1Department of Radiation Oncology, Aja University of Medical Sciences, Iran
2Department of Radiation Oncology, Roshana Radiation Oncology Center, Iran
3Department of Radiation Oncology, Iran University of Medical Sciences, Iran
4Department of Radiation Oncology, Isfahan University of Medical Sciences, Iran
5Department of Medical Physics, Iran University of Medical Sciences, Iran
Submission: May 23, 2018; Published: June 27, 2018
*Corresponding Address: Hamed ghaffari, Department of Medical Physics, School of Medicine, Iran University of Medical Sciences, Junction of Shahid Hemmat and Chamran Expressway, Tehran, Iran, Tel: +989118555899; Email: firstname.lastname@example.org
How to cite this article: SeyedHadi M, Farshid A, Mastaneh S, Elham H, Mina T. et al,. Evaluation of Patient setup Accuracy and Determination of
Optimal Setup Margin for External Beam Radiation therapy using Electronic Portal Imaging Device. Canc Therapy & Oncol Int J. 2018; 11(2): 555808.
Introduction: The setup errors in patient positioning during radiotherapyhave a critical role in the tumor control and overdose of normal tissues. The purpose of this study was to evaluate the patient setup errors and propose optimum planning target volume (PTV) for various treatment sites using electronic portal imaging device (EPID).
Methods:A total of 748 fractions were analyzed from 73 patients treated with 3DCRT for head and neck (H&N), brain, pelvic and prostate. Population systematic and random errors and 3D vector of shifts were calculated. PTV margins for different confidence levels were determined using van Herk’s formula.
Results: Population systematic and random errors were 0.7, 0.86 and 0.6 mm and 0.5, 0.45 and 0.9 mm for H&N cases, 0.61, 0.8 and 0.93 mm and 0.3, 0.4 and 0.72 mm for brain cases, 1.3, 1.26 and 1.51 mm and 0.63, 0.92 and 0.94 mm for pelvic cases and 0.69, 0.86 and 1.0 mm and 2.11, 2.3 and 2.31 mm for prostate cases in vertical, longitudinal and lateral directions, respectively. Three-dimensional vector displacements ≥7 mm was 0 % for brain cases and rare for other treatment sites. PTV(90,95) margins were less than 3 mm for H&N and brain sites and are less than 5 mm for pelvic and prostate cases in three translational directions.
Conclusion:The setup errors depend on the tumor site. The use of image guidance technique is an effective tool for setup verification.
The aim of radiation therapy (RT) is to reliably maximize the dose to the target while minimizing the toxicity to the normal tissues. Therefore, daily treatment setup is considered as a critical requirement in RT for an accurate dose delivery. The planning target volume (PTV) is defined as the clinical target volume (CTV) plus a margin to account for patient positioning uncertainties, beam alignment and organ motion (i.e. setup margin and internal margin). Setup margins have a direct effect on the coverage of target volume. Thus, these should be optimized to prevent inadvertent irradiation of organ at risks (OARs) [1,2].
Setup uncertainties can be divided into two categories: systematic errors and random errors.Whereas the random errors blur the dose distribution, the systematic component of errors leads to a shift of the cumulative dose distribution relative to the target. The systematic errors are reproducible consistent errors, occurring in the same direction and magnitude but random (day-to-day) errors can vary in direction, magnitude and are unpredictable. The systematic errors in contrast to the random errors are more dangerous because they affect all treatment sessions. Thus, systematic errors may lead to the recurrence of the tumor or serious damage in normal organs .
The introduction of image guided RT (IGRT) allows to
reduce the size of PTV-CTV expansion. Recently, image guidance
techniques such as ExacTrac, cone beam computed tomography
(CBCT) and electronic portal imaging devices (EPIDs) provide
the accuracy of patient positioning and target localization for RT
[4-6]. The widespread availability of EPIDs has led to it be an
effective tool to decrease setup errors. Pre-treatment electronic
portal images (EPIs) provide the evaluation of setup errors [7,8].
In this study, setup errors evaluation is doing using a portal
image and a digitally reconstructed radiograph (DRR). The
objective of this study was to quantify the inter-fractional setup
errors and 3D vector lengths and calculate CTV-PTV margin for
different treatment sites such as head and neck (H&N), brain,
pelvic and prostate by electronic portal images guidance and
determine the optimal PTV margins.
Seventy-three patients with cancer treated with 3DCRT for
sites such as head and neck (H&N), brain, prostate and pelvic
at Roshana Radiation Oncology Center, Iran between July 2017
and January 2018 were considered in this retrospective study.
The distribution of patients was 11 patients with H&N tumor,
15 patients with brain tumor, 30 pelvic cancer patients and 17
prostate cancer patients.
At our institution, brain and H&N patients were immobilized
using the 3-point head only (brain patients) and 5-point headneck-
shoulder (H&N patients) thermoplastic mask with a
headrest in the treatment position. One week prior to CT
planning, the prostate cancer patients implanted 3 fiducial
gold markers. The prostate patients had to empty rectum and
had a full bladder (drinking 400-500 mL water 30 minutes
before simulation and treatment sessions) before computed
tomography (CT) planning and daily treatment. For pelvic and
prostate cancer patients, we did not use immobilization device.
All patients were scanned in head first supine position. The
thermoplastic mask or skin of the patients was marked using
radio-opaque labels under laser beams guidance in CT planning
step. Slice thickness was 3 mm in all cases.
CT images were imported into the Varian Eclipse v.13.6
treatment planning software (Varian Medical System Inc, Palo
Alto, CA, USA) for 3DCRT treatment planning. The CTV and OARs
were contoured by the responsible physician. For brain and H&N
plans, PTV was generated with an isotropic margin of 7 mm. For
pelvic cancer cases, CTV-PTV margin of 10 mm all around were
added to the defined CTV. In prostate patients, PTV were defined
as CTV plus an isotropic margin of 7 mm. Prescription dose was
delivered with 3DCRT on the linear accelerator (Varian Clinac
iX) with 6 MV and 18 MV photon beams.
Prior to treatment, patients were positioned with a suitable
immobilization device. Then, they were setup to treatment room
laser and skin marks (or mask marks). Orthogonal portal images
were acquired using a flat panel amorphous silicon digital portal
imaging device with resolution 1024 × 768 pixel. Portal images
were acquired at a dose rate 400 monitor unit (MU) per minute
and 1 MU were delivered per field for portal acquisition. EPID
images were compared to the DRRs (as a reference image)
created for the orthogonal portals at 0ᵒ (anterior) and 90ᵒ
(lateral) using treatment planning software (TPS). Reference
bony landmarks for the comparison of the EPIs and DRRs were
listed in Table 1.
For H&N, brain and pelvic cases, EPIs were performed for
the first 3 consecutive treatment fractions. Online setup error
correction was done for these three fractions. At the fourth
fraction, patients were transferred to the new isocenter, with
average displacements in the first three fractions and were
followed once weekly thereafter. Then, the online setup error
correction would apply if correction was needed. For patients
with prostate cancer, EPIs were carried out several times per
week because we wanted to evaluate the efficacy of fiducial
gold marker-based position verification during prostate EBRT
for the first time at our institution. Therefore, online setup
error correction was carried out for prostate patients. Matching
DRRs and portal images were performed using the anatomy
matching software (ARIA-record & verify system).For study the
setup errors, the displacement in two translational directions
were assessed in each field. The orthogonal portal images were
matched using the visible bony landmarks with their respective
DRRs. The action level was the displacement greater than 2
mm in H&N and brain cases and 3 mm in pelvic and prostate
cases along one direction in which corrected using the linac
couch shifts or by correcting the patient position to match the treatment isocenter. Then, new portal images were acquired.
Patient setup errors were assessed along three translational
directions (vertical (Y), longitudinal (Z) and lateral (X)).
The systematic and random errors were calculated using
the displacement in three translational directions. For H&N,
brain and pelvic cases, the systematic errors were defined as
deviations between the planned patient position and average
patient position of first three consecutive treatment fractions.
The random errors were defined as deviations between
different treatment fractions taken weekly during a course of
the treatment. Standard deviation (SD) of the systematic errors
(Ʃ) and SD of the random errors (σ) were analyzed. For prostate
cancer patients, Ʃ refers to SD of all individual means, and σ is
determined through the root mean square of the individual SD
of all patients . In addition, we quantified the frequency of 3D
vector lengths and calculated the magnitude of 3D vector using
formula (VRT, LONG and LAT are
vertical, longitudinal and lateral setup correction.
In the current study, we analyzed a total of 76, 107, 206 and
359 fractions for H&N, brain, pelvic and prostate, respectively.
The SD of systematic and random errors for different treatment
sites are summarized in Table 2.
The largest magnitude of systematic errors was found in
pelvic cases on the three axial directions relative to other cases.
The smallest systematic errors were observed for H&N in lateral
direction. In addition, the random errors in prostate were large
on the three dimensions in contrast to the other treatment sites.
The random errors in brain had smallest value than that in the
other treatment sites.The frequency of systematic and random
errors for H&N, brain and pelvic cancers in vertical, longitudinal
and lateral directions are shown in Figures 1-3. Overall, most of
the systematic and random displacements were ≤ 3 mm for H&N,
brain and pelvic cancers in the vertical, longitudinal and lateral
The average (± SD) prostate translational shifts per patients
in the three directions are given in Figure 4. The average
prostate shifts were 0.23 mm, 0.14 mm and - 0.42 mm in the
vertical, longitudinal and lateral directions, respectively, as
shown in Figure 4a-4c.Table 3 summarizes the CTV-PTV margins
calculated for achieving adequate coverage with a confidence
level between 90%-99% in each translational direction. Table 4
shows the cumulative percentages of 3D vector lengths in the
setup corrections for the all cases. Three-dimensional vector
displacements ≥7 mm was 0 % for brain cases and rare for other
treatment sites. The frequencies of 3D vector lengths are related
to the treatment site. The percentage of 3D vector lengths in
prostate and pelvic cases is similar. The frequencies of 3D vector
lengths of translational displacements also are similar between
H&N and brain cases. For H&N and brain cases, the distribution
of 3D vector length reduces rapidly from a starting length ≥1 mm
in contrast to pelvic and prostate cases.
Monitoring of patient positioning can be performed by EPID.
Thus, any changes in the treatment isocenter will be corrected. In
the present study, we evaluated the inter-fractional set up errors
for various treatment sites of 73 patients using EPID. In addition,
CTV-PTV margins were calculated with van Herk’s formula .
In our institution, the action level is 2 mm in H&N and brain
cases and 3 mm in pelvic and prostate cases for translational
direction. The results from our study showed that more than
70% of the systematic and random error displacements were
less than 2 mm for H&N and brain sites and less than 3 mm for
pelvic site in three directions (Figures 1-3). For the prostate
cases, about 84%, 77% and 77% of the setup displacements
were less than 3 mm in the vertical, longitudinal and lateral
directions, respectively (the results are not shown). Overall, the
systematic and random errors were small in H&N and brain in
contrast to pelvic and prostate regions because these treatment
sites are rigid and day-to-day variations in set up geometry are
minimal . Previous studies reported that the several factors
associated with fixation can lead to setup uncertainties in H&N
and brain cases. These factors are including swelling in tumor
region and reduction in body countering owing to weighting loss
during radiotherapy that lead to changes in fixation relative to
the onset of treatment [11-14]. Overall, these issues were low at
our institution. Furthermore, if these factors were observed, we
carried out a new CT planning with a change in degree of patient
There are multiple factors which can lead to setup
uncertainties for the pelvic and prostate cases. Target volume
position in pelvic and prostate cancer can change owing to
intestinal movement and vary filling in the bladder and rectum
[14-16]. Meanwhile, skin marks can easily move in these
treatment sites, and can lead to setup error [17,18]. Using thin
lines on the patient’s skin as well as good customized skin
fiducial markers and accuracy of laser room can reduce setup
deviations in theses treatment sites.
We found that in the all treatment sites, the random errors
were greater in the lateral direction compared to two other
directions, as shown in Table 2. This is probably due to the
optical illusion and the inaccuracy in matching laser and line
on the patient body. In a review article, Hurkmans et al. 
reported that the systematic and random errors in the routine
clinical practice can be less than 2 mm (1SD) for H&N, 3 mm (1SD) for pelvic and 2.5 mm (1SD) for prostate. The findings
of our study were in line with Hurkmans et al. study. As shown
in Table 5, the magnitude of systematic and random errors for
pelvic and prostate cases in our study are similar or less than
other studies, while we did not use knee support and foot rest
for pelvic and prostate cancer patients [17,20-22]. Compared to
H&N and brain cases in the current study, these are larger. This
primarily depends on the nature of treatment site and how the
patient is immobilized.
To compare the results with other published study, we have
considered PTV (90, 95)margin. In our study, the calculated CTVPTV
margin for H&N cases in the vertical, longitudinal and lateral
directions were 2.10 mm, 2.47 mm and 2.13 mm, respectively.
In another study, Gupta et al. assessed the setup errors in 25
patients with H&N lesions using a camera-based EPID that
immobilized with thermoplastic mask. The systematic errors
were 0.96 mm, 1.2 mm and 0.98 mm in the vertical, longitudinal
and lateral directions, respectively. The random errors were 1.94
mm, 2.48 mm and 1.97 mm in the vertical, longitudinal and lateral
directions, respectively. They obtained a PTV margin 3.76 mm,
4.74 mm and 3.83 mm for the vertical, longitudinal and lateral
directions . The results of this study are different to the
findings of current work. In the current work, we have changed
the isocenter in the fourth fraction, which resulted in reduction
of the systematic errors. Another difference can be attributed to
the frequency of online verification. Rudat et al.  reported
that setup margin reduces with increasing frequency of online
verification. Gupta et al.  investigated displacements in 93
fractions for 25 patients whereas we evaluated 76 fractions for
11 patients. Furthermore, patients with H&N and brain tumors
in our institution had to have a uniform size of hair throughout
the treatment period. This causes the thermoplastic mask to be
In 2016, Kanakavelu et al. evaluated the setup accuracy
and determined optimal PTV margin for H&N, brain and prostate
using MV CBCT and MV planar imaging. The CTV-PTV margin
was calculated using van Herk’s formula (1.75 mm, 2.98 mm
and 3.45 mm for H&N, 1.86 mm, 3.36 mm and 3.42 mm for brain
and 4.52 mm, 4.56 mm and 5.02 mm for prostate in the vertical,
longitudinal and lateral directions, respectively) . Data of this
study showed PTV margins for H&N and prostate are comparable
to those in the present study. In our institution, prostate cancer
patients had to have a full bladder and empty rectum during CT
planning and daily treatment sessions that lead to a reduction
in the inter-fractional variations in the prostate gland. From
our results, it can be seen that PTV margin in the prostate cases
for different confidence levels in the vertical direction was
less than 5 mm. The reduction in PTV margin in this direction
can be resulted in decrease of the rectal toxicity. Studies have
been shown that reduction in PTV margin for prostate cancer
is possible using the implanted fiducial markers. The CTV-PTV
margin of 5 mm was suggested with fiducial markers . The
results of our study are in line with this suggestion.
Although the systematic errors for brain site in our study are
comparable to Paul et al. study (Table 5), our calculated margin
is tighter. Paul et al. determined the PTV margin for 32 brain
cancer patients with EPID, and reported a margin 3.7 mm, 3.1
mm and 4 mm in the vertical, longitudinal and lateral directions,
respectively . The difference in the calculated PTV margin
in the two studies is mainly due to the random setup errors that
can be attributed to patient-related factors and the tightness of
the thermoplastic mask drown on the head.
Thasanthan et al.  in their work reported that in
50 patients with pelvic lesions, PTV margin in the vertical,
longitudinal and lateral directions are 8.38 mm, 9.33 mm
and 7.56 mm, respectively. As shown in Table 3, our findings
are different with their results. The first two fractions of the
course of treatment were assessed by them . According to
our experience, the magnitude of patient setup errors at initial
treatment sessions is large owing to the lack of patient comfort
in the therapeutic position and immobilization device. Thus, we
think this can be a reason for this difference, as well as the lower
number of investigated fraction for determining the shifts in
study by Thasanthan et al. .
The cumulative frequencies of 3D vector lengths of ≥ 4 mm
were 13.6 %, 9.34 %, 29.61 % and 28.97 % in H&N, brain, pelvic
and prostate cases, respectively. The systematic and random
errors for H&N and brain sites were ≤ 0.93 mm. These results
can be related to using the mask for H&N and brain patients .
From the results, the 3D vector lengths of displacements depend
on the tumor site and were in good agreement with the results of
pervious study that showed the 3D vector length of translational
shift, in contrast with the 3D vector length of rotational shift,
are associated with the treatment area .CTV-PTV margins
for various confidence levels were listed in Table 3. Our setup
with EPID indicated target minimum dose of 99% when using a
target volume to PTV margin less than 4 mm for H&N and brain
sites and less than 6 mm for pelvic and prostate cases to achieve
a 95% confidence level. For tumors that are in regions with high
uncertainty such as prostate, it is more reasonable to consider
upper limit of the calculated margin.
Although, we can reduce setup errors using correction
protocol, those will not eliminate. However, there are the intrafractional
variations and uncertainty in organ delineation
[27-29]. Overall, the results of determination of patient setup
uncertainties related to various factors such as immobilization
device, patient collaboration in the implementation of setup
procedure, the geometrical accuracy of the treatment machine,
accuracy of the lasers in the treatment machine and room, the
image verification system and the time taken to setup of the
patient can also affect the accuracy of the setup. Generally,
we spent an adequate time for the setup of the patient in the
There are two main limitations in this study. First, the intrafractional
setup errors were not considered. Intra-fractional
organ motion and intra-fractional variations in each patient are
important for accurate determination of setup uncertainties.
Second, the rotational setup errors were not evaluated in this
study. Therefore, these data should be analyzed for future
In this study, for first time in our institution, the setup errors
and CTV-PTV margins were determined for various treatment
sites. The setup errors depend on tumor site. The use of image
guidance technique is an effective tool for setup verification.
From our study, the optimal CTV-PTV margin can be acquired.
The reduction in CTV-PTV margins is an effective way to reduce
radiation-related complications in normal tissues.