B. R. Muir

Monte Carlo calculations of kQ, the beam quality conversion factor

PURPOSEnTo use EGSnrc Monte Carlo simulations to directly calculate beam quality conversion factors,kQ, for 32 cylindrical ionization chambers over a range of beam qualities and to quantify the effect of systematic uncertainties on Monte Carlo calculations of kQ. These factors are required to use the TG-51 or TRS-398 clinical dosimetry protocols for calibrating external radiotherapy beams.nnnMETHODSnIonization chambers are modeled either from blueprints or manufacturers users manuals. The dose-to-air in the chamber is calculated using the EGSnrc user-code egs_chamber using 11 different tabulated clinical photon spectra for the incident beams. The dose to a small volume of water is also calculated in the absence of the chamber at the midpoint of the chamber on its central axis. Using a simple equation,kQ is calculated from these quantities under the assumption that W/e is constant with energy and compared to TG-51 protocol and measured values.nnnRESULTSnPolynomial fits to the Monte Carlo calculatedkQ factors as a function of beam quality expressed as %dd(10)x and TPR1020 are given for each ionization chamber. Differences are explained between Monte Carlo calculated values and values from the TG-51 protocol or calculated using the computer program used for TG-51 calculations. Systematic uncertainties in calculated kQ values are analyzed and amount to a maximum of one standard deviation uncertainty of 0.99% if one assumes that photon cross-section uncertainties are uncorrelated and 0.63% if they are assumed correlated. The largest components of the uncertainty are the constancy of W/e and the uncertainty in the cross-section for photons in water.nnnCONCLUSIONSnIt is now possible to calculatekQ directly using Monte Carlo simulations. Monte Carlo calculations for most ionization chambers give results which are comparable to TG-51 values. Discrepancies can be explained using individual Monte Carlo calculations of various correction factors which are more accurate than previously used values. For small ionization chambers with central electrodes composed of high-Z materials, the effect of the central electrode is much larger than that for the aluminum electrodes in Farmer chambers.

Measured and Monte Carlo calculated K(Q) factors: accuracy and comparison.

PURPOSEnThe journal Medical Physics recently published two papers that determine beam quality conversion factors, kQ , for large sets of ion chambers. In the first paper [McEwen Med. Phys. 37, 2179-2193 (2010)], kQ was determined experimentally, while the second paper [Muir and Rogers Med. Phys. 37, 5939-5950 (2010)] provides kQ factors calculated using Monte Carlo simulations. This work investigates a variety of additional consistency checks to verify the accuracy of the kQ factors determined in each publication and a comparison of the two data sets. Uncertainty introduced in calculated kQ factors by possible variation of W/e with beam energy is investigated further.nnnMETHODSnThe validity of the experimental set of kQ factors relies on the accuracy of the NE2571 reference chamber measurements to which kQ factors for all other ion chambers are correlated. The stability of NE2571 absorbed dose to water calibration coefficients is determined and comparison to other experimental kQ factors is analyzed. Reliability of Monte Carlo calculated kQ factors is assessed through comparison to other publications that provide Monte Carlo calculations of kQ as well as an analysis of the sleeve effect, the effect of cavity length and self-consistencies between graphite-walled Farmer-chambers. Comparison between the two data sets is given in terms of the percent difference between the kQ factors presented in both publications.nnnRESULTSnMonitoring of the absorbed dose calibration coefficients for the NE2571 chambers over a period of more than 15 yrs exhibit consistency at a level better than 0.1%. Agreement of the NE2571 kQ factors with a quadratic fit to all other experimental data from standards labs for the same chamber is observed within 0.3%. Monte Carlo calculated kQ factors are in good agreement with most other Monte Carlo calculated kQ factors. Expected results are observed for the sleeve effect and the effect of cavity length on kQ . The mean percent differences between experimental and Monte Carlo calculated kQ factors are -0.08, -0.07, and -0.23% for the Elekta 6, 10, and 25 MV nominal beam energies, respectively. An upper limit on the variation of W/e in photon beams from cobalt-60 to 25 MV is determined as 0.4% with 95% confidence. The combined uncertainty on Monte Carlo calculated kQ factors is reassessed and amounts to between 0.40 and 0.49% depending on the wall material of the chamber.nnnCONCLUSIONSnExcellent agreement (mean percent difference of only 0.13% for the entire data set) between experimental and calculated kQ factors is observed. For some chambers, kQ is measured for only one chamber of each type-the level of agreement observed in this study would suggest that for those chambers the measured kQ values are generally representative of the chamber type.

Beam quality conversion factors for parallel-plate ionization chambers in MV photon beams

PURPOSEnTo investigate the behavior of plane-parallel ion chambers in high-energy photon beams through measurements and Monte Carlo simulations.nnnMETHODSnTen plane-parallel ion chamber types were obtained from the major ion chamber manufacturers. Absorbed dose-to-water calibration coefficients are measured for these chambers and k(Q) factors are determined. In the process, the behaviors of the chambers are characterized through measurements of leakage currents, chamber settling in cobalt-60, polarity and ion recombination behavior, and long-term stability. Monte Carlo calculations of the absorbed dose to the air in the ion chamber and absorbed dose to water are obtained to calculate k(Q) factors. Systematic uncertainties in Monte Carlo calculated k(Q) factors are investigated by varying material properties and chamber dimensions.nnnRESULTSnChamber behavior was variable in MV photon beams, especially with regard to chamber leakage and ion recombination. The plane-parallel chambers did not perform as well as cylindrical chambers. Significant differences up to 1.5% were observed in calibration coefficients after a period of eight months although k(Q) factors were consistent on average within 0.17%. Chamber-to-chamber variations in k(Q) factors for chambers of the same type were at the 0.2% level. Systematic uncertainties in Monte Carlo calculated k(Q) factors ranged between 0.34% and 0.50% depending on the chamber type. Average percent differences between measured and calculated k(Q) factors wereu2009-u20090.02%, 0.18%, andu2009-u20090.16% for 6, 10, and 25 MV beams, respectively.nnnCONCLUSIONSnExcellent agreement is observed on average at the 0.2% level between measured and Monte Carlo calculated k(Q) factors. Measurements indicate that the behavior of these chambers is not adequate for their use for reference dosimetry of high-energy photon beams without a more extensive QA program than currently used for cylindrical reference-class ion chambers.

The central electrode correction factor for high‐Z electrodes in small ionization chambers

PURPOSEnRecent Monte Carlo calculations of beam quality conversion factors for ion chambers that use high-Z electrodes [B. R. Muir and D. W. O. Rogers, Med. Phys. 37, 5939-5950 (2010)] have shown large deviations of kQ values from values calculated using the same techniques as the TG-51 and TRS-398 protocols. This report investigates the central electrode correction factor, Pcel, for these chambers.nnnMETHODSnIonization chambers are modeled and Pcel is calculated using the EGSnrc user code egs_chamber for three cases: in photon and electron beams under reference conditions; as a function of distance from an iridium-192 point source in a water phantom; and as a function of depth in a water phantom on which a 200 kVp x-ray source or 6 MV beam is incident.nnnRESULTSnIn photon beams, differences of up to 3% between Pcel calculations for a chamber with a high-Z electrode and those used by TG-51 for a 1 mm diameter aluminum electrode are observed. The central electrode correction factor for a given value of the beam quality specifier is different depending on the amount of filtration of the photon beam. However, in an unfiltered 6 MV beam, Pcel, varies by only 0.3% for a chamber with a high-Z electrode as the depth is varied from 1 to 20 cm in water. The difference between Pcel calculations for chambers with high-Z electrodes and TG-51 values for a chamber with an aluminum electrode is up to 0.45% in electron beams. The central electrode correction, which is roughly proportional to the chambers absorbed dose sensitivity, is found to be large and variable as a function of distance for chambers with high-Z and aluminum electrodes in low-energy photon fields.nnnCONCLUSIONSnIn this work, ionization chambers that employ high-Z electrodes have been shown to be problematic in various situations. For beam quality conversion factors, the ratio of Pcel in a beam quality Q to that in a Co-60 beam is required; for some chambers, kQ is significantly different from current dosimetry protocol values because of central electrode effects. It would be best for manufacturers to avoid producing ion chambers that use high-Z electrodes.

Monte Carlo calculations for reference dosimetry of electron beams with the PTW Roos and NE2571 ion chambers

PURPOSEnTo investigate recommendations for reference dosimetry of electron beams and gradient effects for the NE2571 chamber and to provide beam quality conversion factors using Monte Carlo simulations of the PTW Roos and NE2571 ion chambers.nnnMETHODSnThe EGSnrc code system is used to calculate the absorbed dose-to-water and the dose to the gas in fully modeled ion chambers as a function of depth in water. Electron beams are modeled using realistic accelerator simulations as well as beams modeled as collimated point sources from realistic electron beam spectra or monoenergetic electrons. Beam quality conversion factors are calculated with ratios of the doses to water and to the air in the ion chamber in electron beams and a cobalt-60 reference field. The overall ion chamber correction factor is studied using calculations of water-to-air stopping power ratios.nnnRESULTSnThe use of an effective point of measurement shift of 1.55 mm from the front face of the PTW Roos chamber, which places the point of measurement inside the chamber cavity, minimizes the difference between R50, the beam quality specifier, calculated from chamber simulations compared to that obtained using depth-dose calculations in water. A similar shift minimizes the variation of the overall ion chamber correction factor with depth to the practical range and reduces the root-mean-square deviation of a fit to calculated beam quality conversion factors at the reference depth as a function of R50. Similarly, an upstream shift of 0.34 rcav allows a more accurate determination of R50 from NE2571 chamber calculations and reduces the variation of the overall ion chamber correction factor with depth. The determination of the gradient correction using a shift of 0.22 rcav optimizes the root-mean-square deviation of a fit to calculated beam quality conversion factors if all beams investigated are considered. However, if only clinical beams are considered, a good fit to results for beam quality conversion factors is obtained without explicitly correcting for gradient effects. The inadequacy of R50 to uniquely specify beam quality for the accurate selection of kQ factors is discussed. Systematic uncertainties in beam quality conversion factors are analyzed for the NE2571 chamber and amount to between 0.4% and 1.2% depending on assumptions used.nnnCONCLUSIONSnThe calculated beam quality conversion factors for the PTW Roos chamber obtained here are in good agreement with literature data. These results characterize the use of an NE2571 ion chamber for reference dosimetry of electron beams even in low-energy beams.

Sci—Thur PM: YIS — 07: Monte Carlo simulations to obtain several parameters required for electron beam dosimetry

When current dosimetry protocols were written, electron beam data were limited and had uncertainties that were unacceptable for reference dosimetry. Protocols for high-energy reference dosimetry are currently being updated leading to considerable interest in accurate electron beam data. To this end, Monte Carlo simulations using the EGSnrc user-code egs_chamber are performed to extract relevant data for reference beam dosimetry. Calculations of the absorbed dose to water and the absorbed dose to the gas in realistic ion chamber models are performed as a function of depth in water for cobalt-60 and high-energy electron beams between 4 and 22 MeV. These calculations are used to extract several of the parameters required for electron beam dosimetry - the beam quality specifier, R50 , beam quality conversion factors, kQ and kR50 , the electron quality conversion factor, kR50 , the photon-electron conversion factor, kecal , and ion chamber perturbation factors, PQ . The method used has the advantage that many important parameters can be extracted as a function of depth instead of determination at only the reference depth as has typically been done. Results obtained here are in good agreement with measured and other calculated results. The photon-electron conversion factors obtained for a Farmer-type NE2571 and plane-parallel PTW Roos, IBA NACP-02 and Exradin A11 chambers are 0.903, 0.896, 0.894 and 0.906, respectively. These typically differ by less than 0.7% from the contentious TG-51 values but have much smaller systematic uncertainties. These results are valuable for reference dosimetry of high-energy electron beams.

TU‐A‐BRB‐11: A New Method for the Measurement of Reference Dosimetric Quantities in Electron Beams

Purpose: To present and confirm the accuracy of a new method to extract data for reference dosimetry in electron beams from depth‐ionization measurements. Methods: Depth‐ionization curves are measured in water using NRC reference ion chambers and several plane‐parallel chambers in 4 to 18 MeV electron beams from the Elekta Precise clinical linear accelerator. To extract useful information from depth‐ionization results, the accuracy of setting the chamber position must be established. For this purpose, differences in the depth at which the chambers ionization reading falls to 50% of its maximum value are used to derive EPOM values relative to those of reference chambers which have been previously established (Lacroix et al., Med. Phys. 37 (2010) 4331). After validation of positioning accuracy, relative ion chamber perturbation factors are determined using the ratio of readings to those of reference ion chambers as a function of mean electron energy, which changes smoothly with depth.Results:Repeatable results for EPOM at the 0.1 mm level indicate little uncertainty from positioning. Different chambers of the same type give EPOM results that differ by less than 0.2 mm, suggesting insignificant chamber‐to‐chamber variability. Values of EPOM at I50 determined for 4 to 18 MeV beams do not exhibit significant variation with beam energy. For well‐behaved chambers, relative perturbation factors at the same mean electron energy are consistent regardless of the incident beam energy. Relative perturbation factors for the same chambers repeated using a different reference chamber after several days are unchanged within 0.3%. Relative perturbation factors for the NACP and Roos chambers agree with published data. Conclusions: These measurements show promising results for the determination of reference dosimetry data from depth‐ionization curves. Ultimately, these relative measurements will be compared against the NRC primary standard calorimeter to get absolute perturbation factors that will prove valuable for updated dosimetry protocols.

TU‐D‐BRB‐06: A Monte‐Carlo Investigation of the Beam Quality Conversion Factor

Purpose: Clinical reference dosimetry based on absorbed dose standards requires the use of a beam quality conversion factor, kQ. This parameter is ionization chamber‐specific and varies with beam quality. Current dosimetry protocols provide values of kQ for commonly used chambers (AAPM TG‐51 (1999), IAEA TRS‐398 (2000)). However, additional chambers are being used clinically for which values are not available. The aim is to obtain values of kQ for various ionization chambers over a range of photon beam quality using Monte Carle (MC) simulations. There is particular interest in obtaining values for those chambers which cannot be calculated using the TG‐51 approach because of missing data, specifically for chambers with a relatively small collecting volume. Method and Materials: Direct MC calculations were made of the absorbed dose to water and the absorbed dose to gas in an ionization chamber at the reference depth in a water phantom. The simulations were performed using the EGSnrc user‐codes cavity and egs_chamber to model the ionization chambers using the extended geometry package. The source input used a collimated point source from tabulated spectra for Cobalt‐60 beams and various clinical linear accelerators. The chamber geometries were modeled using specifications from the various manufacturers manuals. Values for seven of the eleven chambers that were simulated in this study are available in TG‐51. Results: The calculations of values which are available in TG‐51 had a maximum difference of 0.4% from the TG‐51 values. The maximum relative uncertainty in these calculations was 0.2%. In cases where experimental results were available the calculations were compared with measurements. Values were obtained for the chambers not provided in TG‐51. Conclusion: Direct calculations of kQ show agreement with those provided in TG‐51. The simulations are being extended to cylindrical chambers in electron beams and to plane‐parallel chambers in photon beams.

SU‐C‐105‐05: Reference Dosimetry of High‐Energy Electron Beams with a Farmer‐Type Ionization Chamber

PURPOSEnTo investigate gradient effects and provide Monte Carlo calculated beam quality conversion factors to characterize the Farmer-type NE2571 ion chamber for high-energy reference dosimetry of clinical electron beams.nnnMETHODSnThe EGSnrc code system is used to calculate the absorbed dose to water and to the gas in a fully modeled NE2571 chamber as a function of depth in a water phantom. Electron beams incident on the surface of the phantom are modeled using realistic BEAMnrc accelerator simulations and electron beam spectra. Beam quality conversion factors are determined using calculated doses to water and to air in the chamber in high-energy electron beams and in a cobalt-60 reference field. Calculated water-to-air stopping power ratios are employed for investigation of the overall ion chamber perturbation factor.nnnRESULTSnAn upstream shift of 0.3-0.4 multiplied by the chamber radius, r_cav, both minimizes the variation of the overall ion chamber perturbation factor with depth and reduces the difference between the beam quality specifier (R50 ) calculated using ion chamber simulations and that obtained with simulations of dose-to-water in the phantom. Beam quality conversion factors are obtained at the reference depth and gradient effects are optimized using a shift of 0.2r_cav. The photon-electron conversion factor, k_ecal, amounts to 0.906 when gradient effects are minimized using the shift established here and 0.903 if no shift of the data is used. Systematic uncertainties in beam quality conversion factors are investigated and amount to between 0.4 to 1.1% depending on assumptions used.nnnCONCLUSIONnThe calculations obtained in this work characterize the use of an NE2571 ion chamber for reference dosimetry of high-energy electron beams. These results will be useful as the AAPM continues to review their reference dosimetry protocols.

SU‐E‐T‐103: Making Plane‐Parallel Ionization Chambers Available for Reference Dosimetry of High‐Energy Photon Beams

Purpose: Plane‐parallel ionization chambers were not included for reference dosimetry of photon beams in TG‐51 because sufficient data was not available. This work aims to experimentally determine absorbed dose beam quality conversion factors, kQ, for several plane‐parallel ionization chambers in clinical megavoltage photon beams to potentially enable their use for reference dosimetry of high‐energy photon beams and determine if variation in kQ factors for chambers of the same type is considerable. Methods: Measurements traceable to the national standard of absorbed dose were carried out at the National Research Council of Canada with 11 different types of plane‐parallel ion chamber. In most cases, two chambers of each type were obtained from the major ion chamber manufacturers. Chamber stabilization, leakage currents, ion recombination and polarity behavior, as well as chamber‐to‐chamber variations were investigated. Absorbed dose‐to‐water calibration coefficients were measured for the cobalt‐60 irradiator and in 6, 10 and 25 MV photon beams from the Elekta Precise linear accelerator to obtain kQ factors.Results: All chambers investigated gave stable readings within 15 minutes in the cobalt‐60 reference field.Leakage currents were negligible (< 0.05% of the chamber reading) for all chambers. Ion recombination corrections were obtained as a function of dose per pulse; the majority of chambers performed as expected. Polarity corrections were less than 0.1% for most chambers. Values of kQ for chambers of the same type differed by less than 0.4% suggesting that chamber‐to‐chamber variation is not a significant issue Conclusions: Although there are differences between kQ factors determined for chambers of the same type, it is generally not significant. Measurements demonstrate the ability to characterize and use plane‐parallel ion chambers for reference dosimetry of MV photon beams using the TG‐51 protocol. The uncertainty in calibrating a plane‐parallel ion chamber is not significantly greater than for a cylindrical chamber.