Biomed Imaging Interv J 2007; 3(1):e16
© 2007 Biomedical Imaging and
Tomotherapy as a tool in image-guided radiation therapy (IGRT): theoretical and technological aspects
S Yartsev1,*, PhD,
T Kron2, PhD, FCCPM,
J Van Dyk1,3, MSc, FCCPM
1 Physics and Engineering, London Regional
Cancer Program, London Health Sciences Centre, London, Ontario, Canada
2 Department of Physical Sciences, Peter MacCallum Cancer Centre,
East Melbourne, Australia
3 Departments of Oncology and Medical Biophysics, University of
Western Ontario, London, Ontario, Canada
Helical tomotherapy (HT) is a novel treatment approach that
combines Intensity-Modulate Radiation Therapy (IMRT) delivery with in-built
image guidance using megavoltage (MV) CT scanning. The technique utilises a 6
MV linear accelerator mounted on a CT type ring gantry. The beam is collimated
to a fan beam, which is intensity modulated using a binary multileaf collimator
(MLC). As the patient advances slowly through the ring gantry, the linac
rotates around the patient with a leaf-opening pattern optimised to deliver a
highly conformal dose distribution to the target in the helical beam
trajectory. The unit also allows the acquisition of MVCT images using the same
radiation source detuned to reduce its effective energy to 3.5 MV, making the
dose required for imaging less than 3 cGy. This paper discusses the major
features of HT and describes the advantages and disadvantages of this approach
in the context of the commercial Hi-ART system. � 2007 Biomedical Imaging and
Intervention Journal. All rights reserved.
Keywords: Image guidance, helical tomotherapy, radiation
Imaging has always been a necessary prerequisite for
radiation therapy. Presently, an intense interaction between these two fields
of technology is observed. The discovery of X-rays more than a century ago
provided the possibility to locate internal organs in the human body and plan
radiation delivery with rectangular fields using two-dimensional (2D)
transmission images up to the mid-1970s.
The introduction of computed tomography (CT) in clinical
practice resulted in high quality 3D images, which allowed precise definition
of tumour shape and location. This information motivated technology
development, which would allow planning and delivery of radiation in a more
conformal way aiming to give enough dose for disease elimination while sparing
Technological advances in radiation oncology such as
three-dimensional conformal radiation therapy (3DCRT) and intensity-modulated
radiation therapy (IMRT) allow the shaping of the dose distributions in
patients, with a very high degree of conformity and precision . The
application of high-dose gradients provides opportunities for escalating tumour
doses resulting in a better chance of the elimination of cancerous cells while
still sparing healthy, sensitive organs. At the same time, such highly
localised dose distributions may result in a partial target miss and/or risk of
organ damage if on the day of treatment the patient setup and/or anatomy are
different from that of the imaging study used during planning. If changes in
the patient�s anatomy are not detected, the treatment could be compromised .
Several solutions to correct the position of the target
immediately before (or during) treatment have been developed and clinically
implemented including fiducial marker implants [3-6], optical positional
guidance [7,8], MRI , ultrasound [6,10-18], and daily CT imaging [10,18-26].
Each of these techniques has some positive (better targeting, smaller margins)
and negative (increased labor and cost, longer treatment times) features and
their detailed clinical assessments with respect to specific disease sites are
In the current literature, the term� �image-guided radiation
therapy� (IGRT) or IG-IMRT is employed to refer to newly emerging radiation
planning, patient setup and delivery procedures that integrate image-based
tumour definition methods, patient positioning devices and/or radiation
delivery guidance tools . IGRT is a necessary companion of improved
treatment planning and better radiation delivery.
Helical tomotherapy (HT) is a novel radiotherapy concept
that combines elements from a helical CT scanner with a megavoltage (MV) linear
accelerator [28-30]. The idea to include a MV imaging system for setup and dose
verification was already put forward in 1993 in the first publication on
helical tomotherapy . In the initial version of IGRT with on-board MVCT
implemented in the commercially available Hi-ART model, MVCT allows daily
patient setup verification and repositioning. In the future, MVCT will also be
used for imaging patients followed by quick planning for rapid treatment of
emergency cases  and for real-time image collection during treatment
delivery . In this report, the basic principles of imaging with tomotherapy
are discussed. In the companion article, we review the first results of HT use
in clinical practice.
The helical tomotherapy approach to IGRT
The major components of the helical tomotherapy system are
shown schematically in Figure 1. The patient is scanned on a diagnostic
kilovoltage CT (kVCT) unit prior to HT planning and all structures (gross
tumour volume, planning target volume and every sensitive organ that needs to
be protected) should be outlined. Patient CT data and structure set are
transferred to the HT database using DICOM protocol. This information will be
used for inverse planning on the planning station and also as a reference for
image guidance on the operator station where the planning kVCT image is
compared to the MVCT image taken immediately before treatment. Creation of
digitally reconstructed radiographs is not necessary as planning kVCT images
will be directly compared to MVCT verification images.
On the HT unit, a conventional 6 MV linear accelerator and a
detector array system are mounted opposite each other on a ring gantry that
continuously rotates during the imaging and treatment procedures while the
couch translates at a constant speed through the gantry as schematically shown
in Figure 2. The design ensures minimal gantry sag and, provided the unit is
properly aligned, the centre of rotation for radiation and mechanical
components should be within 1 mm . No flattening filter is used and the
X-ray beam with an output of about 10 Gy/min at isocentre is collimated to fan
beam geometry with a width of 40 cm and a fan beam thickness (FBT) variable
from a few millimeters to 50 mm. Orthogonal to the fan beam width is a binary
(i.e. �either open or shut�) multi-leaf collimator (MLC). Its 64 leaves are
divergent with the beam and project to 6.25 mm width at isocentre. The transit
time for the leaves is between 20 and 30 ms for the largest fan beam thickness.
As the unit is specifically designed for IMRT, the leaf thickness (10 cm
tungsten) is thicker than in most conventional MLCs and the overall shielding
of the head is better. Therefore, leakage radiation to the patient is generally
low despite being treated with long beam at times. Jeraj et al found the
out-of-field leakage to be less than 0.1% , which would result in 1% dose
to the periphery of the patient even in long and complex treatments .
For planning and dose delivery, the full gantry rotation is
divided into 51 projections. Each projection is characterised by its own leaf
opening pattern and covers an arc segment of approximately 7�. The available
rotation period may be between 15 and 60 s (typically around 20 s). As such,
each projection takes between 0.2 and 1 s with all leaves shut for a short time
between projections. The delivery assumes constant dose rate of the linac and
no dose feedback servo is employed in the current system. The monitor chambers
are a safety feature that will terminate irradiation if the dose rate is
outside predetermined specifications (typically +/- 5% over 10 s and +/- 50%
over 1 s).
The treatment unit also includes a radiation detector system
at the beam exit side. This is a Xe-filled ionisation chamber array similar to
the ones employed in older diagnostic CT scanners. In practice, it is the
tungsten septa that interact most with the MV beam and the secondary electrons
generated in the tungsten easily reach the cavities where they are detected.
The detector system can be used for acquisition of MVCT scans of the patient in
treatment position. The linear accelerator is de-tuned to 3.5 MV and the pulse
repetition frequency decreased to keep the dose delivered to the patient during
imaging well below 3 cGy. The data acquisition is fast enough to determine the
dose given in individual linac pulses and the detector acquisition system (DAS)
files are a most useful tool for commissioning and quality assurance (QA) of
the unit .
A treatment file for HT consists of some 60,000 numbers,
which specify leaf opening times as a function of gantry position and patient
location in the gantry. Due to this complexity, tomotherapy treatment plans can
only be created in an inverse planning process. Patient CT data and structure
set are transferred to the planning station using DICOM protocol. It is
important to extend the planning CT scan at least 5 cm beyond any potential
target volume, as the dose delivery may be performed using a 5 cm-wide fan
beam. In this case, the ramp up to full dose in the target requires the same
length as the fan beam thickness . The outlining tools in the current
tomotherapy software are limited to contour modifications but the structures
themselves should be created elsewhere. In practice, the number of contours
must be typically larger than in �conventional� IMRT, as no beam directions can
be pre-determined. The planner chooses positions of the movable red lasers
(usually placed on the external marks made during kVCT study), which will be
used for initial positioning of the patient on the treatment couch. The
planning process allows the specification of multiple targets, which is
convenient for simultaneous in-field boost delivery rather than a conventional
treatment course given in multiple phases or for the simultaneous treatment of
multiple isolated lesions. Treatment delivery and planning depends on
parameters specific for HT: fan beam thickness (FBT), pitch factor and
modulation factor (MF). The FBT is chosen by the operator to achieve a
compromise between fast treatment times and dose modulation in the
superior/inferior direction. A large FBT results in larger volumes covered in
any projection and a higher central axis dose output while it reduces the scope
for conformality and detailed dose modulation in cranio/caudal direction of the
patient. As such, the largest FBT of about 50 mm is likely to be used for total
body irradiation and mantle type fields while small FBT of 10 mm or even less
needs to be employed for small brain lesions . The output in the fan beam
drops dramatically below a FBT of 10 mm due to loss of lateral electron
equilibrium and partial source occlusion � therefore, it is unlikely that
smaller FBTs will be used frequently. A different way to improve the modulation
capabilities in the superior/inferior direction is the use of a small pitch
factor. The pitch factor is defined as couch movement per rotation in units of
the FBT. While it is common to use pitch factors of one or higher in diagnostic
CT scanning, the pitch in HT is typically between 0.25 and 0.5 resulting in
overlap between adjacent rotations during the helical delivery. The smaller the
pitch factor, the longer the treatment; however, a small pitch also improves
the capability of dose modulation and the ability to deliver high doses per
fraction. A potential problem with large FBT and large pitch is the dose
distribution away from the central axis. The beam divergence will cause
variations in overlap between adjacent rotations, which increase with distance
from the axis of rotation. This is known as the �thread effect�. Kissick et
al have investigated this question and concluded that a pitch factor of
0.86/integer number (e.g., 0.43, 0.287, 0.215, etc.) minimises the thread
The MF represents the ratio of maximum leaf opening time to
the mean leaf opening time of all MLC leaves, which open in a projection. MF is
proportional to the overall treatment time, and with typical physical
constraints for the tomotherapy delivery, MFs can be selected between 1 and
approximately 6. A small MF results in short treatment times and is adequate
for relatively symmetrical targets close to the central axis of the patient,
e.g., prostate cancer .
The calculation itself is based on a
superposition/convolution dose calculation algorithm  and an iterative least
square optimisation process . The planning procedure starts with a
calculation of the dose distribution produced by all beamlets, which deliver
radiation to the target followed by an optimisation of opening times for each
leaf guided by precedence, importance and penalty factors. The optimisation
results may be quickly modified using the same pre-calculated beamlets and
other sets of important and penalty factors. Usually, it takes a couple of
hours to produce a plan that would satisfy the requirements of the radiation
oncologist. As the tomotherapy environment at present does not allow
multitasking, it is generally recommended for performing the dose calculation
overnight when multiple calculation tasks can be batched. Figure 3 shows an
example of planned dose distribution for an 82-year-old male patient with a
resected large medullary carcinoma of the thyroid with microscopic residual
disease [planning target volume (PTV) = 1932 cm3, target length in
sup/inf direction of 13 cm). A dose of 60 Gy to 90% of the PTV was prescribed
for delivery in 30 fractions according to the plan where trachea, spinal cord
and posterior region were considered sensitive structures with priority to the
sparing of spinal cord and trachea.
MVCT in helical tomotherapy
A patient is initially positioned on the treatment couch
using external markings made during the planning kVCT imaging. Then a MVCT is
acquired. In the imaging mode, the linear accelerator is detuned in order to
improve the soft tissue contrast in such a way that the nominal energy of the
incident electron beam is reduced to 3.5 MeV; the resulting photon spectrum is
compared in Figure 4 with the spectrum for the treatment mode . This photon
beam is collimated by the jaws to a FBT of nominally 4 or 5 mm at the isocenter
in superior/inferior direction and 40 cm width laterally. Due to the use of
megavoltage X-rays, a further reduction of FBT will result in only a marginal
improvement in spatial resolution. Three modes of image acquisition: coarse,
normal and fine, obtained by different pitches (couch movement per gantry
rotation 12, 8 or 4 mm) are available resulting in image reconstruction with
inter-slice distances of 6, 4 and 2 mm. Figure 5 shows MVCT images of a head of
Rando phantom taken in coarse (time required to image 18 cm in
superior/inferior direction in 30 slices was 156.5 s), normal (time required to
image the same volume in 45 slices was 231.5 s) and fine (time required to
image a smaller volume in 80 slices was 406.5 s; 80 is the maximum amount of MVCT
image slices) imaging options. The image reconstruction matrix for the field of
view of 40 cm is 512 (resulting in a 0.78 mm in-plane pixel resolution). The CT
detector used in the HT system has been described in several papers [20,43,44].
This arc-shaped xenon detector has 738 channels, each with two ionisation
cavities filled with xenon gas and divided by 0.32 mm tungsten septa. The
detector array has a 110 cm radius of curvature and 540 out of 738 channels are
used for the MVCT image reconstruction. The source to axis distance is 85 cm
and the source to detector distance is 145 cm.
Usually the MVCT study is performed using a length, which
covers the PTV and/or some specific anatomic landmarks suggested by the
physician. Figure 6 shows typical MVCT/kVCT midline sagittal images on an image
registration display. The current MVCT images are visually evaluated and
registered with the planning kVCT set either automatically or manually. The
automatic mode of registration uses a mutual information algorithm. One may
choose alignment by translation in three directions and add roll, pitch and yaw
displacements as desired. Shifts in superior/inferior and anterior/posterior
directions are introduced by couch displacement. Correction in lateral
direction is done by the radiation therapists using manual fine adjustment on
the treatment couch within the limits of 2.5 cm. Roll correction is accounted
by changing the starting angle for gantry rotation . Pitch and yaw
corrections can only be introduced by moving the patient and these last two
corrections are performed very rarely in clinical practice and only when the
other four displacements are not able to provide sufficient alignment. After
automatic registration, the alignment of fiducial anatomic features as assigned
by a radiation oncologist is checked by the radiation therapists and, if
necessary, manual adjustments of the patient setup are performed.
In principle, the field of view (FOV) of 40 cm available in
the tomotherapy MVCT system may lead to a degradation of image quality because
the tissue outside the FOV is not properly accounted for in the reconstruction
process. The typical result is �bowl� artifacts so regarded because the
reconstructed CT values are increased in the peripheral regions of the images.
Ruchala et al have shown that the voxel-based mutual information
algorithm used by tomotherapy software for registration still provides
successful automatic registration with fields of view down to about one-half of
a patient�s size and limited-slice images .
Concerning setup uncertainties, it is generally accepted
that there are two types, systematic and random. Systematic uncertainties exist
because the acquired 3D image may differ from the average target position and
random uncertainty is the day-to-day deviation from the target average position
. Boswell et al compared automatic tomotherapy setup using MVCT to
an optically-guided patient positioning system using an anthropomorphic head
phantom and found net translational differences between the optical camera and
tomotherapy software automatic registration results to be within 2.3 mm in 878
of 900 registration trials . Setup corrections for real patients may be
much larger because alignments of organs vary from day to day: the detected
maximum setup deviation was 3 mm for patients fixated with the body frame and 6
mm for patients positioned in the vacuum pillow .
Performance characteristics of MVCT on Hi-Art tomotherapy
system were reported by Meeks et al . They studied image noise and uniformity,
spatial resolution, contrast properties and multiple scan average dose with a
Cardinal Health AAPM CT Performance Phantom (Cardinal Health, Hicksville, NY),
which is an acrylic cylinder� (21.6 cm in diameter and 31.75 cm in length) with
inserts. The images were very uniform with an uniformity index greater than 95%
and no statistically significant difference as a function of an equivalent
reconstruction matrix or pitch. Typical noise standard deviations are 2-4%,
which are only slightly worse than that for diagnostic CT. The visible
resolution for the 512 matrix images was approximately 1.25 mm. The contrast
resolution e.g., ability to distinguish between muscle tissue with electron
density of 3.44-3.48 (1023 electrons/cm3 from the surrounding adipose
tissue with 3.18 (1023 electrons/cm3) is clinically an important
characteristic: in general, the need for high resolution is not as pressing as
low-contrast detectability . A� MVCT scan with the dose of 1.1 cGy allows a
clear identification of the prostate and rectum because their electron
densities are on the order of 8-10% different from the surrounding region .
By increasing the imaging dose, it is possible to improve the contrast e.g., an
8 cGy scan made it possible to delineate regions with the contrast about 2%
. This is currently not an option that the user can select in clinical
mode. An experimental study comparing MVCT with conventional diagnostic CT
scans in dogs with spontaneous tumours concluded that the MVCT image quality is
sufficiently good to allow three-dimensional setup verification .
A system of the complexity of a helical tomotherapy unit
obviously requires a significant amount of QA. At present, it is left to the
user to determine the level of QA as no widely accepted protocol for HT QA
exists at present. The suggestion of a QA program for HT is beyond the scope of
the present review: see relevant publications [50,51]. The manufacturer
acknowledges the need for patient specific QA and it is suggested that the dose
distribution for every patient is verified prior to treatment. To this end, a
special phantom (�cheese phantom� shown in Figure 7) and a QA module in the
planning software is included in the purchase of a HT unit. The QA module for
planning allows the calculation of the dose distribution, which would be
achieved if the patient plan was delivered onto a phantom of the user�s choice.
The software is an integral part of the planning station, which makes QA a
natural flow of the planning process.
The typical QA process requires the user to verify the
absolute dose to at least one point using an ionisation chamber, and the dose
distribution in a relevant plane of the phantom using radiographic film. After
digitisation, the dose distribution from the film can be directly imported into
the planning software and quantitative comparisons can be made with the
verification plan using dose profiles and gamma evaluation [52,53].
Recently, Kron et al have proposed an in vivo quality
assurance procedure for treatments on the tomotherapy unit . In this
method, a film is placed between the patient and the couch top during treatment
as can be seen with a phantom example in Figure 8. Tomotherapy Inc. provides a
�dose delivery quality assurance� (DQA) module, which re-calculates the dose
distribution one would get by delivering the patient treatment sequence onto a
selected phantom. It is possible to import MVCT study performed immediately
before patient treatment i.e., before the film exposure, as a �phantom�. This
allows calculation of the dose from the optimised open leaf sinogram for the
same patient and utilises the dose comparison tool available in the DQA
software as illustrated in Figure 9.
In Table 1, we summarize the principle features of helical
tomotherapy and compare them with characteristics of conventional radiotherapy
units using linear accelerators. In the near future, it is the intent that MVCT
will be used also for reconstruction of the dose actually delivered and for
planning and re-planning with real-time image collection during treatment
Helical tomotherapy is a new concept in radiation therapy
combining IMRT treatment, 3-D inverse treatment planning and 3-D MVCT imaging
in one integrated machine. All these components are uniquely designed for IMRT.
The complexity of the delivery process only allows inverse treatment planning
but delivers highly conformal dose distributions. Treatment planning studies
demonstrate dose homogeneity and conformal avoidance capabilities as two of the
major strong points of the system. One of the most important features of the HT
concept is the on-board MVCT image acquisition system. It allows not only the
verification of patient positioning but constitutes a powerful QA tool, which ultimately
will yield the reconstruction of the dose as it was actually delivered to the
patient on every occasion of a fractionated course of treatment.
This study was conducted with the support of the Ontario Institute for Cancer Research through
funding provided by the government of Ontario.
Figure 1 The schematic components of the tomotherapy unit.
Figure 2 Schematic drawing of a helical tomotherapy unit.
Figure 3 Planned dose distributions in axial and sagittal views of a medullary carcinoma of the thyroid. Note the conformal avoidance of trachea and spinal cord in a patient with microscopic residual disease after resecting medullary carcinoma of the thyroid.
Figure 4 Typical photon beam spectra of helical tomotherapy for two operational modes: treatment mode and MVCT imaging mode. While in the treatment mode, the incident electron energy is approximately 5.7 MeV; in MVCT imaging mode, it is reduced to about 3.5 MeV corresponding to the average photon energies of 1.5 MeV and 1.0 MeV, respectively. Reproduced from  with permission.
Figure 5 Example of coarse (6 mm interslice distance), normal (4 mm interslice distance) and fine (2 mm interslice distance) options for MVCT imaging of the same slice on a tomotherapy unit.
Figure 6 Example of sagittal view of MVCT (green) and kVCT (grey) registration.
Figure 7 The �cheese� phantom for tomotherapy delivery quality assurance process. Shown is a sheet of EDR2 film (Eastman Kodak Co. Rochester, NY) on the lower half of the phantom. It will be covered with the other half and both half cylinders can be fixed against each other using rubber ties. In the foreground of the photo is the Exradin A1SL ion chamber (Standard Imaging, Middleton, WI), which is used to verify the absolute dose delivered in at least one of the holes drilled in the phantom.
Figure 8 Typical setup of a �patient� (head of Rando phantom) on the top of the film used for in vivo dosimetry on tomotherapy unit in London, Ontario, Canada.
Figure 9 Example of in vivo dosimetry using MVCT study as a �phantom�: a) The thin isodose lines represent the dose from the in vivo dosimetry film, the thick dotted ones are calculated for the MVCT data on the same day they were imported as a phantom in the DQA software, b) A dose profile comparison along the line shown in (a).
Table 1 Comparison of helical tomotherapy to conventional linac based radiation therapy (RT).
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|Received 19 December 2006; accepted 30 January 2007
Correspondence: Department of Physics & Engineering, London Regional Cancer Program, London Health Sciences Centre, 790 Commissioners Road East, London, Ontario, Canada N6A 4L5. Tel.: +1-5196858600; Fax: +1-5196858658; E-mail: email@example.com (Slav Yartsev).
Please cite as: Yartsev S, Kron T, Van Dyk J,
Tomotherapy as a tool in image-guided radiation therapy (IGRT): theoretical and technological aspects, Biomed Imaging Interv J 2007; 3(1):e16
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