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Evaluation of the Abilities of kV-kV Radiographic Matching and CBCT Matching to
Detect Known Offsets
Tomi Ogunleye, M.S., Ian Crocker, M.D., and Eric Elder, Ph.D.
Department of Radiation Oncology, Emory University School of Medicine, Atlanta, Georgia
INTRODUCTION
CONCLUSION
In the past few years the usage of conformal and IMRT treatments
has been increasing rapidly. These treatments employ the use of
tighter margins around a target volume and increase the need for
the minimization of patient positioning and setup errors. The use
of Image Guided Radio-Therapy (IGRT) along with radio-opaque
markers implanted in a target volume for prostate treatments or
the use of bone anatomy in brain patients allow for an
unprecedented degree of precision. Patient positioning with IGRT
is necessary in these cases to ensure proper target volume
coverage and to reduce toxicity to nearby critical volumes.
Orthogonal kV-kV matching using Varian’s On Board Imaging
(OBI) and cone beam computed tomography (CBCT) are two
methods of IGRT used in our clinic. OBI allows for patient
positioning by using information from two orthogonal images taken
in treatment position and comparing them with digitally
reconstructed radiographs created from the treatment plan. This is
also sometimes referred to as 2-D matching. With CBCT (3-D
matching) matching, a kV CT image is acquired with the patient in
treatment position and volumetric matching is done comparing the
acquired CT with the planning CT. Matching is possible on the
sagittal, axial, and coronal planes. The purpose of this study was
to evaluate the ability of kV-kV orthogonal radiograph matching
using an on board imager (OBI) and linac based cone beam
computed tomography (CBCT) to accurately detect sub-centimeter
offsets during IGRT involving bony anatomy of the head as a
reference.
Based on our tests, CBCT was slightly more accurate than kVkV matching in generating shifts that corresponded with the
random offsets applied to the phantom. Both systems differed
from the known offset by an average magnitude greater than
1.0mm. This may be in part due to the fact that the smallest
shift in any direction possible with these two systems is 1.0mm.
There is also some uncertainty built in due to the subjectivity of
performing some of these matches. In this case we did not
observe a difference between the auto-matches performed by
the system and a user manual match. Further tests will include
a third, objective electromagnetic localization system to
determine the offsets.
Figure 3: CBCT Matching interface. On the overlay, the
planning CT is shown in red and the acquired CBCT image in
green.
MATERIALS & METHODS
A CIRS Radiosurgery Head Phantom containing bone and
soft tissue equivalent materials was mounted to a Newport
460P XYZ micrometer stage with sub-millimeter indexing
accuracy in all three orthogonal directions. This system in
turn was attached firmly to the linac treatment couch.
Orthogonal OBI kV radiographs and the couch lateral,
longitudinal and vertical drives were used to position the
phantom to a starting position in agreement with a CT
simulator produced digital reconstructed radiograph. A CBCT
was performed to confirm proper initial alignment at the
treatment position. Once this initial position was found, the
linac couch was locked to prevent any further motion. Thirtyone random offsets in the x, y and z directions were
generated using a Microsoft Excel spreadsheet. The shifts
were limited to ±1.0 cm in any one direction from the initial
setup position. The offsets were performed with the stage
micrometers. After the application of each offset, orthogonal
radiograph and CBCT alignment procedures were followed
and performed by a user blinded to the offset values. At no
time were the calculated shifts applied to the phantom.
Figure 1: The CIRS Head Phantom and Newport 460 XYZ
Micrometer on the treatment couch.
RESULTS (cont.)
The known offsets were compared to the values of the kV-kV
match and the CBCT match. The kV-kV alignment differed from
the known offset by a magnitude of 1.7 ± 0.6 mm averaged
over 31 measurements. The same comparison using CBCT
revealed a difference magnitude of 1.5 ± 0.7 mm. Evaluation of
all 93 pairs of OBI and CBCT shifts made for each random
offset, 46 (49%) of them were equal, 44 (47%) of them differed
by 1.0mm and 3 (3%) were different by 2.0mm. None were
greater than 2.0mm.
Table 1: Listing of differences on each axis and
corresponding vector magnitude between the applied
random offset and the respective shift made by OBI and
CBCT.
Figure 2: OBI Matching interface. On the overlay, the DRR is
shown in green and the acquired kV image in red.
♦ American Society for Therapeutic Radiology and Oncology (ASTRO) – Boston, MA – September 21 - 25, 2008