Measurement of ionization chamber absorbed dose  factors in megavoltage photon beams

McEwen, M

doi:10.1118/1.3375895

Cited by 126 publications

(195 citation statements)

References 36 publications

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“…The results in this study are supported by recent work by McEwen, (16) who reported recombination factors for different chambers as a function of dose per pulse. McEwen's data for an Exradin A‐12 chamber are shown in Fig.…”

Section: Discussionsupporting

confidence: 90%

Ion recombination correction factors () for Varian TrueBeam high‐dose‐rate therapy beams

Kry

Popple

Molineu

et al. 2012

J Applied Clin Med Phys

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Ion recombination is approximately corrected for in the Task Group 51 protocol by Pion, which is calculated by a two‐voltage measurement. This measurement approach may be a poor estimate of the true recombination, particularly if Pion is large (greater than 1.05). Concern exists that Pion in high‐dose‐per‐pulse beams, such as flattening filter free (FFF) beams, may be unacceptably high, rendering the two‐voltage measurement technique inappropriate. Therefore, Pion was measured for flattened beams of 6, 10, 15, and 18 MV and for FFF beams of 6 and 10 MV. The values for the FFF beams were verified with 1/V versus 1/Q curves (Jaffé plots). Pion was also measured for electron beams of 6, 12, 16, 18, and 20 MeV on a traditional accelerator, as well as on the high‐dose‐rate Varian TrueBeam accelerator. The measurements were made at a range of depths and with PTW, NEL, and Exradin Farmer‐type chambers. Consistent with the increased dose per pulse, Pion was higher for FFF beams than for flattening filter beams. However, for all beams, measurement locations, and chambers examined, Pion never exceeded 1.018. Additionally, Pion was always within 0.3% of the recombination calculated from the Jaffé plots. We conclude that ion recombination can be adequately accounted for in high‐dose‐rate FFF beams using Pion determined with the standard two‐voltage technique.PACS numbers: 87.56.‐v, 87.56.Da

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Section: Discussionsupporting

confidence: 90%

Ion recombination correction factors () for Varian TrueBeam high‐dose‐rate therapy beams

Kry

Popple

Molineu

et al. 2012

J Applied Clin Med Phys

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“…32 However, after a 1-kGy warm-up, A26 IC readings showed a 0.2 and 0.1% relative standard deviation for 3 3 3-and 10 3 10-cm 2 fields, respectively. W1 PSD measurements showed a relative standard deviation of 0.1% for both 3 3 3-and 10 3 10-cm 2 fields.…”

Section: Detector Physical Characterizationmentioning

confidence: 99%

Dosimetric characterization and behaviour in small X-ray fields of a microchamber and a plastic scintillator detector

Pasquino

Cutaia

Radici

et al. 2017

BJR

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Objective: The aim of this work was to investigate the main dosimetric characteristics and the performance of an A26 Exradin ionization microchamber (A26 IC) and a W1 Exradin plastic scintillation detector (W1 PSD) in small photon beam dosimetry for treatment planning system commissioning and quality assurance programme. Methods: Detector characterization measurements (short-term stability, dose linearity, angular dependence and energy dependence) were performed in water for field sizes up to 10 3 10 cm 2 . Polarity effect (P pol ) was examined for the A26 IC. The behaviour of the detectors in small field relative dosimetry [percentage depth dose, dose profiles often called the off-axis ratio and output factors (OFs)] was investigated for field sizes ranging from 1 3 1 to 3 3 3 cm 2 . Results: Results were compared with those obtained with other detectors we already use for small photon beam dosimetry. A26 IC and W1 PSD showed a linear dose response. While the A26 IC showed no energy dependence, the W1 PSD showed energy dependence within 2%; no angular dependence was registered. P pol values for A26 IC were below 0.9% (0.5% for field size .2 3 2 cm 2 ). A26 IC and W1 PSD depth-dose curves and lateral profiles agreed with those obtained with an EDGE diode. No differences were observed among the detectors in OF measurement for field sizes larger than 1 3 1 cm 2 , with average differences ,1%. For field sizes ,1 3 1 cm 2 , the effective volume of ionization chamber and non-water equivalence of EDGE diode become significant. A26 IC OF values were significantly lower than EDGE diode and W1 PSD values, with percentage differences of about 223 and 213% for the smallest field, respectively. W1 PSD OF values lay between ion chambers and diode values, with a maximum percentage difference of about 210% with respect to the EDGE diode, for a 6 3 6-mm 2 field size. Conclusion: The results of our investigation confirm that A26 IC and W1 PSD could play an important role in small field relative dosimetry. Advances in knowledge: Dosimetric characteristics of Exradin A26 ionization microchamber and W1 plastic scintillation detector for small field dosimetry.

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“…Although the data obtained here suggest long-term stability of certain chamber types, there are other reasons (e.g., issues with chamber settling to a stable reading or ion recombination behavior as pointed out by McEwen 14 and others) for rejecting the use of these chambers as reference-class detectors. In addition, ion chamber types that do not meet the 0.3% threshold selected as an indicator of stability may still be appropriate for other purposes (e.g., relative dosimetry).…”

Section: Manufacturermentioning

confidence: 76%

Ion chamber absorbed dose calibration coefficients, N_D,w, measured at ADCLs: Distribution analysis and stability

Muir

2015

Medical Physics

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Purpose: To analyze absorbed dose calibration coefficients, N D, w , measured at accredited dosimetry calibration laboratories (ADCLs) for client ionization chambers to study (i) variability among N D, w coefficients for chambers of the same type calibrated at each ADCL to investigate ion chamber volume fluctuations and chamber manufacturing tolerances; (ii) equivalency of ion chamber calibration coefficients measured at different ADCLs by intercomparing N D, w coefficients for chambers of the same type; and (iii) the long-term stability of N D, w coefficients for different chamber types by investigating repeated chamber calibrations. Methods: Large samples of N D, w coefficients for several chamber types measured over the time period between 1998 and 2014 were obtained from the three ADCLs operating in the United States. These are analyzed using various graphical and numerical statistical tests for the four chamber types with the largest samples of calibration coefficients to investigate (i) and (ii) above. Ratios of calibration coefficients for the same chamber, typically obtained two years apart, are calculated to investigate (iii) above and chambers with standard deviations of old/new ratios less than 0.3% meet stability requirements for accurate reference dosimetry recommended in dosimetry protocols. Results: It is found that N D, w coefficients for a given chamber type compared among different ADCLs may arise from differing probability distributions potentially due to slight differences in calibration procedures and/or the transfer of the primary standard. However, average N D, w coefficients from different ADCLs for given chamber types are very close with percent differences generally less than 0.2% for Farmer-type chambers and are well within reported uncertainties. Conclusions: The close agreement among calibrations performed at different ADCLs reaffirms the Calibration Laboratory Accreditation Subcommittee process of ensuring ADCL conformance with National Institute of Standards and Technology standards. This study shows that N D, w coefficients measured at different ADCLs are statistically equivalent, especially considering reasonable uncertainties. This analysis of N D, w coefficients also allows identification of chamber types that can be considered stable enough for accurate reference dosimetry. [http://dx

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Measurement of ionization chamber absorbed dose factors in megavoltage photon beams

Cited by 126 publications

References 36 publications

Ion recombination correction factors () for Varian TrueBeam high‐dose‐rate therapy beams

Ion recombination correction factors () for Varian TrueBeam high‐dose‐rate therapy beams

Dosimetric characterization and behaviour in small X-ray fields of a microchamber and a plastic scintillator detector

Ion chamber absorbed dose calibration coefficients, N_D,w, measured at ADCLs: Distribution analysis and stability

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Measurement of ionization chamber absorbed dose factors in megavoltage photon beams

Cited by 126 publications

References 36 publications

Ion recombination correction factors () for Varian TrueBeam high‐dose‐rate therapy beams

Ion recombination correction factors () for Varian TrueBeam high‐dose‐rate therapy beams

Dosimetric characterization and behaviour in small X-ray fields of a microchamber and a plastic scintillator detector

Ion chamber absorbed dose calibration coefficients, ND,w, measured at ADCLs: Distribution analysis and stability

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Ion chamber absorbed dose calibration coefficients, N_D,w, measured at ADCLs: Distribution analysis and stability