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

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

Commissioning and verification of the collapsed cone convolution superposition algorithm for SBRT delivery using flattening filter‐free beams

“…For the 10 MV FFF beam, ${\mathrm{normalP}}_{\text{ion}}$ was 1.008 at a dose rate of 2400 MU/min and 1.0026 with a dose rate of 600 MU/min. A decrease in ion collection efficiency was observed due to the higher dose rates of the FFF beams, which has also been observed in other reports 1 , 2 , 17 …”

Ion recombination and polarity corrections for small‐volume ionization chambers in high‐dose‐rate, flattening‐filter‐free pulsed photon beams

“…Behavior differences were noted between the three chamber classes: Farmer‐type, scanning, and microionization chambers. The Farmer chamber, consistent with the literature, exhibited near‐ideal recombination and polarity behavior in both conventional and high‐dose rate, FFF photon beams.…”

Characteristics of flattening filter free beams at low monitor unit settings

“…Although FFF beams used a higher dose per pulse than the 6X beam, the variation in P ion was small, as supported by previous investigations of the FFF beams of TrueBeam TM . [30][31][32] Typical linacs generate 1-3 patterns of photon beam energy with the same number of flattening filters. In contrast, some of newer commercial linacs can generate flattened beams and several patterns of FFF beam energy because FFF beams do not require flattening filters.…”