Kris Armoogum

1
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

2
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.

3
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
presentation1. 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, 162516282008.
4
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.

5
Some Background

• 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.

6
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.
7
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.
8
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.
9
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.
10
Arguments in Favour of Tomo
The maximum field size for typical accelerators
without the need for junctions is less than
4040 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.
11
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
• Imaging and patient positioning
• 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.

12
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.
13
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.
14
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)

15
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.

16
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.

17
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.

18
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.

19
Tomo Rebuttal

• VMAT is limited by the number of MUs used,
  typically 500700, 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

20
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.

21
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.

22
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.

23
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
  beam-stopper 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.

24
Mobile Tomotherapy

• Same machine
• Same planning SW
• Same reliability

• Tomotherapy trailer
• Portable clinic
• Stairs, ramp, walkways
• External shielding and fencing

25
Go forth and discuss
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