CROSS REFERENCE TO RELATED APPLICATIONS

[0001 ] This PCT Application claims priority to US Provisional Application Serial No. 61/487,961 filed May 19, 201 1, which application is incorporated herein by reference as if fully set forth in their entirety.

STATEM ENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

[0002] This invention was made with government support under Contract No. DE-AC02- 05CH1 1231 awarded by the United States Department of Energy to The Regents of the University of California for management and operation of the Lawrence Berkeley National Laboratory. The government has certain rights in this invention.

BACKGROUND

Field

[0003] The present disclosure relates to toroidal electromagnets and specifically to winding of toroidal magnets to obtain specified field properties.

Description of Related Art

[ 0004] The main goal of radiation cancer therapy is to use radiation to kil l cancer tumor tissue while minimizing the damage to healthy tissue. Radiation therapy, together with chemotherapy and surgery, is one of the three most common treatment modalities for cancer tumors. [0005] At present, the most common forms of radiation therapy are charged particle accelerator-based and high energy X-ray therapy. Compared with X-rays, ions have physical and biological properties that in many ways make them advantageous for use in radiotherapy. As a result ions are a rapidly growing area for particle accelerator- based radiation therapy. As of January 201 1 nearly 80,000 patients have been treated worldwide with ions, Of those, approximately 90% were treated with protons and 10% were treated with carbon.

[0006] Ion beam therapy facilities are more costly and complex to construct and operate than X-ray facilities. A typical ion beam therapy center, with 4 treatment rooms, is on the scale of 100,000 square feet of which a large portion is the size of the accelerator and beam delivery system. A proton-only center is about 125 MS and a facility that can provide both protons and carbon ion therapy center costs approximately 100 M$ more. The increased cost of the carbon- proton versus proton only facilities is mainly due to the higher beam energies and thus larger facilities that are needed for carbon to reach the same penetration depth. The typical penetration depth range desired for an ion beam therapy facility is from 3cm to 30cm. The carbon nucleus contains 12 nucleons (μ). To reach 30cm depth in water requires a carbon beam of 430 MeV/nucleon versus only 250 MeV/μ for proton beams. The magnetic rigidity of a 430 MeV/μ carbon ion is approximately 2.5 times higher than a 250 MeV/μ proton ion.

[0007] U.S. Patent No. 7,889,046 to Meinke discloses a conductor assembly for winding conductors about a tubular shape that includes curved sections. A primar objective of the di sclosure is to provide a generally dipole fiel d over 80% of the radius of the central aperture of the tubular assembly, and where higher order pole moments in this region are at least 10 ³ smaller than the dominant dipole field within the 80% radius aperture. The design process, however, does not take into account distortion of the ion beam traversing the curved tubular path, being concerned only with converging on a field distribution that is mainly dipole in nature, nor does it take into account that there is beam-shape distortion due to strong sextupole components in the fringe-field region at both ends of the dipole magnet toroid, whose effect is greatly pronounced when the beam is widely steered at the upstream of the dipole magnet. [0008] Thus, a configuration of electromagnet coil windings on toroidal segments of an ion beam delivery system that includes specified higher order multipole moments of magnetic field that provides a compound function of near point-to-parallel optics while at the same time reducing the beam-shape distortion is very desirable.

SUMMARY

[0009] In an aspect of the disclosure a toroidal magnet includes a toroidal surface geometry to define placement of windings for an electromagnet, and an interior space of the toroidal surface adapted to pass a beam of ions on a specified curved path defined by the toroidal geometr and a specified magnetic field provided by the electromagnet, wherein the electromagnet windings comprise a superposition of two solenoid-like windings oppositely skewed with respect to the central axis of the toroid aperture, the windings being modulated to produce the specified field, and wherein the specified field has a specified gradient.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] FIG. 1 illustrates a magnet layout of an isocentric gantry with bending magnets (BM), quadrupole magnets (Q), horizontal and vertical scanning magnets (Sv and Sh) used in a simulation, in accordance with the disclosure.

[0011] FIG. 2 illustrates an initial coil design concept with the coils touching on the inner and outer radius, in accordance with the disclosure.

[0012] FIG. 3 illustrates the field gradient along the mid-plane for the initial winding scheme of FIG. 2.

[0013] FIG, 4 illustrates the field gradient along the mid plane for an ideal field starting from the initial coil design of FIG. 2 and modulating the windings. [0014] FIG. 5 illustrates the field gradient along the mid plane of the initial winding of FIG. 2 with iron added around the torus.

[0015] FIG. 6 illustrates the set of horizontal and vertical scanning magnet settings for the simulation, in accordance with the disclosure,

[0016] FIG. 7 illustrates the beam shapes and positions resulting in the simulation of FIG, 6.

[0017] FIG. 8 illustrates the coordinate system for determining the position of windings around the tonis, in accordance with the disclosure.

[0018] FIG . 9 illustrates a winding solution determined in accordance with the disclosure.

[0019] FIG. 10 illustrates the resulting vertical magnetic field contour and profile across the mid-plane of the torus, in accordance with the disclosure.

[0020] FIG, 11 illustrates ion beam shapes and location at the patient position with optimized modulated windings resulting from the field contour of FIG, 10, in accordance with the disclosure.

DETAILED DESCRIPTION

[0021 ] Various aspects of the present invention will be described herein with reference to drawings that are schematic illustrations of idealized configurations of the present invention, As such, variations from the shapes of the illustrations as a result, for example, manufacturing techniques and/or tolerances, are to be expected, Thus, the various aspects of the present invention presented throughout this disclosure should not be construed as limited to the particular shapes of elements (e.g., regions, layers, sections, substrates, etc.) illustrated and described herein but are to include deviations in shapes that result, for example, from ma ufacturing. By way of example, an element illustrated or described as a rectangle may have rounded or curved features and/or a gradient concentration at its edges rather than a discrete change from one element to another. Thus, the elements illustrated in the drawings are schematic in nature and their shapes are not intended to illustrate the precise shape of an element and are not intended to limit the scope of the present invention.

[0022] It will he understood that when an element such as a region, layer, section, substrate, or the like, is referred to as being "on" another element, it can be directly on the other element or intervening elements may also be present, in contrast, when an element is referred to as being "directly on" another element, there are no intervening elements present. It will be further understood that when an element is referred to as being "formed" on another element, it can be grown, deposited, etched, attached, connected, coupled, or otherwise prepared or fabricated on the other element or an intervening element, in addition, when a first element is "coupled" to a second element, the first element may be directly connected to the second element or the first element may be indirectly connected to the second element with intervening elements between the first and second elements.

[0023] Furthermore, relative terms, such as "lower" or "bottom" and "upper" or "top," may be used herein to describe one element's relationship to another element as illustrated in the drawings. It will be understood that relative terms are intended to encompass different orientations of an apparatus in addition to the orientation depicted in the drawings. By way of example, if an apparatus in the drawings is turned over, elements described as being on the "lower" side of other elements would then be oriented on the "upper" side of the other elements. The term "lower" can therefore encompass both an orientation of "lower" and "upper," depending of the particular orientation of the apparatus. Similarly, if an apparatus in the drawing is turned over, elements described as "below" or "beneath" other elements would then be oriented "above" the other elements. The terms "below" or "beneath" can therefore encompass both an orientation of above and below,

[0024] Unless otherwise defined, all terms (including technical and scientific terms) used herein have the same meaning as commonly understood by one of ordinary _' skill in the art to which this invention belongs. It will be further understood that terms, such as those defined in commonly used dictionaries, should be interpreted as having a meaning that is consistent with their meaning in the context of the relevant art and this disclosure. [0025] As used herein, the singular forms "a," "an," and "the" are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms "comprise," "comprises," and/or "comprising," when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. The term "and/or" includes any and all combinations of one or more of the associated listed items.

[0026] In a modem multi-room accelerator-based ion therapy center, the accelerator contributes a relatively small amount to the cost and size of the facility. The size of the facility is largely determined by the size of the beam delivery and patient treatment rooms. Most of the treatment rooms are equipped with gantries. Gantries are beam lines that rotate around the patient allowing one to deliver the beam to the patient with an arbitrary angle without having to tilt the patient, but the patient table may be rotated horizontally about a vertical axis, thus allowing the beam to be directed at the tumor from any angle. By changing the angle of beam entry during a course of treatment, damage to surrounding healthy tissue may be minimized. Furthermore, by maintaining the patient in a horizontal position, the tumor is less likely to shift relative to the external markers, making beam function more efficient and accurate. The gantry typically includes toroidal sections for beam steering for delivery to a specific site within the patient.

[0027] Active scanning refers to a type of ion beam deliver} _' where a small sized ("pencil") ion beam is scanned over the tumor volume in all 3 dimensions (two transverse and depth). Scanning the transverse direction is achieved using fast scanning magnets and the depth change is made by changing the particle beam energy. With passive scanning degraders are used to adjust the depth. Active 3D scanning provides more flexibility than passive scanning in conforming to the tumor shape and minimizing the dose to normal tissue.

[0028] A good field region is defined as the size of the transverse region that can be scanned without moving the gantry. If the tumor is larger than the good field region, scanning the entire tumor requires moving the gantry, which may have adverse consequences for accuracy and coverage. [0029] If the scanning magnets are located close to the patient the beam will enter the patient at a diverging angle and the skin will receive a higher dose than the tumor. Since the skin is very radiation sensitive, this is not ideal, Moving the scanning magnets away from the patient reduces the relative skin dose. The distance from the scanning magnets to the tumor is called the source to axis disiance (S AD). The S AD is infinite when the beams are entering the body at a perpendicular angle (i.e., substantially collimated). This means the scanning magnets are effectively an infinite distance away.

[0030] The effect of the SAD on increased skin dose is purely a geometrical effect and can be quite large. For instance when treating a deep seated tumor at 30 cm the difference in skin dose for scanning magnets at a 2 meter SAD versus an infinite SAD would be a 40% increase in skin dose. In principle it is possible to have even lower doses if the beams converge when entering the body. But typically the SAD is positive or infinite.

[0031] The challenges of having a large SAD, like a having a large good field region size, can lead to increases in the cost, size and complexity of the gantry.

[0032] FIG. I illustrates a magnet layout 100 for an isocentric gantry. An isocentric gantry is one where the focal point of the beam remains fixed as the gantry rotates. The magnet layout consists of quadruple magnets (Q) for focusing, bending magnets (BM), with bending angles such as 45 and 90 degrees, and vertical and horizontal scanning magnets (Sv, Sh). The scanning magnets are placed upstream of the last 90 degree bend magnet. The scanning magnets deflect the beam. The attraction of having an isocentric gantry is that the patient does not need to be moved vertically as the gantry rotates, i.e., the beam should always intersect the tumor regardless of beam direction,

[0033] The angular deflection at the scanning magnets translates to a position offset at the patient. In some gantr systems the offset is purely position and there is no angle direction change in the beam. In other words the SAD is effectively infinite. To achieve an infinite SAD value the linear magnet optics between the scanning magnets and the patient need to be carefully designed. This is accomplished by adjusting the horizontal and vertical transverse focusing of the bend. [0034] The attraction of this concept is that the gantr is relatively radially compact with an infinite SAD. The main drawback of this concept is, because the beam is being scanned inside the final 90 degree magnet, the magnet needs to have a wide aperture in both transverse directions, As a result this magnet is considerably larger and heavier than any of the other elements in the beam line, In the Heidelberg Ion Therapy (HIT) facility, the 90 degree magnet is 90 tons, or 65% of the weight of the entire rotating transfer system, The magnet drives the size and weight of the gantry. The problem is not unique to isocentric gantries but applies to any gantry where the beam is being scanned inside of the magnet. This is the motivation for exploring large aperture magnet concepts with smaller size and weight.

[0035] In an aspect of the disclosure, the desired fields are not a pure dipole, or nearly pure dipole, as disclosed by Meinke, The desired fields are driven by the beam properties, which include (1) bendmg the beam 90 degrees, (2) large aperture (to accommodate scanning the beam inside the magnet BM/90 for as large a good field region as possible), (3) quickly ramping the field strength (to quickly scan the beam energy over a large range for penetration depth, e.g., 3 cm to 30 cm), on the order of 100 nis, (4) point-to-parallel focusing from the scanning magnets to the patient, consistent with the desire for large SAD, and (5) keeping the dose distribution constant when scanning the tumor to accurately control the dose distribution inside the tumor volume.

[0036] With regard to point (4) this is achieved by introducing focusing in the bend by either adjusting the bend edge angles at the entrance and exit of the bending magnet BM/90, or by adding a gradient field to the body of the magnet to make the beam parallel at the patient. In other words the linear transfer function from the scanning magnets to the patient is adjusted to point-to-parallel,

[0037] By superposing two solenoid-like thin windings that are oppositely skewed (tilted) with respect to a cylindrical axis, the combined current density on the surface can provide a resulting magnetic field in the bore that is a pure dipole, Good field quality at the straight section is achieved and over the magnet ends harmonics naturally integrate to zero due to the opposite skew. The simplicity of this design is especially suitable for low cost superconducting accelerator magnets. In a straight tube the azimuthal magnetic field components (parallel to the axis of the cylinder) of the two oppositely skewed windings cancel, leaving a pure dipole moment of double Attorney Docket No.: IB-3054 PCT Lawrence Berkeley National Laboratory strength transversely. This pure dipole condition, however, does not hold for a curved section of torus with simple skewed solenoid windings.

[0038] In an aspect of the disclosure, the concept is extended by placing the windings on a toroidal geometry suitable for a curved magnet, wherein the field gradient is intentionally not pure dipolar to maintain the integrity of the beam profile across the aperture. This is accomplished by modulating the winding placement from that of a simple skew solenoid winding pattern. A feature of the magnet includes maintaining vertical as well as horizontal (i.e., radial) focus in a toroidal magnet (where the toroidal mid-plane is assumed in the horizontal plane) to ensure that the ion beam, when swept by the scanning magnets, emerge from the exit aperture of BM/90 and arri e at the patient target site as a pencil beam of substantially constant direction (i.e.. all pencil beams across the aperture are substantially parallel) and substantially constant intensity.

[0039] In an example corresponding to the magnet layout of FIG. 1, the magnetic field at the center of the mid-plane of the torus is nominally 5.0 tesla (T). A winding solution will be one that includes proper gradients in the field that will satisf the five beam properties mentioned above. For example, a compact winding scheme was chosen that requires each turn to remain in contact with its neighboring turns on the mid-plane of bo th the inner radius and outer radius of the torus bu t not anywhere else. FIG. 2 illustrates the winding concept. For illustrative purposes only, a 180 degree segment of a torus is shown. However, in use, only a 90 degree segment, BM/90, would be used. BM/90 is a hollow tube, and the windings are placed on the exterior surface. Diverging entry arrows indicate the various directions that the scanning magnets Sv and Sh may insert the ion beam into the BM/90, and parallel arrows exiting the torus segment indicate that the ion beams emerge parallel, where the lateral displacement of each beam is determined by the entry angle at the entrance. The objective is to tailor the magnetic field distribution within the torus by design of the windings so that the lateral displacement of the beam as a function of insertion angle is well behaved, and the beam coUimation at the exit of the torus, and at the tumor site, is also well behaved and collimated.

[0040] A computer simulation program places wires along such a path on the exterior of the torus and calculates the magnetic field using the Biot-Savart law. Additional simulation calculations include the use of ferromagnetic material. Such a winding scheme resulted in the vertical field increasing at larger radius, i.e. by a positive field gradient (see FIG. 3).

[0041] This positive field gradient is not ideal to meet the parailel-to-point requirement in both planes, the reason being that the magnet is sector bent so the edge angles provide horizontal focusing with little vertical focusing. With the addition of the positive field gradient, the magnet becomes vertically defocusing. There should be a negative gradient so that the combination of edge focusing and negative gradient provides net focusing in both the horizontal and vertical plane at the patient target position.

[0042] To meet this condition the magnet should have a negative field gradient value that is 2.26 T/m (see FIG. 4) for the example magnet layout of FIG. 1, i.e., having a nominal 5 T field at the center of the mid-plane of the torus (i.e., the gradient should be roughly equal and opposite to that in Fig, 3), In an attempt to reverse the field gradient a simulated elliptically shaped iron toroid was placed asymmetrically around the coils with more iron closer to the inner radius and further from the outer radius, This changes the gradient, however a second order (sextupole) gradient term was undesirably added (see FIG. 5). This is an initial case in which properties of the beam dynamics may be simulated and toroidal winding schemes optimized to improve the field profile to maintain beam quality .

[0043] In a simulation of the system representative of the magnet layout 100 of FIG. 1 , the quadrupole magnets Q1-Q6 and die two 45 deg magnets BM/45 are modeled and assumed to deliver a carbon beam from the synchrotron source to the gantry entrance (BM/90) with emittances of 1 mm-mrad (horizontal) and 5 mm-mrad (vertical), and a momentum spread of 0,2% (which is typical of a carbon beam synchrotron). The mid-plane field of the two BM/45 and BM/90 bending magnets are assumed to be 5 T. The quadrupole magnets are design to insure a rotation invariant optics beam train.

[0044] For a ±2 -mm beam spot at the (patient) target, the initial beam sizes are taken to be 2 mm and 10 mm in the horizontal and vertical plane respectively and the divergence angle is 0.5 mrad in both planes. A fixed number of simulated carbon ion particles (1000) with a Gaussian distribution in a six-dimensional phase space consisting of horizontal and vertical displacement, horizontal and vertical divergence angle, relative momentum spread, and longitudinal position spread in time.

[0045] A grid of simulated horizontal and vertical "kick" angle settings for the scanning magnets for insertion of the beam in S /90 is shown in FIG. 6. The beam is deflected at various angles at the position of the steering magnets Sv, Sh. Ideally the beam centroids must show a linear dependence on kick angles while at the same time the shape of the beam should not change. FIG. 7 shows the beam spots in the transverse plane at the patient position for an initial design of the 90 degree magnet BM/90. However, FIG. 7 shows a large distortion of the beam shape and also significant deviation from linearity. This is considered mainly due to large sextupole components in the main body of the magnet.

[0046] The sextupole field may be treated as a thin lens, therefore an "optical lens-like" solution may be sought in a winding optimization to cancel the sextupole field component. An algorithm, such as a Genetic Algorithm (GA), may be used to find the winding path of the coil on the surface of the torus. To use a GA, to search for winding solutions, the winding path of the coil on the surface of torus is parametrized. To simplify the description of the winding, the simple toroidal coordinate system (R; φ; Θ) is used, where R is the radius of the torus bore (called minor radius), φ is the toroidal angle and Θ is the poloidal angle shown in Fig. 8. The simplicity of using this coordinate system is the result of that the constant R forms the surface of the torus. Therefore, we only need two parameters φ and Θ to describe the winding.

[0047] Given a constant bore radius R, the relationship(s) found between φ and Θ, i.e., φ ^::: f(0), will determine the possible winding paths of the coil on the surface of the torus. Now the objective is to determine what kind of winding relation φ = ί ^" (θ) generates not only the dipole field but also the desired magnitudes of quadrupole and sextupole fields (or any higher order field, for that matter). It is well known in the art that for a straight cylinder the cosine-theta like current distribution on the cross-section of the cylinder will generate multipole fields, i.e., cos Θ gives dipole field, cos 2Θ gives quadrupole field, and so on. Using the same methodology, a following winding relation for a torus is described by: φ = θ/η + a _© sin Θ r ai sin 2Θ + a ₂ sin 3Θ + .. , ,

I I where n is the coefficient determining the number of turn of the coil on a 2π torus, and ao; ai ; a ₂ , ... are the coefficients determining the multipole field components. For a given set of coefficients n, ao; ai ; a ₂ , as well as the coil current I, the magnetic field inside the torus can be numerically evaluated using the Biot-Savart law,

[0048] The search for what values the coefficients n, ao; ai ; a ₂ , .... and current I should have in order to generate required magnetic field is performed by GA, Concisely stated, GA is a method to find solutions by mimicking the process of natural evolution, such as inheritance, mutation, selection and crossover. Given a specific problem to solve, the implementation of GA begins with a set of trial solutions (cal led population) which are typically randomly generated. Then, the metric functions (the fitness functions) of these trial solutions are quantitatively evaluated. According to their fitness, the promising candidates are kept and used to reproduce a new population using a crossover operator. To diversify the population, usually mutations are introduced during the reproduction. This is motivated by a hope that the new generation will be better than the old one. It is repeated until a maximum number of generations is reached or optimal solutions are found. GAs have proven to be an enormously powerful and successful problem-solving strategy, and have been used in a wide variety of fields.

[0049] Using GA to solve for the winding coefficients n, ao; ai ; a ₂ , and current I for a given required magnetic field, the metric function used may be, for example, the differences between the calculated multipole field strength and the required ones found by the beam dynamic study described above. The differences are minimized during the evolution process, Multipole solutions are found when the differences between the solution magnetic fields and the required fields are within tolerances. In one example, application of the GA to the magnet layout and beam dynamics of the simulation yields a solution of coefficients n, ao; ai ; a? , and current I to produce a winding path solution on the torus, as shown in FIG. 8. There are two windings in opposite directions to cancel the axial components of the solenoid fields while doubling the transverse field. The magnetic field of this winding solution across the bore of the torus is shown in Fig. 9. The magnetic field of this simulation solution meets the tolerance requirements. As distinguished from Meinke, the field maintains a specified uniformity of gradient across a larger cross-section of the torus aperture (i.e., greater than 80% of the toroid radius R) to achie ve a large good field region size that is nearly equal to the torus aperture.

[0050] Furthermore, Meinke does not mention the importance of keeping the windings convex on the inner portion of the torus surface (the surface facing toward the central axis of the toroid, i.e., the axis about which φ is specified). Large Lorentz forces are likely to develop during operation of the solenoid, and it is important to maintain tension on the winding of the solenoid wires to help maintain position stability. The winding solutions, such as that shown in FIG. 8, maintain a convex path on both the inner and outer edges of the torus. The net benefit is a magnet that may be more compact, cost effective and better designed to handle the large Lorentz forces that develop during operation.

[0051] The calculated wire path can then be used in calculating the field directly from Biot- Savart law and compared with expected harmonics, The magnetic field of this winding solution across the bore of the torus is shown in Fig. 9. The magnetic field of this solution meets the tolerance requirements. Furthermore, the calculated field may then be applied to the particle beam simulation to evaluate the change in the amount of distortion of the beam at the various combinations of horizontal and vertical scanning angles from Sv and Sh. FIG. 11 shows an expected scanning pattern of carbon ion beams for an optimized winding solution on the tonis, which is to be compared to the horizontal and vertical angular steering pattern of FIG. 6. The coil optimization resulted in nearly linear position response to scanning magnet changes and small distortion within the scanning range. The enhancement of the beam shape is mainly due to the removal of the sextupole in the main body of the toroidal magnet.

[0052] The solution presented is only one of a possible number of solutions that meet the field requirements. There may be many other solutions with different combination of (I; n; a<>; a.; a ₂ ) values. Optimization explorations may search for other solutions based on, for example, practicality (e.g., easier winding prescriptions, looser tolerances, lower stress, lower stored energy, etc.), to meet a specified set of requirements in addition to the field profile.

[0053] The various aspects of this disclosure are provided to enable one of ordinary skill in the art to practice the present invention. Modifications to various aspects of forming nanostructures electrodes presented throughout this disclosure will be readily apparent to those skilled in the art of nanotechnology, particle accelerators, catalytic chemistry, applications to other technical arts, and the concepts disclosed herein may be extended to such other applications. Thus, the claims are not intended to be limited to the various aspects of an ion accelerator presented throughout this disclosure, but are to be accorded the full scope consistent with the language of the claims. All structural and functional equivalents to the elements of the various aspects described throughout this disclosure that are known or later come to be known to those of ordinary skill in the art. are expressly incorporated herein by reference and are intended to be encompassed by the claims. Moreover, nothing disclosed herein is intended to be dedicated to the public regardless of whether such disclosure is explicitly recited in the claims. No claim element is to be construed under the provisions of 35 U.S.C. § 112, sixth paragraph, unless the element is expressly recited using the phrase "means for" or, in the case of a method claim, the element is recited using the phrase "step