Third party EPID with IGRT capability retrofitted onto an existing medical linear accelerator
1 Radiation Therapy, Raleigh Regional Cancer Center, Beckley, West Virginia, United States
2 Radiologic Technology, Mountain State University, Beckley, West Virginia United States
Radiation therapy requires precision to avoid unintended
irradiation of normal organs. Electronic Portal Imaging Devices (EPIDs), can
help with precise patient positioning for accurate treatment. EPIDs are now
bundled with new linear accelerators, or they can be purchased from the Linac
manufacturer for retrofit. Retrofitting a third party EPID to a linear
accelerator can pose challenges. The authors describe a relatively inexpensive
third party CCD camera-based EPID manufactured by TheraView (Cablon Medical
B.V.), installed onto a Siemens Primus linear accelerator, and integrated with
a Lantis record and verify system, an Oldelft simulator with Digital Therapy
Imaging (DTI) unit, and a Philips ADAC Pinnacle treatment planning system
(TPS). This system integrates well with existing equipment and its software can
process DICOM images from other sources. The system provides a complete imaging
system that eliminates the need for separate software for portal image viewing,
interpretation, analysis, archiving, image guided radiation therapy and other image
management applications. It can also be accessed remotely via safe VPN tunnels.
TheraView EPID retrofit therefore presents an example of a less expensive
alternative to linear accelerator manufacturers� proprietary EPIDs suitable for
implementation in third world countries radiation therapy departments which are
often faced with limited financial resources. � 2009 Biomedical Imaging
and Intervention Journal. All rights reserved.
Keywords: Portal Imaging; Simulation; EPID; IMRT; IGRT
The aim of external beam radiotherapy is to deliver a
tumoricidal radiation dose while minimizing dose to the surrounding normal
tissues, which requires accurate and reproducible field placement. Verifying
patient position and fields historically has been accomplished via exit
radiation portal films. Ideally, films should be performed every day before the
treatment [1-4], but this process ties up departmental personnel, the film
processor, and the treatment machine for several minutes, while the patient
must remain immobile on the treatment table.
The electronic portal imaging device (EPID)  is a
relatively new development in portal imaging. Boyer et al  and
Munro  have written comprehensive reviews of EPIDs, and, briefly, they
consist of an image acquisition unit fitted to the linear accelerator (Linac),
and a component that digitizes and displays these images on a computer screen.
The unit should provide high resolution and high contrast images, to allow
rapid verification of treatment field shape and position immediately after the
patient's X-ray exposure. Recent developments include the software to analyze
portal images and compare them with treatment planning images for setup
accuracy and localization.
Digital X-ray images have several advantages over X-ray
films: less handling, less radiation to the patient, more convenient patient
management, immediate image viewing, computer-aided analysis, and storage in
digital format rather than in film stacks. EPIDs provide images with better
visibility and review accuracy than do films in the megavoltage X-ray energy
The main idea behind portal imaging is to ensure the
patient is in the correct position during treatment, and regular portal
imaging protocols reduce the size and frequency of field placement errors.
Computer algorithms for detecting field displacements are better than manual
approaches . This process requires a reference image which shows the
patient in the correct position, and the treatment portal image(s)
for comparison. The reference image may be a simulator X‑ray,
another portal image, a digitally reconstructed radiograph or a
digitally reconstructed portal image. Making image registration a
routine process in clinical practice requires an integrated system
that combines the functions of preparation of the reference image,
portal image field edge detection, field edge matching, anatomy
matching and presentation of results . Another important application for
portal imaging is to verify beam collimation and block/MLC shape.
The global definition of Image Guided Radiation Therapy
(IGRT) involves the entire position verification process from portal image
acquisition to image registration by aid of computer software. The process
requires the importation of the DICOM coordinate system from the CT or virtual
simulation, recording of the prescribed coordinate offsets from the simulation
origin. After the images have been acquired, the position offsets are
calculated instantly to provide the necessary treatment couch shifts
information for re-positioning of the treatment area. After re-positioning, the
re-verification images may be taken again and the circle is repeated according
to departmental protocol. Consequently, an ideal IGRT system should be capable
of importing the set up coordinates, recording of current target localization
offsets at the time of image acquisition and thereafter the application of the
total offsets to the treatment couch position.
There are a wide range of problems [11-12] that third
world countries face in radiation therapy and imaging facilities. The clinical
relevance of portal imaging technology in developing countries constitute a
subset of these problems and faces stiff challenges because of the unfavorable
economic situation. Moreover, even the institutions that can afford to purchase
these state-of-the-art technologies in developing countries may not do so
because of limited knowledge of their potential clinical benefits and
Adopting EPID technology can pose problems for small
radiation therapy centers, since many operate machines manufactured before
EPIDs became standard equipment, or did not purchase EPID technology on more
recently acquired Linacs. Adding an EPID to an existing Linac can be
challenging for several reasons.
Firstly, the cost of a Linac manufacturer's proprietary
add-on EPID can represent more than half the cost of a new Linac (see Table 1
for more details). Secondly, there can be hardware and software compatibility
issues with third party products. Thirdly, most add-on EPID systems do not come
with complete surrogate  based- image guided radiation therapy (IGRT)
portal image management software. Finally, there can be logistic issues with
the new EPID retrofit and existing Linac service maintenance contracts due to any
required modifications of the Linac. In evaluating an add-on EPID system to
meet portal imaging needs, to increase efficiency, and to keep the radiation
therapy center competitive, it is necessary to evaluate the quality of imaging,
initial purchase cost, and total cost of ownership of several systems.
Review of EPI systems
The three major radiation therapy digital imaging
technologies include camera based detectors [13-19], liquid ion
chambers [20-21] and solid-state amorphous silicon detectors [22-23]. The
earliest EPIDs were camera based. Here, the X-ray beam excites a metal
fluorescent phosphorus screen, which converts X-rays to light, and the image is
transferred to a high-resolution charge-coupled device (CCD) digital camera via
a high reflectance mirror positioned at a 45-degree angle under the
fluoroscopic screen. The camera control unit transforms the digital image
gathered from the CCD camera via a fiberoptic-linked  datastream fed
directly to a fiberlink personal computer interface (PCI) in the host computer
to be processed by the digital image processor. This processor digitizes
images, and an appropriate number of frames are averaged to reduce artifact and
produce a final display image.
There are cooled and non-cooled CCD camera systems commercially
available for non-radiation therapy imaging applications. The cooled CCD camera
can handle very high signals per pixel, without compromising the low-level
imaging performance. They are designed to allow detection of small differences
in light intensity. Consequently, details of very low contrast images can be
seen against much brighter backgrounds, without saturating the higher intensity
areas. This capacity of detecting low contrast images, with the high
sensitivity and wide dynamic range of the CCD, lead in performance indices for
cooled CCD cameras that surpass the non-cooled CCD camera imaging systems .
Cooled CCD camera systems employ thermoelectric primary cooling with either air
or water-cooled heat exchangers. The ability to remove excess heat from the
heat sink depends on the method of cooling. The temperature of the cooling air
or water influences the lowest operating temperature. The CCD array can be
cooled to -75�C with air-cooling. Water-cooling can push the array temperature
to -90�C, with an increase in the lifespan of water-cooled CCD camera compared
to air cooled  cameras.
Liquid ion chamber arrays EPIDs have slow scan speed. As a result they have limited use in verification of
dynamic techniques, such as intensity modulated radiotherapy where
the dose rate can be varied during the treatment due to the variation in Linac
output . Because of this limited application, the authors did not pursue
them further in this work.
Camera based detectors are cheaper and more durable than
amorphous silicon detectors, but have relatively lower image resolution since
they use phosphor screens  with lower light collection capability  to
capture the image for the camera. There is also poor optical coupling between
the light emitter and camera system . Flat-panel amorphous silicon imagers
of the same screen yield higher quality images than CCD imagers at 2-4
monitor-units (MU) exposures, but are susceptible to radiation damage to the
peripheral electronics . There is also need to calibrate and regularly
re-calibrate these detectors for dark current and flood-field uniformity.
The lifespan of an amorphous silicon-based system can be
shorter than expected with intensive IGRT imaging needs, because radiation
affects the leakage current of the diodes employed in amorphous silicon
detector systems, degrading the system performance  and accounting for the
shorter lifespan compared with water-cooled camera based detector systems.
Financial calculations for the US case (based on 2008
medical billing reimbursement guidelines) for camera-based portal imaging
indicate that an add-on EPID should pay for itself within two years of
ownership. Additionally, the replacement cost of an amorphous silicon detector
represents about 80% of the cost of a new purchase. The replacement cost for
the CCD camera head would be less � about 7.5% of the cost of a new camera
based system, and because of its durability, could be expected to have a
service lifespan comparable to even a new Linac. In evaluating an add-on
system, it is desirable to select one with a lifespan comparable to the
remaining years of service of the Linac. Thus, a camera-based system is more
appealing from a financial point of view.
Materials and methods
Selecting an add-on EPID
One of the categories of portal imaging devices retrofits
to consider is the amorphous silicon proprietary system. The Siemens Primus aSi
flat-panel proprietary system commercially available is called OPTIVUETM500 (Siemens
Medical Solutions USA, Inc, 51 Valley Stream Parkway, Malvern PA 19355). This
system is Linac gantry base-mounted. Its purchasing cost as an add-on EPID was
greater, as seen in Table 1. Furthermore this system does not come with
surrogate based IGRT software suite. Thus additional third party IGRT software
would have to be purchased at an additional cost. As mentioned above, the
lifespan of aSi EPID may decrease with the intensive use required to meet IGRT
needs, however, over five years service are possible with less intensive usage
and optimal maintenance.
The second category of add-ons is the camera based portal
imaging system. There are two camera based EPID systems commercially available
to choose from for retrofitting on to Siemens Primus Linac. One is the Siemens
Primus Linac-specific LCD camera-based system, which requires a purchase of
additional image management software to use for surrogate based IGRT. This
system known as Beamview TI Plus� (Siemens Medical Solutions USA, Inc, 51
Valley Stream Parkway, Malvern PA 19355), includes a retractable and
collapsible Linac gantry head-mounted detector assembly with collision
detection, which can be used at any gantry angle. Beamview TI Plus� is based on
target integration technology, making it capable of acquiring portal images
with a very low number of Monitor Units. The Beamview Plus� electronic portal
imaging device has been evaluated against conventional radiographic films and
found to provide significantly "visible" or better images .
The other camera based portal imaging system is a third
party non-Linac-specific system called TheraView system. The TheraView system (Cablon
Medical B.V. Klepelhoek 11, 3833 GZ Leusden, The Netherlands) is Linac gantry
base-mounted with collision detection. It is a water cooled CCD camera system
and comes with a complete software imaging suite for surrogate based IGRT image
review and manipulation. Its potential use for in vivo dosimetric
applications have been explored recently .
The third category of the portal imaging to consider is a luminescence
based system. There was only one system in this category that was commercially
available at the time of the implementation of this project. This is not a
retrofit system since it does not require mounting on the Linac. This system,
the Kodak 2000RT CR Plus system (Eastman Kodak Comp., Rochester, NY, USA) uses EC-L cassettes with metal phosphor plates for luminescence radiography. It is
a mobile system that requires no mounting. After portal irradiation, the EC-L
cassette with metal phosphor plates would be physically transferred by a
therapist to a computed radiography (CR) digitizer where laser scanning of the
irradiated phosphor causes luminescence, whose detection is used to form a
digital image . The images are then electronically submitted to a computer
for review. Additional software at an additional cost from another third party
vendor is required to perform surrogate based IGRT image review and
Unlike conventional radiographic film, the metal phosphor
plate used with this imaging system is reusable and no film processor is
required; the quality of digital images from this system is comparable to
conventional film images , but unlike amorphous silicon or camera-based
systems, the imaging is not "real time," since cassettes must physically
be manipulated before images are available for review.
�In all the three categories of EPID retrofits considered,
the TheraView system was the most economical of all the four systems (refer to
Table 1 for details on different types of EPIDs).
TheraView portal imaging system
The TheraView Imaging System is manufactured in the Netherlands. Since Dutch power ratings differ from those in the US, factory engineers must
verify that the system's power rating meets US specifications before
importation. The system consists of a water-cooled, telescoping, motorized
image detector with a CCD digital camera. It has a scintillator screen size of
40 cm by 40 cm with a resolution of about 0.78 mm. The camera has 1024 by 1024,
12-bit resolution with a 28 cm by 28 cm field of view. The hardware mounts
easily at the base of the Siemens Primus gantry (See Figures 1a & b), at
the same location where the proprietary Siemens Primus EPID is usually mounted.
The pendant cord passes through the existing conduit to the modulator and is
easily mounted as seen in Figures 1a and 1b. The small remote hand pendant can
be mounted anywhere within the treatment room. The camera unit weighs about 80
kg, which is less than the weight of proprietary aSi EPID, and there is no
interference with the existing Siemens Primus Linac. (See Table 2 for technical
specifications of EPID systems considered). The Theraview EPID image
characteristics have been described before .
The TheraView hardware installation process took
approximately 1.5 days over a weekend. The team comprised four individuals -
two factory-based engineers and two local technicians, one familiar with the
Siemens Primus Linac and the other being the local TheraView systems imaging
engineer who would provide the maintenance services. In addition, after the
system was connected to the local area network, the software engineers in the Netherlands were able to offer remote trouble shooting and guidance services to the team
through the VPN tunnel, which avoided the need for local IT personnel. The
software can be customized to users� needs at any time without having to wait
for the release of a newer version.
Networking and integration with the existing equipment
The DICOM-compatible TheraView portal imaging system was
networked on the existing intranet. The Oldelft simulator has Digital Therapy
Imaging (DTI), a digital image acquisition, processing, and review system for
image intensifier-based X-ray procedures adapted for use with a Simulix-HP
simulator. The images can be stored on a hard disc with the Simulix positions
and patient data, and can be retrieved for review, digital imaging processing,
text and line annotation, printing and transmission to the department network
or PACS. The DTI unit was configured to communicate with the TheraView system.
The TheraView system connects to DTI and imports the DICOM
images. These images can then be used as reference images for comparison with
the EPID images during the initial treatment verification (see Figure 2) using
the TheraView system.
The TheraView system can accept beam�s eye view field
shapes as bitmap or DICOM images from the Philips Pinnacle ADAC treatment
planning system. This involves configuring the TheraView system to receive
images from the treatment planning system (TPS).
Portal image processing
Manipulation of EPID images beyond the screen display
requires additional software in many other EPID systems. Many cancer centers
utilize record and verify systems, like IMPAC or Lantis, for the physicians to
review and approve the portal images. Vendors provide non-numerical image
management modules associated with their existing record and verify systems at
an additional cost, as high as 30% of the cost of an add-on EPID equipped with
image management tools. Within the TheraView system, there is a software
component (called TargetCheck) used to numerically check and compare the
on-line beam�s eye view fields with the chosen reference image (either a DRR or
a simulation image)  and to automatically analyze the field shapes and the
position of the patient based upon anatomic landmarks or implanted fiducial
markers [35-37]. Figure 2 compares an image acquired using the EPID (left) and
a simulator DTI image (right). Figure 3 compares an image acquired using the
EPID (left) and a DRR (right). The system then reports the deviations and the
shifts instructions required to match the EPID image to the reference image,
based on the tolerances preset by the physician and physicist. The deviations
over the course of treatments can then be displayed (shown in Figure 4) or
printed as numerical values for the patient's record. The software allows
graphic display of the deviations with time/fractions. Thus, even in the
absence of the physician, treatment can proceed provided shifts have been made
by the therapist to satisfy the pre-set tolerances.
Remote image manipulation through the internet and system maintenance via
Authorized personnel can access the database remotely via
the internet through a VPN tunnel, which allows remote trouble-shooting of
system troubles, and allows physicians to review portal images from any
computer terminal with internet connectivity. It is important to remember that
computers used for remote diagnostics or physician image review that are
outside the intranet and have static IP addresses pose security issues, and
care must be taken to ensure that the port is closed immediately after use, so
that the port is open only when in use by authorized individuals.
The authors have installed a less expensive third party
EPID onto an existing Siemens Primus Linac. This system can also be installed
on Linacs manufactured by different companies such as Varian and Elekta. The
TheraView electronic portal imaging system integrates well with existing
equipment. It offers a complete imaging system that eliminates the need for
separate software for portal image viewing, interpretation, analysis,
archiving, and other image management applications. It runs on a Linux
platform, and uses DICOM capability to import treatment planning images,
simulation images and scanned film images for comparison with
electronically acquired portal images. It has secure internet access through a
VPN tunnel that allows remote trouble shooting by factory personnel as well as
remote portal image review by the physician.
The major drawback with this system is with the image
quality which is relatively poor compared to other non-camera based EPIDs due
to loss of image resolution inherent in the CCD technology such as afterglow of
previous images in the current image, "dead" pixels and specifically
for Linac systems problems synchronizing with gun pulses and the so-called
A comprehensive research work is in progress to determine
the proper commissioning and QA procedures especially for Theraview system to
come up with appropriate consistent parameters for MTF, spatial resolution,
contrast resolution, signal to noise, typical exposure settings, positional
accuracy and reproducibility, effect of the installation on machine
There are ongoing efforts to improve the quality of images
in this system. Additionally, dosimetric applications such as measuring the
entrance and exit patient doses are currently being tested as are
software modules for filmless machine quality assurance, like isocenter and
radiation/light field match.
Figure 1 TheraView EPID unit installed at the base of Siemens Primus Linac. (a) The EPID camera unit is in position to acquire images. (b) TheraView EPID unit installed at the base of Siemens Primus Linac. EPID camera unit is in retracted position.
Figure 2 TheraView imaging system software. EPID (left) and conventional DTI (right) images for a head and neck treatment as shown. Bony landmarks were used for field matching.
Figure 3 TheraView imaging system software. EPID (left) and DRR (right) images with gold markers implanted into the prostate are shown.
Figure 4 Graphic display of set up deviations over the course of 42 IGRT treatments for a head and neck cancer patient.
Table 1 These are turn-key costs associated with different EPID systems that the authors evaluated as of year 2005. There was a one year warranty on all the systems. The table indicates the imaging technology used by the system, and whether the base purchase price included the image manipulation software. Prices for the different systems in this table represent quotations provided to the authors at the time the different systems were being evaluated, and may differ from current prices.
Table 2 Technical specifications of different types of portal imaging technologies.
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|Received 30 October 2008; received in revised form 4 June
2009, accepted 23 June 2009
Correspondence: Raleigh Regional Cancer Center and Mountain State University, 275 Dry Hill Road, Beckley WV 25801 USA. E-mail: email@example.com (Dan Odero).
Please cite as: Odero DO, Shimm DS,
Third party EPID with IGRT capability retrofitted onto an existing medical linear accelerator, Biomed Imaging Interv J 2009; 5(3):e25