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ASSESSMENT OF A RADIOTHERAPY PATIENT CRANIAL
IMMOBILIZATION DEVICE USING DAILY ON-BOARD KILOVOLTAGE
IMAGING
JOSEPH HARMON, DEREK VAN UFFLEN, SUSAN LARUE
The purpose of this study was to utilize state-of-the-art on-board digital kilovoltage (kV) imaging to determine
the systematic and random set-up errors of an immobilization device designed for canine and feline cranial
radiotherapy treatments. The immobilization device is comprised of a custom made support bridge, bite block,
vacuum-based foam mold and a modified thermoplastic mask attached to a commercially available head rest
designed for human radiotherapy treatments. The immobilization device was indexed to a Varian exact couch-
top designed for image guided radiation therapy (IGRT). Daily orthogonal kV images were compared to
Eclipse treatment planning digitally reconstructed radiographs (DRRs). The orthogonal kV images and DRRs
were directly compared online utilizing the Varian on-board imaging (OBI) system with set-up corrections
immediately and remotely transferred to the treatment couch prior to treatment delivery. Off-line review of 124
patient treatments indicates systematic errors consisting of þ 0.18mm vertical, þ 0.39mm longitudinal and
�0.08mm lateral. The random errors corresponding to 2 standard deviations (95% CI) consist of 4.02mm
vertical, 2.97mm longitudinal and 2.53mm lateral and represent conservative CTV to PTV margins if kV OBI
is not available. Use of daily kV OBI along with the cranial immobilization device permits reduction of
the CTV to PTV margins to approximately 2.0mm. Veterinary Radiology & Ultrasound, Vol. 50, No. 2, 2009,
pp 230–234.
Key words: immobilization device, IGRT, kilovoltage, on-board imaging, radiation therapy, repositioning
accuracy.
Introduction
EFFECTIVE PLANNING AND delivery of external beam ra-
diation requires thorough knowledge and understand-
ing of patient set-up uncertainties. ICRU1 Report no. 62
(supplement to ICRU Report no. 50) defines a planning
target volume (PTV) generated by adding a safety margin
to the clinical tumor volume (CTV) to account for inter-
fractional and intrafractional movement.
When considering intracranial radiotherapy using robust
immobilization methods, a stable treatment couch and
proper anesthesia, intrafractional variability is negligible.
However, interfractional movement can be significant,
depending on the design of the immobilization device,
repeatability and stability of couch position, accuracy of
set-up lasers and type/availability of portal imaging. The
advantage of kV vs. MV imaging for daily set-up includes
less excess dose accumulation, less interobserver alignment
variability,2,3 improved soft tissue contrast and, depending
on the specific imager design, the potential for improved
spatial resolution. Minimizing the CTV to PTV margin
benefits the patient by minimizing the volume of healthy
tissue irradiated, thus reducing complications. This is of
particular importance in cranial radiotherapy treatments
due to the proximity of the treated volume to critical
structures such as the spinal cord, brainstem, parotid
glands, optic nerves, and inner ear. The reduced PTV
margin may also permit dose escalation.
The goal of this study was to determine the appropriate
PTV margin for intracranial treatments using a newly de-
veloped indexable cranial fixation device. The fixation
device was designed for a wide range of patient sizes, the
ability to permit anesthesia ventilation during treatment,
ease of initial and daily set-up, compatibility with the in-
dexed couch top, and suitability for kV imaging. Tools
used for determining the repositioning accuracy and pre-
cision of the device include orthogonal kV images obtained
with on-board imaging (OBI).
Materials and Methods
Nineteen dogs and three cats were treated with the cra-
nial fixation device with a total of 124 individual treatment
sessions being delivered. Patient fractionation ranged
from 1 to 21 fractions and included three-D radiation
therapy, stereotactic radiosurgery, intensity-modulated ra-
diation therapy, and intensity-modulated radiosurgery.
Address correspondence and reprint requests to Joseph Harmon, at theabove address. E-mail: [email protected]
Received June 5, 2008; accepted for publication September 15, 2008.doi: 10.1111/j.1740-8261.2009.01522.x
From the Environmental & Radiological Health Sciences, ColoradoState University, 1681 Campus Delivery, Fort Collins, CO 80523.
230
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The distribution of patient tumor locations included 10
involving the pituitary, two involving the nasal area, and
the remaining 10 distributed throughout the brain.
Components of the immobilization device include a
baseplate, vacuum bead style moldable cushion, custom
upper jaw support bridge, bite block and thermoplastic
mask. The baseplate (Civco model MT-20100CF�) is made
of carbon fiber and attaches (indexes) to a wide variety of
couch tops using a locking bar. A carbon fiber laminate
support bridge was designed and built to rigidly attach to
the base plate using an inner set of pins already existing in
the base plate. For human use, the inner set of pins in the
baseplate are used for a removable headrest. Because pre-
vious animal immobilization studies4–6 have illustrated the
importance of using a bite block for reproducible cranial
fixation, the support bridge was designed with three holes
across the top to permit a custom thermoplastic bite block
(made from thermoplastic pellets, Civco model MT-APS
3A�) to be easily attached and removed between treatments.
Together, the support bridge and bite block secure the
patient’s maxilla during treatment in the prone, head-first,
position. The evacuated bead style cushion (Civco model
MT-VL-37) adapts to the shape of the patient and base
plate during evacuation and supports the patient’s mandi-
ble and neck, as well as providing additional lateral posi-
tioning stability for the upper torso. Finally, a
thermoplastic mask (Civco model MT-APU) is affixed to
the base plate over the top of the immobilization assembly
to secure the patient to the bite block and cushion. The
mask is modified by cutting a triangle-shaped section out
of the cranial end thus permitting undisturbed access to
ventilation tubing during application or removal of the
mask (Fig. 1).
Each patient received an initial CT simulation scan using
a Picker PQ-2000 scanner, typically using a 2mm slice
thickness to balance the need for image contrast and res-
olution of the digitally reconstructed radiograph (DRR).
To ensure the base plate was properly aligned with the
longitudinal axis of the scanning couch, a Varian carbon
fiber OBI couch insert was attached to the Picker couch top
using custom designed adapters. The use of the OBI couch
insert permitted indexing during the scanning process iden-
tical to that during the treatment process. During the CT
simulation process, CT set-up lasers were used to identify a
relative set-up (simulation) isocenter. The location of the
simulation isocenter was marked on the mask and also
identified by use of 1.5mm metal bead markers (Spee-D-
Mark model SDM-BB15�).All treatment plans were performed using a Varian
Eclipse treatment planning system (TPS) using software
version 8.1 and the Analytical Anisotropic Algorithm for
the photon dose distributions. During the planning process,
the treatment isocenter was determined and both dorsal and
lateral DRRs generated for the treatment position. DRRs
were optimized to enhance visualization of bony structures.
On the first day of treatment, using a Varian Trilogy
with 6MV photon beam, each patient was placed within
the immobilization device in the same manner used during
simulation. The treatment room set-up lasers and patient
mask with BBs were then used to place the simulation
isocenter at the location of the treatment room (linac)
isocenter. Appropriate couch shifts (vertical, lateral, and
longitudinal) generated by the treatment planning system
were then applied to the couch to place the treatment plan
isocenter at the treatment room isocenter.
Orthogonal digital radiographs were used to compare
the actual treatment position to the position indicated by
the DRR of the treatment planning system. Typically,
pulsed fluoroscopy mode with automatic brightness con-
trol was used to quickly select optimum technique settings
for the relevant anatomy. The pulsed fluoroscopy images
contain 1024� 768 pixels of image data collected in a
2 � 2 binned mode with an aSi imaging panel. A high
resolution, single exposure, 1 � 1 binned mode is also
available providing 2048� 1536 for select patients that re-
quire higher resolution. For this study, all images were
obtained in the 2 � 2 binned mode to quickly acquire
images. Required couch shifts were determined using
extensive manual and/or automatic matching tools built
into the Varian OBI kV–kV analysis software. Evaluation
Fig. 1. (A) Illustration of cranial immobilization system components 365 � 274mm (96 � 96DPI). (B) Example of cranial immobilization system in use.
�Civco, Orange City, IA.
231CRANIAL IMMOBILIZATION DEVICEVol. 50, No. 2
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of patient set-up was determined by comparing unique bony
landmarks using both image overlay and ‘‘spyglass’’ meth-
ods of image comparison. The spyglass is particularly useful
as it permits one to see the DRR within a user defined,
moveable, rectangular portal within the acquired kV OBI
image. Rotation of the patient, if required, was also an op-
tion available during the matching process. Figure 2 illus-
trates the kV–kV matching workspace with an exaggerated
shift. As illustrated in the left lateral view, there is a need to
raise the treatment couch top to overlay the anatomy shown
in the actual kV image with that in the treatment plan
DRR. Similarly, as illustrated in the dorsal view, there is a
need to shift the treatment couch top to the right to overlay
the actual kV image and treatment plan DRR anatomy.
Once a suitable image match was found, the corre-
sponding couch shifts were remotely transferred to the
Fig. 2. Exaggerated example of vertical and lateral shift between digitally reconstructed radiograph (DRRs) and kV radiographs. Spyglass tool is shown.
Fig. 3. Example of daily treatment position corrections for a single patient based on orthogonal kV imaging.
232 HARMON ET AL. 2009
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treatment couch. The final couch position for the first
treatment session was used as the reference condition for
all subsequent treatment sessions. Thus, on each subse-
quent treatment session, the patient was placed in the im-
mobilization device, indexed to the original couch notches
and the couch returned to the session 1 coordinates. A set
of orthogonal images are then acquired, compared with the
original TPS DRRs, and any required shifts are applied
remotely. The Varian ARIA patient management system
stores all paired images and records each session’s shift
values for follow-up analysis.
Results
A review of the couch shift required to place the patient
in the correct treatment position was performed for all
patient treatment sessions. An example of the typical couch
shifts during a fractionated treatment course is shown in
Fig. 3. Each shift represents the difference between the
reference (day 1) couch position and the final treatment
couch position. In this example, the vast majority of the
shifts fall within 2mm. A summary of all 124 treatment
sessions is shown in Table 1. The average shift and rotation
values represent the overall set-up accuracy and reflect any
systematic errors during set-up. Regardless of the immo-
bilization device design, systematic errors are possible if the
imaging device is not properly calibrated. Thus, a rigorous
quality assurance procedure is followed based on manu-
facturer specifications and published protocols7 to insure
submillimeter accuracy. The precision of the immobiliza-
tion system represents the random errors associated with
the immobilization system. The precision values presented
in Table 1 correspond to two standard deviations (the 95%
confidence interval).
Because the soft tissue in the neck area does not permit
rigid fixation as the bite block does, we evaluated the data
to determine if a trend existed relating magnitude of av-
erage error with distance of treatment isocenter from the
support bridge. Figure 4 presents the position error as a
function of distance of treatment isocenter from the infe-
rior edge of the support bridge. The data does reveal a
slight trend of increasing error (from approximately
0.5mm error at a distance of 4 cm to approximately
1mm error at a distance of 10 cm) in the vertical and lon-
gitudinal direction but not with rotation or lateral direc-
tion. Because the device has been used for a wide range of
patient sizes, we also investigated average error as a func-
tion of patient weight. Figure 5 presents average position
error as a function of patient weight and indicates very
little correlation between patient weight and error except
possibly in the vertical direction where the error actually
decreases from approximately 1mm at 2kg to approxi-
mately 0mm at 43kg.
Table 1. Summary of Radiotherapy Immobilization Device Patient Set-Up Accuracy and Precision Results (Based on 124 Patient Treatments)
MeasureVertical(mm)
Longitudinal(mm)
Lateral(mm)
Rotation(1)
Average error (accuracy) þ 0.18 þ 0.39 �0.08 �0.0152s (precision) 4.02 2.97 2.53 0.32
Fig. 4. Treatment position corrections vs. distance from support bridge to treatment isocenter.
233CRANIAL IMMOBILIZATION DEVICEVol. 50, No. 2
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Discussion
The results of this study indicate submillimeter patient set-
up accuracy (systematic error) and precision (random error)
on the order of 2–4mm. These values represent a conser-
vative estimate of the magnitude of daily set-up variation
corrected for using kV OBI. To treat the same group of
patients at our institution using the cranial immobilization
device without the use of kV OBI, one would need to ensure
PTV margins account for these set-up variations to ensure
the PTV receives the prescribed dose at least 95% of the
time. Because we were not using lasers for the daily patient
set-up, the variability in couch shift corrections reflect a
combination of the accuracy and precision of the digital
couch readout and variability in patient movement within
the immobilization device. Routine quality assurance testing
indicates the couch repositioning variability to be � 1mm.
Thus, the remaining variability presented in Table 1 reflects
patient movement within the immobilization device.
The data presented in Fig. 4 indicate a slight improvement
in overall accuracy the closer the treatment isocenter is to the
support bridge. The trend is most prominent in the vertical
and longitudinal shift axes and not significant for the lateral
shift and rotation correction. The vertical shift error trend is
not unexpected due to the fact the support bridge with bite
block is secure and acts as a pivot point with variation in
patient soft tissue placement within the vacuum foam mold
causing minor vertical displacement variation. Less correla-
tion is noted between positioning error and patient weight as
demonstrated in Fig. 5 thus permitting use of the device on a
wide range of patient sizes.
We have found the cranial fixation device to be easy to
use and, so far, one size support bridge has worked well
with all patients treated. Use of this cranial immobilization
device at an institution with accurate and precise couch
repositioning but without access to kV OBI would require
CTV to PTVmargins ranging from 2 to 4mm to ensure the
PTV received the prescribed dose at least 95% of the time.
Utilizing both daily kV OBI and this immobilization device
permits one to comfortably reduce the CTV to PTV
margin to 2mm for most patients.
REFERENCES
1. International Commission on Radiation Units and Measurements.Prescribing, recording, and reporting photon beam therapy (supplement toICRU Report 50), ICRU Report 62. Bethesda, MD: ICRU Publications,1999.
2. Pisani L, Lockman D, Jaffray D, et al. Setup error in radiother-apy: on-line correction using electronic kilovoltage and mega-voltage radiographs. Int J Radiat Oncol Biol Phys 2000;47:825–839.
3. Mechalakos J, Hunt M, Lee M, Hong L, Ling C, Amols H. Using anonboard kilovoltage imager to measure setup deviation in intensity-modu-lated radiation therapy for head-and-neck patients. J Appl Clin Med Phys2007;8:28–44.
4. Kippenes H, Gavin PR, Sande RD, Rogers D, Sweet V. Compar-ison of the accuracy of positioning devices for radiation therapy of canineand feline head tumors. Vet Radiol Ultrasound 2000;41:371–376.
5. Kippenes H, Gavin PR, Sande RD, Rogers D, Sweet V. Accuracy ofpositioning the cervical spine for radiation therapy and the relationship toGTV, CTV and PTV. Vet Radiol Ultrasound 2003;44:714–719.
6. Bley CR, Blattmann H, Roos M, Sumova A, Kaser-Hotz B. Assess-ment of a radiotherapy patient immobilization device using single plane portradiographs and a remote computed tomography scanner. Vet RadiolUltrasound 2003;44:470–475.
7. Yoo S, Kim G, Hammoud R, et al. A quality assurance program forthe on-board imager. Med Phys 2006;33:4431–4447.
Fig. 5. Treatment position corrections vs. mass of patient. Also shown is frequency distribution of patient mass.
234 HARMON ET AL. 2009