. Dose profiles from the treatment planning system were also compared to the measured dose profiles. Centering and full width half maximum (FWHM) metrics were taken directly from the profiler software.ResultsThe transmission of a 16MeV beam through a 12 mm thick layer of tungsten ball bearings agreed within 1% of a 15 mm thick Cerrobend block (measured with an ion chamber array). The radiation fields shaped by ball bearing filled 3D printed cutout were centered within 0.4 mm of the planned outline, whereas the Cerrobend cutout fields had shift errors of 1–3 mm, and shape errors of 0.5–2 mm. The average shift of Cerrobend cutouts was 2.3 mm compared to the planned fields (n = 5). Beam penumbra of the 3D printed cutouts was found to be equivalent to the 15 mm thick Cerrobend cutout. The beam profiles agreed within 1.2% across the whole 30 cm profile widths.ConclusionsThis study demonstrates that with a proper quality assurance procedure, 3D-printed cutouts can provide more accurate electron radiotherapy with reduced toxicity compared to traditional Cerrobend methods.

Factor 10 Expedience of Monthly Linac Quality Assurance via an Ion Chamber Array and Automation Scripts.

Purpose:While critical for safe and accurate radiotherapy, monthly quality assurance of medical linear accelerators is time-consuming and takes physics resources away from other valuable tasks. The previous methods at our institution required 5 hours to perform the mechanical and dosimetric monthly linear accelerator quality assurance tests. An improved workflow was developed to perform these tests with higher accuracy, with fewer error pathways, in significantly less time.Methods:A commercial ion chamber array (IC profiler, Sun Nuclear, Melbourne, Florida) is combined with automation scripts to consolidate monthly linear accelerator QA. The array was used to measure output, flatness, symmetry, jaw positions, gated dose constancy, energy constancy, collimator walkout, crosshair centering, and dosimetric leaf gap constancy. Treatment plans were combined with automation scripts that interface with Sun Nuclear’s graphical user interface. This workflow was implemented on a standard Varian clinac, with no special adaptations, and can be easily applied to other C-arm linear accelerators.Results:These methods enable, in 30 minutes, measurement and analysis of 20 of the 26 dosimetric and mechanical monthly tests recommended by TG-142. This method also reduces uncertainties in the measured beam profile constancy, beam energy constancy, field size, and jaw position tests, compared to our previous methods. One drawback is the increased uncertainty associated with output constancy. Output differences between IC profiler and farmer chamber in plastic water measurements over a 6-month period, across 4 machines, were found to have a 0.3% standard deviation for photons and a 0.5% standard deviation for electrons, which is sufficient for verifying output accuracy according to TG-142 guidelines. To minimize error pathways, automation scripts which apply the required settings, as well as check the exported data file integrity were employed.Conclusions:The equipment, procedure, and scripts used here reduce the time burden of routine quality assurance tests and in most instances improve precision over our previous methods.

OA240] Comparison between four routine control devices detecting photon beam energy changes

Purpose The quality assurance (QA) program of a LINAC includes the control of the stability of the photon beam energy, through a control variable, the beam quality BQ. Our department owns four devices for this task. In this work a method to compare the performance of these QA devices is designed and applied. Methods Two photon beams with energies 6 and 15 MV, produced by a Siemens Artiste LINAC, were used to analyse the response of the following QA devices: StarTrack (Iba Dosimetry), QuickCheck and StarCheck (PTW) and IC Profiler (Sun Nuclear). They determine the BQ indirectly through their own control variable, BQ dev which shall be related to the BQ measured in reference conditions. To determine this relationship, the photon energy was modified by varying the bending magnet intensity I BM For each value of I BM , three PDD curves in water and three measurements for each device were taken. Through the measurements taken before and assuming that they follow a normal distribution, pairs of values of BQ and BQ dev were generated using Monte Carlo simulation methods. The pairs of simulated values are related between them by the relationship obtained previously. By comparing if the pair of values are inside or outside the tolerance interval, each simulation is classified as true positive TP, false positive FP, true negative TN or false negative FN. The tolerance interval chosen was 0.5%. Summing all the TP, FP, TN and FN, two parameters can be calculated: sensitivity and specificity. These parameters will allow to compare the performance between devices. Results For all devices, the relationship between BQ dev and BQ is linear, with a determination coefficient r 2 ≥ 0.9 . The QuickCheck and IC Profiler devices showed the best performance, with a sensitivity and a specificity higher than 95%. The StarTrack device had a good sensitivity, but a specificity of around 85%, for both energies. The StarCheck device showed a decent sensitivity, but a specificity of 67.7% and 78.7% for each energy, respectively. Conclusions The QuickCheck and IC Profiler devices showed the best performance, followed by StarTrack. The StarCheck device had a specificity much lower than the other three devices.

A proposed method for linear accelerator photon beam steering using EPID.

Beam steering is the process of calibrating the angle and translational position with which a linear accelerator's (linac's) electron beam strikes the x‐ray target with respect to the collimator rotation axis. The shape of the dose profile is highly dependent on accurate beam steering and is essential for ensuring correct delivery of the radiotherapy treatment plan. Traditional methods of beam steering utilize a scanning water tank phantom that makes the process user‐dependent. This study is the first to provide a methodology for both beam angle steering and beam translational position steering based on EPID imaging of the beam and does not require a phantom. Both the EPID‐based beam angle steering and beam translational steering methods described have been validated against IC Profiler measurement. Wide field symmetry agreement was found between the EPID and IC Profiler to within 0.06 ± 0.14% (1 SD) and 0.32 ± 0.11% (1 SD) for flattened and flattening‐filter‐free (FFF) beams, respectively. For a 1.1% change in symmetry measured by IC Profiler the EPID method agreed to within 0.23%. For beam translational position steering, the EPID method agreed with IC Profiler method to within 0.03 ± 0.05 mm (1 SD) at isocenter. The EPID‐based methods presented are quick to perform, simple, accurate and could easily be integrated with the linac, potentially via the MPC application. The methods have the potential to remove user variability and to standardize the process of beam steering throughout the radiotherapy community.

Independent validation of machine performance check for the Halcyon and TrueBeam linacs for daily quality assurance.

PurposeTo evaluate the ability of the machine performance check (MPC) on the Halcyon to detect errors, with comparison with the TrueBeam.Methods MPC is an automated set of quality assurance (QA) tests that use a phantom placed on the couch and the linac's imaging system(s) to verify the beam constancy and mechanical performance of the Halcyon and TrueBeam linacs. In order to evaluate the beam constancy tests, we inserted solid water slabs between the beam source and the megavoltage imager to simulate changes in beam output, flatness, and symmetry. The MPC results were compared with measurements, using two‐dimensional array under the same conditions. We then studied the accuracy of MPC geometric tests. The accuracies of the relative gantry offset and couch shift tests were evaluated by intentionally inserting phantom shifts, using a rotating or linear motion stage. The MLC offset and absolute gantry offset tests were assessed by miscalibrating these motions on a Halcyon linac.ResultsFor the Halcyon system, the average difference in the measured beam output between the IC Profiler and MPC, after intentional changes, was 1.3 ± 0.5% (for changes ≤5%). For Halcyon, the MPC test failed (i.e., prevented treatment) when the beam symmetry change was over 1.9%. The accuracy of the MLC offset test was within 0.05 mm. The absolute gantry offset test was able to detect an offset as small as 0.02°. The accuracy of the absolute couch shift test was 0.03 mm. The accuracy of relative couch shift test of Halcyon was measured as 0.16 mm.ConclusionWe intentionally inserted errors to evaluate the ability of the MPC to identify errors in dosimetric and geometric parameters. These results showed that the MPC is sufficiently accurate to be effectively used for daily QA of the Halcyon and TrueBeam treatment devices.

SU‐F‐T‐71: A Practical Method for Evaluation of Electron Virtual Source Position

Purpose:Since electrons are easily scattered, the virtual source position for electrons is expected to locate below the x‐ray target of Medical Linacs. However, the effective SSD method yields the electron virtual position above the x‐ray target for some applicators for some energy in Siemens Linacs. In this study, we propose to use IC Profiler (Sun Nuclear) for evaluating the electron virtual source position for the standard electron applicators for various electron energies.Methods:The profile measurements for various nominal source‐to‐detector distances (SDDs) of 100–115 cm were carried out for electron beam energies of 6–18 MeV. Two methods were used: one was to use a 0.125 cc ion chamber (PTW, Type 31010) with buildup mounted in a PTW water tank without water filled; and the other was to use IC Profiler with a buildup to achieve charge particle equilibrium. The full width at half‐maximum (FWHM) method was used to determine the field sizes for the measured profiles. Backprojecting (by a straight line) the distance between the 50% points on the beam profiles for the various SDDs, yielded the virtual source position for each applicator.Results:The profiles were obtained and the field sizes were determined by FWHM. The virtual source positions were determined through backprojection of profiles for applicators (5, 10, 15, 20, 25). For instance, they were 96.415 cm (IC Profiler) vs 95.844 cm (scanning ion chamber) for 9 MeV electrons with 10×10 cm applicator and 97.160 cm vs 97.161 cm for 12 MeV electrons with 10×10 cm applicator. The differences in the virtual source positions between IC profiler and scanning ion chamber were within 1.5%.Conclusion:IC Profiler provides a practical method for determining the electron virtual source position and its results are consistent with those obtained by profiles of scanning ion chamber with buildup.

SU-G-BRB-05: Automation of the Photon Dosimetric Quality Assurance Program of a Linear Accelerator

Purpose: To develop an automated method to calculate a linear accelerator (LINAC) photon radiation field size, flatness, symmetry, output and beam quality in a single delivery for flattened (FF) and flattening-filter-free (FFF) beams using an ionization chamber array. Methods: The proposed method consists of three control points that deliver 30×30, 10×10 and 5×5cm2 fields (FF or FFF) in a step-and-shoot sequence where the number of monitor units is weighted for each field size. The IC Profiler (Sun Nuclear Inc.) with 5mm detector spacing was used for this study. The corrected counts (CCs) were calculated and the locations of the maxima and minima values of the first-order gradient determined data of each sub field. Then, all CCs for each field size are summed in order to obtain the final profiles. For each profile, the radiation field size, symmetry, flatness, output factor and beam quality were calculated. For field size calculation, a parameterized gradient method was used. For method validation, profiles were collected in the detector array both, individually and as part of the step-and-shoot plan, with 9.9cm buildup for FF and FFF beams at 90cm source-to-surface distance. The same data were collected with the device (plus buildup) placed on a movable platform to achieve a 1mm resolution. Results: The differences between the dosimetric quantities calculated from both deliveries, individually and step-and-shoot, were within 0.31±0.20% and 0.04±0.02mm. The differences between the calculated field sizes with 5mm and 1mm resolution were ±0.1mm. Conclusion: The proposed single delivery method proved to be simple and efficient in automating the photon dosimetric monthly and annual quality assurance.

Time-resolved beam symmetry measurement for VMAT commissioning and quality assurance.

In volumetric‐modulated arc therapy (VMAT) treatment delivery perfect beam symmetry is assumed by the planning system. This study aims to test this assumption and present a method of measuring time‐resolved beam symmetry measurement during a VMAT delivery that includes extreme variations of dose rate and gantry speed. The Sun Nuclear IC Profiler in gantry mount was used to measure time‐resolved in‐plane and cross‐plane profiles during plan delivery from which symmetry could be determined. Time‐resolved symmetry measurements were performed throughout static field exposures at cardinal gantry angles, conformal arcs with constant dose rate and gantry speed, and during a VMAT test plan with gantry speed and dose rate modulation. Measurements were performed for both clockwise and counterclockwise gantry rotation and across four Varian 21iX linacs. The symmetry was found to be generally constant throughout the static field exposures to within 0.3% with an exception on one linac of up to 0.7%. Agreement in symmetry between cardinal angles was always within 1.0% and typically within 0.6%. During conformal arcs the results for clockwise and counterclockwise rotation were in agreement to within 0.3%. Both clockwise and counterclockwise tended to vary in similar manner by up to 0.5% during arc consistent with the cardinal gantry angle static field results. During the VMAT test plan the symmetry generally was in agreement with the conformal arc results. Greater variation in symmetry was observed in the low‐dose‐rate regions by up to 1.75%. All results were within clinically acceptable levels using the tolerances of NCS Report 24 (2015).PACS number(s): 87.55.Qr

SU‐E‐T‐645: Qualification of a 2D Ionization Chamber Array for Beam Steering and Profile Measurement

Purpose:Establish a procedure for beam steering and profile measurement using a 2D ionization chamber array and show equivalence to a water scanning system.Methods:Multiple photon beams (30×30cm2 field) and electron beams (25×25cm2 cone) were steered in the radial and transverse directions using Sun Nuclear's IC PROFILER (ICP). Solid water was added during steering to ensure measurements were beyond the buildup region. With steering complete, servos were zeroed and enabled. Photon profiles were collected in a 30×30cm2 field at dmax and 2.9 cm depth for flattened and FFF beams respectively. Electron profiles were collected with a 25×25cm2 cone and effective depth (solid water + 0.9 cm intrinsic buildup) as follows: 0.9 cm (6e), 1.9 cm (9e), 2.9 cm (12e, 16e, 20e). Profiles of the same energy, field size and depth were measured in water with Sun Nuclear's 3D SCANNER (3DS). Profiles were re‐measured using the ICP after the in‐water scans. Profiles measured using the ICP and 3DS were compared by (a) examining the differences in Varian's “Point Difference Symmetry” metric, (b) visual inspection of the overlaid profile shapes and (c) calculation of point‐by‐point differences.Results:Comparing ICP measurements before and after water scanning showed very good agreement indicating good stability of the linac and measurement system. Comparing ICP Measurements to water phantom measurements using Varian's symmetry metric showed agreement within 0.5% for all beams. The average magnitude of the agreement was within 0.2%. Comparing ICP Measurements to water phantom measurements using point‐by‐point difference showed agreement within 0.5% inside of 80% area of the field width.Conclusion:Profile agreement to within 0.5% was observed between ICP and 3DS after steering multiple energies with the ICP. This indicates that the ICP may be used for steering electron beams, and both flattened and FFF photon beams.Song Gao: Sun Nuclear's invitation of speak and financial support for attending the 8th QA &amp; Dosimetry Symposium in Orlando, Florida 2015.

SU‐E‐P‐24: Simplified EDW Profile Measurements Using Two Commonly Available Detector Arrays

Purpose:Enhanced dynamic wedge (EDW) profiles are needed as part of the commissioning of a treatment planning system. This work compares the acquisition of EDW profiles using a linear diode array (LDA) with two commonly used detector arrays available in the clinics, with the goal of identifying the simplest approach for these measurements.Methods:The measurements of EDW profiles were performed on a Varian TrueBeam linear accelerator for 6, 10, and 18 MV photon beams for all seven wedge angles at four depths. The measurements were done using the LDA 99 in Blue Phantom2 (IBA Dosimetry), and IC Profiler and MapCHECK2 (Sun Nuclear) in solid water phantoms. The water phantom was set up at 100 cm SSD, whereas the two other devices were set up at 75 cm due to the size limitations of the devices. The largest possible field size was used. The average and maximum percentage differences were examined within the central 90% of the field and in the penumbra.Results:Dose profiles measured with IC Profiler were in a good agreement with LDA 99 data. The average percentage difference within the field did not exceed 0.5% for all energies. MapCHECK2 measurements matched well with LDA 99 for 10 and 18 MV (within 0.3%) with discrepancies of up to 1.4% observed for the 6 MV beam. The maximum percentage differences for both devices in the penumbra exhibited larger variations than LDA 99 results due to differences in detector spacing and high dose gradient, as expected.Conclusion:Common linac QA devices such as IC Profiler or MapCHECK2 provide EDW beam profile data of reasonable accuracy as compared to measurements performed using a linear diode array in a water phantom, saving the expense and time involved in acquiring and setting up a LDA.