2.2. Stereotactic radiotherapy

The relative biological effectiveness (RBE)-weighted dose of 50 Gy [23] (5x10 Gy) was prescribed to the 80 % isodose encompassing the Planning Target Volume (PTV) according to the International Commission of Radiation Units and Measurements (ICRU) Report 91 guidelines [30]. PTV included GTV with a 2-mm isotropic margin derived upon a multidisciplinary local consensus based on the assessment of uncertainties. At least 98 % of PTV received 95 % of the prescribed dose, resulting in a PTV near-maximum dose of 62.5 Gy (RBE-weighted = 1.0). Dose was calculated in Accuray Multiplan™ (Accuray, Sunnyvale, CA, USA) and delivered with a robotic CyberKnife M6 using either an iris collimator (range: 7.5 to 20 mm) or a multi-leaf collimator. To compensate for intrafraction patient motion, the 6D skull tracking method was used with tight rotational and translational boundaries. To mitigate eye setup errors, gating using a modified version of the Rotterdam Gill-Thomas-Cosman frame [31] with the camera and LED placed on an arch attached to a double-shell mask was employed.