197. Stereotactic Body Radiotherapy: Comparison between two IGRT systems

Abstract

Purpose The aim of this work is to compare the residual setup errors measured with ExacTrac X-ray 6 degree-of-freedom (6D) and kilo-voltage cone-beam computed tomography (CBCT) for patients receiving stereotactic body radiotherapy (SBRT) at Fondazione IOM in collaboration with REM Radioterapia. The first device employs an infrared optical system that performs patients pre-positioning through body markers and isocenter coordinates, and an X-ray imaging system that verifies target position using bone anatomy. CBCT imaging allows positioning verification using not only bone anatomy but also soft tissues. Methods On a TrueBeam-Novalis STx treatment unit, from January to November 2017, 45 patients with extracranial tumours were treated with SBRT for a total of 53 lesions (some patients having more than one lesion) and 152 treatment sessions. Prescribing doses ranged from 24 to 60 Gy in 3 fractions or 20 Gy in a single fraction and planning techniques were dynamic conformal arc therapy or hybrid intensity modulated radiotherapy, with no-coplanar fields. All patients were initially located using personalized immobilization systems and setup corrections were determined and corrected by means of registrations of ExacTrac X-ray images with the corresponding digitally reconstructed radiographs using the ExacTrac 6D fusion. At the end of each treatment session, with the couch at 0°, the residual setup errors were determined by means of registrations of CBCT images with the planning CT using online 3D fusion and for each session, displacements were evaluated to analyse the residual setup errors. Results A modest difference in residual setup errors was found between ExacTrac system and CBCT (figure below). The average of the observed residual displacements for the three space directions, in absolute value, was 1.5 mm in vertical and in lateral directions, 1.2 mm in longitudinal direction. Conclusion This work showed the good agreement on the setup accuracy between ExacTrac X-ray and CBCT for patients treated with SBRT. The two systems present comparable precision, but ExacTrac offers additional benefits such the capability to quantify all rotational errors, fastest automated positioning in 6D even for no-coplanar fields and smaller doses, representing a valid alternative to CBCT with complementary information in image-guided stereotactic radiotherapy.