important role in achieving optimal dose distribution for patients. Geometrical

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1 White Paper Agility * Intelligent Design Kevin Brown, Global Vice President Scientific Research Giulia Thompson, Senior Research Physicist Elekta, Ltd Abstract Multileaf collimators (MLCs) have a variety of characteristics that play an important role in achieving optimal dose distribution for patients. Geometrical and physical characteristics (such as transmission, leaf width, penumbra and leaf speed) interact with one another to achieve optimal performance. These have been taken into account in the development of Agility, which has been designed to meet the demands of modern clinical radiotherapy. This has been accomplished through a strong understanding of which factors are critical to patient plan optimization and good patient outcome, while maintaining excellent clinical flexibility and patient safety. This document highlights the scientific principles used in the design of Agility and explains their relevance in the clinical environment. *Agility is works in progress and not available for sale or distribution in all regions.

2 Transmission A major characteristic of the Agility multileaf collimator (MLC), and an important design objective, is its inherently low transmission. The scientific evidence for this feature is derived, in part, from the published work of Topolnjak et al 1. Topolnjak and his colleagues evaluated the impact of MLC design on Head and Neck intensity-modulated radiation therapy (IMRT) treatment plans that aimed to minimize the mean dose to the parotid glands. Although this study acknowledged the influence of the specific treatment planning system in use, it also demonstrated that some general conclusions can be applied to different treatment planning systems (with different beam and MLC model implementations) and optimization engines. It showed that, for Head and Neck cases (typical examples of a complex treatment plan), low MLC transmission provided the greatest benefit in reducing the mean dose to the parotid glands (see Figure 1). Furthermore, the authors concluded that MLC transmission can never be too low. This evidence formed the basis for development of the low transmission Elekta MLCi2 and has been a fundamental principle in the design of Agility. In fact, Agility has the lowest leakage in the industry, at less than 0.5% across the entire field 2. This meets the most recent requirements of the international safety standards for radiotherapy accelerators, which recommend that lower limits of MLC leakage should apply for equipment that is used in IMRT treatments 3. Transmission has such a significant effect on treatment plan optimization because it affects the majority of the field. The transmission effect from each field accumulates in the final dose distribution. It is clear that, as treatment techniques become more complex, certain physical characteristics increase in importance while others decrease. Achieving the correct balance has been crucial in the design of Agility. Lower transmission also facilitates the use of more effective modulation to achieve higher quality plans. This is because smaller fields delivering fewer monitor units can be used, without increasing accumulated transmission through the leaves. The low transmission of Agility permits the use of such advanced techniques without compromising the patient dose. Leaf Speed Another important characteristic of Agility is the high speed of the leaves and dynamic leaf guides, which result in a potential effective leaf speed of up to 65mm/s. 2 This characteristic has two immediate benefits for dynamic delivery: increased delivery speed and, by removing the constraints associated with lower leaf speed, the ability to deliver better quality plans through more effective modulation. Clearly, faster leaves result in quicker treatments. The benefit of more effective modulation by altering leaf speed limitations is described by Vorwerk et al 4. Within the limitations of their particular MLC and MLC leaf speed, they concluded that the use of lower dose rates is necessary to achieve optimal plan delivery. However, it is actually more effective modulation that is required. The high leaf speed provided by Agility permits more effective modulation by allowing higher dose rates to be used. This can help to improve plan quality and shortens treatment times. The Agility design uses dynamic leaf guides to assist leaf movement, extending leaf travel to 15cm over the central axis. This minimizes the size of the treatment head and ensures flexibility in clinical approach. It enables seamless delivery across the full 40cm x 40cm field and, since leaf guide movement is synchronized with the leaves, optimal leaf speed is ensured. Figure 1: DVH of the left (LP) and the right (RP) parotid glands for one patient. Calculations were obtained for MLC transmission values of 0%, 0.75% and 1.5% in PLATO TPS. 2

3 Figure 2a. Illustration of Agility leaf guides. Figure 2b. Illustration showing Agility sculpted diaphragms. Agility has the lowest leakage in the industry, at less than 0.5% across the entire field 2 3

4 Orthogonal Diaphragms Schmidhalter et al 5 demonstrated clearly that the use of dynamic orthogonal jaws improves the quality of IMRT delivered plans by reducing unwanted patient dose. The implementation of dynamic tracking jaws and Active Leakage reduction has been an original, constant feature of the Elekta MLCi and MLCi2 designs. To maintain this clinical benefit, Agility incorporates orthogonal dynamic jaws that can travel 12 cm over the central axis, at speeds of up to 90 mm/s. A novel, sculpted shape has been designed for the jaws. This reduces weight, and consequently improves speed, to optimize treatment delivery efficiency. The jaw is thinner where leaves will always provide additional shielding. The leaves can also be positioned intelligently behind the thicker part of the jaw, without closing, ready for the next beam shape. Leaf width The effect of leaf width, when combined with modern delivery techniques, is surprisingly small. When different leaf widths are compared for smoothness of the field edge (for a single field), smaller leaf widths appear to have superior dose distributions. However, curative patients are rarely treated with a single radiation field. In fact, fields from many different directions are generally employed in complex clinical techniques. The smearing effect produced by multiple fields on the scalloped field edges (defined by the finite width of the leaves) was identified by Mohan in the early days of MLC development 6. The modern volumetric intensity modulated arc therapy (VMAT) technique exploits the beneficial effect of such multiple beam directions. However, the definition of the field edge from one beam angle is affected by the dose passing through from another beam angle. In addition, when using IMRT, many different shaped fields are superimposed and the resulting dose distribution is calculated from the average of many leaf positions. IMRT and VMAT dose optimization algorithms account for the physical characteristics of the collimator, minimizing the effect of machine parameters on the final dose distribution. Finally, as recognized by Mohan, the actual dose distribution is also affected by positional uncertainties and patient motion, further reducing the effect of leaf width on the plan. The published evidence for the effect of leaf width is not consistent. Most, if not all, publications consider the effect of leaf width in isolation from positional uncertainties in the planning study (i.e. considering only static CT scans and contours). This limitation was acknowledged by Fiveash et al 7. Their planning study suggested that some improvement in the dose distributions could be achieved with the use of a higher-resolution dynamic MLC. However, they also acknowledged that patient set-up error could potentially remove the predicted benefits of finer leaves by increasing the effective penumbra. Leaf width affects the part of the field that is most influenced by these positional uncertainties (i.e. the target periphery), which reduces confidence in its benefits. In contrast, lower transmission affects those parts of the field that are not confounded by these uncertainties. Many reports conclude that the dosimetric benefits of finer leaf widths are small and that the clinical benefits are uncertain. For example, Burmeister et al 8 found that, in many cases, no apparent clinically significant differences were found in plans calculated using a 1 cm or 5 mm leaf width and, in fact, the 1 cm width plans often presented the advantage of reduced complexity (and hence reduced treatment time and whole body dose). While some advocate the use of 5 mm leaves, we anticipate that others will continue to support the use of 1 cm leaves, in view of the clinical and dosimetric uncertainties discussed above. Figure 3: DVH of the left (LP) and the right (RP) parotid glands for one patient. Calculations were obtained for MLC transmission values of 2.5mm, 5mm and 10mm in PLATO TPS. 4

5 Notably, the planning study of Topolnjak et al 1, which concluded that 5 mm leaves are better than 10 mm leaves (in the absence of any uncertainties) (see Figure 3), also stipulated that 2.5 mm leaves do not offer any further improvement for Head and Neck treatments. It should also be noted that degradation to dose-volume histograms (DVH) due to leaf width is more significant when set-up uncertainties are taken into account. The benefit of smaller leaf size is even less clear for 2.5 mm leaves. For example, Ku et al 9 stated that, for Stereotactic Body Radiation Therapy (SBRT), 2.5 mm leaves offer only very minor improvements. In addition, Boyd et al 10, in a study of radiosurgery treatment planning for arteriovenous malformations, concluded that the absolute improvements noted in these dosimetric parameters are admittedly small and the clinical significance uncertain. Therefore, since the gains are small for a limited number of techniques (and the importance of penumbra increases when treating such cases), we concluded that the use of 2.5 mm leaves can be restricted to an add-on collimator. Penumbra As a property of the field edge, similar considerations apply to the penumbra as to the effect of leaf width. One of the few penumbra studies performed (Topolnjak et al 1 ) concluded that smaller penumbra is beneficial. However, the benefit was small and the range of penumbra that was analyzed extended to larger values than are typical of actual devices. Additional literature on the clinical benefit of smaller penumbra is scant. This may be because it is technically difficult to make radical changes to the geometry without severely compromising other fundamental constraints of the linac, such as patient clearance. In addition, the relative contributions to penumbra at treatment depth are dominated by scatter and sourcecollimator distance, and are only slightly affected by the collimator design. At Elekta, we concluded that good MLC penumbra performance should be maintained without compromising other higher priority design considerations, such as patient clearance. The taller leaves, which were designed to satisfy the criteria of low transmission, also help to maintain consistent penumbra over the extended leaf over-travel of 15 cm. Flexibility Another important consideration is maximization of MLC flexibility. To achieve this, Agility has been designed with a compact outline, while maintaining market-leading patient clearance. This enables as many techniques to be used as possible; ensures optimal dose distribution; and minimizes the need to move the patient to avoid collisions. In addition, the dynamic leaf guides allow the leaves to move continuously over their full travel range. This enables seamless delivery of the full field (40cm x 40cm) IMRT without the need to use split fields (which could potentially extend the treatment time). Accuracy Leaf positioning accuracy is a major feature of Agility. It is a consequence of MLC integration into the linac control system and the real time leaf monitoring/ positioning system. To ensure that Agility achieves a 1 mm dynamic tolerance, and the ability to maintain this tolerance even with enhanced leaf speeds, Elekta engineers were given a blank sheet of paper and encouraged to design a brand new system. They concluded that an optical system was the only technology capable of achieving the specifications required and so they concentrated on enhancing the existing system. In the new optical system, separate light sources are used to supply the patient light-field and to illuminate the leaves. These two sources consist of LEDs operating at different wavelengths, which enable independent set-up optimization. The Rubicon optical leaf positioning system uses near UV light from an LED source. This produces near infra red fluorescence when shone on the ruby tips of the MLC leaves. Using special filters, a camera detects this near IR fluorescence. The resulting data is used to reliably monitor and accurately position the leaves. This new system is inherently safe, requiring no additional fail safe devices. Another important consideration is maximization of MLC flexibility. To achieve this, Agility has been designed with a compact outline, while maintaining market-leading patient clearance. 5

6 Leaf positioning accuracy is a major feature of Agility. It is a consequence of MLC integration into the linac control system and the real time leaf monitoring/ positioning system. Figure 4. Illustration showing Rubicon, the new optical positioning device for Agility. Conclusion Agility has been designed carefully and thoroughly, taking into account the most advanced physics and clinical research. Exceedingly low transmission, high speed, optimal leaf width, orthogonal tracking jaws, consistently good penumbra, maximum flexibility and accuracy have all been incorporated, resulting in a product that is able to deliver the most sophisticated radiation treatments in the minimum time. 6

7 References [1] R Topolnjak, UA van der Heide, GJ Meijer, B van Asselen, CPJ Raaijmakers and JJW Lagendijk, Influence of the linac design on intensity-modulated radiotherapy of head-and-neck plans, Phys. Med. Biol. 52, (2007) [2] VP Cosgrove et al., Physical characterisation of a new concept design of an Elekta radiation head with integrated 160-leaf multi-leaf collimator, Poster presentation, ASTRO 51st annual meeting (2009) [3] International Electrotechnical Commission, Final Draft International Standard IEC , Ed.3, SC 62C, Particular requirements for the basic safety and essential performance of electron accelerators in the range 1 MeV to 50 MeV (2010) [4] H Vorwerk, D Wagner and CF Hess, Impact of different leaf velocities and dose rates on the number of monitor units and the dose-volume-histograms using intensity modulated radiotherapy with sliding-window technique, Radiation Oncology 3:31, (2008) [5] D Schmidhalter, MK Fix, P Niederer, R Mini and P Manser, Leaf transmission reduction using moving jaws for dynamic MLC IMRT, Med. Phys. 34 (9), (2007) [6] R Mohan, Field Shaping for Three-Dimensional Conformal Radiation Therapy and Multileaf Collimation, Semin. Radiat. Oncol. 5 (2), (1995) [7] JB Fiveash, H Murshed, J Duan, M Hyatt, J Caranto, JA Bonner and RA Popple, Effect of multileaf collimator leaf width on physical dose distributions in the treatment of CNS and head and neck neoplasms with intensity modulated radiation therapy, Med. Phys. 29 (6), (2002) [8] J Burmeister, PN McDermott, T Bossenberger, E Ben-Josef, K Levin and JD Forman, Effect of MLC leaf width on the planning and delivery of SMLC IMRT using the CORVUS inverse treatment planning system, Med. Phys. 31 (12), (2004) [9] L Ku, M Fuss, Impact of Multi-leaf Collimator Leaf Width on Quantitative Dosimetry of Stereotactic Body Radiation Therapy Plans, Int. J. Radiat. Oncol., Biol., Phys 72 (1), S621 (2008) [10] JA Boyd, DW Kim, JJ Meyer, Z Wang, GW Britz, J Wu, F Yin, JP Kirkpatrick, High-definition Micro-multileaf Collimator in Radiosurgery Treatment Planning for Arteriovenous Malformations, Int. J. Radiat. Oncol., Biol., Phys 72 (1), S531 (2008) 7

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