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 0.51 +/- 0.86 mm (lateral), 0.30 +/- 1.55 mm (longitudinal), and -1.63 +/- 1.00 mm (vertical); -0.53 degrees +/- 0.56 degrees (pitch), 0.42 degrees +/- 0.60 degrees (roll), and 0.44 degrees +/- 0.90 degrees (yaw), respectively. These were -0.07 +/- 0.24 mm, -0.07 +/- 0.25 mm, 0.06 +/- 0.21 mm, 0.04 degrees +/- 0.23 degrees, 0.00 degrees +/- 0.30 degrees, and -0.02 degrees +/- 0.22 degrees, respectively, for the postcorrection (XV). Substantial degradation of the treatment plans was observed in D-95 of PTV (2.6% +/- 3.3%; simulated treatment versus treatment planning), D(mi)n of PTV (13.4% +/- 11.6%), and D-min of CTV (2.8% +/- 3.8%, with the maximum error of 10.0%) from XC, while dosimetrically negligible changes (< 0.1%) were detected for both CTV and PTV from XV simulation. 3D angular correction significantly improved CTV dose coverage when the total angular shifts (vertical bar pitch vertical bar + vertical bar roll vertical bar + vertical bar yaw vertical bar) were greater than 2 degrees. 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.