Ion recombination correction factors ([Formula: see text]) for Varian TrueBeam high‐dose‐rate therapy beams

Ion recombination is approximately corrected for in the Task Group 51 protocol by [Formula: see text] , which is calculated by a two‐voltage measurement. This measurement approach may be a poor estimate of the true recombination, particularly if [Formula: see text] is large (greater than 1.05). Concern exists that [Formula: see text] 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, [Formula: see text] 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). [Formula: see text] 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, [Formula: see text] was higher for FFF beams than for flattening filter beams. However, for all beams, measurement locations, and chambers examined, [Formula: see text] never exceeded 1.018. Additionally, [Formula: see text] 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 [Formula: see text] determined with the standard two‐voltage technique. PACS numbers: 87.56.‐v, 87.56.Da