Download evaluation of image-guidance protocols in the treatment of head and

Int. J. Radiation Oncology Biol. Phys., Vol. 67, No. 3, pp. 670 – 677, 2007
Copyright © 2007 Elsevier Inc.
Printed in the USA. All rights reserved
0360-3016/07/$–see front matter
doi:10.1016/j.ijrobp.2006.09.040
CLINICAL INVESTIGATION
Head and Neck
EVALUATION OF IMAGE-GUIDANCE PROTOCOLS IN THE TREATMENT
OF HEAD AND NECK CANCERS
OMAR A. ZEIDAN, PH.D., KATJA M. LANGEN, PH.D., SANFORD L. MEEKS, PH.D.,
RAFAEL R. MANON, M.D., THOMAS H. WAGNER, PH.D., TWYLA R. WILLOUGHBY, M.S.,
D. WAYNE JENKINS, M.D., AND PATRICK A. KUPELIAN, M.D.
Department of Radiation Oncology, M. D. Anderson Cancer Center Orlando, Orlando, FL
Purpose: The aim of this study was to assess the residual setup error of different image-guidance (IG) protocols
in the alignment of patients with head and neck cancer. The protocols differ in the percentage of treatment
fractions that are associated with image guidance. Using data from patients who were treated with daily IG, the
residual setup errors for several different protocols are retrospectively calculated.
Methods and Materials: Alignment data from 24 patients (802 fractions) treated with daily IG on a helical
tomotherapy unit were analyzed. The difference between the daily setup correction and the setup correction that
would have been made according to a specific protocol was used to calculate the residual setup errors for each
protocol.
Results: The different protocols are generally effective in reducing systematic setup errors. Random setup errors
are generally not reduced for fractions that are not image guided. As a consequence, if every other treatment is
image guided, still about 11% of all treatments (IG and not IG) are subject to three-dimensional setup errors of
at least 5 mm. This frequency increases to about 29% if setup errors >3 mm are scored. For various protocols
that require 15% to 31% of the treatments to be image guided, from 50% to 60% and from 26% to 31% of all
fractions are subject to setup errors >3 mm and >5 mm, respectively.
Conclusion: Residual setup errors reduce with increasing frequency of IG during the course of external-beam
radiotherapy for head-and-neck cancer patients. The inability to reduce random setup errors for fractions that
are not image guided results in notable residual setup errors. © 2007 Elsevier Inc.
Image guidance, Head-and-neck cancers, Megavoltage CT, Alignment protocols, TomoTherapy.
that are subject to interfraction organ motion, such as the
prostate, will benefit most from the daily use of IG.
However, the gain is less obvious for targets that are less
mobile.
Targets in the head-and-neck region are, in general, assumed to be fairly rigidly attached to bony anatomy; and the
main source of uncertainty in positioning these patients is
setup error. Typically, the setup uncertainty is minimized
with head or head-and-shoulder masks that help to reposition and to immobilize the patient. The improvement in
setup accuracy that can be achieved for immobilized headand-neck patients with daily IG is not clear. It is the aim of
this study to assess the residual setup errors in immobilized
head-and-neck patients for imaging protocols that use IG
less often than daily. Using the daily alignment data from 24
head-and-neck cancer patients, eight protocols were retrospectively replayed. The protocols range from using no IG
INTRODUCTION
The increased use of conformal irradiation techniques has
been accompanied by the development of image-guidance
(IG) techniques to improve patient positioning (1– 6). All IG
techniques are designed to integrate into the treatment process, and the ease of operation of some of the developed
techniques allows their daily use. However, each technique
is still associated with cost in terms of machine time, patient
imaging dose, or both. This cost must be evaluated in the
context of improved target positioning.
It is clear that the daily use of an accurate and precise IG
technique results in the best possible patient positioning
over the course of treatment. Less frequent use of IG will
reduce the associated costs at the expense of reduced setup
accuracy. The gain in setup accuracy that is expected with
the daily use of IG will depend on the nature and magnitude of setup and organ-motion uncertainties. Targets
Acknowledgments—We thank Women Playing for T.I.M.E for
their support of our research efforts.
Conflict of interest: none.
Received April 19, 2006, and in revised form Sept 20, 2006.
Accepted for publication Sept 26, 2006.
Reprint requests to: Omar A. Zeidan, Ph.D., Department of
Radiation Physics, M. D. Anderson Cancer Center Orlando, 1400
S. Orange Ave., MP 730, Orlando, FL 32806. Tel: (321) 841-7037;
Fax: (407) 649-6895; E-mail: [email protected]
Drs. Sanford L. Meeks and Patrick A. Kupelian have a research
grant from TomoTherapy Inc.
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Image-guidance protocols for head-and-neck radiotherapy
to using IG with every other fraction. The residual setup
errors will quantify the improvement in setup accuracy for
this class of patients that can be achieved with daily IG.
METHODS AND MATERIALS
At the M. D. Anderson Cancer Center Orlando, a commercial
helical tomotherapy unit (Hi*Art II; TomoTherapy Inc., Madison,
WI) (7) has been used to treat patients with head and neck cancer.
Since October 2003, approximately 350 patients have been treated
with this modality. In our current clinical practice, all patients
treated are imaged using megavoltage CT (MVCT) scans before
each fraction. These MVCT scans are used to correct patient
position before each treatment. Daily alignment data were available for 24 head-and-neck cancer patients treated with helical
tomotherapy. The number of fractions per patient ranged from 20
to 35. A total of 802 image-guided treatments were available for
analysis, with an average of 33 treatments per patient. In all
patients, immobilization was achieved by use of a head mask
(MEDTEC, Orange City, IA), which was firmly attached to the
treatment couch in the same position for all patients.
Planning and daily alignments
For treatment-planning purposes, a kVCT scan (3-mm slice
thickness) was obtained of each patient. In preparation for treatment, each patient was initially aligned using room lasers and
permanent marks on the patient mask. A megavoltage (MVCT)
scan was subsequently obtained (3). The MVCT scans are obtained
with a normal or coarse pitch setting that corresponds to nominal
slice thicknesses of 4 and 6 mm, respectively. After the MVCT is
acquired, it is registered with the kVCT image to determine corrections to the patient position. Typically, a bony anatomy– based
automatic fusion algorithm is used. This algorithm is based on an
extracted feature fusion algorithm (8). After this initial registration, the radiation therapists visually inspect the images and have
the option of manually modifying the registration. On days when
the bony anatomy is deformed relative to the kVCT, optimal
alignment throughout the entire region of interest is not possible.
Priority is given to the registration of bony anatomy in the highdose area, i.e., the bony anatomy near the target areas.
Alignment protocols
To determine the accuracy and precision of the patient alignment if IG is used less often than daily, several protocols were
retrospectively replayed using the daily alignment data collected
from the 24 patients.
Protocol A: no imaging
In this protocol, each patient’s position is corrected only for a
machine-specific systematic positioning error. On our TomoTherapy unit, a systematic drop in the couch height exists between
the patient setup location at the virtual isocenter (outside the bore)
and the treatment isocenter (inside the bore). The magnitude of this
systematic couch drop was estimated from the first 100 patients
treated using the machine. The systematic couch shift is 8 mm in
the anterior–posterior (A/P) direction and 2 mm in the superior–
inferior (S/I) direction inferiorly. The systematic shift in S/I is
related to the A/P drop, as the drop is not unidirectional. Using this
protocol, the patient would never be imaged, but after alignment
with the room lasers, the systematic correction in the A/P and S/I
direction would be performed before treatment. The residual po-
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sitioning errors that would have occurred with this protocol can be
determined by subtracting the systematic corrections from the
daily image-guided positioning data. Since no MVCT images are
acquired, this protocol has the least workload.
Protocols B1, B3, B5, and B7: mean shift of initial one,
three, five, or seven fractions
The patient undergoes imaging for the first one, three, five, or
seven fractions. For these initial fractions, the patient position is
corrected according to the MVCT-kVCT registration. After one,
three, five or seven fractions, a mean shift is calculated and applied
to each subsequent fraction. No further images are obtained. Four
different protocols were investigated using this strategy. These
protocols are similar to the no-action–level protocol proposed by
de Boer and Heijmen, except that our protocols call for the patient
position always to be corrected on days when an MVCT image is
obtained (9).
Protocol C: weekly imaging with 3-mm threshold
This protocol is designed to mimic the common clinical practice
of weekly portal imaging. The patient undergoes imaging before
the first fraction. The shifts are applied to the first and all subsequent treatments. Weekly images are acquired. If any shift according to the MVCT-kVCT registration is ⱖ3 mm, the patient position is adjusted for this shift that day and for all subsequent
fractions. If the shift is ⬍3 mm, the daily shifts are not changed.
Protocol D: first five fractions, then subsequent weekly
imaging with patient-specific threshold
The patient undergoes imaging on Days 1 to 5 and is aligned
according to the MVCT-kVCT registration. A mean shift is calculated from these data and is applied on all subsequent treatment
days. From the corrections of the first five fractions, the standard
deviation of the shifts is calculated for each direction. Imaging is
subsequently performed on fraction numbers 10, 15, 20, and so on.
Because the patient shifts are available on these days, the patient
is properly aligned on Days 10, 15, 20, and so on. The initial shifts
calculated from the first five fractions are not changed unless any
patient correction is more than two times the standard deviation of
the shifts from the first 5 days. If the patient shift is ⬎2 standard
deviations, the subsequent fractions are corrected by this new
additional systematic shift for all subsequent fractions.
Protocol E: imaging every other fraction with
a running mean
The patient undergoes imaging every other day. A running
average shift is determined, and the patient’s position is corrected
by this running average on days when no image is obtained.
Frequency of IG
To determine the workload of each protocol, the percentage of
treatments that would have been imaged is scored. Using the
patient data from the current study, in protocol B with n ⫽ 7,168
(7 ⫻ 24) of the 802 treatments would have been image guided.
This corresponds to a frequency of 21%. Although not each
treatment is imaged guided, each fraction is still corrected according to the different IG protocols. For example, in protocol A (no
imaging), the imaging frequency is 0%, but a systematic correction
is still applied at each treatment fraction. In protocol B, the mean
shift of the initial fractions is calculated, and subsequently all
patient positions are corrected by this average correction. In prac-
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tice, these shifts would be implemented after the laser-based patient alignment. Therefore, whereas not each fraction is image
guided, each fraction is nevertheless corrected. This distinction is
important, because it is obvious that with an increased imaging
frequency, the residual setup error averaged over all fractions
decreases. However, the value of any protocol can also be evaluated by separately examining the treatments that are not image
guided. A protocol that is most effective in reducing the residual
positioning errors for treatments that are not image guided may be
preferable.
Data analysis
The difference between the daily alignment and the alignment
that would have been made had a particular image-guidance protocol been followed is calculated for each treatment day and IG
protocol. These data represent the residual setup errors that would
have occurred if the protocols had been used during the treatment.
To score the residual alignment errors in each protocol, population-based parameters are calculated based on the methodology
introduced by van Herk (10). The residual setup error for each
fraction is calculated from the difference between the actual daily
alignment and the alignment according to a given protocol. For
each patient, the systematic setup error, M, and the standard
deviation (sd) can be calculated for each protocol averaged over all
fractions. A population-based mean systematic setup error, m, is
calculated by averaging the systematic setup errors, M1⫺24, from
all patients. The standard deviation of the systematic error, 冱, is
equal to the standard deviation of the patient-specific systematic
errors, M1⫺24. The average random error over the whole patient
population, ␴, is determined by calculating the root-mean-square
of the random setup errors (sd) observed for each patient. In
addition the frequency of the residual positional errors can be
determined for each protocol. Population-based statistics and the
frequency of residual setup errors were separately calculated for
fractions that were not associated with IG to test the predictive quality
of each protocol.
Accuracy and precision of image-guided alignments
For treatments that are accompanied by IG, it cannot necessarily
be assumed that the residual positioning errors are zero after
MVCT-kVCT image registration. To determine the accuracy and
precision of the patient setup after MVCT-kVCT image registration, the alignment process was tested using a rigid wax-filled head
phantom. A kVCT scan of this phantom was obtained. To test the
accuracy of the alignment, the skull phantom was aligned with the
laser using attached fiducial markers. To avoid the known couch
sag between the virtual and the treatment isocenter, the phantom’s
vertical position was aligned at the treatment isocenter (i.e., inside
the bore). This setup assumes that a laser-based alignment at the
treatment isocenter represents an accurate alignment. Using this
setup, the phantom was scanned and the MVCT-kVCT registration
results were noted. This was repeated five times to test the accuracy of the image registration with respect to the laser-based
phantom alignment. A mean setup difference of 0.11, 0.1, and
⫺0.32 mm was found in the anterior–posterior, superior–inferior,
and lateral directions, respectively. Assuming that the reference
position, was the accurate phantom position, these values can be
used to estimate the systematic setup error, M. The standard
deviation of the systematic error, 冱, cannot be estimated, as only
one phantom plan was tested.
To test the precision of the MVCT-kVCT alignment the phan-
Volume 67, Number 3, 2007
tom was aligned at the virtual isocenter using a laser and fiducial
markers. The phantom was scanned and the MVCT scan was
registered with the kVCT scan. The resulting table shifts were
made and the table position was noted. This initial registration was
used to establish a reference table position. The phantom was then
moved by known amounts using table shifts and rescanned. The
second MVCT scan was again registered with the kVCT and the
table position was corrected accordingly. The resulting table positions were compared with the reference table position to determine the reproducibility of the MVCT to kVCT registration process. This was repeated 10 times to determine the precision of the
alignment. The standard deviation of the final table position was
0.2, 0.7, and 0.3 mm in the anterior–posterior, superior–inferior,
and lateral direction, respectively. The values can be used to
estimate the average random error, ␴.
RESULTS
A summary of the alignment data from the daily imageguided sessions is provided in Table 1. These data show the
daily setup corrections that were performed for the 24
patients (802 fractions). The systematic couch drop in the
anterior–posterior direction and the related systematic couch
shift in the superior–inferior direction is manifested in the 8.72
and ⫺2.8 mm respective mean systematic setup corrections,
M. The mean random corrections are in the range of 2 to 3 mm.
These are in agreement with random setup errors reported
in the literature for immobilized head-and-neck patients (11).
Figure 1 shows the mean systematic error, M; the standard deviation of the systematic error, 冱; and the average
random error, ␴, for the residual setup errors after each
protocol is applied retrospectively to the daily setup corrections. The data are presented for all treatments (all TX),
either image guided or not. In addition, the data for the
fractions that were not image guided (non-IGRT) are
shown. The data are plotted against the imaging frequency
associated with each protocol. An analysis of Fig. 1 shows
that all protocols are effective in reducing the mean systematic error, m, to less than 1.5 mm. The patient-to-patient
variation of the systematic error, 冱, does, in general, reduce
with increasing imaging frequency. This component is reduced to ⬍1.5 mm (averaged over all fractions) in all
directions for protocols with imaging frequencies of 9% or
more. A reduction of this variation to ⬃0.5 mm in all directions is observed in the protocol that uses IG in every other
fraction (imaging frequency 50%). If only the treatments
that are not image guided are analyzed, the patient-toTable 1. Mean systematic setup correction, M; standard
deviation of the systematic correction ,冱; and average random
correction, ␴, for the daily setup corrections after daily image
guidance was performed on 24 patients (802 fractions, average
33 per patient)
Direction
M (mm)
冱 (mm)
␴ (mm)
Anterior–posterior
Superior–inferior
Lateral
8.7
⫺2.8
0.0
2.2
2.6
1.6
2.3
2.2
2.3
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Fig. 1. The mean systematic setup error, M; the standard deviation of the systematic error, 冱; and the average random
error, ␴, for the residual setup errors are plotted against the workload for each protocol (a total of eight protocols). Data
are shown for all treatments (all TX) and for the fractions without image guidance (non-IGRT). (a– c) Results for the
three directions. IGRT ⫽ image-guided radiation therapy.
patient variation of the systematic error is larger but generally decreases with increasing imaging frequency. This
means that more frequent imaging reduces the systematic
errors in the nonimage-guided treatments.
In contrast to systematic errors, random errors are more
problematic. Random errors can be secondary to a multitude
of technical, patient, or treatment factors. The mean random
error in patient setup (averaged over all treatments) decreases with increasing imaging frequency, but no reduction
in the random error is seen if the nonimage-guided fractions
are examined. The frequencies of errors that are larger than
some tolerance are of interest clinically and allow an intuitive judgment of the patient alignment accuracy. The frequency distribution of residual errors is plotted for all treatment fractions and protocols in Fig. 2. This analysis is
repeated in Fig. 3 for the fractions that were not image
guided. If all treatments are evaluated, most imaging protocols with frequencies of ⬍50% result in misalignment ⬎5
mm in the anterior–superior direction in about 10% of the
fractions. In the superior–inferior and lateral direction the
frequency ranges from about 5–12% and 5–18%, respectively. However, with protocol E (imaging every other
fraction with a running mean), which has an imaging frequency of 50%, the rate of such errors is reduced to 2–3%
in each direction. If a 3-mm margin is used, the frequency
of partial misalignments is still about 9% in all directions if
protocol E (imaging every other fraction with a running
mean) is used.
If only treatments are evaluated that are not accompanied
by IG (Fig. 3), the frequency of residual setup errors varies
less with different IG protocols. The magnitude of the
three-dimensional displacement can be calculated from the
residual directional setup errors. The frequency of this vector is plotted in Fig. 4 for all treatments and the nonimageguided treatments. Table 2 lists the percentage of all treatments in which the three-dimensional residual setup errors
exceed 3, 5, and 10 mm. For protocols that have an imaging
frequency between 15% and 31%, the percentage of treat-
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Fig. 2. The frequency of residual setup errors is shown for all protocols. All treatment fractions are included in this
analysis (i.e., image guidance [IG] and non-IG). (a– c) Results for the three directions. IGRT ⫽ image-guided radiation
therapy; TX ⫽ treatment.
ments that are subject to residual errors ⬎3 mm and 5 mm
ranges from 50% to 60% and 26% to 31%, respectively. If
every other treatment is image guided and the nonimageguided treatment is corrected by a running average, still
about one-third of all treatments is associated with a setup
error of at least 3 mm. Using this protocol, about 10% of all
treatments are limited by setup errors of at least 5 mm. The
percentage of IG for each protocol is shown to illustrate the
tradeoff between residual error frequency and the imaging
workload per protocol.
DISCUSSION
The availability of daily alignment data allows the evaluation of different image-guidance protocols. The gain in
alignment accuracy and precision that is achieved if daily
IG is used can thus be assessed. Furthermore, systematic
and random contributions to the residual setup uncertainty
can be calculated. The above findings, namely, that imaging
protocols are only effective in reducing the systematic component of the setup errors, are in agreement with expectations. This systematic component can be better estimated
with more frequent imaging. The random error component
is only reduced if averaged over all fractions. No reduction in the random error is seen for the fractions that are
not image guided. Residual setup errors in each IG strat-
egy are mostly the result of the random position fluctuations.
The frequency of residual errors allows a judgment of the
clinical impact of each imaging protocol. The frequency of
setup errors that can be tolerated is a clinical decision that
may depend on the specific treatment site and patient-specific
conditions. The ability of each protocol to reduce the frequency of setup errors when IG is not performed can be
deduced from Fig. 3. In each direction, Protocol D (IG in the
first five fractions and weekly thereafter) fairs among the
worst in terms of reducing setup errors. Not surprisingly, the
two protocols with the lowest imaging frequency are also
the least effective in reducing setup errors when IG is not
used. Conversely, the protocol that calls for imaging at
every other fraction (i.e., highest imaging frequency evaluated) is best at reducing setup errors on days when IG is not
performed. If the magnitude of setup errors is evaluated as
a three-dimensional vector, the residual setup errors are
obviously larger than if the three dimensions are evaluated
separately. From Fig. 4a and Table 2 it can be seen that with
an imaging frequency of 50%, nearly 11% of all treatments
are subject to a three-dimensional setup error of ⬎5 mm.
For the same protocol about one-third of all treatments is
limited by setup errors ⬎3 mm.
It is clear that an improvement in setup accuracy and
precision can be achieved if each fraction is accompanied
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Fig. 3. The frequency of residual setup errors is shown for all protocols. Only treatments that are not associated with
image guidance are included in this analysis. (a– c) Results for the three directions. IGRT ⫽ image-guided radiation
therapy; TX ⫽ treatment.
by IG. The phantom tests show submillimeter accuracy and
precision of the MVCT to kVCT registration process. However, it must be kept in mind that the skull phantom is a rigid
body and presents a best-case scenario for MVCT to kVCT
image registration. In reality, patient anatomies may deform
during the course of treatment, and bony anatomy registration in the head-and-neck region is frequently compromised
by changes in the patient’s anatomy. A margin to account
for deformation and other uncertainties in the delivery proccess is necessary and must be added irrespectively of setup
Fig. 4. The frequency of residual three-dimensional setup errors is shown for all protocols. (a and b) Results for all
treatments and the nonimage-guidance (IG) treatments. IGRT ⫽ image-guided radiation therapy; TX ⫽ treatment.
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Table 2. Frequencies of residual errors observed with different protocols exceeding 3, 5, and 10 mm
Protocol
A: No imaging
B1: Initial fraction only
B3: Mean of initial three fractions
B5: Mean of initial five fractions
B7: Mean of initial seven fractions
C: Weekly imaging, 3-mm threshold
D: First five fractions ⫹ weekly imaging, patient-specific
threshold
E: Imaging every other fraction, running mean
IG%
Error ⬎3 mm
Errors ⬎5 mm
Errors ⬎10 mm
0%
3%
9%
15%
21%
20%
31%
72% ⫾ 3%
79% ⫾ 3%
63% ⫾ 3%
58% ⫾ 3%
53% ⫾ 3%
60% ⫾ 3%
50% ⫾ 2%
37% ⫾ 2%
47% ⫾ 2%
31% ⫾ 2%
26% ⫾ 2%
26% ⫾ 2%
31% ⫾ 2%
27% ⫾ 2%
4% ⫾ 1%
5% ⫾ 1%
1% ⫾ 0.5%
1% ⫾ 0.5%
1% ⫾ 0.5%
2% ⫾ 0.5%
4% ⫾ 1%
50%
29% ⫾ 2%
11% ⫾ 1%
0.5% ⫾ 0.3%
The three-dimensional setup error is the vector calculated from the remaining setup errors in the three directions. Data in this table are
calculated for all (i.e., image-guided as well as nonimage-guided, treatments. The uncertainty in the results was estimated by using the square
root of N, where N is the number of observations. The percentage of fractions that are image guided (IG%) according to each protocol is also shown.
errors. This study also inherently assumes that patient setup
based on bony anatomy is optimal. In the head-and-neck
region, organ movement is minimal; however, progressive
deformation of the patient’s soft tissue structures during the
course of treatment has been reported (12). To account for
soft tissue deformation in head-and-neck patients at the time
of patient positioning would require a target structure that is
in its entirety well visible on the daily image. In the headand-neck region, target volumes are frequently complex,
and their identification is not trivial on kVCT scans. MVCT
images are noisier than kVCT images, and the identification
of subtle soft tissue contrasts is difficult, particularly given
that the radiation therapist has to register the MVCT images
in a limited time frame while the patient is waiting to be
treated (13). A bony anatomy– based MVCT to kVCT registration is therefore a reasonable proxy for soft tissue
alignment in the head-and-neck region. Furthermore, patients may move during the actual treatment. The MVCT to
kVCT registration cannot account for this uncertainty, and
its effect must be minimized with the use of appropriate
PTV margins.
The daily use of MVCT guidance is associated with an
increasing patient imaging dose as well as the machine time
per patient. In our clinical practice the acquisition of the
MVCT, image registration, and adjustment of patient alignment adds about 5 to 10 min to the in-room time. In comparison, the typical beam-on time for head-and-neck treatments is
⬍10 min. To estimate the dose from MVCT imaging dose
measurements were performed in the center of a 20-cm
cylindrical acrylic phantom. For images acquired with a
normal or coarse pitch setting, the doses were 0.7 and 0.4
cGy, respectively (13). Contrary to portal imaging, MVCT
dose is distributed throughout the imaged patient anatomy.
This means that anatomy that is removed from traditional
beam ports will receive imaging doses that may not have
been given if traditional portal images had been used. However, this is not likely to be the case in head-and-neck cancer
patients in whom the entire anatomy is included in traditional double-exposure portal images.
The benefit of daily IG for head-and-neck cancer patients
can be assessed from our results. Reducing the imageguidance frequency from daily to 50% results in 11% of all
treatments to be subject to a misalignment error of at least
5 mm. Approximately one-third of all treatments are subject
to a ⱖ5-mm misalignment if protocols are used that use IG
on a weekly basis. The effect of these misalignments on the
target coverage can be mitigated if appropriate PTV margins are used. For patients with target volumes in close
proximity to critical sensitive structures, PTV margins
may have to be small. In these cases daily IG should be
used.
CONCLUSION
Systematic errors throughout the course of treatment can
be reduced with protocols that involve infrequent imaging.
However, random setup errors are not eliminated with any
of the imaging protocols investigated in the current study.
Protocols that use IG for 0 –50% of all treatments have
three-dimensional residual setup errors of ⱖ5 mm in 47–
11% of all treatment. The acceptability of the reduced
setup accuracy and precision that is caused by infrequent
use of IG depends on treatment margins and the proximity of sensitive structures. If target structures are in close
proximity to critical sensitive structures, IG should be
used daily.
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