Dosimetric confirmation of monitor chamber positioning following a carousel pullout on a varian TrueBeam linac

Abstract

Introduction The carousel assembly in a TrueBeam is different from that on the C-Series linacs. In addition to the foils and filters the carousel now positions the monitor chamber and field light using four motorized motion axes. The chamber is positioned in the beam during the Prepare phase and is retracted during the Preview phase. Purpose The Chamber Fault interlock prevents beam-on conditions when the monitor chamber is not correctly positioned. To correct this fault it may be necessary to pullout the carousel assembly, inspect and resolve issues with the chamber moving mechanism. We describe a set of dosimetric measurements serving as baselines and repeated following a carousel pullout. Materials and methods We used the Profiler2 which comprises of two sets of diode arrays. The Profiler is mounted on the collimator head. In addition to the array calibrations which are necessary for relative profile measurements, we obtained absolute dose calibrations for each of the nine energies. Radial and transverse profiles were then acquired at two different depths. Results The CAX doses at each of the depths were used to evaluate both the output and energy constancy and the profiles were used to derive the radial and transverse symmetries. These four dosimetric indicators were compared, following a carousel pullout, to their baseline values. Conclusion We have implemented an efficient and effective set of measurements to dosimetrically characterize each beam. They serve as baselines for evaluating correct monitor chamber placement following repairs involving the carousel. Disclosure None.