Determining charge collection efficiency in parallel-plate liquid ionization chambers

MR-OCTAVIUS 4D with 1500 MR and 1600 MR arrays is suitable for plan QA in a 1.5 T MRI-linac

To ensure the accuracy of radiation delivery to patients in a 1.5 T MRI-linac, the implementation of quality assurance (QA) devices compatible with MR technology is essential. The OCTAVIUS 4D MR, made by PTW (Freiburg, Germany) is designed to ensure consistent and ideal alignment of its detectors with the direction of each beam segment. This study focuses on investigating the fundamental characteristics of the detector response for the OCTAVIUS Detector (OD) 1500 MR and OCTAVIUS 1600 MR when used in the MR-compatible OCTAVIUS 4D. Characteristics examined included short-term reproducibility, dose linearity, field size dependency, monitor unit (MU) rate dependency, dose-per-pulse dependency, and angular dependency. The evaluation of OD 1500 MR also involved measuring 25 clinical treatment plans across diverse target sizes and anatomical sites, including the liver/pancreas, rectum, prostate, lungs, and lymph nodes. One plan was measured with the standard setup and with a 5 cm left offset. The OD 1600 MR was not available for these measurements. The capability of the OD 1500 MR to identify potential errors was assessed by introducing a MU and positional shift within the software. The results demonstrated no significant differences in short-term reproducibility (<0.2%), dose linearity (<1%), field size dependency (<0.7%for field sizes larger than 5 cm × 5 cm), MU rate dependency (<0.8%), dose-per-pulse dependency (<0.4%) and angular dependency (standard deviation<0.5%). All tests of clinical plans were successfully completed. The OD 1500 MR demonstrated compatibility with the standard 95% pass rate when employing a global 3%/3 mm gamma criterion, and a 90% pass rate using a global 2%/2 mm gamma criterion. The detector demonstrated the capacity to measure treatment plans with a 5 cm left offset. With the standard parameters, the gamma test was sensitive to position errors but required an addition tests of mean/median dose or point dose in order to detect small dose difference.

Development of an ultra-thin parallel plate ionization chamber for dosimetry in flash radiotherapy

Background

Conventional air ionization chambers (ICs) exhibit ion recombination correction factors that deviate substantially from unity when irradiated with dose per pulse magnitudes higher than those used in conventional radiotherapy. This fact makes these devices unsuitable for the dosimetric characterization of beams in ultra‐high dose per pulse as used for FLASH radiotherapy.

Purpose

We present the design, development, and characterization of an ultra‐thin parallel plate IC that can be used in ultra‐high dose rate (UHDR) deliveries with minimal recombination.

Methods

The charge collection efficiency (CCE) of parallel plate ICs was modeled through a numerical solution of the coupled differential equations governing the transport of charged carriers produced by ionizing radiation. It was used to find out the optimal parameters for the purpose of designing an IC capable of exhibiting a linear response with dose (deviation less than 1%) up to 10 Gy per pulse at 4 μs pulse duration. As a proof of concept, two vented parallel plate IC prototypes have been built and tested in different ultra‐high pulse dose rate electron beams.

Results

It has been found that by reducing the distance between electrodes to a value of 0.25 mm it is possible to extend the dose rate operating range of parallel plate ICs to ultra‐high dose per pulse range, at standard voltage of clinical grade electrometers, well into several Gy per pulse. The two IC prototypes exhibit behavior as predicted by the numerical simulation. One of the so‐called ultra‐thin parallel plate ionization chamber (UTIC) prototypes was able to measure up to 10 Gy per pulse, 4 μs pulse duration, operated at 300 V with no significant deviation from linearity within the uncertainties (ElectronFlash Linac, SIT). The other prototype was tested up to 5.4 Gy per pulse, 2.5 μs pulse duration, operated at 250 V with CCE higher than 98.6% (Metrological Electron Accelerator Facility, MELAF at Physikalisch‐Technische Bundesanstalt, PTB).

Conclusions

This work demonstrates the ability to extend the dose rate operating range of ICs to ultra‐high dose per pulse range by reducing the spacing between electrodes. The results show that UTICs are suitable for measurement in UHDR electron beams.

Three-voltage linear method to determine ion recombination in proton and light-ion beams

A new practical method to determine the ion recombination correction factor (k _s ) for plane-parallel and Farmer-type cylindrical chambers in particle beams is investigated. Experimental data were acquired in passively scattered and scanned particle beams and compared with theoretical models developed by Boag and/or Jaffé. The new method, named the three-voltage linear method (3VL-method), is simple and consists of determining the saturation current using the current measured at three voltages in a linear region and dividing it by the current at the operating voltage (V) (even if it is not in the linear region) to obtain k _s . For plane-parallel chambers, comparing k _s -values obtained by model fits to values obtained using the 3VL-method, an excellent agreement is found. For cylindrical chambers, recombination is due to volume recombination only. At low voltages, volume recombination is too large and Boag's models are not applicable. However, for Farmer-type chambers (NE2571), using a smaller voltage range, limited down to 100 V, we observe a linear variation of k _s with 1/V ² or 1/V for continuous or pulsed beams, respectively. This linearity trend allows applying the 3VL-method to determine k _s at any polarizing voltage. For the particle beams used, the 3VL-method gives an accurate determination of k _s at any polarizing voltage. The choice of the three voltages must to be done with care to ensure to be in a linear region. For Roos-type or Markus-type chambers (i.e. chambers with an electrode spacing of 2 mm) and NE2571 chambers, the use of the 3VL-method with 300 V, 200 V and 150 V is adequate. A difference with the 2V-method and some 3V-methods in the literature is that in the 3VL-method the operational voltage does not have to be one of the three voltages. An advantage over a 2V-method is that the 3VL-method can inherently verify if the linearity condition is fulfilled.

Correction for volume recombination in liquid ionization chambers at high dose-per-pulse

PURPOSE

To determine the volume recombination at high dose-per-pulse in liquid ionization chambers (LIC) and to ascertain whether existing calculation methods verified in air-filled chambers may be used to calculate a correction factor.

METHODS

Two LICs, one filled with 2,2,4-trimethylpentane (isooctane) the other with tetramethylsilane (TMS), were irradiated in a pulsed, 20 MeV electron beam. Via reference measurements with a Faraday cup, the saturation correction for volume recombination was determined for dose-per-pulse values ranging from about 5 mGy to 1 Gy for both chambers at a pulse duration of 693 ns. In addition, the isooctane chamber was irradiated with pulses of varying duration, ranging from 5 ps to 10 ms, at a dose-per-pulse of about 76.5 mGy. The dose-per-pulse-dependent measurements were compared to calculations based on Boag's models (with and without a free electron fraction), the two-dose-rate method, and a numerical calculation. The pulse duration dependent measurements were compared only to a numerical calculation that iteratively calculates the charge transport and loss in a 1D model of an ionization chamber.

RESULTS

In TMS only Boag's model with a free electron fraction and the numerical calculation are in good agreement with the experimental data. However, in isooctane, good agreement is observed between the experimental data, the numerical calculation as well as the two-dose-rate method, and Boag's model including a free electron fraction. Only Boag's model without a free electron fraction shows a good agreement with lesser extend. Furthermore, the pulse duration-dependent data for isooctane are well described by the numerical model.

CONCLUSION

With isooctane as an active medium, a LIC could be directly used in a field with high dose-per-pulse utilizing the well-established two-dose-rate method to correct the volume recombination. In addition, pulsed fields with variable pulse duration are easily modeled for this medium using a numerical calculation. Other media, as exemplified by the TMS-filled chamber, might require additional considerations, such as including a fraction of free electrons in the consideration of volume recombination.

Evaluating small field dosimetry with the Acuros XB (AXB) and analytical anisotropic algorithm (AAA) dose calculation algorithms in the eclipse treatment planning system

Background:

An increasing number of external beam treatment modalities including intensity modulated radiation therapy, volumetric modulated arc therapy (VMAT) and stereotactic radiosurgery uses very small fields for treatment planning and delivery. However, there are major challenges in small photon field dosimetry, due to the partial occlusion of the direct photon beam source’s view from the measurement point, lack of lateral charged particle equilibrium, steep dose-rate gradient and volume averaging effect of the detector response and variation of the energy fluence in the lateral direction of the beam. Therefore, experimental measurements of dosimetric parameters such as percent depth doses (PDDs), beam profiles and relative output factors (ROFs) for small fields continue to be a challenge.

Materials and Methods:

In this study, we used a homogeneous water phantom and the heterogeneous anthropomorphic stereotactic end-to-end verification (STEEV) head phantom for all dose measurements and calculations. PDDs, lateral dose profiles and ROFs were calculated in the Eclipse Treatment Planning System version 13·6 using the Acuros XB (AXB) and the analytical anisotropic algorithms (AAAs) in a homogenous water phantom. Monte Carlo (MC) simulations and measurements using the Exradin W1 Scintillator were also accomplished for four photon energies: 6 MV, 6FFF, 10 MV and 10FFF. Two VMAT treatment plans were generated for two different targets: one located in the brain and the other in the neck (close to the trachea) in the head phantom (CIRS, Norfolk, VA, USA). A Varian Truebeam linear accelerator (Varian, Palo Alto, CA, USA) was used for all treatment deliveries. Calculated results with AXB and AAA were compared with MC simulations and measurements.

Results:

The average difference of PDDs between W1 Exradin Scintillator measurements and MC simulations, AAA and AXB algorithm calculations were 1·2, 2·4 and 3·2%, respectively, for all field sizes and energies. AXB and AAA showed differences in ROF of about 0·3 and 2·9%, respectively, compared with W1 Exradin Scintillator measured values. For the target located in the brain in the head phantom, the average dose difference between W1 Exradin Scintillator and the MC simulations, AAA and AXB were 0·2, 3·2 and 2·7%, respectively, for all field sizes. Similarly, for the target located in the neck, the respective dose differences were 3·8, 5·7 and 3·5%.

Conclusion:

In this study, we compared dosimetric parameters such as PDD, beam profile and ROFs in water phantom and isocenter point dose measurements in an anthropomorphic head phantom representing a patient. We observed that measurements using the W1 Exradin scintillator agreed well with MC simulations and can be used efficiently for dosimetric parameters such as PDDs and dose profiles and patient-specific quality assurance measurements for small fields. In both homogenous and heterogeneous media, the AXB algorithm dose prediction agrees well with MC and measurements and was found to be superior to the AAA algorithm.

Pretreatment Dose Verification in Volumetric Modulated Arc Therapy Using Liquid Ionization Chamber

Purpose:

The purpose of the present study was to evaluate the practicability of liquid ionization chamber (LIC) for pretreatment dose verification of the advanced radiotherapy techniques such as volumetric modulated arc therapy (VMAT).

Materials and Methods:

The dosimetric characteristics of LIC such as repeatability, sensitivity, monitor unit linearity, dose rate dependence, angular dependence, voltage-current response, and output factors were investigated in 6 MV therapeutic X-ray beams. The LIC was cross-calibrated against 0.125-cc air-filled thimble ionization chamber. A dedicated dosimetry insert made up of Perspex to incorporate the LIC at proper location in the intensity-modulated radiation therapy thorax phantom was locally fabricated. The collection efficiency and ion recombination correction factor was determined using the two-dose rate method. Pretreatment dose verification measurement of VMAT treatment plans were carried out using the liquid ionization chamber as well as small volume (0.125 cc) air-filled thimble ionization chamber. The measured dose values by the two dosimeters and TPS calculated dose at a given point were compared.

Results:

The relative percentage differences between the TPS calculated and measured doses were within ± 1.57% for LIC and ± 2.21% for 0.125 cc ionization chamber, respectively.

Conclusions:

The measured dose values by the two dosimeters and TPS calculated dose at a given point were found comparable suggesting that the LIC could be a good choice of dosimeter for pretreatment dose verification in VMAT.

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

PURPOSE

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

Recombination in liquid-filled ionization chambers beyond the Boag limit

PURPOSE

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.

METHODS

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.

RESULTS

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.

CONCLUSIONS

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.

Study of the PTW microLion chamber temperature dependence

The use of liquid ionization chambers in radiotherapy has grown during the past few years. While for air ionization chambers the k(TP) correction for air mass density due to pressure and temperature variations is well known, less work has been done on the case of liquid ionization chambers, where there is still the need to take into account the influence of temperature in the free ion yield. We have measured the PTW microLion isooctane-filled ionization chamber temperature dependence in a ~ ±10 °C interval around the standard 20 °C room temperature for three operation voltages, including the manufacturer recommended voltage, and two beam qualities, (60)Co and 50 kV x-rays. Within the measured temperature range, the microLion signal exhibits a positive linear dependence, which is around 0.24% K(-1) at 800 V with (60)Co irradiation. This effect is of the same order of magnitude as the T dependence found in air ionization chambers, but its nature is completely different and its sign opposite to that of an air chamber. Onsager theory has been used to model the results and is consistent with this linear behaviour. However, some inconsistencies in the modelling of the 50 kV x-ray results have been found that are attributed to the failure of Onsager's isolated pair assumption for such radiation quality.