SU-E-T-74: Assessing Effects of Ion Collection Efficiency in Flattening Filter-Free (FFF) Beams on Three TrueBeam Machines

Abstract

IntroductionA TrueBeam linear accelerator (TB-LINAC) is a new generation digital linear accelerator designed to deliver both standard flattened and flattening filter-free (FFF) beams. It has been reported that incomplete ion collection efficiency of an ion chamber occurs in FFF beams; however, recent works reported considerable differences in variation of ion collection efficiency. To ensure accurate dosimetry, we further investigated the ion collection efficiency under FFF conditions on three TB-LINAC units.MethodsEach TB-LINAC is capable of delivering photon beams from 4MV to 15MV, electron beams from 6MeV to 22MeV, and 6MV-FFF and 10MV-FFF photon beams. Ion collection efficiency of an ionization chamber for various dose rates at various locations under 6MV, 10MV, 6MV FFF and 10MV FFF conditions was measured, quantified as Pion. The standard Boag's two-voltage technique was used to calculate Pion.All these measurements were conducted in a standard Wellhofer water tankwith a compact ion-chamber (CC13, IBA dosimetry, Germany). ResultsVariations of ion collection efficiency of the chamber CC13 are less than 0.3% in standard 6MV and 10MV from 5cm depth to the 35cm depth under 40×40cm2 under the dose rates ranging from 100MU/min to 600MU/min. Variations of Pion of the chamber CC13 are 1.2% (from 1.015±0.002 to 1.002±0.001) under 6MV FFF and 2.0% (from 1.023±0.006to 1.002±0.001) from dmax to the 18cm-off-axis point at 35cm depth under 40×40cm2. Variations of ion collection efficiency are less than 0.1% across dose rates from 400MU/min to 1400MU/min in 6MV FFF and less than 0.2% across dose rates from 400MU/min to 2400MU/min in 10MV FFF at various locations.ConclusionVariations of Pion in FFF beams are much greater than those in the corresponding flatten beams. Given the approximate 2% variation in 10MV FFF, proper correction may be considered for the incomplete ion collection efficiency in 10MV FFF.