METHODS
In the present study, 62 EPI images belonging to 13 head and neck patients were evaluated separately by
four therapists as offline, and the amount of shift in the center of fields was determined. CBCT obtained
at the same time with the EPI images was accepted as reference, and the amount of shift in the center of
fields was compared separately for each therapist with the results of EPI.
RESULTS
According to our results, the amount of shift in the center of fields had changed between therapists with
0?9.4 mm in the reference of CBCT. The probability of shifting center of fields to be >3 mm was 60% for
the first therapist, 35% for the second therapist, 63% for the third therapist, and 50% for the fourth therapist.
The probability of shifting center of fields to be >5 mm was 24%, 8%, 27%, and 14.5%, respectively.
Analysis of variance for repeated measures test was applied to center shift values, and there were a significant
difference in the groups (sig. <0.05) and a significant difference between the groups (sig. <0.05).
CONCLUSION
The usage of CBCT for the verification of treatment fields eliminates the differences in interpersonal
evaluation. CBCT improves the set-up accuracy such that planning tumor volume expansion margin
can be safely dropped. Therefore, CBCT should be the preferred imaging modality in intensity-modulated
radiation therapy planning.
Keywords: CBCT; head and neck; IGRT
In daily practice, with an increasing number of patients
undergoing IGRT, some procedures can give
charge to radiation therapists (RTTs). RTTs must be
aware of the potential dosimetric impact of position verification
procedures, as well as their influence on required
margins for HNC radiation therapy. Several studies show
that the correctness of the interpretation of RTTs can be
improved with proper education and training and may
come to a level comparable to radiologists.[
Studies have shown that using volumetric imaging
(cone beam computed tomography (CBCT)) instead of
2D imaging improves the dosimetric results. Li et al. determined
that 3D imaging is more likely to detect shifts
>3 mm than 2D imaging (18% vs. 11%).[
The set-up accuracy is important in IMRT to avoid
the geographical misses increasing the risk of recurrence.
In radiation therapy, the set-up errors in the
treatment fields are determined by using portal imaging
and CBCT. The images obtained during treatment are compared with the planning images to verify the
treatment fields.
In the present study, we aim to determine the electronic
portal imaging (EPI) evaluation differences between
the therapists in the reference of CBCT.
Treated patients received their prescribed doses between 10 and 35 fractions. For each patient, megavoltage (MV)-CBCT and electronic portal imaging device (EPID) images were acquired 2 times/week. CBCT was obtained at the same time with the EPI images.
Electronic Portal Imaging
Before radiotherapy, two orthogonal EPID images (AP
or PA and left to right) were acquired using an Optivue
1000ART amorphous silicon flat panel detector. The
flat panel has a sensitive area of 409.6×409.6 mm and
an imaging matrix of 1024×1024 with a resolution of
0.4×0.4 mm2. The images were acquired using coherence
therapist software. The bone tissues were marked
on the digitally reconstructed radiograph. Thereafter,
manual adaptation of the bony anatomy was conducted
by RTTs. The set-up error was recorded in two directions
in each image, and the error was calculated in
three dimensions as shown in Figure
Cone Beam Computed Tomography
MV-CBCT images were obtained by using the X-ray
beam with 4 MV energy produced for only imaging. The Optivue flat panel detector was attached to
the linac on the opposite side of the treatment head
at a distance of 1450 mm. Eight MU half circle scan
protocol with 200 projections over an arc of 200 was
used for obtaining images. By using the flat panel detector,
image projections were obtained, and filtered
back projection algorithm was used to acquire threedimensional
volumetric CT image set. CBCT images
were automatically registered to the planning scan using
mutual information-based registration algorithm
for bone, air, and soft tissue. For checking positional
errors, CBCT images were matched with the planning
CT. The set-up error was recorded in three directions
as shown in Figure
Patient Data
A total of 62 EPI images belonging to 13 head and neck
patients treated in our department were evaluated separately
by four therapists as offline, and the amount
of shift in the center of fields was determined. CBCT
obtained at the same time with the EPI images was accepted
as reference, and the amount of shift in the center
of fields was compared separately for each therapist
with the results of EPI.
The Statistical Package for Social Sciences (SPSS Inc., Chicago, IL, USA) version 22.0 was used for statistical analysis. Analysis of variance for repeated measures (ANOVA) test was applied to center shift values. A p value of <0.05 was considered to be significant.
The probability of shifting center of fields to be >3
mm was 60% for the first therapist, 35% for the second
therapist, 63% for the third therapist, and 50% for the
fourth therapist. The probability of shifting center of
fields to be >5 mm was 24%, 8%, 27%, and 14.5%, respectively.
ANOVA test was applied to center shift values
by using the SPSS program, and there were no significant
difference in each RTT group (sig. >0.05) (Fig.
There are multiple types of technology that can be used for IGRT. The technologies that might be used for CBCT can include EPIDs, stereoscopic kV imaging, CBCT, CT-on-rails, MV-CT, X-ray real-time tracking systems combining 2D orthogonal kV and infrared, ultrasound, magnetic resonance imaging, and electromagnetic systems.
The type of system used will depend on the resources of departments and the accuracy of the type of treatments that need to be delivered.
The movements of tumor and tissues may be significant from second to second, day to day, week to week, or longer. The predictable, irregular, or permanent movements may exist in the therapeutic region. Some of them will be significant, whereas others not. Imaging before treatment can overcome movement problems and increase awareness of the range of organ motion, set-up errors, and changes in tumor size and shape that can occur in clinical practice. Keeping patients immobile during treatment, reducing organ movement, and optimizing irradiated volumes provide conformation of the dose around the tumor, achieving greater healthy tissue sparing.
X-ray beams are used to obtain images with good
image quality and adequate for detecting bone structures
in EPID.[
In the present study, set-up differences between
RTTs have been evaluated for head and neck patients.
We have performed on 13 patients and 62 fractions to
determine the evaluation differences of EPI images between
therapists. Although the EPID technology has
been very much improved, it is still not easy to decide
the real location of the bony or soft tissues with EPID.
There are many clinics that treat their patients with
IMRT technique without using CBCT imaging. However,
because of high-dose gradients in IMRT treatments,
the set-up accuracy is very important, and the
only usage of immobilization materials (e.g., mask and
index bar) is not enough to avoid geographical misses.
In addition, the evaluation of the EPI images is highly
dependent on who evaluates them. The usage of CBCT
for the verification of treatment fields eliminates the differences
in interpersonal evaluation. CBCT improves
the set-up accuracy such that planning tumor volume
(PTV) expansion margin can be safely dropped. Therefore,
CBCT should be the preferred imaging modality in IMRT planning. Our results suggest that PTV margins
can be safely reduced if daily CBCT is possible.
Hawkins et al. have also reached the same conclusion
in their study.[
Peer-review: Externally peer-reviewed.
Conflict of Interest: None declared.
Ethics Committee Approval: None declared.
Financial Support: None declared.
Authorship contributions: Concept ? E.K., A.H.; Design
? E.K.; Supervision ? A.H.; Materials ? M.A., A.H.; Data collection
&/or processing ? B.B., M.A.G., A.K.; Analysis and/
or interpretation ? E.K.; Literature search ? B.D.; Writing ?
E.K.; Critical review ? A.H.
