Radiation hardness of open Fabry-Pérot microcavities.

High-finesse microcavities offer a platform for compact, high-precision sensing by employing high-reflectivity, low-loss mirrors to create effective optical path lengths that are orders of magnitude larger than the device geometry. Here, we investigate the radiation hardness of Fabry-Pérot microcavities formed from dielectric mirrors deposited on the tips of optical fibers. The microcavities are irradiated under both conventional (∼ 0.1 Gy/s) and ultrahigh (FLASH, ∼ 20 Gy/s) radiotherapy dose rates. Within our measurement sensitivity of ∼ 40 ppm loss, we observe no degradation in the mirror absorption after irradiation with over 300 Gy accumulated dose. This result highlights the excellent radiation hardness of the dielectric mirrors forming the cavities, enabling new optics-based, real-time, in-vivo, tissue-equivalent radiation dosimeters with ∼ 10 micron spatial resolution (our motivation), as well as other applications in high-radiation environments.

Accurate machine-specific reference and small-field dosimetry for a self-shielded neuro-radiosurgical system.

BACKGROUND: The newly available ZAP-X stereotactic radiosurgical system is designed for the treatment of intracranial lesions, with several unique features that include a self-shielding, gyroscopic gantry, wheel collimation, non-orthogonal kV imaging, short source-axis distance, and low-energy megavoltage beam. Systematic characterization of its radiation as well as other properties is imperative to ensure its safe and effective clinical application. PURPOSE: To accurately determine the radiation output of the ZAP-X with a special focus on the smaller diameter cones and an aim to provide useful recommendations on quantification of small field dosimetry. METHODS: Six different types of detectors were used to measure relative output factors at field sizes ranging from 4 to 25 mm, including the PTW microSilicon and microdiamond diodes, Exradin W2 plastic scintillator, Exradin A16 and A1SL ionization chambers, and the alanine dosimeter. The 25 mm cone served as the reference field size. Absolute dose was determined with both TG-51-based dosimetry using a calibrated PTW Semiflex ion chamber and measurements using alanine dosimeters. RESULTS: The average radiation output factors (maximum deviation from the average) measured with the microDiamond, microSilicon, and W2 detectors were: for the 4 mm cone, 0.741 (1.0%); for the 5 mm cone: 0.817 (1.0%); for the 7.5 mm cone: 0.908 (1.0%); for the 10 mm cone: 0.946 (0.4%); for the 12.5 mm cone: 0.964 (0.2%); for the 15 mm cone: 0.976 (0.1%); for the 20 mm cone: 0.990 (0.1%). For field sizes larger than 10 mm, the A1SL and A16 micro-chambers also yielded consistent output factors within 1.5% of those obtained using the microSilicon, microdiamond, and W2 detectors. The absolute dose measurement obtained with alanine was within 1.2%, consistent with combined uncertainties, compared to the PTW Semiflex chamber for the 25 mm reference cone. CONCLUSION: For field sizes less than 10 mm, the microSilicon diode, microDiamond detector, and W2 scintillator are suitable devices for accurate small field dosimetry of the ZAP-X system. For larger fields, the A1SL and A16 micro-chambers can also be used. Furthermore, alanine dosimetry can be an accurate verification of reference and absolute dose typically measured with ion chambers. Use of multiple suitable detectors and uncertainty analyses were recommended for reliable determination of small field radiation outputs.

A novel calorimeter for synchrotron produced monochromatic x-ray beams.

BACKGROUND: Electron synchrotrons produce x-ray beams with dose rates orders of magnitude greater than conventional x-ray tubes and with beam sizes on the order of a few millimeters. These characteristics put severe challenges on current dosimeters to accurately realize absorbed dose or air kerma. PURPOSE: This work seeks to investigate the suitability of a novel aluminum-based calorimeter to determine absorbed dose to water with an uncertainty significantly smaller than currently possible with conventional detectors. A lower uncertainty in the determination of absolute dose rate would impact both therapeutic applications of synchrotron-produced x-ray beams and research investigations. METHODS: A vacuum-based calorimeter prototype with an aluminum core was built, matching the beam profile of the 140 keV monochromatic x-ray beam, produced by the Canadian Light Source Biomedical Imaging and Therapy beamline. The choice of material and overall calorimeter design was optimized using FEM thermal modeling software while Monte Carlo radiation transport simulations were used to model the impact of interactions of the radiation beam with the detector components. RESULTS: Corrections for both the thermal conduction and radiation transport effects were of the order of 3% and the simplicity of the geometry, combined with the monochromatic nature of the incident x-ray beam, meant that the uncertainty in each correction was ≤0.5%. The calorimeter performance was found to be repeatable over multiple irradiations of 1 Gy at the ± 0.6% level, and no systematic dependence on environmental effects or total dose was observed. CONCLUSION: The combined standard uncertainty in the determination of absorbed dose to aluminum was estimated to be 0.8%, indicating that absorbed dose to water, the ultimate quantity of interest, could be determined with an uncertainty on the order of 1%. This value is an improvement over current techniques used for synchrotron dosimetry and comparable with the state-of-the art for conventional kV x-ray dosimetry.

A First Look at Equity, Diversity, and Inclusion of Canadian Medical Physicists: Results From the 2021 COMP EDI Climate Survey.

PURPOSE: In 2021, the Canadian Organization of Medical Physicists (COMP) conducted its first equity, diversity, and inclusion Climate Survey. The membership's experiences of inclusion, belonging, professional opportunities, discrimination, microaggressions, racism, and harassment in their professional lives are presented. METHODS AND MATERIALS: The ethics-reviewed survey was distributed in English and French to full members of COMP. Participants responded to questions covering demographics and professional climate. Simple descriptive statistics were used to measure frequency of responses. Data pertaining to impressions on the climate within the profession were compared using nonparametric statistical tests. RESULTS: The survey was distributed to 649 eligible members; 243 (37%) responded, and 214 (33%) provided full response sets. From the full response sets, findings showed that in general, age, highest academic degree, and racial and ethnic distribution trends of medical physicists were comparable with previously collected data and/or the Canadian population. The experiences of respondents relating to harassment in the workplace and perception of climate are reported and provide a useful benchmark for future assessments of interventions or training programs. In the workplace, fewer women (58%) reported having professional opportunities compared with men (70%). The survey also found that 17% of respondents (most of whom were women) directly or indirectly experienced sexual harassment in the workplace within the past 5 years. Finding that 23% of survey respondents identified as having a disability is a valuable reminder that accommodations in the workplace are necessary for more than 1 in every 5 medical physicists working in clinics. CONCLUSIONS: This study provided insight into the diversity and experiences of medical physicists in Canada. The majority of respondents had positive perceptions about their professional environment. However, equity-lacking groups were identified, such as women, underrepresented minorities, Indigenous peoples, and people with visible and invisible disabilities.

Uncertainties Associated with Clonogenic Assays using a Cs-137 Irradiator and Ir-192 Afterloader: A Comprehensive Compilation for Radiation Researchers.

Clonogenic assays are the gold standard for measuring cell clonogenic survival and enable quantification of a cell line's radiosensitivity through the calculation of the surviving fraction, the ratio of cell clusters (colonies) formed after radiation exposure compared to the number formed without exposure. Such studies regularly utilize Cs-137 irradiators. While uncertainties for specific procedural aspects have been described previously, a comprehensive review has not been completed. We therefore quantified uncertainties associated with clonogenic assays performed using a Cs-137 Shepherd irradiator, and a recently established brachytherapy afterloader in vitro radiation delivery apparatus (BAIRDA), through a series of experiments and a literature review. The clonogenic assay is subject to uncertainties that affect the determination of the surviving fraction (e.g., accuracy of the number of cells seeded, potential effects of hypothermia, and the threshold number of cells for a cluster to be identified as a colony). Furthermore, dose delivery uncertainties related to both the Cs-137 irradiator and BAIRDA were also quantified. The combined standard (k = 1) uncertainty was ± 6.0% in the surviving fraction for the Cs-137 irradiator (±6.3% for BAIRDA), up to ± 2.2% in the dose delivered by the Cs-137 irradiator, and up to ± 4.3% in the dose delivered by BAIRDA. The largest individual uncertainties were associated with the number of cells seeded on a plate (3.4%) and inter-observer variability in counting (4.1%), suggesting that effective reduction of uncertainties in the conduct of the clonogenic assay may provide the greatest relief on the uncertainty budget. Finally, measurable impact on experimental findings was assessed by applying this uncertainty to clonogenic assays of SW756 cells using either a Cs-137 irradiator or BAIRDA, introducing a maximum shift in the reported radiobiological parameters α/ß and T1/2 of 0.3 Gy and 0.4 h, respectively, while the 95% confidence interval increased by 0.5 Gy and decreased by 0.4 h, respectively. Though the overall impact on radiobiological parameter estimation was small, the individual uncertainties could have a significant influence in other applications of in vitro experiments in radiation biology. Hence, better understanding of the uncertainties associated with both clonogenic assays and the radiation source used can improve the accuracy of experimental analysis and reproducibility of the results.

Calorimeter measurements of absolute dose in aluminum, a surrogate of bone, to validate dose-to-medium in Acuros XB.

Objective. While the accuracy of dose calculations in water with Acuros XB is well established, experimental validation of dose in bone is limited. Acuros XB reports both dose-to-medium and dose-to-water, and these values differ in bone, but there are no reports of measurements of validation in bone. This work compares Acuros XB calculations to measurements of absolute dose in aluminum (medium similar to bone). The validity of using selected relative dosimeters in aluminum is also investigated.Approach. A calorimeter with an aluminum core embedded in an aluminum phantom was selected as bone surrogate for the measurement of absolute dose. Matching the medium of the core to the medium of the phantom allowed eliminating the calculation of the conversion between media. The dose was measured at the fixed depth of 3.3 cm in aluminum (∼9 g·cm-2) with 6X, 10X, 6FFF and 10FFF photon beams from a TrueBeam Varian linac. In addition, experimental cross-calibration between water and aluminum was performed for an IBA CC13 ionization chamber, a PTW microDiamond and EBT3 Gafchromic film.Main results. Calculations with Acuros XB dose-to-medium in aluminum differed from the calorimetry data by -2.8% to -3.5%, depending on the beam. Use of dose-to-water would have resulted in about 39% discrepancy. The cross calibration coefficient between water and aluminum yielded values of about 0.87 for the CC13 chamber, 0.91 for the microDiamond, and 0.88 for the film, and independent of the beam within about ±1%.Significance. It was demonstrated the value of the dose-to-medium in aluminum (surrogate of bone) computed with Acuros XB is close to the value of the absolute dose measured with a calorimeter, and there is a significant discrepancy when dose-to-water is used instead. The use of an ionization chamber, a microDiamond and Gafchromic film in aluminum required a considerable correction from calibration in water.

Extraction of depth-dependent perturbation factors for silicon diodes using a plastic scintillation detector.

PURPOSE: This work presents the experimental extraction of the perturbation factor in megavoltage electron beams for three models of silicon diodes (IBA Dosimetry, EFD and SFD, and the PTW 60012 unshielded) using a plastic scintillation detector (PSD). METHODS: The authors used a single scanning PSD mounted on a high-precision scanning tank to measure depth-dose curves in 6-, 12-, and 18-MeV clinical electron beams. They also measured depth-dose curves using the IBA Dosimetry, EFD and SFD, and the PTW 60012 unshielded diodes. The authors used the depth-dose curves measured with the PSD as a perturbation-free reference to extract the perturbation factors of the diodes. RESULTS: The authors found that the perturbation factors for the diodes increased substantially with depth, especially for low-energy electron beams. The experimental results show the same trend as published Monte Carlo simulation results for the EFD diode; however, the perturbations measured experimentally were greater. They found that using an effective point of measurement (EPOM) placed slightly away from the source reduced the variation of perturbation factors with depth and that the optimal EPOM appears to be energy dependent. CONCLUSIONS: The manufacturer recommended EPOM appears to be incorrect at low electron energy (6 MeV). In addition, the perturbation factors for diodes may be greater than predicted by Monte Carlo simulations.

Measurement of ionization chamber absorbed dose k(Q) factors in megavoltage photon beams.

PURPOSE: Absorbed dose beam quality conversion factors (k(Q) factors) were obtained for 27 different types of ionization chamber. The aim was to obtain objective evidence on the performance of a wide range of chambers currently available, and potentially used for reference dosimetry, and to investigate the accuracy of the k(Q) calculation algorithm used in the TG-51 protocol. METHODS: Measurements were made using the 60Co irradiator and Elekta Precise linac facilities at the National Research Council of Canada. The objective was to characterize the chambers over the range of energies applicable to TG-51 and determine whether each chamber met the requirements of a reference-class instrument. Chamber settling, leakage current, ion recombination and polarity, and waterproofing sleeve effects were investigated, and absorbed dose calibration coefficients were obtained for 60Co and 6, 10, and 25 MV photon beams. Only thimble-type chambers were considered in this investigation and were classified into three groups: (i) Reference chambers ("standard" 0.6 cm3 Farmer-type chambers and their derivatives traditionally used for beam output calibration); (ii) scanning chambers (typically 0.1 cm3 volume chambers used for beam commissioning with 3-D scanning phantoms); and (iii) microchambers (very small volume ion chambers (< or = 0.01 cm3) used for small field dosimetry). RESULTS: As might be expected, 0.6 cm3 thimble chambers showed the most predictable performance and experimental k(Q) factors were obtained with a relative uncertainty of 0.1%. The performance of scanning and microchambers was somewhat variable. Some chambers showed very good behavior but others showed anomalous polarity and recombination corrections that are not fully explained at present. For the well-behaved chambers, agreement between measured and calculated k(Q) factors was within 0.4%; for some chambers, differences of more than 1% were seen that may be related to the recombination/polarity results. Use of such chambers could result in significant errors in the determination of reference dose in the clinic. CONCLUSIONS: Based on the experimental evidence obtained here, specification for a reference-class ionization chamber could be developed that would minimize the error in using a dosimetry protocol with calculated beam quality conversion factors. The experimental k(Q) data obtained here for a wide range of thimble chambers can be used when choosing suitable detectors for reference dosimetry and are intended to be used in the upcoming update/addendum to the AAPM TG-51 dosimetry protocol.

Zero-shift thimble ionization chamber.

PURPOSE: Verify experimentally the theoretical prediction of F. Tessier and I. Kawrakow [Med. Phys. 37, 96-107 (2010)] that it is possible to design a thimble ionization chamber with no shift in its effective point of measurement (EPOM), i.e., a chamber that provides a measure of the dose to the medium at the location of its central axis. METHODS: Measure dose from a 25 MV photon beam incident on water with an Exradin A1SL ionization chamber inside a thin sleeve (as a means of effectively increasing the thimble wall thickness). The depth-dose curve is compared to that obtained using a well-characterized PTW Roos parallel-plate chamber. RESULTS: With an appropriate increase in thimble wall thickness, the EPOM shift of the Exradin A1SL vanishes. Further increase of the wall thickness yields a chamber with a positive (downstream) shift in its point of measurement. CONCLUSIONS: It is possible to design a thimble ionization chamber with a zero EPOM shift by adjusting the wall thickness.

Extraction of depth-dependent perturbation factors for parallel-plate chambers in electron beams using a plastic scintillation detector.

PURPOSE: This work presents the experimental extraction of the overall perturbation factor PQ in megavoltage electron beams for NACP-02 and Roos parallel-plate ionization chambers using a plastic scintillation detector (PSD). METHODS: The authors used a single scanning PSD mounted on a high-precision scanning tank to measure depth-dose curves in 6, 12, and 18 MeV clinical electron beams. The authors also measured depth-dose curves using the NACP-02 and PTW Roos chambers. RESULTS: The authors found that the perturbation factors for the NACP-02 and Roos chambers increased substantially with depth, especially for low-energy electron beams. The experimental results were in good agreement with the results of Monte Carlo simulations reported by other investigators. The authors also found that using an effective point of measurement (EPOM) placed inside the air cavity reduced the variation of perturbation factors with depth and that the optimal EPOM appears to be energy dependent. CONCLUSIONS: A PSD can be used to experimentally extract perturbation factors for ionization chambers. The dosimetry protocol recommendations indicating that the point of measurement be placed on the inside face of the front window appear to be incorrect for parallel-plate chambers and result in errors in the R50 of approximately 0.4 mm at 6 MeV, 1.0 mm at 12 MeV, and 1.2 mm at 18 MeV.

The European Joint Research Project UHDpulse - Metrology for advanced radiotherapy using particle beams with ultra-high pulse dose rates.

UHDpulse - Metrology for advanced radiotherapy using particle beams with ultra-high pulse dose rates is a recently started European Joint Research Project with the aim to develop and improve dosimetry standards for FLASH radiotherapy, very high energy electron (VHEE) radiotherapy and laser-driven medical accelerators. This paper gives a short overview about the current state of developments of radiotherapy with FLASH electrons and protons, very high energy electrons as well as laser-driven particles and the related challenges in dosimetry due to the ultra-high dose rate during the short radiation pulses. We summarize the objectives and plans of the UHDpulse project and present the 16 participating partners.

Treatment head disassembly to improve the accuracy of large electron field simulation.

PURPOSE: The purposes of this study are to improve the accuracy of source and geometry parameters used in the simulation of large electron fields from a clinical linear accelerator and to evaluate improvement in the accuracy of the calculated dose distributions. METHODS: The monitor chamber and scattering foils of a clinical machine not in clinical service were removed for direct measurement of component geometry. Dose distributions were measured at various stages of reassembly, reducing the number of geometry variables in the simulation. The measured spot position and beam angle were found to vary with the beam energy. A magnetic field from the bending magnet was found between the exit window and the secondary collimators of sufficient strength to deflect electrons 1 cm off the beam axis at 100 cm from the exit window. The exit window was 0.05 cm thicker than manufacturer's specification, with over half of the increased thickness due to water pressure in the channel used to cool the window. Dose distributions were calculated with Monte Carlo simulation of the treatment head and water phantom using EGSnrc, a code benchmarked at radiotherapy energies for electron scatter and bremsstrahlung production, both critical to the simulation. The secondary scattering foil and monitor chamber offset from the collimator rotation axis were allowed to vary with the beam energy in the simulation to accommodate the deflection of the beam by the magnetic field, which was not simulated. RESULTS: The energy varied linearly with bending magnet current to within 1.4% from 6.7 to 19.6 MeV, the bending magnet beginning to saturate at the highest beam energy. The range in secondary foil offset used to account for the magnetic field was 0.09 cm crossplane and 0.15 cm inplane, the range in monitor chamber offset was 0.14 cm crossplane and 0.07 cm inplane. A 1.5%/0.09 cm match or better was obtained to measured depth dose curves. Profiles measured at the depth of maximum dose matched the simulated profiles to 2.6% or better at doses of 80% or more of the dose on the central axis. The profiles along the direction of MLC motion agreed to within 0.16 cm at the edge of the field. There remained a mismatch for the lower beam energies at the edge of the profile that ran parallel to the direction of jaw motion of up to 1.4 cm for the 6 MeV beam, attributed to the MLC support block at the periphery of the field left out of the simulation and to beam deflection by the magnetic field. The possibility of using these results to perform accurate simulation without disassembly is discussed. Phase-space files were made available for benchmarking beam models and other purposes. CONCLUSIONS: The match to measured large field dose distributions from clinical electron beams with Monte Carlo simulation was improved with more accurate source details and geometry details closer to manufacturer's specification than previously achieved.

Determination of Wair in high-energy electron beams using graphite detectors.

PURPOSE: ICRU Report 90 on Key Data for Ionizing-Radiation Dosimetry: Measurement Standards and Applications (2014) has reaffirmed the recommended value of the mean energy required to create an ion pair in air, Wair , to be 33.97(12) eV. The report also indicates that this "constant" of radiation dosimetry is energy independent above 10 keV, since there is no theoretical or experimental evidence to the contrary. The goal of this investigation is to obtain additional experimental determinations of Wair in high energy beams and thus to verify the suggested energy independence. METHODS: Wair can be evaluated by combining ionometric and calorimetric measurements with a calculated ratio of the absorbed dose in the ion chamber air cavity and that of the calorimeter absorbing element. In this investigation, a graphite parallel plate chamber and a graphite calorimeter were used and the dose ratio was calculated using the EGSnrc Monte Carlo code. Measurements were made in electron beams from the NRC Vickers linear accelerator at two incident energies, 20 and 35 MeV. A range of average energies at the measurement point were obtained by inserting graphite plates in the primary beam. RESULTS: The average value of Wair obtained in this investigation is 33.85(18) eV which is consistent with the recommended value of 33.97(12) eV where the number in brackets represents the combined standard uncertainty of the value, referring to the corresponding last digits. The individual values of Wair do not show any statistically significant energy dependence. CONCLUSION: The overall combined uncertainty of 0.5% meets the original target of the investigation. A larger-scale investigation, involving more individual energy points and a wider range of electron energies is required to go further and, for example, comment on the Wair energy dependency question raised by Tessier et al. [Med. Phys. 2018;45:370-381].

An experimental and computational investigation of the standard temperature-pressure correction factor for ion chambers in kilovoltage x rays.

For ion chambers with cavities open to the surrounding atmosphere, the response measured at a given temperature and pressure must be corrected using the standard temperature-pressure correction factor (P(TP)). A previous paper based solely on Monte Carlo simulations [D. J. La Russa and D. W. O. Rogers, Med. Phys. 33, 4590-4599 (2006)] pointed out the shortcomings of the P(TP) correction factor when used to correct the response of non-air-equivalent chambers for low-energy x-ray beams. This work presents the results of several experiments that corroborate these calculations for a number of ion chambers. Monte Carlo simulations of the experimental setup revealed additional insight into the various factors affecting the extent of the breakdown of P(TP), including the effect of impurities and the sensitivity to chamber dimensions. For an unfiltered 60 kV beam, the P(TP)-corrected response of an NE 2571 ion chamber measured at 0.7 atm was 2.5% below the response measured at reference conditions. In general, Monte Carlo simulations of the experimental setup using EGSnrc were within 0.5% of measured values. EGSnrc-calculated values of air kerma calibration coefficients (N(K)) at low x-ray energies are also provided as a means of estimating the level of impurities in the chambers investigated. Calculated values of N(K) normalized to the value measured for a 250 kV beam were obtained for three chambers and were within 1% of experiment with one exception, the Exradin A12 in a 50 kV beam.

Technical Note: On the use of cylindrical ionization chambers for electron beam reference dosimetry.

PURPOSE: To investigate the use of cylindrical chambers for electron beam dosimetry independent of energy by studying the variability of relative ion chamber perturbation corrections, one of the main concerns for electron beam dosimetry with cylindrical chambers. METHODS: Measurements are made with sets of cylindrical and plane-parallel reference-class chambers as a function of depth in water in 8 MeV and 18 MeV electron beams. The ratio of chamber readings for similar chambers is normalized in a high-energy electron beam and can be thought of as relative perturbation corrections. Data are plotted as a function of mean electron energy at depth for a range of depths close to the phantom surface to R80 , the depth at which the ionization falls to 80% of its maximum value. Additional, similar measurements are made in a Virtual Water® phantom with cylindrical chambers at the reference depth in a 4 MeV electron beam. RESULTS: The variability of relative ion chamber perturbation corrections for nominally identical cylindrical Farmer-type chambers is found to be less than 0.4%, no worse than plane-parallel chambers with similar specifications. CONCLUSIONS: This work discusses several issues related to the use of plane-parallel ion chambers and suggests that reference-class cylindrical chambers may be appropriate for reference dosimetry of all electron beams. This would simplify the reference dosimetry procedure and improve accuracy of beam calibration.

Examining the influence of humidity on reference ionization chamber performance.

PURPOSE: International dosimetry protocols require measurements made with a vented ionization chamber to be corrected for the influence of air density by using the standard temperature-pressure correction factor. The effect of humidity, on the other hand, is generally ignored with the provision that the relative humidity (RH) is within certain limits (typically 20% to 80%). However, there is little experimental data in the published literature as to the true effect of humidity on modern reference-class ionization chambers. This investigation used two different radiation beams - a Co-60 irradiator and an Sr-90 check source - to examine the effect of humidity on several Farmer-type ionization chambers. METHODS: An environmental cabinet controlled the humidity. For the Co-60 beam, the irradiation was external, whereas for the Sr-90 measurements, the source itself was placed within the cabinet. Extensive measurements were carried out to ensure that the experimental setup provided reproducible readings. Four chamber types were investigated: IBA FC65-G, IBA FC65-P, PTW 30013 and Exradin A19. The different wall materials provided potentially different mechanical responses (i.e., in terms of expansion/contraction) to the water content in the air. The relative humidity was varied between 8% and 98% and measurements were made with both increasing and decreasing humidity to investigate any possible hysteresis effects. RESULTS: Measurements in Co-60 were consistent with the published data obtained with primary standard cavity chambers in ICRU Report 31 (i.e., a very small variation <0.1% for RH >10%). The measurements in the Sr-90 field showed no dependence with the relative humidity, within the measurement uncertainties (0.05%, k = 1). Very good repeatability of the ionization current was obtained over successive wet/dry cycles, no hysteresis was observed, and there was no dependence on chamber type. CONCLUSION: These results indicate that humidity has no significant effect on these particular types of ionization chambers, consistent with recommendations in current megavoltage dosimetry protocols.

Quantitative ionization chamber alignment to a water surface: Performance of multiple chambers.

PURPOSE: The purpose of this study was to experimentally examine the reliability of the gradient chamber alignment point (gCAP) determination method for accurately identifying water surface location with a range of ionization chambers (ICs). MATERIALS AND METHODS: Twelve cylindrical ICs were scanned from depth through a water surface into air using a customized high-accuracy scanning system which allows for accurate alignment of the IC with respect to the true water surface. Thirteen other cylindrical ICs and five parallel-plate ICs were scanned using a standard commercially available scanning system. The thirty different ICs used in this study represent 22 different IC models. Measurements were taken with different radiation field parameters such as incident photon beam energies and field sizes. The effects of scan direction and water surface tension were also investigated. The depth at which the gradient of the relative ionization was maximized and discontinuous, the gCAP, was found for each curve. Each measured gCAP depth was compared with the theoretically expected gCAP location, the depth at which the submerged IC outer radius (OR) coincides with the water surface. RESULTS: When scanning an IC from in water to air, the only parameter that affects the gCAP location is the IC OR. The gCAP location corresponds with the IC central axis positioned at a depth equal to the IC OR within the 0.1 mm measurement scan resolution for all eighteen ICs studied with the commercially available system. Using the customized scanning system, all but three ICs were identified exhibiting a gCAP within the scan resolution, with the other three within 0.25 mm of the expected location. This discrepancy was not observed in the same IC model when using the conventional scanning system. Altering the beam energy from 6 to 25 MV did not alter the gCAP location, nor did variations in the radiation field size or scan parameters. In-air IC response is proportional to the IC wall thickness. CONCLUSION: The water-to-air scanning method coupled with gCAP analysis identifies the alignment of the IC OR to the water surface within the scanning resolution for all ICs studied. The gCAP method can precisely and reproducibly align the physical center of a given cylindrical IC with the water surface, be applied prospectively or retrospectively, and provides the prospect for automated water surface identification for scanning systems. The gCAP method eliminates the visual subjectivity inherent to current IC-to-water surface alignment techniques, has been validated with a wide variety of commercially available ICs, and should be independent of the scanning system used for data acquisition.

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

PURPOSE: 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. METHODS: Measurements are made in electron beams with energies between 8 MeV and 22 MeV 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 18 MeV and 22 MeV beams. An insulated enclosure with fine temperature control is used to maintain a constant temperature (drifts less than 0.1 mK/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. RESULTS: 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 22 MeV 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 10 yr 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. CONCLUSIONS: 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.

An empirical method for the determination of wall perturbation factors for parallel-plate chambers in high-energy electron beams.

The calibration of ion chambers in high-energy electron beams in terms of absorbed dose to water at the National Physical Laboratory requires knowledge of the ratio of perturbation factors in graphite and water phantoms. During a review of data required for the NPL calibration procedure an empirical model was developed to calculate the perturbation due to the rear wall, pwall, of a well-guarded ion chamber in a high-energy electron beam. The overall uncertainty in this method is estimated to be 0.4%, which is the lowest value reported to date. The model reproduces measured data at the 0.1% level or better and indicates that the NACP ion chamber has a nonzero perturbation factor in electron beams due to backscatter from the rear wall. The effect is small (<0.5%) at high energies (R50>4 cm, E0>10 MeV) but becomes large at low energies-up to 1.4% at E0=4 MeV (R50=1.2 cm). The model indicates that there is a nonzero correction for the NACP chamber in both a graphite and water phantom and that material adjacent to the air cavity has a significant effect on the measured ionization. These values are consistent with previous measurements and recent Monte Carlo calculations. The model could be used in the design of ion chambers and in the estimation of corrections for non-homogeneous systems, especially in the absence of accurate Monte Carlo simulations.