Download Reliability Study of ExacTrac® System Image Isocenter Using an On

Reliability Study of ExacTrac® System Image Isocenter Using an On-demand QA procedure
Brainlab ExacTrac® system is a widely used IGRT system that uses IR camera
technique to guide the initial patient setup, and uses kV x-ray imaging technique for target
localization. It is important to know the accuracy of such system not only depends on the
accuracy of image registration through bony anatomy or fiducial marks, but also depends on the
assumption that the geometric relationship between ExacTrac® and treatment unit is known and
reliable. Brainlab advises to calibrate the ExacTrac® isocenter using the room laser and
provides a calibration phantom for the isocenter calibration and daily verification. Since the
room laser could be deviated from the linac radiation isocenter during the calibration and drift
over time. The ExacTrac® isocenter needs to be verified to be coincident with the radiation
isocenter after calibration and periodically.
A simple process was developed in this work to quickly and accurately verify the
congruence of ExacTrac® image isocenter with the linac radiation isocenter. In the procedure, a
metal ball was aligned with the room laser. By fixing the ball in the space and measuring the
positions of ExacTrac® image isocenter and linac radiation isocenter relative to the ball, the
deviations between two isocenters was easily calculated. The ExacTrac® image isocenter
relative to the ball was measured using the manufacture provided “Detect Winston-Lutz Pointer”
test (Figure 1), in which kV images of the ball are projected to flat panels and the distance from
the center of the ball to the image isocenter was measured. The linac radiation isocenter
relative to the ball was measured by taking the portal image of the ball at 4 gantry angles
(0,90,180 and 270). An in-house developed software was used to identify the center of the ball
and the center of the radiation [1]. This process was used as part of weekly QA to ensure the
congruence of ExacTrac isocenter with the radiation isocenter and to study the reliability of the
ExacTrac isocenter.
Figure 3 shows the deviation of image isocenter from radiation isocenter. The position
error are -0.27±0.30, 0.52±0.24, 0.18±0.22 mm in the lateral, vertical and longitudinal directions,
and the max deviation were 0.86, 0.89 and 0.54 mm respectively. These verification data
showed the ExacTrac® image isocenter can be very reliable with sub-millimeter accuracy. The
quick QA procedure can be applied on-demand to ensure the accuracy of ExacTrac® isocenter.
1
Lateral Deviation: ‐0.27±0.30 mm
0
‐1
Vertical Deviation: 0.52±0.24 mm
1
‐1
b
Figure 1. Ball position relative to
ExacTrac image isocenter
Reference
[1] Du W, Yang JN, Chang EL, Luo D,
McAleer M, Shiu A, Martel MK.J Appl
Clin Med Phys 11(4):15-26, 11/2010
Figure 2.
Detection of
radiation
isocenter (a) and
metal ball (b)
in portal film
1
Longitudinal Deviation:0.18±0.22
mm
‐1
Figure 3. Deviation of ExacTrac®
image isocenter from linac radiation
isocenter