SU‐E‐T‐430: Quantifying Alterations in Flattened and Flattening‐Filter‐Free Beam Characteristics from a Gantry Mounted, in Vivo Beam Delivery Verification System on a TrueBeam Linear Accelerator

Purpose: Planning and delivery of radiation therapy are becoming more complex with the wide use of intensity‐modulated radiation therapy (IMRT) and volumetric modulated arc therapy (VMAT). In vivo quality assurance of field modulation is possible using a novel multiwire “harp” ionization chamber array DAVID (PTW, Freiburg, Germany) positioned at the treatment head. The purpose of this work is to pre‐validate its use in vivo by characterizing potential dosimetric changes in the beam introduced by the device.Methods: Beam profiles for 6, 10, 15 MV with flattening filter and 6 MV flattening‐filter‐free (FFF) were acquired for square field sizes of 5, 10, 20, and 35 cm at 5.8 cm depth on a TrueBeam linear accelerator (Varian, Palo Alto, CA). Profiles were acquired in the target‐gun, left‐right, and diagonal directions using the StarCheck Maxi ion chamber array (PTW, Freiburg, Germany) with and without DAVID in place. Ion chamber spacing was 3 mm. Attenuation was measured by calculating the transmission factor at the central chamber with and without DAVID. Results: For beams with a flattening filter, transmission factors ranged from 95–96% with 15 MV to 92–94% with 6 MV. For the 6 MV FFF, the transmission factor was 91–92%. Beam profiles were minimally affected by DAVID, demonstrating at most 1–2% difference at 35 cm field size, for the higher energy beams. For 6 MV and 6 MV FFF and field sizes less than 20 cm, profiles agreed within 1% even in penumbral regions Conclusions: Except for a correctable transmission factor, the presence of DAVID minimally influences dosimetric profiles. Transmission factors range from 92%–96% for conventional beams, but can be lower for FFF beams. In future work, percent depth dose and electron contamination will be measured to examine spectral influences of DAVID on FFF beams