Abstract
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
Highlights
The current standard for beam calibration in North America is the American Association of Physicists in Medicine Task Group (TG) 51 protocol.[1]
For the conventional flattened photon beams examined in this study, the mean and range of the measured Pion values are presented in Table 1 for the 6, 10, 15, and 18 MV X-ray beams
The value of Pion increased as the dose per pulse at 10 cm depth increased
Summary
The current standard for beam calibration in North America is the American Association of Physicists in Medicine Task Group (TG) 51 protocol.[1]. 319 Kry et al: Ion recombination correction for high-dose-rate beams measured ionization readings MH and ML, respectively). Equation [1] is an approximation of the recombination effects that assumes a linear relationship between 1/M and 1/V. While this is reasonably accurate when there is little recombination,(1,13) the true relationship is known to be nonlinear,(12) and the true recombination may be different than predicted with the two-voltage technique. That error corresponded to a 20% difference in dose to water measured by several ion chambers, as compared to a doseper-pulse independent dosimeter
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