Biomed Imaging Interv J 2007; 3(2):e23
© 2007 Biomedical Imaging and
Dose optimisation during imaging in radiotherapy
PhD, DipRP, FCCPM
Christian Medical College, Vellore, India
The desire to increase the precision in radiotherapy
delivery has led to the development of advanced imaging systems such as
amorphous silicon (a-Si)-based electronic portal imaging, and kV and MV cone
beam CT. These are used prior to the delivery of radiation to visualise the
organ to be treated and to ensure that the patient setup and treatment delivery
are accurate. However, little attention has been given to the dose received by
adjacent normal tissues during these imaging procedures. Though these doses are
very small compared to the dose delivered during radiotherapy, the involvement
of normal tissues and the concern that these could increase the probability of
stochastic effect, mainly the induction of secondary malignancy, cannot be
ignored. This article reviews some work on the doses received during imaging in
radiotherapy and the methods to optimise the same. � 2007 Biomedical Imaging
and Intervention Journal. All rights reserved.
Keywords: Animal irradiator; rodent irradiation;
radiobiology; radiation beam measurement
Recent technological developments have instilled
considerable interest in advanced radiotherapy techniques such as Three
Dimensional Conformal Radiotherapy (3D-CRT) and Intensity Modulated
Radiotherapy (IMRT). These techniques have enabled dose escalation to the
clinical target volume (CTV) and dose reduction to normal tissues as well as to
the surrounding critical organs thus leading to better tumour control
probability (TCP) and lower normal tissue complication probability (NTCP). The
success of these techniques depends heavily on the accuracy in targeting the
CTV, and achieving the aimed sparing of the normal tissues.� Conventionally,
external markers made on the patient surface are used to direct the radiation
beam to the target volume. This could result in a geometrical miss as the
relationship between the external marker and the CTV could have been lost
during the time between planning and the treatment, and during the course of
the treatment. In order to achieve accurate targeting during treatment, the
organ to be treated is visualised using portal imaging. Portal imaging is the
use of a therapeutic X-ray or gamma ray beam to form an image of the area being
irradiated and its main application is to analyse the patient setup during
treatment and account for the uncertainty in treatment delivery. It is well
known that the images produced with MV beam suffer from low subject contrast
compared to the images produced with kV X-ray beams. This is mainly due to the
fact that the MV beams interact with the patient by Compton scattering that
depends on the density rather than the atomic number. In addition to this, many
other factors contribute to the poor quality of images in portal imaging and
these include the performance of image receptor, scatter due to patient
thickness, source size, etc. .
Portal images have been defined as three types: localization
radiograph, verification film and double exposure . X-ray film is the
traditional medium for portal imaging. Historically, portal imaging has been
performed with industrial films  mainly for complex-shaped beams such as the
one used in Mantel Technique for Hodgkin�s disease. A few centres have also
been using the Computed Radiography (CR) technology with the Photostimulable
Phosphor Plate (PSP) for MV portal imaging in radiation therapy . The main
advantage of this method is that the images are made available in digital
format. Electronic portal imaging (EPI) was introduced in the early 1980s when
the late Norman Baily demonstrated the use of a fluoroscopic system to acquire
MV transmission images. Since then electronic portal imaging devices have
undergone significant developments from CCD-based imaging devices to a-Si flat
panel devices. Electronic portal imaging devices (EPID) have many potential
advantages over traditional X-ray films for portal imaging. The images obtained
are immediately available and can be used interactively to adjust patient or
field position during radiotherapy. The images are digital, which aids image
processing, contrast enhancement and image matching. Moreover, digital
archiving saves space and allows for rapid recall of images over a network .
These developments in portal imaging have resulted in the development of 3D
imaging for positioning.
The advantage of obtaining full volumetric information with
single rotation of the source and the flat panel detector with cone beam CT has
led to its introduction as an image guidance system in radiation therapy. Cone
beam CT generated with the MV beam is used to obtain patient setup information
in 3D by registering these images with the planning CT images in radiation
therapy. A few vendors provide cone beam CT obtained with kV beam for setup
verification. This has been made possible by having a kV X-ray tube and a flat
panel detector 90o to the treatment beam .
The requirement of radiation protection as per the
international basic safety standards (BSS 115) is that �any medical exposure
should be justified by weighing the diagnostic or therapeutic benefits they
produce against the radiation detriment they might cause by taking into account
the benefits and risks of available alternative techniques that do not involve
medical exposure�. In radiation therapy, it is generally assumed that the
potential of radiation dose delivered during imaging for verification and
localisation does not add to the patient�s burden because the doses from such
exposures are very small compared with the intended therapy dose.� This is true
when considering imaging of the target volume that receives the intended
prescribed dose. However, imaging during radiotherapy for setup verification
results in dose exposure to normal tissues outside the tumor volume. In this
paper, the dose received due to portal imaging, and during a few newer image guidance
techniques, such as MV and kV cone beam CT, and the methods that have been
suggested to optimise the dose to normal tissues during imaging in
radiotherapy, are reviewed.
Dose optimisation during portal imaging in radiation therapy
It has been concluded from early studies that portal films
are essential for accurate delivery of radiation therapy, and frequent filming
may be required to decrease the frequency of localisation and field design
error . Portal images are acquired either by single exposure technique or by
double exposure technique. In single exposure technique, the treatment beam is
used to image the region to be treated and is used when adequate landmarks are
available for verification within this region. The dose delivered during this single
exposure portal imaging is usually adjusted from the treatment dose and this
does not deliver dose to normal tissues. When adequate landmarks are not
available within the treatment region, a double exposure technique is used. In
this double exposure technique, a field at least 5cm larger than the area to be
treated is also imaged in addition to the region to be treated. This results in
delivering dose outside the tumour volume and thus increases the probability of
stochastic effect. The risk of cancer induction is additive and the concomitant
dose from the double exposure portal images adds to the dose received by the
patient due to leakage and scatter radiation . Several studies have been
performed to increase the image quality during portal imaging and thus reduce
the dose during portal imaging. Crooks and Fallore have estimated the dose
during the portal imaging with three different films viz. CEA TLF, CEA TVS and
Kodak EC-L films and the dose during portal imaging is estimated as 1.2 cGy,
15.9 cGy and 1.5 cGy for the CEA TLF, CEA TVS and Kodak EC-L films,
respectively . The authors have concluded that the CEA TVS film could be
used for single exposure, where it could be taken out during the exposure
(treatment) and CEA TLF or the Kodak EC-L films for double exposure as they
require very low dose to get optimally-exposed images.
A few authors have tried portal imaging with computed
radiography (CR), a non-film-based system used to obtain high quality portal
images. In this system, the film is replaced with a photostimulable phosphor
plate . In addition to the advantages of digital imaging, this technique
produces excellent images with radiation exposures of 1 and 2 monitor units
only. The use of CR for portal imaging is also being tried in the author�s
institute where PSP plates are used with a CR reader for obtaining portal
images. Single and double exposure images obtained with 1 MU exposure are shown
in Figure 1a and Figure 1b, respectively.
Jaffray et al have suggested a method to reduce the dose
during double exposure and enhance the image quality by using a dual-beam
system consisting of a kV X-ray tube mounted on the gantry of a medical linear
accelerator thus mixing low- and high-energy beams for double exposure
technique . Both the kV and MV images are collected with a fluoroscopic
imaging system that uses a low-noise CCD camera to accumulate the light emitted
from a phosphor screen. The authors have concluded that the quality of the
dual-beam image is similar to the prescription (simulation) image, contains a
larger anatomical region, and delivers a lower integral dose to the patient.
Radiation dose delivered during double exposure portal
imaging for pediatric radiotherapy has been evaluated by Kudchadker et al
by conducting a retrospective review of the port film dose for 56 consecutive
pediatric patients who underwent definitive radiation therapy . The mean
total port dose varied from a maximum of 46 cGy for the brain to a minimum of
17 cGy for the thorax. The mean total port dose as a percentage of prescribed
dose was less than 1.25% for all locations in this study. However, most of the
port dose is a result of the open-field dose from the double-exposure technique.
Kudchadker et al suggest that care should be exercised while exposing
port films of pediatric patients to minimise both the number of films and the
radiation dose without compromising the quality of treatment delivery. The
specific suggestion of the authors is to minimise the number of monitor units
used to image regions outside the treatment field to reduce the risk of
development of secondary neoplasms.
Development of Electronic Portal Imaging devices and Cone
Beam CT systems
The requirement for better visualisation of MV images and
enhanced image quality for verification of setup for conformal radiotherapy has
led to the development of online electronic portal imaging devices, which have
replaced the conventional film-based portal imaging .� The main advantage
of this type of portal imaging over film is the immediate (online) availability
of images that enables verification and correction of patient setup before
The availability of digital images from the EPID makes image
processing, contrast enhancement and automatic comparison with planning images
(simulator images or DRR) possible. The development of the EPID has undergone
several significant changes starting from video camera- and mirror-based EPIDs,
to the recent a-Si based flat panel EPIDs. The construction, development and
the physics of these portal imaging devices have been discussed in several
publications [1, 3, 11]. The other major advantage particularly with the a-Si
flat panel EPIDs is the considerable low MUs required to produce the portal
images. A primus linear accelerator fitted with an a-Si flat panel portal
imaging system is shown in Figure 2. A portal image acquisition mode has also
been developed for the PortalVision� that allows one to take portal images with
reduced dose while keeping good image quality . Moreover, the introduction
of the a-Si flat panel imaging devices and the interest to have 3D verification
images have led to the development of cone beam CT-based Image Guided
Radiotherapy system (IGRT). The availability of full volumetric information of
the patient anatomy with single rotation of the source and the detector has led
to the development and clinical implementation of both the MV and kV cone beam
CT for image guidance for precision radiotherapy. MV cone beam CT has been
developed using the treatment beam and the EPID [13-15]. In order to increase
the image quality kV cone beam CT has been developed with a kV X-ray tube and
an a-Si flat panel detector fixed at 90o to the MV source [5, 16].
An Elekta linear accelerator with integrated kV CBCT system is shown in Figure
3. The imaging performance of both kV and MV cone beam CT has been compared
Dose during MV and kV CBCT
Although the clinical use of cone beam CT in image guidance
has improved the accuracy in patient setup and radiation delivery, the dose
delivered during the image guidance is of significant concern, as the volume of
the tissue irradiated is much more than the volume to be treated and several
times higher than the volume irradiated during the double exposure technique of
portal imaging. Moreover, repeated imaging using the cone beam CT through
several fractions during the course of the treatment is likely to result in a
significant dose to normal tissues that can increase the probability of
MV cone beam CT
MV cone beam CT is simple to use, as it does not require
additional hardware except for a-Si flat panel detector with high detection
quantum efficiency. However, relatively lesser image quality compared to kV
cone beam CT images and radiation dose during the cone beam CT are of great concern.
Initial attempts to generate MV cone beam CT with about 90o
projections� resulted in a central dose of about 90 cGy . As one cannot
employ full-field imaging on a regular basis with the MV cone beam CT doses
mentioned above, attempt has been made to image the region of interest
(GTV/PTV) using conformal beams and to improve the image quality with a-Si
flat-panel EPID that has higher detective quantum efficiency . The dose
during the MV cone beam CT was reduced by Seppi et al by developing an
image receptor that uses a flat-panel imaging system consisting of a
conventional flat-panel sensor attached to a thick CsI scintillator . The
scintillator consists of individual CsI crystals 8 mm thick and 0.38 mm by 0.38
mm pitch. With this projection, images could be obtained with one accelerator
pulse delivering as little as 0.023 cGy per image. They could generate MV CT
images with soft tissue contrast with irradiations as small as 16 cGy. Further
dose reduction during MV cone beam CT has been achieved by windowing the
dose-pulse rate of 6 MV Primus accelerator beam to expose an a-Si flat panel by
using only 0.02 to 0.08 MUs per image and produce low-noise 3D MV cone beam CT
images without pulsing artifacts with a total delivered dose that ranged from 5
to 15 cGy .
kV cone beam CT
Imaging using a kV beam is superior to the MV imaging as
photoelectric interaction, which depends on the atomic number, is dominant in
this energy range and hence gives better subject contrast resulting in clear
differentiation between bone and tissue. Although the dose to the patient due
to a kV image-guided radiotherapy is small compared to that of the MV image
guidance, a large volume of the normal tissue is involved in the cone beam CT
imaging and the repeated use of this modality for image guidance on a daily
basis may contribute significant dose to normal tissue. The dose during a kV cone
beam CT and the dose at the centre and surface of the body phantom have been
estimated as 1.6 cGy and 2.3 cGy for a typical imaging protocol using full
rotation scan, with a technique setting of 120 kVp and 660 mAs . Similarly,
the dose to the surface of the eyes during a kV cone beam CT head scan and the
dose to the surface of the breast and the contra-lateral breast during a kV
cone beam CT breast scan have been estimated with humanoid phantoms, and
methods have also been suggested to reduce the dose during a kV cone beam CT
. Using a lesser number of projections to reduce the dose and the impact on
the image quality has also been studied . The possibility of achieving an
ultra-low dose of about 1 mGy per scan has been reported by attempting to
generate low exposure volumetric X ray CT by reducing the number of exposures
Imaging during radiation therapy has a significant role as
it reduces the setup error and avoids geographical miss. Portal imaging and 3D
image guided therapy with cone beam CT have increased the accuracy of treatment
delivery considerably. However, one should remember that a significant amount
of normal tissue is irradiated in this process and could increase the
probability of stochastic effect. Hence, it is necessary to develop protocols
to optimally use the imaging techniques for treatment delivery in radiotherapy
that could significantly reduce the risk of stochastic effect.
Figure 1 (a) Single exposure port film obtained with Computed Radiography (CR) for 6 MV X-rays; (b) Double exposure port film obtained with Computed Radiography (CR) for 6 MV X-rays.
Figure 2 Primus linear accelerator with a-Si flat panel electronic portal imaging device.
Figure 3 kV X-ray source and flat panel detector fixed at 90� to treatment beam of a linear accelerator for image guidance with kV cone beam CT.
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|Received 8 December 2006; accepted 12 January 2007
Correspondence: Department of Radiotherapy, Christian Medical College, Hospital Vellore 632004, Tamil Nadu, India. E-mail: firstname.lastname@example.org (B. Paul Ravindran).
Please cite as: Ravindran P,
Dose optimisation during imaging in radiotherapy, Biomed Imaging Interv J 2007; 3(2):e23
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