Implementation and validation of a new fixation system for stereotactic radiation therapy: An analysis of patient immobilization.

Abstract

Stereotactic radiation therapy is an established treatment technique for intracranial malignancies. We evaluated a new intracranial immobilization system with an emphasis on determining the intrafraction motion and the correlation of this motion with treatment time. Patients were immobilized using the trUpoint ARCH fixation system (CIVCO Medical Solutions). We collected data from 85 lesions in 73 patients treated between November 2011 and December 2013. Sixty-nine of 73 patients (95%) used the complete mask system; for the remaining 4 patients, the system had to be adapted. Patients were treated using volumetric modulated arc therapy stereotactic radiation therapy on a TrueBeam linear accelerator (Varian Medical Systems, Palo Alto, CA). Fraction doses of 2-8 Gy were applied in 4-30 fractions. Daily cone beam computed tomography imaging was performed before the treatment and was matched to the reference computed tomography using a 6-degrees-of-freedom automatching procedure. Additionally, posttreatment cone beam computed tomography scans were performed to assess intrafraction motion for 67 patients (375 fractions). The average 3-dimensional setup error was 2.1 ± 2.9 mm. The mean pitch and roll was -0.1 ± 0.7° and 0.2 ± 0.7°. A total of 98.0% of the pitch values and 98.9% of the roll values were <1.5°. Mean intrafractional motion was 0.51 mm (±0.27) and mean treatment time was 10.1 minutes (±1.4). The maximum intrafractional motion was 2.0 mm in the longitudinal direction; 95% of the total shifts were <1.4 mm. The linear regression showed a weak but significant influence (R(2) = 0.26, P = .01) of the treatment time on the total intrafractional shift. The new intracranial immobilization system appears to be robust in terms of setup accuracy, intrafraction motion, and repositioning of the mask system.