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

Abstract

Dosimetric consequences of positional shifts were studied using frameless Brainlab ExacTrac X-ray system for hypofractionated (3 or 5 fractions) intracranial stereo-tactic 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° ± 0.56° (pitch), 0.42° ± 0.60° (roll), and 0.44°± 0.90° (yaw), respectively. These were -0.07 ± 0.24 mm, -0.07 ± 0.25 mm, 0.06± 0.21 mm, 0.04° ± 0.23°, 0.00° ± 0.30°, and -0.02° ± 0.22°, respectively, for the postcorrection (XV). Substantial degradation of the treatment plans was observed in D95 of PTV (2.6% ± 3.3%; simulated treatment versus treatment planning), Dmin of PTV (13.4% ± 11.6%), and Dmin 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 (|pitch| + |roll| + |yaw|) 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.

Figure 1

Change in PTV dose coverage $((\text{Planned}\hspace{0.17em}\text{dose}‐\text{Simulated}\hspace{0.17em}\text{dose})/\text{Planned}\hspace{0.17em}\text{dose}\times 100\%)$ with respect to the mean $3\mathrm{D}\hspace{0.17em}\text{vector}(\surd ({\mathrm{x}}^2+{\mathrm{y}}^2+{\mathrm{z}}^2\left)\right)$ translational error of each patient for XC: (a) $\mathrm{\Delta }{\mathrm{D}}_{95}$, (b) $\mathrm{\Delta }{\mathrm{D}}_{\text{min}}$, (c) $\mathrm{\Delta }{\mathrm{D}}_{\text{max}}$, and (d) $\mathrm{\Delta }{\mathrm{D}}_{\text{mean}}$.

Figure 2

Change in PTV dose coverage $((\text{Planned}\hspace{0.17em}\text{dose}‐\text{Simulated}\hspace{0.17em}\text{dose})/\text{Planned}\hspace{0.17em}\text{dose}\times$ with respect to the mean 3D vector $3\mathrm{D}\hspace{0.17em}\text{vector}(\surd ({\mathrm{x}}^2+{\mathrm{y}}^2+{\mathrm{z}}^2\left)\right)$ translational error of each patient for XV: (a) $\mathrm{\Delta }{\mathrm{D}}_{99}$, (b) $\mathrm{\Delta }{\mathrm{D}}_{\text{min}}$, (c) $\mathrm{\Delta }{\mathrm{D}}_{\text{max}}$, and (d) $\mathrm{\Delta }{\mathrm{D}}_{\text{mean}}$.

Figure 3

Change in CTV dose coverage $((\text{Planned}\hspace{0.17em}\text{dose}‐\text{Simulated}\hspace{0.17em}\text{dose})/\text{Planned}\hspace{0.17em}\text{dose}\times$ with respect to the mean 3D vector $3\mathrm{D}\hspace{0.17em}\text{vector}(\surd ({\mathrm{x}}^2+{\mathrm{y}}^2+{\mathrm{z}}^2\left)\right)$ translational error of each patient for XC: (a) ΔD₉₉, (b) $\mathrm{\Delta }{\mathrm{D}}_{\text{min}}$, (c) $\mathrm{\Delta }{\mathrm{D}}_{\text{max}}$, and (d) $\mathrm{\Delta }{\mathrm{D}}_{\text{mean}}$.

Figure 4

Change in CTV dose coverage $((\text{Planned}\hspace{0.17em}\text{dose}‐\text{Simulated}\hspace{0.17em}\text{dose})/\text{Planned}\hspace{0.17em}\text{dose}\times$ with respect to the mean 3D vector $3\mathrm{D}\hspace{0.17em}\text{vector}(\surd ({\mathrm{x}}^2+{\mathrm{y}}^2+{\mathrm{z}}^2\left)\right)$ translational error of each patient for XV: (a) ΔD₉₉, (b) $\mathrm{\Delta }{\mathrm{D}}_{\text{min}}$, (c) $\mathrm{\Delta }{\mathrm{D}}_{\text{max}}$, and (d) $\mathrm{\Delta }{\mathrm{D}}_{\text{mean}}$.

Figure 5

Effect of angular correction by ${\mathrm{D}}_{\text{min}}$ coverage change of (a) PTV and (b) CTV with respect to magnitude of angular deviation using XC simulation. The positive value means the 6D translational and rotational correction can improve the ${\mathrm{D}}_{\text{min}}$ coverage more than 3D translational correction only does.