Liquid-filled Ionization Chambers Research Articles

Evaluation of 2D ion chamber arrays for patient specific quality assurance using a static phantom at a 0.35 T MR-Linac

IntroductionPatient specific quality assurance (QA) in MR-Linacs can be performed with MR-compatible ion chamber arrays. However, the presence of a static magnetic field can alter the angular response of such arrays substantially. This works investigates the suitability of two ion chamber arrays, an air-filled and a liquid-filled array, for patient specific QA at a 0.35 T MR-Linac using a static phantom. MethodsIn order to study the angular response, the two arrays were placed in a static, solid phantom and irradiated with 9.96 × 9.96 cm2 fields every 10° beam angle at a 0.35 T MR-Linac. Measurements were compared to the TPS calculated dose in terms of gamma passing rate and relative dose to the central chamber. 20 patient specific quality assurance plans were measured using the liquid-filled array. ResultsThe air-filled array showed asymmetric angular response changes of central chamber dose of up to 18% and down to local 3 mm / 3% gamma rates of 20%, while only minor differences within 3% (excluding parallel irradiation and beams through the couch edges) were found for the liquid-filled ion chamber array without rotating the phantom. Patient plan QA using the liquid-filled array yielded a median local 3 mm / 3% 3D gamma passing rate of 99.8% (range 96.9%–100%) ConclusionA liquid-filled ionization chamber array in combination with a static phantom can be used for efficient patient specific plan QA in a single measurement set-up in a 0.35 T MR-Linac, while the air-filled ion chamber array phantom shows large angular response changes and has its limitations regarding patient specific QA measurements

The Evolution of Lateral Dose Distributions of Helium Ion Beams in Air: From Measurement and Modeling to Their Impact on Treatment Planning

To start clinical trials with the first clinical treatment planning system supporting raster-scanned helium ion therapy, a comprehensive database of beam characteristics and parameters was required for treatment room-specific beam physics modeling at the Heidelberg Ion-Beam Therapy Center (HIT). At six different positions in the air gap along the beam axis, lateral beam profiles were systematically measured for 14 initial beam energies covering the full range of available energies at HIT. The 2D-array of liquid-filled ionization chambers OCTAVIUS from PTW was irradiated by a pencil beam focused at the central axis. With a full geometric representation of HIT’s monitoring chambers and beamline elements in FLUKA, our Monte Carlo beam model matches the measured lateral beam profiles. A second set of measurements with the detector placed in a water tank was used to validate the adjustments of the initial beam parameters assumed in the FLUKA simulation. With a deviation between simulated and measured profiles below ±0.8 mm for all investigated beam energies, the simulated profiles build part of the database for the first clinical treatment planning system for helium ions. The evolution of beamwidth was also compared to similar simulations of the clinically available proton and carbon beam. This allows a choice of treatment modality based on quantitative estimates of the physical beam properties. Finally, we investigated the influence of beamwidth variation on patient treatment plans in order to estimate the relevance and necessary precision limits for lateral beam width models.

Online Recombination Correction in Dosimetry with Arrays of Ionization Chambers: Application to FFF and UHDR Radiotherapy

Recombination of charge carriers can affect the response of ionization detectors used for the dosimetry of radiotherapy fields. In this work, we present a method for correcting online the recombination effects in arrays of ionization chambers irradiated with time-varying dose rates. The method is based on the characterization of the dose rate/recombination response of the detector, and the measurement of the instant ionization current in the detector, rather than the integrated charge. The proposed method was investigated with simulations of the response of different air and liquid ionization chambers in situations where recombination can be large. In addition, we experimentally investigated the application of the method with an in-house-developed liquid-filled ionization chamber. The proposed online correction method can compensate for recombination losses and seems feasible to implement in the software of ionization arrays/detectors used for the dosimetry of radiotherapy fields.

Tungsten carbide and LMPA electron cutouts: comparison and validation using Monte Carlo modelling and measurement of dose

Electron applicator cutouts for radiation therapy electron beam shaping are typically cast from Low Melting Point Alloys (LMPA), such as cerrobend. In this work, we describe the Monte Carlo modelling of novel 3D printed cutouts based on tungsten carbide powder and the resulting dose profiles subject to Elekta Agility electron beams. Cerrobend cutouts were also modelled using the Monte Carlo code EGSnrc. Cerrobend and tungsten carbide cutouts were found to have the same dose profiles within model variance. Computed profiles and percentage depth dose (PDDs) curves of the Elekta Agility accelerator model using standard cutouts in water were found to agree with water tank measurements using gamma criteria of 2%/2mm. The Monte Carlo computed dose profiles of the tungsten carbide cutouts in a polystyrene phantom were also found to agree with liquid-filled ionization chamber array measurements using gamma criteria of 2%/2mm. We conclude that the tungsten carbide cutouts are clinically equivalent to LMPA cutouts.

On-line beam monitoring and dose profile measurements of a [formula omitted]Pb beam of 150 GeV/n with a liquid-filled ionization chamber array

In this study, the possibility of on-line monitoring of an ultra-high-energy (UHE) beam of 150 GeV/n lead (208Pb) ions using a liquid-filled ionization chamber array (OCTAVIUS 1000SRS, PTW Freiburg, Germany) is evaluated. The applicability of the array has been tested for the investigated beam, where adequate adjustments of the array have been made for measurements under very low fluxes of 102 to 104 ions / cm2/s. The results have been compared to measurements obtained using the standard beam line detectors, such as a scintillator and a delay wire chamber (DWC). Since these detectors were situated further upstream from the experimental position, measurements with these devices may not be representative of the beam characteristics at setup due to the interactions of the primary beam particles within these detectors and the subsequent transport of these particles between these detectors and the setup. The main advantage of the application of the liquid-filled array is its portability so that it can be placed directly at the position of the experimental setup to capture the beam characteristics. A comparison between the measured beam profile and independent film measurements has demonstrated its reliability for analysing the beam characteristics. In this context, the 2D array has been implemented to measure on-line not only the 2D beam profile but also the total imparting particle fluence of the UHE lead beam by using the measured dose and considering the calibration of the detector.

195. Electronic Brachytherapy in clinical practice: Implementation and comparison against conventional techniques and electron montecarlo algorithm

Purpose Electronic Brachytherapy (EBX), a technique that utilizes miniaturized X-Rays source, is gaining ground in radiotherapy treatment in presence of small superficial lesions. This work focuses on acceptance test, commissioning, clinical implementation and dosimetric features of the novel EBX system Esteya® (Elekta AB, Stockholm, Sweden). Methods Flatness and symmetry of X-ray beams have been evaluated according IEC60976 using a high definition 2D array equipped of liquid filled ionization chambers (SRS 1000, PTW, Germany). Half Value Layer (HVL), PDD and absolute dose have been measured for each applicator with a soft X-ray parallel plate chamber (T34013 PTW, Germany) and solid water according to IAEA TRS 398 [1] . Dose distributions have been compared with the ones calculated for conventional electron treatments. Calculations have been carried out in a virtual water phantom by electron montecarlo (eMc) algorithm implemented in a commercial TPS (Monaco v. 5.11, Elekta) for each applicator. Results Flatness, symmetry and penumbra showed excellent performance even if compared with eMC plans. Build up absence and PDDs slopes allow good homogeneity for coverage in superficial targets. Conclusions Shielding requirements, patient compliance and global management of Electronic brachytherapy sources are well known advantages [2] . This study demonstrated that EBX could be a valid alternative to electron treatment in case of small skin lesions also from a dosimetric point of view.

Development and clinical characterization of a novel 2041 liquid-filled ionization chambers array for high-resolution verification of radiotherapy treatments.

The aim of this study was to present a novel 2041 liquid-filled ionization chamber array for high-resolution verification of radiotherapy treatments. The prototype has 2041 ionization chambers of 2.5 × 2.5 mm2 area filled with isooctane. The detection elements are arranged in a central square grid of 43 × 43, totally covering an area of 107.5 × 107.5 mm2 . The central inline and cross-line are extended to 227 mm and the diagonals to 321 mm to be able to perform profile measurements of large fields. We have studied stability, pixel response uniformity, dose rate dependence, depth and field size dependence and anisotropy. We present results for output factors, tongue-and-groove, garden fence, small field profiles, irregular fields, and verification of dose planes of patient treatments. Comparison with other detectors used for small field dosimetry (SFD, CC13, microDiamond) has shown good agreement. Output factors measured with the device for square fields ranging from 10 × 10 to 100 × 100 mm2 showed relative differences within 1%. The response of the detector shows a strong dependence on the angle of incident radiation that needs to be corrected for. On the other hand, inter-pixel relative response variations in the 0.95-1.08 range have been found and corrected for. The application of the device for the verification of dose planes of several treatments has shown gamma passing rates above 97% for tolerances of 2% and 2 mm. The verification of other clinical fields, like small fields and irregular fields used in the commissioning of the TPS, also showed large passing rates. The verification of garden fence and tongue-and-groove fields was affected by volume-averaging effects. The results show that the liquid filled ionization chamber prototype here presented is appropriate for the verification of radiotherapy treatments with high spatial resolution. Recombination effects do not affect very much the verification of relative dose distributions. However, verification of absolute dose distributions may require normalization to a radiation field which is representative of the dose rate of the treatment delivered.

Verification of high-dose-rate brachytherapy treatment planning dose distribution using liquid-filled ionization chamber array.

PurposeThis study aims to investigate the dosimetric performance of a liquid-filled ionization chamber array in high-dose-rate (HDR) brachytherapy dosimetry. A comparative study was carried out with air-filled ionization chamber array and EBT3 Gafchromic films to demonstrate its suitability in brachytherapy.Material and methodsThe PTW OCTAVIUS detector 1000 SRS (IA 2.5-5 mm) is a liquid-filled ionization chamber array of area 11 x 11 cm2 and chamber spacing of 2.5-5 mm, whereas the PTW OCTAVIUS detector 729 (IA 10 mm) is an air vented ionization chamber array of area 27 x 27 cm2 and chamber spacing of 10 mm. EBT3 films were exposed to doses up to a maximum of 6 Gy and evaluated using multi-channel analysis. The detectors were evaluated using test plans to mimic a HDR intracavitary gynecological treatment. The plan was calculated and delivered with the applicator plane placed 20 mm from the detector plane. The acquired measurements were compared to the treatment plan. In addition to point dose measurement, profile/isodose, gamma analysis, and uncertainty analysis were performed. Detector sensitivity was evaluated by introducing simulated errors to the test plans.ResultsThe mean point dose differences between measured and calculated plans were 0.2% ± 1.6%, 1.8% ± 1.0%, and 1.5% ± 0.81% for film, IA 10 mm, and IA 2.5-5 mm, respectively. The average percentage of passed gamma (global/local) values using 3%/3 mm criteria was above 99.8% for all three detectors on the original plan. For IA 2.5-5 mm, local gamma criteria of 2%/1 mm with a passing rate of at least 95% was found to be sensitive when simulated positional errors of 1 mm was introduced.ConclusionThe dosimetric properties of IA 2.5-5 mm showed the applicability of liquid-filled ionization chamber array as a potential QA device for HDR brachytherapy treatment planning systems.

18. Metallic implants management in radiotherapy treatment planning process

Introduction Treatment planning in the presence of metallic implants may lead to several issues from contouring to dose calculation. The aim of this study was to control if extended CT scale technique (−10240 UH; 30710 HU) is a suitable alternative to standard CT scale technique for dose calculation in presence of high-Zmaterials. Then the efficiencies of two commercially available dose prediction algorithms, Accuros XB™ (AXB) and Analytical Anisotropic Algorithm™ (AAA) (Varian Medical Systems) were compared in this condition. This work will use for implementation of spinal stereotactic body radiation therapy (SBRT) with metallic fixations. Methods First an extended scale Hounsfield units (HU) conversion curve to relative electron density and to mass density was created based on density phantom (CIRS model 062). Uniformity, noise, low contrast and spatial resolution were evaluated for extended CT scale technique with manufactured CT phantom (GE Healthcare). Images were analyzed with ImageJ. Then measurements were performed using a True Beam™ linac (Varian Medical Systems) with energies of 6 MV flattening filter and flattening filter free for static fields and VMAT. Deposited dose through a titanium implant included in a water-equivalent phantom was evaluated. Two dimensional measurements were also performed at different depths in RW3’s plates with the implant placed above it. Calculations dose were made with Eclispe TPS (Varian Medical Systems) using AXB and AAA. EBT3 radiochromic films (Gafchromic) and liquid-filled ionization chamber array SRS 1000 (PTW Freiburg) were used. Analyses were performed with FilmQAPro (Ashland) et VeriSoft (PTW Freiburg). Results The extended scale HU conversion curve to relative electron density and to mass density show a good agreement to standard scale with 2% maximum variance. Image quality was steady. However low contrast detectability slightly decreased without significant clinical influence. AXB agrees better with measurements than AAA in presence of metallic implant. Conclusions Using extended CT scale technique can potentially improve efficiency in presence of metallic implants in the treatment planning process. In this condition, AXB improves dose calculation in comparison to AAA.

The "collimator monitoring fill factor" of a two-dimensional detector array, a measure of its ability to detect collimation errors.

Two-dimensional detector arrays are routinely used for constancy checks and treatment plan verification in photon-beam radiotherapy. In addition to the spatial resolution of the dose profiles, the "coverage" of the radiation field with respect to the detection of any beam collimation deficiency appears as the second characteristic feature of a detector array. The here proposed "collimator monitoring fill factor" (CM fill factor) has been conceived to serve as a quantitative characteristic of this "coverage". The CM fill factor is defined as the probability of a 2D array to detect any collimator position error. Therefore, it is represented by the ratio of the "sensitive area" of a single detector, in which collimator position errors are detectable, and the geometrical "cell area" associated with this detector within the array. Numerical values of the CM fill factor have been Monte Carlo simulated for 2D detector arrays equipped with air-vented ionization chambers, liquid-filled ionization chambers and diode detectors and were compared with the "FWHM fill factor" defined by Gago-Arias etal. (2012). For arrays with vented ionization chambers, the differences between the CM fill factor and the FWHM fill factor are moderate, but occasionally the latter exceeds unity. For narrower detectors such as liquid-filled ionization chambers and Si diodes and for small sampling distances, large differences between the FWHM fill factor and the CM fill factor have been observed. These differences can be explained by the shapes of the fluence response functions of these narrow detectors. A new parameter "collimator monitoring fill factor" (CM fill factor), applicable to quantitate the collimator position error detection probability of a 2D detector array, has been proposed. It is designed as a help in classifying the clinical performance of two-dimensional detector arrays in photon-beam radiotherapy.

LET Monitoring Using Liquid Ionization Chambers

Relative biological effectiveness (RBE) is an important quantity in planning particle beam cancer therapy. In general, the RBE describes the biological effectiveness of a given primary beam with respect to a reference photon irradiation. RBE varies not only for different primary beams but also with depth in the target for a given beam modality. It is not a quantity that easily lends itself to measurements or computation as it depends on many biological and physical quantities. Numerous experiments in vitro using various cell lines and irradiation modalities have shown that a general relationship between RBE and the physical quantity Linear Energy Transfer (LET) exists. Several groups have proposed including LET in the radiation therapy treatment planning instead of the more complicated and elusive RBE. It has been shown that LET is an important quantity to consider in treating radio-resistant tumors. The concept of LET painting has been proposed with the goal of improving tumor control probability (TCP) for hypoxic tumors by focusing high LET radiation on the hypoxic region of the tumor while restricting the surrounding normal tissue to low LET radiation. In order to properly incorporate LET in clinical treatment, it is important to be able to experimentally measure and verify LET distribution. We propose a novel method for measuring LET using a dual chamber methodology exploiting the difference in the observed recombination between air filled ionization chambers (IC) and liquid filled ionization chambers (LIC). The resulting difference in the measured signals will be used to directly extract the relative LET of an actual treatment beam in real time. This paper describes our initial studies of this method, presents preliminary results, and discusses further improvements toward a practical real-time LET measuring device.

Recombination in liquid-filled ionization chambers beyond the Boag limit.

The high mass density and low mobilities of charge carriers can cause important recombination in liquid-filled ionization chambers (LICs). Saturation correction methods have been proposed for LICs. Correction methods for pulsed irradiation are based on Boag equation. However, Boag equation assumes that the charge ionized by one pulse is fully collected before the arrival of the next pulse. This condition does not hold in many clinical beams where the pulse repetition period may be shorter than the charge collection time, causing overlapping between charge carriers ionized by different pulses, and Boag equation is not applicable there. In this work, the authors present an experimental and numerical characterization of collection efficiencies in LICs beyond the Boag limit, with overlapping between charge carriers ionized by different pulses. The authors have studied recombination in a LIC array for different dose-per-pulse, pulse repetition frequency, and polarization voltage values. Measurements were performed in a Truebeam Linac using FF and FFF modalities. Dose-per-pulse and pulse repetition frequency have been obtained by monitoring the target current with an oscilloscope. Experimental collection efficiencies have been obtained by using a combination of the two-dose-rate method and ratios to the readout of a reference chamber (CC13, IBA). The authors have also used numerical simulation to complement the experimental data. The authors have found that overlap significantly increases recombination in LICs, as expected. However, the functional dependence of collection efficiencies on the dose-per-pulse does not change (a linear dependence has been observed in the near-saturation region for different degrees of overlapping, the same dependence observed in the nonoverlapping scenario). On the other hand, the dependence of collection efficiencies on the polarization voltage changes in the overlapping scenario and does not follow that of Boag equation, the reason being that changing the polarization voltage also affects the charge collection time, thus changing the amount of overlapping. These results have important consequences for saturation correction methods for LICs. On one hand, the two-dose-rate method, which relies on the functional dependence of the collection efficiencies on dose-per-pulse, can also be used in the overlapping situation, provided that the two measurements needed to feed the method are performed at the same pulse repetition frequency (monitor unit rate). This result opens the door to computing collection efficiencies in LICs in many clinical setups where charge overlap in the LIC exists. On the other hand, correction methods based on the voltage-dependence of Boag equation like the three-voltage method or the modified two-voltage method will not work in the overlapping scenario due to the different functional dependence of collection efficiencies on the polarization voltage.

SU-F-T-584: Investigating Correction Methods for Ion Recombination Effects in OCTAVIUS 1000 SRS Measurements

Purpose: PTW's Octavius 1000 SRS array performs IMRT QA measurements with liquid filled ionization chambers (LICs). Collection efficiencies of LICs have been shown to change during IMRT delivery as a function of LINAC pulse frequency and pulse dose, which affects QA results. In this study, two methods were developed to correct changes in collection efficiencies during IMRT QA measurements, and the effects of these corrections on QA pass rates were compared. Methods: For the first correction, Matlab software was developed that calculates pulse frequency and pulse dose for each detector, using measurement and DICOM RT Plan files. Pulse information is converted to collection efficiency and measurements are corrected by multiplying detector dose by ratios of calibration to measured collection efficiencies. For the second correction, MU/min in daily 1000 SRS calibration was chosen to match average MU/min of the VMAT plan. Usefulness of derived corrections were evaluated using 6MV and 10FFF SBRT RapidArc plans delivered to the OCTAVIUS 4D system using a TrueBeam equipped with an HD- MLC. Effects of the two corrections on QA results were examined by performing 3D gamma analysis comparing predicted to measured dose, with and without corrections. Results: After complex Matlab corrections, average 3D gamma pass rates improved by [0.07%,0.40%,1.17%] for 6MV and [0.29%,1.40%,4.57%] for 10FFF using [3%/3mm,2%/2mm,1%/1mm] criteria. Maximum changes in gamma pass rates were [0.43%,1.63%,3.05%] for 6MV and [1.00%,4.80%,11.2%] for 10FFF using [3%/3mm,2%/2mm,1%/1mm] criteria. On average, pass rates of simple daily calibration corrections were within 1% of complex Matlab corrections. Conclusion: Ion recombination effects can potentially be clinically significant for OCTAVIUS 1000 SRS measurements, especially for higher pulse dose unflattened beams when using tighter gamma tolerances. Matching daily 1000 SRS calibration MU/min to average planned MU/min is a simple correction that greatly reduces ion recombination effects, improving measurements accuracy and gamma pass rates. This work was supported by PTW.

SU-F-T-576: Characterization of Two Dimensional Liquid Filled Detector Array(SRS 1000) in High Precision Cyberknife Robotic Radiosurgery System

Purpose: The aim of the study is to characterize a two dimensional liquid filled detector array SRS 1000 for routine QA in Cyberknife Robotic Radiosurgery system. Methods: SRS 1000 consists of 977 liquid filled ionization chambers and is designed to be used in small field SRS/SBRT techniques. The detector array has got two different spacial resolutions. Till field size of 5.5×5.5 cm the spacial resolution is 2.5mm (center to center) and after that till field size of 11 × 11 cm the spacial resolution is 5mm. The size of the detector is 2.3 × 2.3 0.5 mm with a volume of .003 cc. The CyberKnife Robotic Radiosurgery System is a frameless stereotactic radiosurgery system in which a LINAC is mounted on a robotic manipulator to deliver beams with a high sub millimeter accuracy. The SRS 1000's MU linearity, stability, reproducibility in Cyberknife Robotic Radiosurgery system was measured and investigated. The output factors for fixed and IRIS collimators for all available collimators (5mm till 60 mm) was measured and compared with the measurement done with PTW pin-point ionization chamber. Results: The MU linearity was measured from 2 MU till 1000 MU for doserates in the range of 700cGy/min – 780 cGy/min and compared with the measurement done with pin point chamber The MU linearity was with in 3%. The detector arrays stability and reproducibility was excellent and was withinin 0.5% The measured output factors showed an agreement of better than 2% when compared with the measurements with pinpoint chamber for both fixed and IRIS collimators with all available field sizes. Conclusion: We have characterised PTW 1000 SRS as a precise and accurate measurement tool for routine QA of Cyberknife Robotic radiosurgery system.

Investigating ion recombination effects in a liquid-filled ionization chamber array used for IMRT QA measurements.

PTW's Octavius 1000 SRS array performs IMRT quality assurance (QA) measurements with liquid-filled ionization chambers (LICs) to allow closer detector spacing and higher resolution, compared to air-filled QA devices. However, reduced ion mobility in LICs relative to air leads to increased ion recombination effects and reduced collection efficiencies that are dependent on Linac pulse frequency and pulse dose. These pulse parameters are variable during an IMRT delivery, which affects QA results. In this study, (1) 1000 SRS collection efficiencies were measured as a function of pulse frequency and pulse dose, (2) two methods were developed to correct changes in collection efficiencies during IMRT QA measurements, and the effects of these corrections on QA pass rates were compared. To obtain collection efficiencies, the OCTAVIUS 1000 SRS was used to measure open fields of varying pulse frequency, pulse dose, and beam energy with results normalized to air-filled chamber measurements. Changes in ratios of 1000 SRS to chamber measured dose were attributed to changing collection efficiencies, which were then correlated to pulse parameters using regression analysis. The usefulness of the derived corrections was then evaluated using 6 MV and 10FFF SBRT RapidArc plans delivered to the OCTAVIUS 4D system using a TrueBeam (Varian Medical Systems) linear accelerator equipped with a high definition multileaf collimator. For the first correction, matlab software was developed that calculates pulse frequency and pulse dose for each detector, using measurement and DICOM RT Plan files. Pulse information is converted to collection efficiency, and measurements are corrected by multiplying detector dose by ratios of calibration to measured collection efficiencies. For the second correction the MU/min in the daily 1000 SRS calibration was chosen to match the average MU/min of the volumetric modulated arc therapy plan. Effects of the two corrections on QA results were examined by performing 3D gamma analysis comparing predicted to measured dose, with and without corrections. Collection efficiencies correlated linearly to pulse dose, while correlations with pulse frequency were less defined, generally increasing as pulse frequency decreased. After complex matlab corrections, average 3D gamma pass rates improved by [0.07%,0.40%,1.17%] for 6 MV and [0.29%,1.40%,4.57%] for 10FFF using [3%/3 mm,2%/2 mm,1%/1 mm] criteria. Maximum changes in gamma pass rates were [0.43%,1.63%,3.05%] for 6 MV and [1.00%,4.80%,11.2%] for 10FFF using [3%/3 mm,2%/2 mm,1%/1 mm] criteria. On average, pass rates of simple daily calibration corrections were within 1% of complex matlab corrections. OCTAVIUS 1000 SRS ion recombination effects have little effect on 6 MV measurements. However, the effect could potentially be clinically significant for higher pulse dose unflattened beams when using tighter gamma tolerances, especially when small aperture sizes are used, as is common for SRS/SBRT. In addition, ion recombination effects are strongly correlated to changing MU/min, therefore MU/min used in daily 1000 SRS calibrations should be matched to the expected average MU/min of the IMRT plan.

Detecting MLC errors in stereotactic radiotherapy plans with a liquid filled ionization chamber array.

Quality assurance of stereotactic radiotherapy demands the use of equipment with the highest resolution and sensitivity available. This study examines the sensitivity of a commercially available liquid-filled ionization chamber array--the Octavius 1000 SRS (PTW, Frieburg, Germany) for detecting small (sub-millimetre) multi-leaf collimator (MLC) alignment errors in static square fields (side length 16-40 mm). Furthermore, the effectiveness of detecting small MLC errors in clinical stereotactic radiotherapy patient plans using the device was also evaluated. The commonly used gamma pass rate metric (of the measurements compared with treatment planning system generated results) was used. The gamma pass rates were then evaluated as a function of MLC position error (MLC error size 0.1-2.5 mm). The detector array exhibited a drop in pass rate between plans without error and those which had MLC errors induced. For example a drop in pass rate of 4.5% (gamma criteria 3%, 1 mm) was observed when a 0.8 mm error was introduced into a 16 mm square field. Furthermore the drop in pass rate increased as the MLC position error increased. This study showed that the Octavius 1000 SRS array could be a useful tool for applications requiring the detection of small geometric delivery uncertainties.

Dose rate dependence of the PTW 60019 microDiamond detector in high dose-per-pulse pulsed beams

Recombination effects can affect the detectors used for the dosimetry of radiotherapy fields. They are important when using ionization chambers, especially in liquid-filled ionization chambers, and should be corrected for. The introduction of flattening-filter-free accelerators increases the typical dose-per-pulse used in radiotherapy beams, which leads to more important recombination effects.Diamond detectors provide a good solution for the dosimetry and quality assurance of small radiotherapy fields, due to their low energy dependence and small volume. The group of Università di Roma Tor Vergata has developed a synthetic diamond detector, which is commercialized by PTW as microDiamond detector type 60019. In this work we present an experimental characterization of the collection efficiency of the microDiamond detector, focusing on high dose-per-pulse FFF beams. The collection efficiency decreases with dose-per-pulse, down to 0.978 at 2.2 mGy/pulse, following a Fowler–Attix-like curve. On the other hand, we have found no significant dependence of the collection efficiency on the pulse repetition frequency (or pulse period).

A numerical model of initial recombination for high-LET irradiation: Application to liquid-filled ionization chambers

Abstract In this paper we present a numerical model of initial recombination in media irradiated with high linear energy transfer (LET) ions, which relies on an amorphous track model of ionization of high LET particles, and diffusion, drift and recombination of ionized charge carriers. The model has fundamental applications for the study of recombination in non-polar liquids, as well as practical ones, like in modelling hadrontherapy dosimetry with ionization chambers. We have used it to study the response of liquid-filled ionization chambers to hadrontherapy beams: dependence of initial recombination on ion species, energy and applied external electric field.

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.

Characterization of tetramethylsilane for liquid-filled ionization dosimeters: Ion mobilities, free-ion yield and general recombination

Liquid-filled ionization chambers (LICs) are interesting detectors for the dosimetry of radiotherapy beams due to their water-equivalent response and high spatial resolution. Isooctane is the liquid most often used as an active medium, but other hydrocarbons, particularly tetramethylsilane (TMS), can be suitable for dosimetry.In this work we present a characterization of TMS (Merck, NMR calibration grade, purity>99.7%) for its use in LICs. The characterization consisted of measuring ion mobilities, using low dose 6MV photon pulses from a medical linac, and free-ion yield, using a continuous cobalt-60 beam (the reference beam quality used in radiotherapy dosimetry). Those values were then used to model general recombination in a TMS-filled LIC.Measured ion mobilities, (2.6±0.3)×10−8 and (3.6±0.4)×10−8m2V−1s−1, are similar to mobilities in isooctane, and two- to three-fold lower than some values reported for TMS. Such discrepancy can probably be attributed to the presence of different impurities. On the other hand, free-ion yield values obtained are approximately two-fold higher than for isooctane, in agreement with published data. Such high free-ion yield values result in a higher signal-to-noise ratio and may allow even better spatial resolution to be obtained with TMS-filled LICs. However, it comes at the cost of higher recombination effects that can compromise the operation of the chamber. Such high recombination and the low boiling point of TMS (≃28°C) make isooctane-filled LICs preferable to TMS-filled LICS for radiotherapy applications.