Patent Number: 059206012
Section: description

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Reference will now be made to the drawings in which the various elements of the present invention will be given numeral designations and in which the invention will be discussed so as to enable one skilled in the art to make and use the invention. It is to be understood that the following description is only exemplary of the principles of the present invention, and should not be viewed as narrowing the appended claims. In General A sketch of a proposed system for neutron delivery is shown in FIG. 1. A superconducting cyclotron produces a proton beam, which is subsequently directed through a system of bending magnets to a target assembly where neutrons are produced. This target assembly is laminated and has other design features to enhance the production of neutrons having a spectrum that will simultaneously induce a desired fast-neutron dose-depth profile in hydrogenous tissue at the isocenter as well as a desired thermal-neutron flux profile in the same hydrogenous target volume at the isocenter. This specially tailored neutron beam is achieved in the present invention by a specially designed accelerator target and/or by selective neutron filtering downstream from the target itself, but within the target assembly shown in FIG. 1. The preferred target assembly has several elements: first is a proton-neutron conversion region composed in the currently-preferred embodiment of laminated beryllium and tungsten, followed by a neutron filtering subassembly composed of a suitable material having neutron cross-section characteristics such that the downstream spectrum emerging from the filter has an operator adjustable bimodal shape as a function of energy as the result of the action of the target and/or filter. The target filter assembly is followed in turn by suitable flattening and wedge filters, composed in the currently preferred embodiment of tungsten or iron. Downstream of the target-filter assembly is a multi-segment collimator to further tailor the neutron beam prior to delivery to the patient at the isocenter. As shown, all major components of the system are preferably housed within a balanced rotating structure that can be mounted in a large circular opening in the interior wall of the building where the systems is installed. The mounting wall will generally be quite thick, in order to provide the necessary level of neutron shielding. The device can be mounted on roller bearings, with a ring gear wrapped circumferentially around the periphery of the housing or circumferentially within the opening within the wall where the system is installed. Either way, small drive motors engage the large ring gear to facilitate rotation of the entire assembly about the isocenter to within the required precision. An alternative embodiment of this invention is illustrated in FIG. 3. In FIG. 3, the cyclotron itself is located just behind the rotating assembly. This is a less-preferred embodiment at this time because somewhat more space is required. Additionally, the cyclotron could not be used to counterbalance the weight of the target and collimator assembly. Thus, this embodiment requires the installation of additional counterweights in the rotating structure itself. An alternate approach within this invention is to sequentially expose the target volume of the patient with two or more neutron beams having different energy levels to cumulatively achieve the same result as that achieved by a tailored beam. Ease of treatment will normally make a system having a tailored neutron beam the system of choice. In Detail As illustrated in FIG. 1 and FIG. 2, a neutron delivery system 10 provides improved capability for tumor control during medical therapy. The neutron delivery system 10 includes the following parts: a cyclotron 14 for producing a proton beam 16; magnets 18; and at least one target 22. The magnets 18 are for directing the proton beam 16 into the target 22 so that the target produces at least one neutron beam 26 when impacted by the proton beam. The neutron beam 26 has a bimodal energy spectrum that can be used with both fast-neutron therapy and boron neutron capture therapy. The delivery system 10 also includes a collimator 30 to further tailor the neutron beam 26 prior to delivery to a patient. The delivery system 10 is housed on a balanced rotating structure 34. Although the neutron delivery system 10 of FIG. 1 shows a cyclotron 14, the delivery system may have various embodiments. More specifically, the cyclotron 14 of FIG. 1, may be replaced by a compact superconducting cyclotron, or other means for producing a proton or deuteron beam. However a user chooses to produce the proton beam 16, the neutron delivery system 10 should enable a user to produce proton beams with various energy levels. A preferred energy range of a proton beam is in the 50-70 MeV range. When a proton beam 16 in the 50-70 MeV range is produced, the neutron delivery system 10 typically uses magnets 18 to create a bending magnet system that directs the proton beam 16 into the target 22. Once the proton beam 16 is directed into the target 22, the target will produce the neutron beam 26. The target 22 is capable of producing at least one neutron beam having a high energy component in the 30-60 MeV range and a low-energy component in the 10 KeV to 2 MeV range. As shown in FIG. 4, the target 22 is composed of a plurality of layers. Although in a preferred embodiment the target 22 has two layers, i.e., a layer of beryllium 40 and a layer of tungsten 44, in another embodiment, the target has a third layer of carbon 48. The layer of beryllium 40 is about five millimeters thick and the layer of tungsten 44 is about four millimeters thick. The layer of carbon may be placed between the two other layers. In addition, rather than a beryllium layer 40, the layer may be made of lithium or other material to produce a neutron beam. Different targets and projectiles will produce different types of neutron sources. These layers create the bimodal energy effect in the neutron beam(s) when first the beryllium layer 40 and subsequently the tungsten layer 44 of the target 22 are impacted by the proton beam 16. One effect is to cause a single neutron beam to be produced and this is tailored in a manner commonly understood by those skilled in the art to change the spectrum and/or intensity in a desired manner. In another embodiment, the bimodal effect of the target 22 can be accomplished with a target that has the following combination of layers: a proton-neutron conversion region; a neutron filtering subassembly; a spectral filter for modifying the neutron beam; and a plurality of filters for flattening and wedging purposes. The proton-neutron conversion region is often made up of a layer of beryllium operably attached to a layer of tungsten. The layer of beryllium can have a thickness of between about 3 to about 10 millimeters while the layer of tungsten can have a thickness of between about 1 to about 7 millimeters. These are only suggested layer thicknesses and are clearly approximations that can be modified and still produce desirable results. The neutron filtering subassembly is only limited in that it should be made of a material having neutron cross section characteristics such that the beam is modified in a desired manner upon passage through the filtering subassembly. For example if the system is to be used in a particular case for fast-neutron therapy without neutron capture augmentation it may be desirable to preferably attenuate the low-energy component of the bimodal spectrum by use of a filtering subassembly composed of a hydrogeneous material such as polyethylene. The spectral filter for producing a neutron beam from the neutron filtering subassembly can be adjustable by an operator. The operator has the option of producing a spectrum having a bimodal shape or otherwise. The tailored neutron beam is typically collimated to transverse dimensions of from 5 to 30 centimeters by from 5 to 30 centimeters. In addition, when the collimated neutron beam impinges on the treatment volume, the half-value depth of the fast-neutron dose profile is typically from 17 to 21 centimeters, and a scalar thermal-neutron fluency field of from 2 to 5.times.10.sup.10 neutron per square centimeter (2200 meters per second equivalent) per 100 centigrays of fast neutron dose is simultaneously generated at a 5 centimeter depth on-axis. It should be noted that the spectral filter means may also be made of tungsten, bismuth, and/or iron, depending on the desired effect. The bimodal shape of the spectrum is a function of energy as the result of the action of the target and/or filter. The filters for flattening and wedging purposes are such that the neutron beam is flattened and tilted. The plurality of filters for flattening and wedging purposes may be composed of tungsten or iron. The filters may also include a hydrogenous material for reducing the low-energy component of the spectrum. This hydrogenous material is commonly polyethylene. The collimator 30 of FIG. 1, to further tailor the neutron beam prior to delivery to a patient, may be a multi-segment collimator wherein the neutron beam may be further tailored prior to delivery to the patient. Of course, the beam should be delivered to the patient at an isocenter 18. Additionally, the collimator 30 may be made of iron and/or bismuth. The neutron delivery system is housed in a balanced rotating structure 34. One possible embodiment for housing the system is a rotating isocentric gantry structure. The gantry structure contains all system components. These components are held in a manner such that the tailored neutron beam from the collimator is easily moved for exposing the target to be treated in different directions. To accomplish this embodiment, the balanced rotating structure for housing the system has numerous parts that must work together. First, as shown in FIG. 5, the structure has a housing 50 that is mounted in a large circular opening in an interior wall 54 of the building where the system is installed. This interior wall 54 is quite thick so that it is capable of neutron shielding and able to support the weight of the structure. Second, the housing 50 is mounted on roller bearings to assist the housing to rotate within the wall 54. Rotating the housing 50 on the bearings 56 is accomplished with a ring gear 58. The ring gear 58 is wrapped circumferentially around a periphery of the housing 50. The ring gear 58 is operated by at least one is drive motor 59. The drive motor 59 is mechanically coupled to the ring gear 58 to engage the ring gear and facilitate rotation of the balanced rotating structure. The structure as a whole can then be easily rotated about an isocenter 38 (see FIG. 1). One skilled in art and viewing the housing of the present invention would be enabled to make and use the above described ring gear and drive motor for rotating the housing. The method for using a neutron delivery system to provide improved tumor control capability during medical therapy is accomplished through the following steps: producing a proton beam; directing the proton beam into at least one target; creating at least one neutron beam having a bimodal energy spectrum for use with both fast-neutron therapy and boron neutron capture therapy; tailoring the at least one neutron beam by means of a filter collimator system; delivering the tailored at least one neutron beam to a patient; and housing the system in a balanced rotating structure for ease of adjustment. This method may be further defined to specify that the step of producing a proton beam comprises producing a proton beam in the 50-70 MeV energy range. This proton beam may be produced with any particle accelerator, e.g., a typical cyclotron or a superconducting cyclotron. The proton beam is then used to create at least one neutron beam when the proton beam impacts with at least one target. This collision produces a neutron beam that has a bimodal energy spectrum because of the properties of the material in the target. The bimodal energy spectrum is a neutron beam(s) that has neutrons with various energy levels. In other words, at least one neutron beam has a high energy component in the 30-60 MeV range and a low-energy component in the 10 KeV to 2 MeV range. The method described above may be accomplished by creating a single tailored neutron beam. This single tailored neutron beam is created by exposing a specially designed target with the neutron beam and/or by the use of spectral filters to eliminate or enhance neutrons of certain energies. The above variations are not inclusive. They are only examples of the preferred embodiments. It is to be understood that the above-described arrangements are only illustrative of the application of the principles of the present invention. Numerous modifications and alternative arrangements may be devised by those skilled in the art without departing from the spirit and scope of the present invention and the appended claims are intended to cover such modifications and arrangements.