Will Helical Tomotherapy ultimately
replace linac based IMRT as the best
way to deliver conformal
radiotherapy?
Kris Armoogum
Department of Radiation Physics, Royal Derby Hospital
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Overview
Many physicists and radiation oncologists are convinced
that helical tomotherapy is the be-all and end-all of
intensity modulated radiation therapy (IMRT) delivery
systems.
Linac manufacturers have not stood still and many of them
have developing cone-beam CT and intensity modulated
arc therapy capabilities for their linear accelerators which,
they claim, will provide the ability to deliver IMRT
treatments with versatility and verifiability comparable to
those achieved with Tomotherapy.
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Which is correct?
The premise that helical tomotherapy will ultimately prove
to be the best way to deliver IMRT is the claim debated in
this presentation[1].
With our Department actively engaged in the process of
replacing the Oncor machines, now is good time to have
this discussion.
[1] T. Bichay, D. Cao, and C. G. Orton, “Point/counterpoint. Helical tomotherapy will
ultimately replace linear accelerator based IMRT as the best way to deliver
conformal radiotherapy,” Med. Phys. 35, 1625–1628:2008.
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Some Background
The introduction of IMRT has significantly improved the
ability to deliver a highly conformal radiation dose
distribution to a complex target while minimizing
collateral damage to adjacent tissues.
IGRT further improves this by precisely locating a highly
conformal dose distribution with daily verification and
with the potential for daily correction.
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Some Background
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There are four key elements of highly accurate IMRT and
IGRT:
Stability of the imaging system
Number of available beam directions
Dynamic range of intensities
Position verification
The more stable the system, the sharper the images and the more
accurate beam placement can be. To enhance physical stability, many
imaging systems have adopted a ring gantry doughnut shape, for
example CT units, PET, MRI, gamma cameras, etc.
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Arguments in Favour of Tomo
The ring gantry of a Tomotherapy unit exploits this (ring)
structural stability resulting in an isocentric precision of
0.2 mm, 5x better than typical arm-gantry systems.
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Arguments in Favour of Tomo
It is well recognized that increasing the number of fields
can improve the overall dose conformality. In typical
arm gantry-based IMRT, selection of the most effective
gantry angles may not be obvious.
This can result in the loss of useful directions prior to
the initiation of optimization. In tomotherapy IMRT, the
optimizer has full access to 360° of rotation.
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Arguments in Favour of Tomo
One of the weaknesses of MLCs is that most of them
are motorised making them prone to motor breakdown,
positional inaccuracies, and velocity fluctuations.
However, binary MLCs, such as the 64-leaf system of
tomotherapy, are inherently much more reliable since
the sensors need to read only in open or closed
positions.
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Arguments in Favour of Tomo
In addition, the MLC motion is extremely rapid, opening and closing
within 20 ms, and the dwell time at each position can be
automatically varied from 1 to 400 ms.
The combination of number of control points, gantry directions, and
dwell times yields substantial flexibility in generating an optimized
distribution.
This allows an almost infinite dynamic range of intensities, not only
for every angle, but also for every point in the target volume from
that angle. IMRT without a wide dynamic range of intensities will
always be inferior.
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Arguments in Favour of Tomo
The maximum field size for typical accelerators without
the need for junctions is less than 40×40 cm2. Larger
fields for IMRT require complex junctions and/or
extended SSD.
With Tomotherapy, fields of up to 160 cm in length can
be treated without the need for junctions.
Will be able to use Tomotherapy for total marrow
Irradiation.
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Arguments in Favour of Tomo
The imaging chain of tomotherapy allows a full 38 cm
diameter imaging ring.
The detector (511 Xenon ion chambers) serves a dual
purpose:
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Imaging and patient positioning
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Can obtain quantitative dose values, allowing the
delivery to be validated.
Reconstruction of the actual dose can then be
calculated on the acquired 3D CT data set.
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Summarising the case for Tomo
Given the superior design of the imaging / delivery
hardware, the construction and speed of the MLC, the
integrated design and less QA, it is clear that the
tomotherapy approach to IMRT will lead the way in the
future.
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Arguments Against Tomo
Helical Tomotherapy is an excellent modality for both
IMRT and IGRT.
Using cone-beam CT and arc-based IMRT, linear
accelerators can match Tomotherapy in terms of both
IGRT and IMRT capabilities.
Linear accelerators provide more flexibility than is
available with Tomotherapy.
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Arguments Against Tomo
- A key feature of the helical tomotherapy
system is its ability to deliver highly
conformal treatments. For many treatment
sites such as the prostate, however, it is
unlikely that further clinical benefits will be
realized beyond what is provided by
rotational linacs (VMAT, RapidArc)
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Arguments Against Tomo
- VMAT has the advantage of delivering
non-coplanar arcs (an option not available
with tomotherapy?). For some intracranial
and head-and-neck tumours, the use of
non-coplanar arcs can provide significant
dosimetric benefits due to preferential
sparing of adjacent OARs.
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Arguments Against Tomo
- Tomotherapy provides MV fan-beam CT
scanning while conventional linear accelerators
can provide kilovoltage cone-beam CT.
- The fan-beam approach used by tomotherapy
has improved scatter rejection that reduces
image noise. The use of kV imaging in most
linacs, however, is advantageous because the
lower beam energy results in improved soft
tissue contrast.
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Arguments Against Tomo
- Tomotherapy systems are dedicated specifically
to IMRT and IGRT and cannot match the
versatility of a linear accelerator(?) For some
patients, the delivery of 3D conformal treatments
on a linear accelerator provides a more efficient
solution than is available with Tomotherapy.
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Arguments Against Tomo
- Linacs also provide the ability to deliver electron
fields. For many superficial targets, the use of
electrons from a linear accelerator is clearly a
better choice for its simplicity of dose delivery as
well as its higher skin dose and sharper dose
fall-off beyond the target.
- For the ability to deliver a wide range of
treatments ranging from palliation to the most
complex IMRT plans, linacs will continue to
provide the most efficient and flexible solution.
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Tomo: Rebuttal
- VMAT is limited by the number of MUs used,
typically 500–700, resulting in poor modulation.
- Some simple mathematics demonstrates the
limitations of a motorised leaf in VMAT delivery:
In a typical 7° arc of 1.17 s, the leaves can move
no more than 2.3 cm; at best a modulation factor
of 2, or about 50x less than the comparable
modulation factor in Tomotherapy
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Tomo: Rebuttal
- It is correct in that non-coplanar arcs are not
possible in Tomotherapy. However, the
availability of hundreds of thousands of
beamlets can overcome much of this limitation
even in very complex targets adjacent to OARs.
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Tomo: Rebuttal
- There is also the considerable potential for
radiobiological gain. In Tomotherapy every cell
receives its full complement of dose in less than
2 minutes. In conventional accelerators the time
from first to last photon may be 20 min or more
allowing significant tumour cell recovery.
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Tomo: Rebuttal
- It is claimed that conventional linacs are more
versatile in that they can treat non-complex sites
such as those normally treated with electrons.
- However, superficial treatments for skin lesions
have been carried out with Tomotherapy with
excellent results, in certain cases superior to
conventional electrons.
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Other arguments for Tomo
- The price is now comparable to the cost of a
rotational capable linac.
- Less functionality in Tomotherapy but this
translates into shorter commissioning time and
reduced QA burden.
- Can use existing linac bunker. Inherent beamstopper in Tomo.
- Can (possibly) use Mobile Tomotherapy instead
of an overspill bunker when replacing Oncors.
Could be a cheaper alternative and improve
patient throughput and staff skill levels.
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Mobile Tomotherapy
• Same machine
• Same planning SW
• Same reliability
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Tomotherapy trailer
Portable clinic
Stairs, ramp, walkways
External shielding and
fencing
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Go forth and discuss…
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