Evaluation of Initial Setup Accuracy and Intra-fraction Motion for Spine SBRT Using Stereotactic
Body Frames.
We have quantified initial setup accuracy and intra-fraction motion when using stereotactic body
frames (SBF) for spine SBRT. Either the commercial Elekta SBF or an in-house SBF (BHSSBF) were used. The BHS-SBF is shown in figure 1. Patients were treated using VMAT arcs.
The Brainlab ExacTrac system was used for patient positioning. Lasers calibrated to the frame
coordinate system were used to position the patient in the frame. The ExacTrac system would
then drive the frame into position using IR markers identified on the planning CT. kV x-rays
were acquired with the ExacTrac system and an automatic 6-D bony anatomy fusion performed
(Figure 2). The calculated translations & rotations give the initial setup accuracy of the frame.
Figure 1: BHS-SBF with lasers used to
position the patient in the frame attached. IRplatforms are used to localize the frame in
the room coordinates with ExacTrac.
Figure 2: ExacTrac 6-D bony anatomy
fusion window showing calculated
translations and rotations.
Figure 3 shows the initial setup error results for the 31 fractions treated. No significant
systematic error was observed and there was no difference between the Elekta and BHS-SBF.
The patient was shifted based on the fusion and verification imaging was acquired confirming
positioning within 1 mm / 1 deg. The imaging/fusion process was repeated between treatment
arcs and at the end of the fraction. Intra-fraction motion was determined from shifts between
image sets. Figure 4 shows a histogram of the 3-D intra-fraction shift magnitude observed. The
mean 3-D intra-fraction magnitude was 0.6 mm with a 95% certainty of 1.4 mm which compares
favorably with our previously published results for mask based cranial radiosurgery1.
SBRT Spine Initial Positioning
Rotation
80.00%
20.00%
Figure 3: Setup error prior to image guidance.
2
1.
8
1.
6
1.
4
0.00%
-8
1
10.00%
-6
-10
1
30.00%
1.
2
-4
40.00%
0.
8
-2
VRT Translation BHS
LNG Translation BHS
LAT Translation BHS
VRT Angle BHS
LNG Angle BHS
LAT Angle BHS
50.00%
0
-3.5
0
60.00%
0.
6
2
70.00%
VRT Translation Elekta
LNG Translation Elekta
LAT Translation Elekta
VRT Angle Elekta
LNG Angle Elekta
LAT Angle Elekta
0.
4
4
Mean=0.6 mm
95% certainty = 1.4 mm
90.00%
6
(%)
Shift (mm/degrees)
Translation 8
Cumulative Intra-Fx 3-D Shift Magnitude Histogram
100.00%
0.
2
10
Intra-Fraction 3-D Shift Magnitude (mm)
Figure 4: 3-D intra-fraction shift magnitude.
Ramakrishna N et al. A clinical comparison of patient setup and intra-fraction motion using
frame-based radiosurgery versus a frameless image-guided radiosurgery system for intracranial
lesions. Radiother Oncol. 2010 Apr;95(1):109-15. Epub 2010 Jan 28.