ESTRO 2021 Abstract Book

S1444

ESTRO 2021

Conclusion The on-gantry EPID is very reliable for fast daily QA of an MR linac and is an important tool for monitoring changes on the system. Beam performance in terms of output and profiles was very stable, and only small variations were observed for MLCs, jaws, and especially gantry angle over a 22 month period of clinical use. PO-1719 Incorporation of an agile test into the linacs QA to verify the beam focal spot A. Garcia Sanz 1 , R. Ayala Lázaro 1 , M.J. Garcia Hernández 1 , J.M. Becerro Morgado 1 , Á. Soza Marañón 1 , S. Gómez Cores 1 , E. Martínez Gómez 1 1 H.G.U. Gregorio Marañón, Servicio de Dosimetría y Radioprotección, Madrid, Spain Purpose or Objective Beam focal spot is an important parameter to be considered on linacs QA tests because deviations from its optimal position can alter the geometric and dosimetric properties of the beam, and therefore, compromise the accuracy of advanced radiotherapy techniques such as SBRT. However, currently, verification of the beam focal spot is not explicitly endorsed in the linacs QA international protocols, due to available tests were tedious and time consuming. Following a recently proposed methodology (Chojnowski 2018 and 2020), we have detected and corrected a misalignment of the beam focal spot in one of our Elekta Versa HD TM in a simple and straightforward manner. This test has been implemented into our routine QA procedure. Materials and Methods Potential beam focal spot deviations from the collimator axis of rotation are evaluated performing the Winston Lutz (WL) test with the Ball-Bearing (BB) phantom (Elekta AB). Data analysis has been performed using pyQAserver software and the sequential test has been programmed into the QAtrack+ platform. If the beam focal spot is misaligned, a drift of the field CAX respect to the BB position is observed at each collimator rotation. This is caused by the fact that the MLC and diaphragms are placed at different heights of the beam line. Correction of focal spot misalignment was carried out employing the ICProfiler TM detector attached to the gantry with the Gantry Mounting Fixture™ (GMF). Maintaining the gantry at 0°, measurements of the beam centre were collected at opposite collimator angles (0° and 180°). Discrepancies observed between both beam centres were reduced iteratively in real-time by adjusting the parameters 2T I Ctrl and 2T Bal of the linac.

Results