Mid-Treatment Intrafraction Motion Assessment During LINAC-Based Stereotactic Radiosurgery: A Single Institution Experience of Over 600 Treatments

Purpose/Objective(s) Intrafraction motion, if not mitigated, may pose a significant challenge to the delivery of frameless intracranial stereotactic radiosurgery. The current study presents a 7-year mid-treatment intrafraction motion assessment experience on 300 patients treated with a relocatable mask system. Materials/Methods 668 lesions, each treated using a mono-isocentric technique, comprised the current retrospective analysis from April 2012 to April 2019. Patient immobilization was by means of a 3-point hybrid thermoplastic mask system (Orfit industries). Orthogonal stereoscopic were acquired pre-beam and approximately mid-way through each treatment session, and at the same nominal 0° couch angle, for intrafraction motion assessment on a six-degree of freedom robotically actuated couch system. Results Mean/median time between each stereoscopic image pair was 6.05/6.10 s (SD = 1.10; range: 3.90–7.98). Mean/median translational motion (SD) was 0.19/0.12 mm (max = 1.97; SD = 0.22), 0.25/0.17 mm (max = 1.99; SD = 0.27), and 0.24/0.16 mm (max = 2.24; SD = 0.27) in the antero-posterior, cranio-caudal and medio-lateral directions, respectively. Mean/median 3D displacement 0.46/0.35 mm (max = 2.31; SD = 0.39). Mean/median rotational was 0.20/0.16o (max = 1.11; SD = 0.17), 0.15/0.10o (max = 1.09; SD = 0.16), and 0.21/0.14o (max = 2.07; SD = 0.24) in the yaw, roll and pitch directions, respectively. Frequencies of 2D and 3D intrafraction translational motion exceeding 1 mm/1.5 mm/2 mm were 5.24%/1.50%/0.15% and 7.78%/3.14%/1.20%, respectively. Rotational motion greater than 1o/1.5o/2o were 1.95%/0.60%/0.15%, respectively. No correlation between laterality/anatomic location/time vs. intrafraction motion was observed (P > 0.1). Conclusion Mid-treatment intrafraction motion assessment revealed 1 mm or greater shift in 3D for 7.78% of cases. This frequency was small, but non-trivial. Our findings have implications on PTV margin setting and underscore the importance of intrafraction motion monitoring. Author Disclosure R. Boopathy: None. C.M. Kato: None. K. Gallagher: None. J.J. Jaboin: None. C. Kubicky: None. C.M. Marquez: None. M. Fuss: None. J.A. Tanyi: None.