ESTRO 2021 Abstract Book

S209

ESTRO 2021

harmonization of MPE training, in line with EU guidelines. [1] Guidelines on medical physics expert. European Commission. Radiat Prot 174, 2014. [2] EFOMP Policy statement 12.1: Recommendations on Medical Physics Education and Training in Europe 2014; Caruana CJ, Christofides S, Hartmann GH. Phys Med. 2014 Sep;30(6):598-603

OC-0302 FFF beam steering discrepancies not identified by traditional physics workflow J. McAloney 1 1 Riverina Cancer Care Centre, Radiation Oncology, Wagga Wagga, Australia

Purpose or Objective During patient QA for a SBRT spine patient a transverse beam positional error was identified for a FFF beam resulting in dose impinging on the spinal cord. This plan was re-planned using a conventional flat beam while the issue was investigated. It was found that a large portion of the dose was originating from particular gantry angles where the lookup table had been poorly established. In Elekta machines there is no additional beam steering for positional corrections in the transverse direction as there is in the radial direction, as such beam position is tied directly to symmetry. Asymmetry from reference conditions is provided as feedback in the form of beam steering error. Traditional cardinal angle-only Winston-Lutz tests did not identify this issue as it relies on discrepancies between the cardinal angles to be accurately reported via the steering error parameter. For this patient case the reported steering error was insignificant at these angles. An initial investigation in to how the error sensitivity is established via Elekta protocols using an EPID revealed large penumbra shifts for small reported steering errors. This prompted a more in-depth investigation into the impact that said insensitivity has on beam data and patient plans, covered in this presentation. See figure below for EIPD example of beam shifts.

Materials and Methods To quantify the impact of the reported beam steering error the physicist – with the on-site engineer – established a series of transverse steering currents yielding steering errors approximately ±0.5 and ±1 away from reference as set points for testing. These tests consisted of physics data and patient QA results. The physics data involved using a scanning water tank to acquire profiles and point dose measurements – adjusted based on the profiles to ensure measurement at the centre of the radiation field – to both determine magnitude of beam shifts and calculate small field output factors following IAEA TRS-483 formalism, repeated for each setpoint and compared to reference data. The patient QA results involved re-measuring a series of patient plans that use FFF beams at each setpoint on the Delta4 dosimetry system and comparing the new gamma metric to that of the original QA result. Results The physics data tests showed large beam shifts for small beam steering errors, see figure below, as well as changing the output factor of the smallest field, 5 mm x 5 mm square, by ~6%. The patient QA tests resulted in patient plans no longer considered a pass in QA, and in some cases resulting in unacceptable doses in critical OARs.