Kris Armoogum Department of Radiation Physics, Royal Derby Hospital

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Kris ArmoogumDepartment of Radiation Physics, Royal Derby Hospital

Kris ArmoogumDepartment of Radiation Physics, Royal Derby Hospital

Will Helical Tomotherapy ultimately replace linac based IMRT as the best

way to deliver conformal radiotherapy?

Will Helical Tomotherapy ultimately replace linac based IMRT as the best

way to deliver conformal radiotherapy?

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OverviewOverviewMany 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.

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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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.

Which is correct?Which is correct?

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Some BackgroundSome 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.

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 BackgroundSome 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.

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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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.

Arguments in Favour of Tomo

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Arguments in Favour of TomoIt 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 TomoOne 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 TomoIn 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 TomoThe 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 TomoThe 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.

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Summarising the case for Tomo

Given the superior design of the imaging / delivery Given the superior design of the imaging / delivery

hardware, the construction and speed of the MLC, the hardware, the construction and speed of the MLC, the

integrated design and less QA, it is clear that the integrated design and less QA, it is clear that the

tomotherapy approach to IMRT will lead the way in the tomotherapy approach to IMRT will lead the way in the

future.future.

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Arguments Against TomoHelical 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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- 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)

- 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)

Arguments Against Tomo

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- 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.

- 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.

Arguments Against Tomo

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- 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.

- 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.

Arguments Against Tomo

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- 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.

- 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.

Arguments Against Tomo

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- 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.

- 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.

Arguments Against Tomo

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- 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

- 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

Tomo: Rebuttal

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- 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.

- 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.

Tomo: Rebuttal

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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.

- 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.

- 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 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.

- 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.

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Mobile Tomotherapy

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

fencing

• Same machine• Same planning SW• Same reliability

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Go forth and discuss…

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