ESTRO 2021 Abstract Book

S1473

ESTRO 2021

Fig. 2. AlignRT deviations with isocentric couch rotation.

Conclusion The QA results of AlignRT allow to guarantee the submillimeter accuracy required for stereotactic treatments.

PO-1749 Initial experience with Surface-guided radiation therapy (SGRT) in SRS treatments P. Sánchez-Rubio 1 , R. Rodríguez-Romero 2 , M. Pinto-Monedero 1 , J. Martínez-Ortega 1 , M. Torres-López 1 1 hospital Universitario Puerta De Hierro Majadahonda, Medical Physics, Majadahonda, Spain; 2 HOSPITAL UNIVERSITARIO PUERTA DE HIERRO MAJADAHONDA , MEDICAL PHYSICS, MAJADAHONDA, Spain Purpose or Objective SGRT allows patient positioning and intrafraction control without using ionizing radiation or fiducial marks. Its use is mainly documented for breast treatments, or in the case of stereotaxic radiosurgery, its accuracy is validated from phantom-based studies. The aim of this work is to analyze the accuracy of a SGRT system for SRS, as well as its ability to intrafraction control from real patient treatment data. Materials and Methods Since may 2020, date of beginning of SRS treatments with the SGRT system AlignRT ( VisionRT ), 36 MV images, 24 CBCTs and 133 SGRT monitoring sessions were analyzed, corresponding to 11 patients immobilized with Encompass TM SRS Fiberplast open masks ( Qfix ) and treated with VMAT or dIMRT and 6FFF energy in one fraction on a Varian TrueBEAM with HD 120 MLC and Perfect Pitch TM table (6 DoF). An initial CBCT was performed and, prior to each field/arc with table rotation, an AP MV image was taken. During treatment, new CBCTs were acquired when AlignRT detected, for more than one second, some translation and/or rotation greater than 0.3 mm or 0.3º, respectively. After the CBCT/MV shifts were applied, a new reference surface capture was acquired. To evaluate the accuracy of AlignRT, for initial setup and intrafraction control, the corrections detected were compared with those found by the IGRT systems. The mean translations/rotations monitoring by AlignRT during the treatment were also calculated. Results The average difference and its standard deviation between AlignRT and CBCT corrections, ∆ SGRT - INITIAL CBCT , were 0.62±1.16, 1.01±1.19, -0.26±0.94 mm vert, lat. and long, respectively; 0.03º±0.59º, 0.26º±0.83º, 0.19º±0.74º pitch, roll and yaw, respectively, being the longitudinal correction the one with the greatest divergence, similar to the results of other authors. Such differences decreased in the intrafraction control, being ∆ SGRT - CBCT INTRAFRAX : -0.37± 0.67, 0.08±0.55, -0.26±0.31 mm vert, lat. and long, respectively; 0.05º ± 0.56º, -0.35º±0.37º, -1.16º±2º pitch, roll and yaw, respectively. The discrepancies between the two systems could be due to the disparity in the ROI used to register in the CBCT or AlignRT. Fig.1 shows that AlignRT proposes corrections greater than the MV image for table angles greater than ±50º, especially in lateral, with a maximum average difference, ∆ SGRT– MV , of -0.61 mm in lateral and 2.39º of yaw for table 80º- 85º. For a better comparison of both systems in the 6 DoF, orthogonal images should be acquired, preferably kV-MV (currently under study)