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Background: Reference dosimetry in ultra‐high dose rate (UHDR) beamlines is significantly hindered by limitations in conventional ionization chamber design. In particular, conventional chambers suffer from severe charge collection efficiency (CCE) degradation in high dose per pulse (DPP) beams. Purpose: The aim of this study was to optimize the design and performance of parallel plate ion chambers for use in UHDR dosimetry applications, and evaluate their potential as reference class chambers for calibration purposes. Three chamber designs were produced to determine the influence of the ion chamber response on electrode separation, field strength, and collection volume on the ion chamber response under UHDR and ultra‐high dose per pulse (UHDPP) conditions. Methods: Three chambers were designed and produced: the A11‐VAR (0.2–1.0 mm electrode gap, 20 mm diameter collector), the A11‐TPP (0.3 mm electrode gap, 20 mm diameter collector), and the A30 (0.3 mm electrode gap, 5.4 mm diameter collector). The chambers underwent full characterization using an UHDR 9 MeV electron beam with individually varied beam parameters of pulse repetition frequency (PRF, 10–120 Hz), pulse width (PW, 0.5–4 µs), and pulse amplitude (0.01–9 Gy/pulse). The response of the ion chambers was evaluated as a function of the DPP, PRF, PW, dose rate, electric field strength, and electrode gap. Results: The chamber response was found to be dependent on DPP and PW, and these dependencies were mitigated with larger electric field strengths and smaller electrode spacing. At a constant electric field strength, we measured a larger CCE as a function of DPP for ion chambers with a smaller electrode gap in the A11‐VAR. For ion chambers with identical electrode gap (A11‐TPP and A30), higher electric field strengths were found to yield better CCE at higher DPP. A PW dependence was observed at low electric field strengths (500 V/mm) for DPP values ranging from 1 to 5 Gy at PWs ranging from 0.5 to 4 µs, but at electric field strengths of 1000 V/mm and higher, these effects become negligible. Conclusion: This study confirmed that the CCE of ion chambers depends strongly on the electrode spacing and the electric field strength, and also on the DPP and the PW of the UHDR beam. A significant finding of this study is that although chamber performance does depend on PW, the effect on the CCE becomes negligible with reduced electrode spacing and increased electric field. A CCE of ≥95% was achieved for DPPs of up to 5 Gy with no observable dependence on PW using the A30 chamber, while still achieving an acceptable performance in conventional dose rate beams, opening up the possibility for this type of chamber to be used as a reference class chamber for calibration purposes of electron FLASH beamlines. [ABSTRACT FROM AUTHOR]

Ion chambers are required for calibration and reference dosimetry applications in radiation therapy (RT). However, exposure of ion chambers in ultra‐high dose rate (UHDR) conditions pertinent to FLASH‐RT leads to severe saturation and ion recombination, which limits their performance and usability. The purpose of this study was to comprehensively evaluate a set of commonly used commercially available ion chambers in RT, all with different design characteristics, and use this information to produce a prototype ion chamber with improved performance in UHDR conditions as a first step toward ion chambers specific for FLASH‐RT. The Advanced Markus and Exradin A10, A26, and A20 ion chambers were evaluated. The chambers were placed in a water tank, at a depth of 2 cm, and exposed to an UHDR electron beam at different pulse repetition frequency (PRF), pulse width (PW), and pulse amplitude settings on an IntraOp Mobetron. Ion chamber responses were investigated for the various beam parameter settings to isolate their dependence on integrated dose, mean dose rate and instantaneous dose rate, dose‐per‐pulse (DPP), and their design features such as chamber type, bias voltage, and collection volume. Furthermore, a thin parallel‐plate (TPP) prototype ion chamber with reduced collector plate separation and volume was constructed and equally evaluated as the other chambers. The charge collection efficiency of the investigated ion chambers decreased with increasing DPP, whereas the mean dose rate did not affect the response of the chambers (± 1%). The dependence of the chamber response on DPP was found to be solely related to the total dose within the pulse, and not on mean dose rate, PW, or instantaneous dose rate within the ranges investigated. The polarity correction factor (Ppol) values of the TPP prototype, A10, and Advanced Markus chambers were found to be independent of DPP and dose rate (± 2%), while the A20 and A26 chambers yielded significantly larger variations and dependencies under the same conditions. Ion chamber performance evaluated under different irradiation conditions of an UHDR electron beam revealed a strong dependence on DPP and a negligible dependence on the mean and instantaneous dose rates. These results suggest that modifications to ion chambers design to improve their usability in UHDR beamlines should focus on minimizing DPP effects, with emphasis on optimizing the electric field strength, through the construction of smaller electrode separation and larger bias voltages. This was confirmed through the production and evaluation of a prototype ion chamber specifically designed with these characteristics. [ABSTRACT FROM AUTHOR]

Purpose: The purpose of this study is to perform an extensive investigation of an approximately 2.5 mm long 90Sr/90Y source designed for treating wet age-related macular degeneration. Methods: As part of this investigation, a NIST-traceable absorbed dose to water calibration technique was established, and a source deployment verification test was developed. The influence of treatment cannula construction tolerance on the measurements as well as the dose delivered to the patient was investigated using the Monte Carlo code MCNP5. Variation between production cannulae was quantified experimentally using a well-type ionization chamber, and additional measurements along with Monte Carlo calculations of the collimating insert used for source deployment verification were performed to validate the model. Results: Maximum variation in the integrated target dose was seen when the source was shifted laterally within the treatment cannula. For the well chamber measurements, the observed standard deviation in ionization current for a single source placed in different reference cannulae was ±0.3%, with a maximum observed range of less than ±0.5%. Clinical cannulae in the collimating insert showed an average of 17.8%±0.4% of the reference signal when sources were fully deployed compared to 18.5% predicted by Monte Carlo calculations. This discrepancy has been attributed primarily to construction of the collimator since the collimation gap was observed to be approximately 0.025–0.075 mm smaller than specified. Construction tolerance of the well chamber insert as well as position tolerance of the cannula tip were both investigated, and their influence on the predicted signal was quantified. Additional measurements along with Monte Carlo based calculations of the collimating insert with polyethylene spacers added to the setup were performed to validate the Monte Carlo model. The shimmed Monte Carlo and measured data agree to within 1%, which is a magnitude difference of approximately 0.1% of the reference signal. Conclusions: This investigation confirms that the signal for an acceptably deployed source in the collimating insert is between 17.5% and 21.5% of the reference signal, as calculated using Monte Carlo models. Clinical cannulae for which the source deployment verification measurement falls outside the acceptable range should not be used to treat patients. [ABSTRACT FROM AUTHOR]

A correction to the article "Experimental Determination of the Radial Dose Function of Sr/90Y IVBT Sources" that was published in the January 30, 2007 issue is presented.

Purpose: The purpose of this study is to perform an extensive investigation of an approximately 2.5 mm long 90Sr/90Y source designed for treating wet age-related macular degeneration., Methods: As part of this investigation, a NIST-traceable absorbed dose to water calibration technique was established, and a source deployment verification test was developed. The influence of treatment cannula construction tolerance on the measurements as well as the dose delivered to the patient was investigated using the Monte Carlo code MCNP5. Variation between production cannulae was quantified experimentally using a well-type ionization chamber, and additional measurements along with Monte Carlo calculations of the collimating insert used for source deployment verification were performed to validate the model., Results: Maximum variation in the integrated target dose was seen when the source was shifted laterally within the treatment cannula. For the well chamber measurements, the observed standard deviation in ionization current for a single source placed in different reference cannulae was +/-0.3%, with a maximum observed range of less than +/-0.5%. Clinical cannulae in the collimating insert showed an average of 17.8% +/-0.4% of the reference signal when sources were fully deployed compared to 18.5% predicted by Monte Carlo calculations. This discrepancy has been attributed primarily to construction of the collimator since the collimation gap was observed to be approximately 0.025-0.075 mm smaller than specified. Construction tolerance of the well chamber insert as well as position tolerance of the cannula tip were both investigated, and their influence on the predicted signal was quantified. Additional measurements along with Monte Carlo based calculations of the collimating insert with polyethylene spacers added to the setup were performed to validate the Monte Carlo model. The shimmed Monte Carlo and measured data agree to within 1%, which is a magnitude difference of approximately 0.1% of the reference signal., Conclusions: This investigation confirms that the signal for an acceptably deployed source in the collimating insert is between 17.5% and 21.5% of the reference signal, as calculated using Monte Carlo models. Clinical cannulae for which the source deployment verification measurement falls outside the acceptable range should not be used to treat patients.

A series of measurements were undertaken using both high sensitivity radiochromic film and new lithium fluoride thermoluminescent dosimeters in a liquid water medium to define the radial dose function of 90Sr/90Y beta emitting intravascular brachytherapy sources more accurately. These measurements of a single 5 French source pellet served to verify current Monte Carlo transport models and extrapolation chamber measurements of the radial dose function, thus providing the recommended independent published measurements for g(r) of these sources. A slight deviation in the published radial dose function at depth leads the authors to recommend that treatment planning be performed using updated g(r) values from current Monte Carlo transport models verified by measurements such as those shown in this investigation.