Ion Recombination Correction Factors (Pion) for Varian TrueBeam High Dose Rate Therapy Beams
S Kry1*, R Popple2, A Molineu1, D Followill1, (1) MD Anderson Cancer Ctr., Houston, TX, (2) Univ Alabama Birmingham, Birmingham, ALSU-E-T-375 Sunday 3:00:00 PM - 6:00:00 PM Room: Exhibit Hall
Purpose: Ion recombination is approximated in TG-51 by Pion, which is calculated by a 2-voltage measurement. This approximation is sufficiently accurate for the calibration of beams only when Pion is less than 1.05. High-dose per pulse beams, such as flattening filter free beams, may increase Pion to an unacceptable level for the 2-voltage measurement approach to be adequate.
Methods: Pion was measured for flattened beams of 6, 10, 15, and 18 MV, as well as flattening filter free beams of 6 and 10 MV. Pion was also measured for electron beams of 6-18 MeV at standard as well as high dose rate (1000 MU/min). The measurements were made at a range of depths and with a variety of clinical ion chambers, including PTW, NEL, and Exradin chambers. 1/V versus 1/I curves were generated for the scenarios generating the maximum measured Pion values in order to confirm the accuracy of the 2-voltage technique.
Results: Consistent with the increased dose per pulse, Pion was higher for FFF beams than for FF beams. However, for all beams, measurement locations, and chambers examined, Pion never exceeded 1.018. Compared to the 1/V versus 1/I results, the 2 voltage technique was always accurate within 0.3%.
Conclusions: Recombination can be adequately accounted for in high dose rate flattening-filter free beams using the standard 2-voltage technique to determine Pion.