Dosimetric effects of positioning shifts using 6D‐frameless stereotactic Brainlab system in hypofractionated intracranial radiotherapy

Dosimetric consequences of positional shifts were studied using frameless Brainlab ExacTrac X‐ray system for hypofractionated (3 or 5 fractions) intracranial stereotactic radiotherapy (SRT). SRT treatments of 17 patients with metastatic intracranial tumors using the stereotactic system were retrospectively investigated. The treatments were simulated in a treatment planning system by modifying planning parameters with a matrix conversion technique based on positional shifts for initial infrared (IR)‐based setup (XC: X‐ray correction) and post‐correction (XV: X‐ray verification). The simulation was implemented with (a) 3D translational shifts only and (b) 6D translational and rotational shifts for dosimetric effects of angular correction. Mean translations and rotations (± 1 SD) of 77 fractions based on the initial IR setup (XC) were [Formula: see text] (lateral), [Formula: see text] (longitudinal), and [Formula: see text] (vertical); [Formula: see text] (pitch), [Formula: see text] (roll), and [Formula: see text] (yaw), respectively. These were [Formula: see text] , [Formula: see text] , [Formula: see text] , [Formula: see text] , [Formula: see text] , and [Formula: see text] , respectively, for the postcorrection (XV). Substantial degradation of the treatment plans was observed in [Formula: see text] of PTV ([Formula: see text]; simulated treatment versus treatment planning), [Formula: see text] of PTV ([Formula: see text]), and [Formula: see text] of CTV ([Formula: see text] , with the maximum error of 10.0%) from XC, while dosimetrically negligible changes [Formula: see text] were detected for both CTV and PTV from XV simulation. 3D angular correction significantly improved CTV dose coverage when the total angular shifts [Formula: see text] were greater than 2°. With the 6D stereoscopic X‐ray verification imaging and frameless immobilization, submillimeter and subdegree accuracy is achieved with negligible dosimetric deviations. 3D angular correction is required when the angular deviation is substantial. A CTV‐to‐PTV safety margin of 2 mm is large enough to prevent deterioration of CTV coverage. PACS number: 87.55.dk