Linac Output Research Articles

A diamond detector based dosimetric system for instantaneous dose rate measurements in FLASH electron beams.

A reliable determination of the instantaneous dose rate (I-DR) delivered in FLASH radiotherapy treatments is believed to be crucial to assess the so-called FLASH effect in preclinical and biological studies. At present, no detectors nor real-time procedures are available to do that in Ultra High Dose Rate (UH-DR) electron beams, typically consisting of μs pulses characterized by I-DRs of the order of MGy/s. A dosimetric system is proposed possibly overcoming the above reported limitation, based on the recently developed flashDiamond (fD) detector (model 60025, PTW-Freiburg, Germany).
Approach. A dosimetric system is proposed, based on a flashDiamond detector prototype, properly modified and adapted for very fast signal transmission. It was used in combination with a fast transimpedance amplifier and a digital oscilloscope to record the temporal traces of the pulses delivered by an ElectronFlash linac (SIT S.p.A., Italy).
The proposed dosimetric systems was investigated in terms of the temporal characteristics of its response and the capability to measure the absolute delivered dose and instantaneous dose rate (I DR). A "standard" flashDiamond was also investigated and its response compared with the one of the specifically designed prototype.
Main Results. Temporal traces recorded in several UH-DR irradiation conditions showed very good signal to noise ratios and rise and decay times of the order of a few tens ns, faster than the ones obtained by the current transformer embedded in the linac head. By analyzing such signals, a calibration coefficient was derived for the fD prototype and found to be in agreement within 1% with the one obtained under reference 60Co irradiation. I-DRs as high as about 2 MGy/s were detected without any undesired saturation effect. Absolute Dose Per Pulse (DPP) values extracted by integrating the I DR signals were found to be linear up to at least 7.13 Gy and in very good agreement with the ones obtained by connecting the fD to a UNIDOS electrometer (PTW-Freiburg, Germany). A good short term reproducibility of the linac output was observed, characterized by a pulse-to-pulse variation coefficient of 0.9%. Negligible differences were observed when replacing the fD prototype with a standard one, with the only exception of a somewhat slower response time for the latter detector type.
Significance. The proposed fD-based system was demonstrated to be a suitable tool for a thorough characterization of UH-DR beams, providing accurate and reliable time resolved I-DR measurements from which absolute dose values can be straightforwardly derived.
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Evaluation of an EPID in vivo monitoring system using local and external independent audit measurements.

The aim of this work was to evaluate the SunCHECK PerFRACTION, the software for in vivo monitoring using EPID images. First, the PerFRACTION ability to detect errors was investigated simulating two situations: (1) variation of LINAC output and (2) variation of the phantom thickness. An ionization chamber was used as reference to measure the introduced dose variations. Both tests used EPID in integrated mode (absolute dose). Second, EPID measurements in integrated mode were carried out during an independent Brazilian governmental audit that provided four phantoms and TLDs. PerFRACTION calculated the absolute dose on EPID plane, and it compared with predicted calculated dose for every delivered plan. The dose deviations reported using PerFRACTION were compared with dose deviations reported by the independent audit. Third, an end-to-end test using a heterogeneous phantom was performed. A VMAT plan with EPID in cine mode was delivered. PerFRACTION calculated the mean dose on CBCT using EPID information and log files. The calculated doses at four different points were compared with ionization chambers measurements. About the first test, the largest difference found was 1.2%. Considering the audit results, the variations detected by TLD measurements and by PerFRACTION dose calculation on EPID plane were close: 12 points had variations less than 2%, 2 points with variation between 2% and 3%, and 2 points with deviations greater than 3% (max 3.7%). The end-to-end tests using a heterogeneous phantom achieved dose deviation less than 1.0% in the water-equivalent region. In the mimicking lung region, the deviations were higher (max 7.3%), but in accordance with what is expected for complex situations. The tests results indicate that PerFRACTION dose calculations in different situations have good agreement with standard measurements. Action levels were suggested for absolute dose on EPID plane as well as 3D dose calculation on CBCT using PerFRACTION.

Strip detector array (SDA) for beam monitoring in radiotherapy: reconstruction of MLC parameters from multiple projections of flux

Objective. In this paper we propose and investigate a new detector with multiple strip detector arrays (SDA) for monitoring MLC shaped x-ray beams for radiotherapy treatment. Approach. Each SDA measures 1D dose profiles equivalent to dose projections. The goal of such a detector is to determine individual MLC leaf positions as well as the Monitor Units (MU) per MLC segment during radiotherapy. In the present work we investigate an optimal SDA detector configuration and reconstruction algorithm. We determine the accuracy of SDA for different treatment sites (spine, pelvis, retroperitoneum, prostate, brain SRT, SRS, lung and head and neck). We perform a simulation study accounting for different type of MLC leaf positional errors: random MLC leaf, systematic for the whole leaf bank and systematic for an individual leaf. In a similar fashion, we also account for errors in Monitor Units per segment. Main results. We demonstrate that for a broad range of IMRT treatment plans a robust reconstruction of errors is achievable with only 3 projections (3 sets of SDA oriented at at 0°, 45° and 135°). The SDA is capable of capturing both systematic errors in leaf banks and individual leaves as well as random errors sufficient for practical clinical purposes. Significance. These features of the SDA detector makes it suitable for real-time Quality Control of MLC collimated linac output.

Assessing the accuracy of electronic portal imaging device (EPID)-based dosimetry: I. Quantities influencing long-term stability.

The aim of this study is to reduce the uncertainty associated with determining dose-to-water using an amorphous silicon electronic portal imaging detector (EPID) under reference conditions by identifying and accounting for operational and environmental factors influencing the long-term stability of EPID response. Measurements of the EPID relative response, corrected for variations in linear accelerator (linac) output, were performed regularly over a period of 12 months. For every acquired image set, measurements of detector supply voltages, internal operating temperature, and ambient environmental conditions were obtained. Pearson r correlation coefficients were then calculated for each pair of variables, a subset of which were fitted using multiple linear regression to develop a predictive model of EPID response. Principal component analysis was performed on the dataset to reveal the internal structure of the data in a way that best accounts for the observed variations. The +5.5V power supply voltage, internal operating temperature, and the accumulated dose absorbed in EPID were identified as having the greatest influence on the long-term stability of EPID response. By correcting for the combined effect of these variables, the mean difference in linac output as measured by the EPID relative to a reference class chamber improved from -0.46% to 0.23% over the period of the study. This work suggests that the stability of an EPID over a period of a year can be improved by a factor of two by monitoring and accounting for the effects of variations in power supply voltage, internal temperature of the detector, and accumulated absorbed dose.

PO-1585 A fast and robust procedure for independent regular Linac output quantification

PO-1585 A fast and robust procedure for independent regular Linac output quantification

A modified formalism for electron beam reference dosimetry to improve the accuracy of linac output calibration.

To present and demonstrate the accuracy of a modified formalism for electron beam reference dosimetry using updated Monte Carlo calculated beam quality conversion factors. The proposed, simplified formalism allows the use of cylindrical ionization chambers in all electron beams (even those with low beam energies) and does not require a measured gradient correction factor. Data from a previous publication are used for beam quality conversion factors. The formalism is tested and compared to the present formalism in the AAPM TG-51 protocol with measurements made in Elekta Precise electron beams with energies between 4MeV and 22MeV and with fields shaped with a 10×10cm2 clinical applicator as well as a 20×20cm2 clinical applicator for the 18MeV and 22MeV beams. A set of six ionization chambers are used for measurements (two cylindical reference-class chambers, two scanning-type chambers and two parallel-plate chambers). Dose per monitor unit is derived using the data and formalism provided in the TG-51 protocol and with the proposed formalism and data and compared to that obtained using ionization chambers calibrated directly against primary standards for absorbed dose in electron beams. The standard deviation of results using different chambers when TG-51 is followed strictly is on the order of 0.4% when parallel-plate chambers are cross-calibrated against cylindrical chambers. However, if parallel-plate chambers are directly calibrated in a cobalt-60 beam, the difference between results for these chambers is up to 2.2%. Using the proposed formalism and either directly calibrated or cross-calibrated parallel-plate chambers gives a standard deviation using different chambers of 0.4%. The difference between results that use TG-51 and the primary standard measurements are on the order of 0.6% with a maximum difference in the 4MeV beam of 2.8%. Comparing the results obtained with the proposed formalism and the primary standard measurements are on the order of 0.4% with a maximum difference of 1.0% in the 4MeV beam. The proposed formalism and the use of updated data for beam quality conversion factors improves the consistency of results obtained with different chamber types and improves the accuracy of reference dosimetry measurements. Moreover, it is simpler than the present formalism and will be straightforward to implement clinically.

Technical Note: Time-gating to medical linear accelerator pulses: Stray radiation detector.

CCD cameras are employed to image scintillation and Cherenkov radiation in external beam radiotherapy. This is achieved by gating the camera to the linear accelerator (Linac) output. A direct output signal line from the linac is not always accessible and even in cases where such a signal is accessible, a physical wire connected to the output port can potentially alter Linac performance through electrical feedback. A scintillating detector for stray radiation inside the Linac room was developed to remotely time-gate to linac pulses for camera-based dosimetry. A scintillator coupled silicon photomultiplier detector was optimized and systematically tested for location sensitivity and for use with both x rays and electron beams, at different energies and field sizes. Cherenkov radiation emitted due to static photon beams was captured using the remote trigger and compared to the images captured using a wired trigger. The issue of false-positive event detection, due to additional neutron activated products with high energy beams, was addressed. The designed circuit provided voltage >2.5V even for distances up to 3m from the isocenter with a 6MV, 5×5cm beam, using a Ø3×20mm3 Bi4 Ge3 O12 (BGO) crystal. With a larger scintillator size, the detector could be placed even beyond 3m distance. False-positive triggering was reduced by a coincidence detection scheme. Negligible fluctuations were observed in time-gated imaging of Cherenkov intensity emitted from a water phantom, when comparing directly connected vs this remote triggering approach. The remote detector provides untethered synchronization to linac pulses. It is especially useful for remote Cherenkov imaging or remote scintillator dosimetry imaging during radiotherapeutic procedures when a direct line signal is not accessible.

Technical Note: EPID's response to 6MV photons in a strong, parallel magnetic field.

Electronic portal imaging devices (EPIDs) are potentially useful for dosimetric verification in integrated MRI-linac systems. This work presents the reproducibility, linearity, image lag, and radiation field profiles in a conventional EPID, with and without a 0.5T parallel magnetic field present in a 6MV photon beam. An aS500 EPID was modified to function in strong magnetic fields. All measurements were made using the linac-MR installed at the Cross Cancer Institute. The EPID remained stationary on the couch between the measurements made with and without magnetic field. We measured short-term reproducibility of dark and flood fields, signal linearity from 1 to 500MU irradiations, and image lag post 100MU irradiation. An ion chamber was used to measure any linac output variations to correct the EPID signal due to these variations for the duration of experiment. X-axis and Y-axis radiation field profiles were obtained from the EPID image resulting from a 10×10cm2 radiation field delivery. The average pixel value (±standard deviation) of flood field with and without magnetic fields were 57,876±379 and 57,703±366, respectively, and the corresponding average dark field pixel values were -32.05±0.85 and -32.19±0.97. The maximum difference in image linearity data with and without magnetic field is 0.2% which is well within the measurement uncertainty of 0.65%. Similarly, the image lag curves, with and without the magnetic field, were nearly identical. The first measured point, with mean lag signal of 1.44% without and 1.41% with magnetic field, shows that the largest difference is well below the uncertainty in the EPID signal measurement. The radiation field profiles obtained with and without magnetic fields were nearly identical; 91.3% of the X-axis and 95.2% of the Y-axis profile points pass a gamma criterion of 1% and 1mm. A conventional EPID imager with a 0.1cm copper plate responds to 6MV photons similarly irrespective of the strong magnetic field being off or on in the parallel orientation to the radiation beam. Thus, the EPID is a potentially useful tool for pretreatment dosimetric verification in linac-MR systems using parallel magnetic field.

The Influence of Brass Compensator Thickness and Field Size on Neutron Contamination Spectrum in 18MV Elekta SL 75/25 Medical Linear Accelerator with and without Flattening Filter: A Monte Carlo Study

One of the most significant Intensity Modulated Radiation Therapy treatment benefits is a high target to normal tissue dose ratio. To improve this advantage, an additional accessory such as a compensator is used to deliver doses. Compensator-based IMRT treatment is usually operated with an energy higher than 10 MV. Photoneutrons, which have high linear energy transfer and radiobiological effectiveness, are produced by colliding high-energy photon beams with linear accelerator structures, then they deliver the unwanted doses to patients and staff. Therefore, the neutron energy spectra should be determined in order to calculate and reduce the photoneutron risk. We have conducted a comprehensive and precise study on the influence of brass compensator thickness and field size on neutron contamination spectrum in an Elekta SL 75/25 medical linear accelerator with and without the flattening filter by Monte Carlo method. MCNPX MC Code version 2.6.0 was utilized to simulate the detailed geometry of Elekta SL 75/25 head components based on Linac's manual. This code includes an important feature to simulate the photo-neutron interactions. Photoneutrons spectrum was calculated after the Linac output benchmarking based on tuning the primary electron beam. Based on the Friedman and Wilcoxon nonparametric tests results (P<0.05), photoneutron fluence directly depends on the field size and compensator thickness. Moreover, the unflattened beam provides lower photoneutron fluence than the flattened beam. Photoneutrons fluence is not negligible in compensator-based IMRT treatment. However, in order to optimize treatment plans, this additional and unwanted dose must be accounted for patientss.

Collimator scatter factor: Monte Carlo and in-air measurements approaches

BackgroundLinac output as a function of field sizes has a phantom and a head scatter component. This last term can be measured in-air with appropriate build-up ensuring a complete electron equilibrium and the absence of the contaminant electrons. Equilibrium conditions could be achieved using a build-up cap or a mini-phantom. Monte Carlo simulations in a virtual phantom mimicking a mini-phantom were analysed with the aim of better understanding the setup conditions for measuring the collimator scatter factor that is the head scatter component of the linac output factors.MethodsBeams of 6 and 15 MV from a TrueBeam, with size from 4 × 4 to 40 × 40 cm2 were simulated in cylindrical acrylic phantoms 20 cm long, of different diameters, from 0.5 to 4 cm, with the cylinder axis coincident with the beam central axis. The PRIMO package, based on PENELOPE Monte Carlo code, was used. The phase-space files for a Varian TrueBeam linac, provided by the linac vendor, were used for the linac head simulation. Depth dose curves were analysed, and collimator scatter factors estimated at different depth in the different phantom conditions.Additionally, in-air measurements using acyrilic and brass build-up caps, as well as acrylic mini-phantom were acquired for 6 and 18 MV beams from a Varian Clinac DHX.ResultsThe depth dose curves along the cylinders were compared, showing, in each phantom, very similar curves for all analysed field sizes, proving the correctness in estimating the collimator scatter factor in the mini-phantom, provided to position the detector to a sufficient depth to exclude electron contamination. The results were confirmed by the measurements, where the acrylic build-up cap showed to be inadequate to properly estimate the collimator scatter factors, while the mini-phantom and the brass caps gave reasonable measurements.ConclusionA better understanding of the beam characteristics inside a virtual mini-phantom through the analysis of depth dose curves, showed the critical points of using the acrylic build-up cap, and suggested the use of the mini-phantom for the collimator scatter factor measurements in the medium-large field size range.

Two-argument total scatter factor for small fields simultaneously collimated by MLC and jaws: application to stereotactic radiosurgery and radiotherapy

Direct use of the total scatter factor (Stot) for independent monitor unit (MU) calculations can be a good alternative approach to the traditional separate treatment of head/collimator scatter (Sc) and phantom scatter (Sp), especially for stereotactic small fields under the simultaneous collimation of secondary jaws and tertiary multileaf collimators (MLC). We have carried out the measurement of Stot in water for field sizes down to 0.5 × 0.5 cm2 on a Varian TrueBeam STx medical linear accelerator (linac) equipped with high definition MLCs. Both the jaw field size (c) and MLC field size (s) significantly impact the linac output factors, especially when c s and s is small (e.g. s < 5 cm). The combined influence of MLC and jaws gives rise to a two-argument dependence of the total scatter factor, Stot(c,s), which is difficult to functionally decouple. The (c,s) dependence can be conceived as a set of s-dependent functions (‘branches’) defined on domain [smin, smax = c] for a given jaw size of c. We have also developed a heuristic model of Stot to assist the clinical implementation of the measured Stot data for small field dosimetry. The model has two components: (i) empirical fit formula for the s-dependent branches and (ii) interpolation scheme between the branches. The interpolation scheme preserves the characteristic shape of the measured branches and effectively transforms the measured trapezoidal domain in (c,s) plane to a rectangular domain to facilitate easier two-dimensional interpolation to determine Stot for arbitrary (c,s) combinations. Both the empirical fit and interpolation showed good agreement with experimental validation data.

A novel method for sub-arc VMAT dose delivery verification based on portal dosimetry with an EPID.

The EPID-based sub-arc verification of VMAT dose delivery requires synchronization of the acquired electronic portal images (EPIs) with the VMAT delivery, that is, establishment of the start- and stop-MU of the acquired images. To realize this, published synchronization methods propose the use of logging features of the linac or dedicated hardware solutions. In this study, we developed a novel, software-based synchronization method that only uses information inherently available in the acquired images. The EPIs are continuously acquired during pretreatment VMAT delivery and converted into Portal Dose Images (PDIs). Sub-arcs of approximately 10 MU are then defined by combining groups of sequentially acquired PDIs. The start- and stop-MUs of measured sub-arcs are established in a synchronization procedure, using only dosimetric information in measured and predicted PDIs. Sub-arc verification of a VMAT dose delivery is based on comparison of measured sub-arc PDIs with synchronized, predicted sub-arc PDIs, using γ-analyses. To assess the accuracy of this new method, measured and predicted PDIs were compared for 20 clinically applied VMAT prostate cancer plans. The sensitivity of the method for detection of delivery errors was investigated using VMAT deliveries with intentionally inserted, small perturbations (25 error scenarios; leaf gap deviations ≤ 1.5 mm, leaf motion stops during ≤ 15 MU, linac output error ≤ 2%). For the 20 plans, the average failed pixel rates (FPR) for full-arc and sub-arc dose QA were 0.36% ± 0.26% (1 SD) and 0.64% ± 0.88%, based on 2%/2 mm and 3%/3 mm γ-analyses, respectively. Small systematic perturbations of up to 1% output error and 1 mm leaf offset were detected using full-arc QA. Sub-arc QA was able to detect positioning errors in three leaves only during approximately 20 MU and small dose delivery errors during approximately 40 MU. In an ROC analysis, the area under the curve (AUC) for the combined full-arc/sub-arc approach was 0.90. A novel method for sub-arc VMAT dose delivery verification with EPIDs is proposed, using only dosimetric information in acquired EPIs for synchronization. Especially in combination with full-arc QA, the established sensitivity for detection of very small errors is high, with also a high specificity.

Error detection capability of a novel transmission detector: a validation study for online VMAT monitoring

The purpose of this study was to characterize a new single large-area ionization chamber, the integral quality monitor system (iRT, Germany), for online and real-time beam monitoring.Signal stability, monitor unit (MU) linearity and dose rate dependence were investigated for static and arc deliveries and compared to independent ionization chamber measurements. The dose verification capability of the transmission detector system was evaluated by comparing calculated and measured detector signals for 15 volumetric modulated arc therapy plans. The error detection sensitivity was tested by introducing MLC position and linac output errors. Deviations in dose distributions between the original and error-induced plans were compared in terms of detector signal deviation, dose-volume histogram (DVH) metrics and 2D γ-evaluation (2%/2 mm and 3%/3 mm).The detector signal is linearly dependent on linac output and shows negligible (<0.4%) dose rate dependence up to 460 MU min−1. Signal stability is within 1% for cumulative detector output; substantial variations were observed for the segment-by-segment signal.Calculated versus measured cumulative signal deviations ranged from −0.16%–2.25%. DVH, mean 2D γ-value and detector signal evaluations showed increasing deviations with regard to the respective reference with growing MLC and dose output errors; good correlation between DVH metrics and detector signal deviation was found (e.g. PTV Dmean: R2 = 0.97). Positional MLC errors of 1 mm and errors in linac output of 2% were identified with the transmission detector system.The extensive tests performed in this investigation show that the new transmission detector provides a stable and sensitive cumulative signal output and is suitable for beam monitoring during patient treatment.

Electron beam water calorimetry measurements to obtain beam quality conversion factors.

To provide results of water calorimetry and ion chamber measurements in high-energy electron beams carried out at the National Research Council Canada (NRC). There are three main aspects to this work: (a) investigation of the behavior of ionization chambers in electron beams of different energies with focus on long-term stability, (b) water calorimetry measurements to determine absorbed dose to water in high-energy beams for direct calibration of ion chambers, and (c) using measurements of chamber response relative to reference ion chambers, determination of beam quality conversion factors, kQ , for several ion chamber types. Measurements are made in electron beams with energies between 8MeV and 22MeV from the NRC Elekta Precise clinical linear accelerator. Ion chamber measurements are made as a function of depth for cylindrical and plane-parallel ion chambers over a period of five years to investigate the stability of ion chamber response and for indirect calibration. Water calorimetry measurements are made in 18MeV and 22MeV beams. An insulated enclosure with fine temperature control is used to maintain a constant temperature (drifts less than 0.1mK/min) of the calorimeter phantom at 4°C to minimize effects from convection. Two vessels of different designs are used with calibrated thermistor probes to measure radiation induced temperature rise. The vessels are filled with high-purity water and saturated with H2 or N2 gas to minimize the effect of radiochemical reactions on the measured temperature rise. A set of secondary standard ion chambers are calibrated directly against the calorimeter. Finally, several other ion chambers are calibrated in the NRC 60 Co reference field and then cross-calibrated against the secondary standard chambers in electron beams to realize kQ factors. The long-term stability of the cylindrical ion chambers in electron beams is better (always <0.15%) than plane-parallel chambers (0.2% to 0.4%). Calorimetry measurements made at 22MeV with two different vessel geometries are consistent within 0.2% after correction for the vessel perturbation. Measurements of absorbed dose calibration coefficients for the same secondary standard chamber separated in time by 10yr are within 0.2%. Drifts in linac output that would affect the transfer of the standard are mitigated to the 0.1% level by performing daily ion chamber normalization measurements. Calibration coefficients for secondary standard ion chambers can be achieved with uncertainties less than 0.4% (k=1) in high-energy electron beams. The additional uncertainty in deriving calibration coefficients for well-behaved chambers indirectly against the secondary standard reference chambers is negligible. The kQ factors measured here differ by up to 1.3% compared to those in TG-51, an important change for reference dosimetry measurements. The measurements made here of kQ factors for eight plane-parallel and six cylindrical ion chambers will impact future updates of reference dosimetry protocols by providing some of the highest quality measurements of this crucial dosimetric parameter.

High dose‐per‐pulse electron beam dosimetry — A model to correct for the ion recombination in the Advanced Markus ionization chamber

The purpose of this work was to establish an empirical model of the ion recombination in the Advanced Markus ionization chamber for measurements in high dose rate/dose-per-pulse electron beams. In addition, we compared the observed ion recombination to calculations using the standard Boag two-voltage-analysis method, the more general theoretical Boag models, and the semiempirical general equationpresented by Burns and McEwen. Two independent methods were used to investigate the ion recombination: (a) Varying the grid tension of the linear accelerator (linac) gun (controls the linac output) and measuring the relative effect the grid tension has on the chamber response at different source-to-surface distances (SSD). (b) Performing simultaneous dose measurements and comparing the dose-response, in beams with varying dose rate/dose-per-pulse, with the chamber together with dose rate/dose-per-pulse independent Gafchromic™ EBT3 film. Three individual Advanced Markus chambers were used for the measurements with both methods. All measurements were performed in electron beams with varying mean dose rate, dose rate within pulse, and dose-per-pulse (10-2 ≤mean dose rate ≤103 Gy/s, 102 ≤mean dose rate within pulse ≤107 Gy/s, 10-4 ≤dose-per-pulse ≤101 Gy), which was achieved by independently varying the linac gun grid tension, and the SSD. The results demonstrate how the ion collection efficiency of the chamber decreased as the dose-per-pulse increased, and that the ion recombination was dependent on the dose-per-pulse rather than the dose rate, a behavior predicted by Boag theory. The general theoretical Boag models agreed well with the data over the entire investigated dose-per-pulse range, but only for a low polarizing chamber voltage (50V). However, the two-voltage-analysis method and the Burns & McEwen equationonly agreed with the data at low dose-per-pulse values (≤10-2 and ≤10-1 Gy, respectively). An empirical model of the ion recombination in the chamber was found by fitting a logistic function to the data. The ion collection efficiency of the Advanced Markus ionization chamber decreases for measurements in electron beams with increasingly higher dose-per-pulse. However, this chamber is still functional for dose measurements in beams with dose-per-pulse values up toward and above 10Gy, if the ion recombination is taken into account. Our results show that existing models give a less-than-accurate description of the observed ion recombination. This motivates the use of the presented empirical model for measurements with the Advanced Markus chamber in high dose-per-pulse electron beams, as it enables accurate absorbed dose measurements (uncertainty estimation: 2.8-4.0%, k=1). The model depends on the dose-per-pulse in the beam, and it is also influenced by the polarizing chamber voltage, with increasing ion recombination with a lowering of the voltage.

Study of the ionization chamber response to flattening-filter-free photon beams

The commissioning of up-to-date linacs, designed to deliver flattening-filter-free (FFF) photon beams, is performed by measurements using air-vented ionization chambers which are often employed for traditional beams. The aim is to select the ionization chambers which are able to minimize the ion recombination effect, critical for these high intensity beams. In order to provide operators with information about the response to FFF beams of widely used chambers, the recombination correction factor ks and the dose-per-pulse (DPP) were determined in different irradiation conditions. The study, performed by using devices available in most radiotherapy departments, confirmed the reliability of the Boag model for FFF beams as well as with-flattening-filter (WFF) beams. The results showed that ks factor largely depends on the spacing between the collecting electrodes (del)in the ionization chamber and on the DPP of the linac output, which in turn depends on beam energy but is independent of monitor unit rate (MU/min). For plane-parallel and pin-point chambers, whose del is minimal, the ks factor is negligible, whereas for Farmer type chambers the recombination could be important and hence the correction is to be applied (also at voltages higher than the recommended one).

Effects of initial electron beam parameters of a linear accelerator on the properties of bremsstrahlung radiation in a radiotherapy setting

The dependence of the initial electron-beam parameters on absorbed dose distributions have been investigated using a CyberKnife radiotherapy accelerator (Accuray, United States). To describe the initial electron-beam characteristics, simulations of the linear electron accelerator are performed and the electron distributions in the beam of a linac output are analyzed. The radial distributions of electrons are assumed exponential, whereas the energy electron distributions are approximated by monoenergetic and rectangular spectra. There is no significant dependence of depth-dose curves in a phantom on the shape of the electron beam. Importantly, a clear dependence of the radiation field profile on the size of the electron beam is observed not just in the penumbra region, but also in the open part.

O13. A comparison of TPS commissioning dosimeters

Introduction The quality of treatment planning system (TPS) commissioning data largely determines the quality of the derived TPS beam model. Modern treatment techniques require a very high degree of competence from the dosimetrist, as well as the dosimetry equipment used for TPS commissioning. Once derived, the beam model should be verified with a range of tests other than commissioning procedures to test the beam model against linac output. Generally, dosimeters used for TPS beam data collection differ largely from those used for post-modelling verification measurements; therefore their correlation needs to be investigated. Materials and Methods Reference beam profile commissioning data were acquired with a Diamond detector (PTW-60019) for field sizes 2 × 2, 3 × 3 and 5 × 5 cm2 at depths 5 and 10 cm in water. Measurements were repeated with a 2D diode array (SunNuclear Mapcheck2) and compared to reference data. Default array spatial resolution of 1 cm was optimized to 1 mm by manual stepping of the device; therefore only measured (non-interpolated) data was gathered and collated using Excel VBA. A 2D gamma analysis with criteria 1%/1 mm was used for quantitative comparisons. Results Comparison of datasets revealed γ 1 in the open and penumbra regions for all scans evaluated. Mapcheck generally yielded a sharper penumbra (0.5 mm) while the largest differences were seen outside the field with Mapcheck demonstrating a 1.5% larger relative dose. Such small deviations can be attributed to differences in the detector sensitive volumes and dosimeter setup. Further, reducing the amount of steps per array profile by two-thirds (from 10 step increments to 3) did not influence the results significantly. Conclusion Results showed with increased resolution the sensitivity of the array are comparable to a diamond detector for small field profiles. Excellent overall agreement was found between these. However, the dosimetric accomplishment may be weakened by acquisition and processing workload therefore optimization thereof was demonstrated.

SU‐F‐T‐308: Mobius FX Evaluation and Comparison Against a Commercial 4D Detector Array for VMAT Plan QA

Purpose:To evaluate the use of MobiusFX as a pre‐treatment verification IMRT QA tool and compare it with a commercial 4D detector array for VMAT plan QA.Methods:15 VMAT plan QA of different treatment sites were delivered and measured by traditional means with the 4D detector array ArcCheck (Sun Nuclear corporation) and at the same time measurement in linac treatment logs (Varian Dynalogs files) were analyzed from the same delivery with MobiusFX software (Mobius Medical Systems). VMAT plan QAs created in Eclipse treatment planning system (Varian) in a TrueBeam linac machine (Varian) were delivered and analyzed with the gamma analysis routine from SNPA software (Sun Nuclear corporation).Results:Comparable results in terms of the gamma analysis with 99.06% average gamma passing with 3%,3mm passing rate is observed in the comparison among MobiusFX, ArcCheck measurements, and the Treatment Planning System dose calculated. When going to a stricter criterion (1%,1mm) larger discrepancies are observed in different regions of the measurements with an average gamma of 66.24% between MobiusFX and ArcCheck.Conclusion:This work indicates the potential for using MobiusFX as a routine pre‐treatment patient specific IMRT method for quality assurance purposes and its advantages as a phantom‐less method which reduce the time for IMRT QA measurement. MobiusFX is capable of produce similar results of those by traditional methods used for patient specific pre‐treatment verification VMAT QA. Even the gamma results comparing to the TPS are similar the analysis of both methods show that the errors being identified by each method are found in different regions. Traditional methods like ArcCheck are sensitive to setup errors and dose difference errors coming from the linac output. On the other hand linac log files analysis record different errors in the VMAT QA associated with the MLCs and gantry motion that by traditional methods cannot be detected.

TH-CD-201-09: High Spatial Resolution Absorbed Dose to Water Measurements Using Optical Calorimetry in Megavoltage External Beam Therapy

Purpose: To develop and implement a high spatial resolution calorimeter methodology to measure absorbed dose to water (ADW) using phase shifts (PSs) of light passing through a water phantom and to compare measurements with theoretical calculations. Methods: Radiation-induced temperature changes were measured using the PSs of a He-Ne laser beam passing through a (10×10×10) cm3 water phantom. PSs were measured using a Michelson interferometer and recording the time-dependent fringe patterns on a CCD camera. The phantom was positioned at the center of the radiation field. A Varian 21EX was used to deliver 500 MU from a 9 MeV beam using a (6×6) cm2 cone. A 127cm SSD was used and the PSs were measured at depths ranging from of 1.90cm to 2.10cm in steps of 0.05cm by taking profiles at the corresponding rows across the image. PSs were computed by taking the difference between pre- and post-irradiation image frames and then measuring the amplitude of the resulting image profiles. An amplitude-to-PS calibration curve was generated using a piezoelectric transducer to mechanically induce PSs between 0.05 and 1.50 radians in steps of 0.05 radians. The temperature dependence of the refractive index of water at 632.8nm was used to convert PSs to ADW. Measured results were compared with ADW values calculated using the linac output calibration and commissioning data. Results: Milli-radian resolution in PS measurement was achieved using the described methodology. Measured radiation-induced PSs ranged from 0.10 ± 0.01 to 0.12 ± 0.01 radians at the investigated depths. After converting PSs to ADW, measured and calculated ADW values agreed within the measurement uncertainty. Conclusion: This work shows that interferometer-based calorimetry measurements are capable of achieving sub-millimeter resolution measuring 2D temperature/dose distributions, which are particularly useful for characterizing beams from modalities such as SRS, proton therapy, or microbeams.