Department of Radiation Physics, Royal Derby Hospital 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

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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)

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.

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.

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.

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.

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

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.

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.

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.

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.

Mobile Tomotherapy Same machine Same planning SW Same reliability Tomotherapy trailer Portable clinic Stairs, ramp, walkways External shielding and fencing

Go forth and discuss… www.medicalphysicist.co.uk