Biomed Imaging Interv J 2007; 3(1):e30
© 2007 Biomedical Imaging and
Measurements of patientís setup variation in intensity-modulated radiation therapy of head and neck cancer using electronic portal imaging device
N Naiyanet1, MSc,
S Oonsiri2, MSc,
C Lertbutsayanukul*,1, MD,
S Suriyapee1, MEng
1 Department of Radiology, Faculty of Medicine,
Chulalongkorn University, Bangkok, Thailand
2 Department of Radiology, King Chulalongkorn Memorial Hospital,
Purpose: To measure the interfraction setup variation
of patient undergoing intensity-modulated radiation therapy (IMRT) of head and
neck cancer. The data was used to define adequate treatment CTV-to-PTV margin.
Materials and methods: During March to September
2006, data was collected from 9 head and neck cancer patients treated with
dynamic IMRT using 6 MV X-ray beam from Varian Clinac 23EX. Weekly portal
images of setup fields which were anterior-posterior and lateral portal images
were acquired for each patient with an amorphous silicon EPID, Varian aS500.
These images were matched with the reference image from Varian Acuity simulator
using the Varis vision software (Version 7.3.10). Six anatomical landmarks were
selected for comparison. The displacement of portal image from the reference
image was recorded in X (Left-Right, L-R), Y (Superior-Inferior, S-I) direction
for anterior field and Z (Anterior-Posterior, A-P), Y (S-I) direction for
lateral field. The systematic and random error for individual and population
were calculated. Then the population-based margins were obtained.
Results: A total of 135 images (27 simulation images
and 108 portal images) and 405 match points was evaluated. The systematic error
ranged from 0 to 7.5 mm and the random error ranged from 0.3 to 4.8 mm for all
directions. The population-based margin ranged from 2.3 to 4.5 mm (L-R), 3.5 to
4.9 mm (S-I) for anterior field and 3.4 to 4.7 mm (A-P), 2.6 to 3.7 mm (S-I)
for the lateral field. These margins were comparable to the margin that was
prescribed at the King Chulalongkorn Memorial Hospital (5-10 mm) for head and
Conclusion: The population-based margin is less than
5 mm, thus the margin provides sufficient coverage for all of the patients. ©
2007 Biomedical Imaging and Intervention Journal. All rights reserved.
Keywords: Electronic portal imaging; IMRT; CTV-to-PTV
margin; head and neck cancer
Radiotherapy for head and neck cancer requires accuracy of
radiation dose to the target volume. Setup reproducibility in the head and neck
area is particularly important due to the proximity of many critical organs.
The introduction of new technology such as intensity modulated radiation
therapy (IMRT) and 3-D conformal radiation therapy poses new challenges for
delivering intended target dose while minimising dose and toxicity to critical
normal structures. This is accomplished by conforming the treatment fields to
the target volume, using appropriate margins to account for treatment
uncertainties. To determine these margins between the clinical target volume
(CTV) and field borders, the concept of the planning target volume (PTV) has
been introduced by the International Commission on Radiation Units and
Measurement (ICRU) . The PTV is the CTV plus a margin to allow for
geometrical uncertainty in its shape and variations in its location relative to
the radiation beams due to organ mobility, organ deformation and patient setup
variations. The common methods to monitor treatment accuracy are visual
comparison of simulation film or DRRs (prescription) and port film (treated) or
electronic portal imaging. However, the image quality of DRR images is not good
enough to set as reference image due to large slice thickness (5mm). We elect
to use simulation image by using conventional simulation to verify setup
isocenter before moving the patient to the treatment room. Megavoltage film
measurements are rather time consuming and not always very accurate.
Significant improvements in both accuracy and efficiency of detecting and
correcting setup errors can, in principle, be achieved by using electronic
portal imaging devices where the setup is verified prior to each treatment and,
in some situations, also during the treatment. Since 2005, EPIDs have become
available in the Division of Radiation Oncology at King Chulalongkorn Memorial Hospital,
so the portal imaging from EPID was used† to check the setup accuracy in this
At present, a CTV-to-PTV margin ranging from 5 mm to 10 mm
is prescribed to patients undergoing IMRT of head and neck cancer at our
division. However, a too small CTV-to-PTV margin will result in a geometrical
miss at some or even all treatment fractions. It, therefore, becomes
increasingly important to define adequate CTV-to-PTV margin. RTOG protocol
H-0022 , suggests using a uniform CTV-to-PTV margin of at least 5 mm
until the institution-specific uncertainty has been evaluated. Therefore, the
purpose of this study is to measure interfraction setup variation in head and
neck cancer patients undergoing IMRT. The data will be used to define adequate
Materials and methods
This study was performed on 9 head and neck cancer patients,
treated with dynamic IMRT, 6 MV X-ray beam from Varian Clinac 23EX of 120
leaves MLC at King Chulalongkorn Memorial Hospital from March 1st to
November 30th, 2006. Treatment fields encompass primary tumour as
well as lymph nodes at risk. All the patients were immobilised with a TYPE-STM
thermoplastic mask covering head, neck and shoulders, which was fixed to the
treatment couch. Prior to treatment, all patients had three images of setup
field, which were two orthogonal, anterior-posterior (AP) and lateral image at
the upper neck level, and the other AP field at the shoulder level. The
simulation images were acquired on the Acuity digital simulator and transferred
into VARiS Vision as the reference images. Weekly portal images of three setup
fields were acquired for each patient with amorphous silicon EPID. All portal
images were matched with the reference images using the VARiS Vision (version
Portal image analysis by anatomical matching
Before collecting the patient data, the quality control of
image software had been performed to verify the accuracy of the software, using
perspex (PMMA) phantom attached with the marker. The images were collected in
anterior and lateral directions for both simulator and EPID. Then the program of
Anatomy Matching was used to verify the accuracy of the program by looking at
the deviation of the marker. The matching showed good agreement with the
deviation within 0.5 mm.
Comparison between a simulator image set as reference image
and a portal image was done using Anatomy Matching. Anatomy Matching is used to
find a small patch of image around each point in the reference that matches an
identical patch in the portal image. In this study, we created an anatomy layer
that was required for the matching process. Anatomical contours of bony
landmarks, which were skull bones, the first cervical vertebral body (C1) and
the fourth cervical vertebral body (C4) for lateral field and mandible,
clavicle and spinous process for anterior field, were drawn manually on each
reference image. Then the system aligned the portal images and the reference
image anatomically according to the defined match points on the matching
anatomy layer. An anatomy match object is produced and superimposed on the
portal image and subsequently shifted until a satisfactory match is achieved.
The patient misalignment is now visible and indicated in the Image Mismatch
panel (Figure 1-3).
Setup error for head-and-neck patients
Displacements of isocenter in X (Left-Right, L-R) and in Y
(Superior-Inferior, S-I) directions were measured on anterior portal images,
whereas, in Z and Y direction were measured on lateral portal images. After the
anatomical matching was performed on the treatment fields for an individual
patient, mismatch data were recorded into a Microsoftģ Excel
The reported X, Y and Z displacement of isocenter between
simulation and treatment was decomposed into the appropriate shifts along each
body axis. In this study, positive shifts correspond to shifts inferior, left
Systematic error and random error for individual
patient and population
For each individual patient, measurement of the displacement
between simulator image and one single treatment session represents the total
variation in patient positioning for the treatment session considered. This
displacement is a combination of both the systematic and the random error.
The systematic error represents displacement that was
persistent during the whole treatment course. For an individual patient, the systematic
error (∑) was calculated as the average displacement of a particular
reference structure and direction between simulation and treatment during the
whole treatment course,
where N represents the total number of portal images
acquired for a particular field and Δ i is the
calculated displacement for the I th treatment fraction.
The random error represents day-to-day variations during the
treatment course. For each individual patient, the random error (σ) was
calculated as the dispersion around the systematic error,
The systematic and random errors for each patient were
calculated, using Eqs. (1) and (2) . For the whole population, the
population systematic errors (Σpop) for a particular isocenter
and dire action were expressed by the standard deviation (SD) from the values
of the average displacement of all individual patients (Σind). While
the population random error was expressed by the SD from all individual random
error (σind) .
According to ICRU report 62 , the CTV-to-PTV margin
should account for internal motion and variations in the size, shape and
position of the CTV (internal margin) and setup uncertainties (setup margin) in
the patientís position relative to the beam. For this study, it was assumed
that the location of the PTV is adequately represented by bony structures, due
to the anatomy in the head and neck region, thus, the internal target motion is
considered negligible. Population-based margins were calculated for all
patients based on the equations of van Herk . To ensure a minimum dose of
95% to the CTV for 90% of the patients, a one-dimensional margin of 1.64∑pop + 0.7σpop
is suggested, where ∑pop and σpop are defined
by Gilbeau . The calculated CTV-to-PTV margins were then compared to a value
5-10 mm based on traditional margins used in the King Chulalongkorn Memorial Hospital.
A total of 135 images (27 simulation images and 108 portal
images) and 405 anatomical matches was evaluated. Table 1 (a)-(c) and Table 2
(d)-(f) represent sample spreadsheets used to calculate deviations along the
L-R, S-I and A-P axes for each patient. The systematic (Σind)
and random (σind) calculated using Eqs. (1) and (2) are also
listed in Table 1 and 2. The individual systematic error (Σind)
ranged from -3.5 to 2.9 mm, -2.8 to -4.5 mm and -7.4 to 2.5 mm along L-R, S-I
and A-P direction, respectively. The individual random error (σind)
ranged from 0.4 to 4.8 mm, 0.4 to 3.8 mm and 0.2 to 3.1 mm along the L-R, S-I
and A-P axes, respectively (data not shown). The population-based margin ranged
from 2.4 to 4.5 mm (L-R), 3.4 to 4.9 mm (S-I) for anterior field and 3.4 to 4.7
mm (A-P), 2.6 to 3.7 mm (S-I) for the lateral field. The summary of the
population-based statistics (Σpop and σpop) and
one-dimensional population-based margins are presented in Table 3 and Table 4.
These CTV-to-PTV margins for head and neck cancer were less
than traditional 5 mm margin used in King Chulalongkorn Memorial Hospital.
The primary objective of the present study was to measure
interfraction setup variation in head and neck cancer patients undergoing IMRT
using an EPID. Displacements of portal images from simulator images, set as
reference images, were measured for calculating both systematic and random
errors. Systematic error can arise from various factors, the most important
being transfer errors from simulator to the treatment unit. Random errors are
related to any accidental error during setup, due to mispositioning of the
patient in the mask, movements of the patient or organ motion in the period
between positioning and start of irradiation or during irradiation.
Prisciandaro et al.  reported systematic errors ranging from -0.3 to
-0.2 mm, -0.2 to 1.1 mm and -0.4 to 1.2 mm and random errors
ranging from 3.0 to 3.6 mm, 2.2 to 3.3 mm and 2.6 to
2.7 mm, along the LĖR, SĖI and AĖP axes, respectively, using TYPE-STM
head/neck shoulder immobilisation systems. While our study has shown the
systematic errors (systematic error ranging from -3.5 to 2.9 mm, -2.8 to -4.5
mm and -7.4 to 2.5 mm and for random errors ranging from 0.4 to 4.8 mm, 0.4 to
3.8 mm and 0.2 to 3.1 mm along the L-R, S-I and A-P axes) that exceed those in
previous work, the random errors are comparable. The impact of systematic
errors is much larger than the impact of random errors. Large systematic errors
lead to a large underdosage for some of the patients while large random errors
lead to a moderate underdosage for a large number of patients.
Based on the results presented in Table 3 and Table 4, the
difference in one-dimensional population-based margins along S-I axis between
anterior (3.4 to 4.9 mm) field and lateral (2.6 to 3.7 mm) field were observed
because the clavicles, chosen for anterior field at the shoulder level were
less stable than anatomical landmarks chosen for lateral field (i.e. skull
bone, C1 and C4).
The secondary objective of the present study was to define
adequate CTV-to-PTV margin for IMRT of head and neck cancer in our department.
Ideally, the CTV-to-PTV margin should be determined solely by the magnitudes of
the uncertainties involved. In practice, the clinician usually also considers abutting
healthy tissues when deciding on the size of the CTV-to-PTV margin .
Stroom et al.  developed a different method for
calculating CTV-to-PTV margin for prostate, cervix and lung cancer cases, which
ensures at least 95% dose to 99% of the CTV. It appears to be equal to about 2∑ + 0.7σ
for all three cases, based on the assumption that the CTV should be adequately
irradiated with a high probability. In clinical practice, one might prefer a
tighter CTV-to-PTV margin near a dose-limiting structure.
In this study, population-based margins were calculated for
all patients based on the equations of van Herk  that suggested a
one-dimensional margin of 1.64∑pop + 0.7σpop
to ensure a minimum dose of 95% to the CTV for 90% of the patients. In the
present study, population-based margins ranging from 2.4 to 4.9 mm were
demonstrated. As compared to our traditional margin of 5 mm in our department,
it seems that we can further decrease the CTV-to-PTV margin to spare more
organs at risk in the future.
In this study, the population-based margin was less than 5
mm, thus the margin provides sufficient coverage for all of the patients. The
result of this study suggests that the determination of setup variation is
important for the assessment of population-based margin calculation to define
adequate CTV-to-PTV margin of head and neck cancer patients and improve the
confidence in patient-specific margin.
Figure 1 (Left) Simulator image of a right lateral setup field with contours outlined skull bones, C1 and C4. (Right) Corresponding treatment portal image matched to skull bones. An additional match was performed on this image to C1 and C4.
Figure 2 (Left) Simulator image of an anterior setup field with contour outlined mandible and spinous process. (Right) Corresponding treatment portal image matched to mandible. An additional match was performed on this image to spinous process.
Figure 2 (Solution) Plain axial CT scan through the superior aspect of the brain.
Figure 3 (Left) Simulator image of an anterior setup field at shoulder level with contour outlined clavicle. (Right) Corresponding treatment portal image matched to clavicle.
Table 1 Example spreadsheets for a right lateral field matched over the course of six fractions to skull bones (a), C1 (b) and C4 (c), respectively. ?AP and ?SI represent the deviations in the A-P and S-I direction of each anatomical landmark between simulation images and portal images. Positive shifts correspond to anterior and inferior shifts while negative shifts correspond to posterior and superior shifts.
Table 2 Example spreadsheets for an anterior field matched over the course of six fractions to mandible (a), clavicle (b) and spinous process (c), respectively. ?LR and ?SI represent the deviations in the L-R and S-I direction of each anatomical landmark between simulation images and portal images. Positive shifts correspond to left shifts and negative shifts correspond to right shifts.
Table 3 Population-based statistics (Spop and spop) and one-dimensional population-based margins (1.64Spop+ 0.7spop) calculated for each anatomical structure of all patients along the A-P and S-I axes for lateral field.
Table 4 Population-based statistics (Spop and spop) and one-dimensional population-based margins (1.64Spop + 0.7spop) calculated for each anatomical structure of all patients along the L-R and S-I axes for anterior field.
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Instructions for use: Treatment Delivery. Image review procedures VARiSVision software Version 7.3.10.
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|Received 30 November 2006; received in revised form 10 April 2007; accepted 19 April 2007
Correspondence: Department of Radiology, Faculty of Medicine, Chulalongkorn University, Rama IV Rd, Bangkok 10330, Thailand. Tel.: +662-2564334; Fax: +662-2564334;
E-mail: firstname.lastname@example.org (C. Lertbutsayanukul).
Please cite as: Naiyanet N, MSc, Oonsiri S, Lertbutsayanukul C, Suriyapee S,
Measurements of patientís setup variation in intensity-modulated radiation therapy of head and neck cancer using electronic portal imaging device, Biomed Imaging Interv J 2007; 3(1):e30
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