8 Implementation of MOD-ISO-CBCT+ QA module from Qualiformed on Elekta Agility

Abstract

Introduction The MOD-ISO-CBCT+ module from QUALIFORNED is dedicated to the QA of isocenter (size and position) and angles position of gantry, collimator and table. For SBRT or non-coplanar treatments some mechanical tolerances could be more restrictive [1] . This module contains phantoms (OTP-ISO+ sphere, OTP-Cross-MV and OTP-Cross-kV plates). The OTP-CROSS plates materialize a fix point coming from the source (MV or kV) [2] . We have studied several possible devices in order to evaluate their user-friendly, safe and efficient aspects. Methods For the OTP-CROSS-kV, we recycled a filter neither used in service or clinical mode. We removed the aluminum part, fixed inside a machined plastic slab to reproducibly insert the OTP-Cross-kV in S position. This filter was painted in red and used in service mode only. For OTP-CROSS-MV, 3 setups were tested: Two with the shadow tray and one with a Mylar crosswire screen. Setup 1: a coded acrylic tray was machined to align OTP-Cross-MV. Setup 2: axes determination for OTP alignment by doing a cross irradiation of a gafchromic film taped to the top acrylic tray. Setup 3: OTP-Cross-MV was taped down on an old Mylar screen, avoiding any contact with the current crosswire, and then it is superimposed, aligned and fixed with nut. Finally these 3 setups were compared by performing QA with the same sphere position. Results No difference was observed, suggesting that our shadow tray and Mylar crosswire screen are well aligned and don’t involve any additional mechanical movement. Setup 1: metal balls of OTP-CROSS-MV appear on the sphere surface, complicating their image detection. Setup 2: The Shadow tray is heavy, but its positioning is reproducible. It is not used clinically, anymore. Setup 3: the old Mylar crosswire screen requires a good alignment with the linac crosswire, which is not removed, meaning no risk for the UV light. Conclusions The motivation of this work was to develop a dedicated QA kit, plug and play, routinely usable, with reproducible positioning, and clinically safe. For the second setup, the difficulty was the accurate axes determination on the top removable acrylic tray and that was solved by irradiating a taped gafchromic film. This setup n°2 is retained. We are considering the introduction of a security key for setup 3. Finally by recycling some linac parts, the cost development is limited. Thanks to Mr. N. Dodd from Elekta for his help and advice.