Patent Publication Number: US-9431138-B2

Title: Method of generating specified activities within a target holding device

Description:
BACKGROUND 
     1. Field 
     The present application relates to methods for the production of brachytherapy and radiography targets. 
     2. Description of Related Art 
     Conventional methods for producing brachytherapy seeds involve non-irradiated wires (e.g., non-irradiated iridium wires) that are subsequently provided with the desired activity. The desired activity may be provided thereto through neutron absorption in a nuclear reactor. 
     Brachytherapy seeds have also been produced from irradiated wires. With regard to the production of the seeds, the irradiation of long wires has been suggested, wherein the irradiated wires are subsequently cut into individual seeds. However, because of flux variations in a reactor, the attainment of seeds with uniform activity is difficult. 
     SUMMARY 
     A method for producing uniform activity targets according to an embodiment of the invention may include arranging a plurality of targets in a holding device having an array of compartments. Each target is assigned to a compartment based on a known flux of a reactor core so as to facilitate an appropriate exposure of the targets to the flux based on target placement within the array of compartments. The holding device is positioned within the reactor core to irradiate the targets. The targets may be formed of the same or different materials and may be placed individually or in groups in the compartments. 
     The targets may be radially arranged such that more targets are grouped together in compartments that are at a greater radial distance from a center of the holding device. The targets may also be axially arranged such that more targets are grouped together in compartments in axial portions of the holding device that are subjected to higher flux during irradiation. Furthermore, more targets may be grouped together in compartments that are in closer proximity to the flux during irradiation. 
     The targets may also be arranged based on their self-shielding properties. For instance, targets with lower self-shielding properties may be grouped together in one or more compartments, while targets with higher self-shielding properties may be separated from each other so as to be grouped in different compartments. 
     The targets may also be arranged based on their different cross sections. For instance, targets having lower cross sections may be arranged in one or more compartments that are in closer proximity to the flux during irradiation. The number of targets in a compartment may be increased so as to decrease a resulting activity of each target in the compartment after irradiation. The method for producing uniform activity targets may further include waiting a predetermined period of time for impurities to decay after irradiation prior to collecting the irradiated targets. 
     A method for producing uniform activity targets according to another embodiment of the invention may include positioning targets within a holding device according to a predetermined or subsequently determined target loading configuration. The determined target loading configuration is based on a required flux for each target in conjunction with a known environment of a reactor core that is used to irradiate the targets. The determined target loading configuration may be in a form of a ring pattern and/or correspond to a shape of a target plate of the holding device. As a result of the determined target loading configuration, a target may be subjected to uniform or non-uniform flux. 
     A method for producing uniform activity targets according to another embodiment of the invention may include arranging a plurality of targets in a holding device having an array of compartments, each target being assigned to a compartment based on a known flux of a reactor core so as to facilitate an appropriate exposure of the targets to the flux based on target placement within the array of compartments. The holding device is positioned within the reactor core to irradiate the targets. The targets may be formed of different natural or enriched neutron-absorption isotopes and may be arranged by isotope type, cross section, and self-shielding properties. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The various features and advantages of the non-limiting embodiments herein may become more apparent upon review of the detailed description in conjunction with the accompanying drawings. The accompanying drawings are merely provided for illustrative purposes and should not be interpreted to limit the scope of the claims. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted. For purposes of clarity, various dimensions of the drawings may have been exaggerated. 
         FIG. 1  is a perspective view of a target holding device according to an embodiment of the invention. 
         FIG. 2  is a partially exploded view of a target holding device according to an embodiment of the invention. 
         FIG. 3  is a perspective view of a target plate according to an embodiment of the invention. 
         FIG. 4  is a plan view of a target plate according to an embodiment of the invention. 
         FIG. 5  is a diagram illustrating a system for mapping the holes of a target plate according to an embodiment of the invention. 
         FIG. 6  is a perspective view of a target plate that has been loaded with targets according to an embodiment of the invention. 
         FIG. 7  is a cross-sectional view of a loaded target holding device, taken along its longitudinal axis, according to an embodiment of the invention. 
         FIG. 8  is a perspective view of a target holder assembly according to an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     It should be understood that when an element or layer is referred to as being “on,” “connected to,” “coupled to,” or “covering” another element or layer, it may be directly on, connected to, coupled to, or covering the other element or layer or intervening elements or layers may be present. In contrast, when an element is referred to as being “directly on,” “directly connected to,” or “directly coupled to” another element or layer, there are no intervening elements or layers present. Like numbers refer to like elements throughout the specification. As used herein, the term “and/or” includes any and all combinations of one or more of the associated listed items. 
     It should be understood that, although the terms first, second, third, etc. may be used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers, and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer, or section from another region, layer, or section. Thus, a first element, component, region, layer, or section discussed below could be termed a second element, component, region, layer, or section without departing from the teachings of example embodiments. 
     Spatially relative terms (e.g., “beneath,” “below,” “lower,” “above,” “upper,” and the like) may be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It should be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the term “below” may encompass both an orientation of above and below. The device may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     The terminology used herein is for the purpose of describing various embodiments only and is not intended to be limiting of example embodiments. 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 “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. 
     Example embodiments are described herein with reference to cross-sectional illustrations that are schematic illustrations of idealized embodiments (and intermediate structures) of example embodiments. As such, variations from the shapes of the illustrations as a result, for example, of manufacturing techniques and/or tolerances, are to be expected. Thus, example embodiments should not be construed as limited to the shapes of regions illustrated herein but are to include deviations in shapes that result, for example, from manufacturing. For example, an implanted region illustrated as a rectangle will, typically, have rounded or curved features and/or a gradient of implant concentration at its edges rather than a binary change from implanted to non-implanted region. Likewise, a buried region formed by implantation may result in some implantation in the region between the buried region and the surface through which the implantation takes place. Thus, the regions illustrated in the figures are schematic in nature and their shapes are not intended to illustrate the actual shape of a region of a device and are not intended to limit the scope of example embodiments. 
     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 example embodiments belong. It will be further understood that terms, including 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 will not be interpreted in an idealized or overly formal sense unless expressly so defined herein. 
     A method according to the present invention enables the production of brachytherapy and/or radiography targets (e.g., seeds, wafers) in a reactor core such that the targets have relatively uniform activity. The targets may be used in the treatment of cancer (e.g., breast cancer, prostate cancer). For example, during cancer treatment, multiple targets (e.g., seeds) may be placed in a tumor. As a result, targets having relatively uniform activity will provide the intended amount of radiation so as to destroy the tumor without damaging surrounding tissues. The device of producing such targets is described in further detail in “BRACHYTHERAPY AND RADIOGRAPHY TARGET HOLDING DEVICE” (HDP Ref.: 8564-000184/US; GE Ref.: 24IG237430), filed concurrently herewith, the entire contents of which are incorporated herein by reference. 
       FIG. 1  is a perspective view of a target holding device according to an embodiment of the invention.  FIG. 2  is a partially exploded view of a target holding device according to an embodiment of the invention. Referring to  FIGS. 1-2 , the target holding device  100  includes a plurality of target plates  102  and a plurality of separator plates  104 , wherein the plurality of target plates  102  and the plurality of separator plates  104  are alternately arranged. The thickness of each of the target plates  102  may be varied as needed to accommodate for the size of the intended targets to be contained therein. Thus, although the lower target plates  102  are shown as being thicker than the upper target plates  102 , the opposite may be true or the target plates  102  may all be of the same thickness. Furthermore, although the target plates  102  are shown as having the same diameter, the target plates  102  may have different diameters (e.g., tapering arrangement) based on reactor conditions and/or intended targets. 
     The alternately arranged target plates  102  and separator plates  104  are sandwiched between a pair of end plates  106 . A shaft  108  passes through the end plates  106  and the alternately arranged target plates  102  and separator plates  104  to facilitate the alignment and joinder of the plates. The joinder of the end plates  106  and the alternately arranged target plates  102  and separator plates  104  may be secured with a nut and washer arrangement although other suitable fastening mechanisms may be used. Furthermore, although the target holding device  100  is shown as having a single shaft  108 , it should be understood that a plurality of shafts  108  may be employed. 
     As shown in  FIG. 2 , each target plate  102  has a plurality of holes/compartments  202  in addition to the central hole for the shaft  108 . The plurality of holes  202  may be provided in various sizes and configurations depending on production requirements. Although the upper and lower target plates  102  are shown as having holes  202  of different sizes and configurations, it should be understood that all the target plates  102  may have holes  202  of the same size and/or configuration. 
     The plurality of holes  202  may extend partially or completely through each target plate  102 . When the holes  202  are provided such that they only extend partially through each target plate  102 , the separator plates  104  may be omitted. In such a case, an upper surface of a target plate  102  would directly contact a lower surface of an adjacent target plate  102 . On the other hand, when the holes  202  are provided such that they extend completely through the target plates  102 , the separator plates  104  are placed between the target plates  102  so as to separate the holes  202  of each target plates  102 , thereby defining a plurality of individual compartments within each target plate  102  for holding one or more targets (e.g., seeds, wafers) therein. 
       FIG. 3  is a perspective view of a target plate according to an embodiment of the invention. Referring to  FIG. 3 , the target plate  102  has a plurality of holes  202  for holding one or more targets (e.g., seeds, wafers) therein during production. The target plate  102  may be formed of a relatively low cross-section material (e.g., aluminum, molybdenum, graphite, zirconium) to allow a higher amount of flux to reach the targets contained therein. For instance, the material may have a cross-section of about 10 barns or less. Alternatively, the target plate  102  may be formed of a neutron moderator material (e.g., beryllium, graphite). Furthermore, the use of materials of relatively high purity may confer the added benefit of lower radiation exposure to personnel as a result of less impurities being irradiated during target production. 
     The upper and lower surfaces of the target plate  102  may be polished so as to be relatively smooth and flat. The thickness of the target plate  102  may be varied to accommodate the targets to be contained therein. Although the target plate  102  is illustrated as being disc-shaped, it should be understood that the target plate  102  may have a triangular shape, a square shape, or other suitable shape. Additionally, it should be understood that the size and/or configuration of the holes  202  may be varied based on production requirements. Furthermore, although not shown, the target plate  102  may include one or more alignment markings on the side surface to assist with the orientation of the target plate  102  during the stacking step of assembling the target holding device  100 . 
       FIG. 4  is a plan view of a target plate according to an embodiment of the invention. Referring to  FIG. 4 , in addition to having a plurality of holes  202 , the target plate  102  may also have sectional markings  402  to assist in the identification of each hole  202 , thereby also facilitating the placement of one or more targets within the holes  202 . Although the holes  202  are illustrated as extending completely through the target plate  102 , it should be understood, as discussed above, that the holes may only extend partially through the target plate  102 . Additionally, although the sectional markings  402  are illustrated as dividing the target plate  102  into quadrants, it should be understood that the sectional markings  402  may be alternatively provided so as to divide the target plate  102  into more or less sections. Furthermore, it should be understood that the sectional markings  402  may be linear, curved, or otherwise provided to accommodate the configuration of the holes  202  in the target plate  102 . 
       FIG. 5  is a diagram illustrating a system for mapping the holes of a target plate according to an embodiment of the invention. Referring to  FIG. 5 , the plurality of holes in a target plate may be divided into four quadrants Q 1 -Q 4 . The plurality of holes in the target plate may also be associated with rows/rings R 1 -R 5 . The holes in each of quadrants Q 1 -Q 4  may be further associated with holes H 1 -H 6 . With such a coordinate system based on quadrants Q 1 -Q 4 , rows R 1 -R 5 , and holes H 1 -H 6 , each hole in the target plate may be properly identified so as to facilitate the strategic placement of one or more targets therein. For instance, the hole identified as Q 2 , R 3 , H 2  is expressly labeled in  FIG. 5  for purposes of illustration. 
     It should be understood that a suitable coordinate system may differ from that shown in  FIG. 5  depending on the size of the holes, the configuration of the holes, the shape of the target plate, etc. For example, an alternate coordinate system may have more or less quadrants, rows, and/or holes than as shown in  FIG. 5 . Furthermore, other grouping methodologies may also be suitable and need not be limited to the methodology exemplified by the quadrants, rows, and holes shown in  FIG. 5 . 
       FIG. 6  is a perspective view of a target plate that has been loaded with targets according to an embodiment of the invention. Referring to  FIG. 6 , the holes  202  of a target plate  102  may be loaded with one or more targets  600 . The targets  600  may be formed of the same material or different materials. The targets  600  may also be formed of natural isotopes or enriched isotopes. For example, suitable targets may be formed of chromium (Cr), copper (Cu), erbium (Er), germanium (Ge), gold (Au), holmium (Ho), iridium (Ir), lutetium (Lu), palladium (Pd), samarium (Sm), thulium (Tm), ytterbium (Yb), and/or yttrium (Y), although other suitable materials may also be used. 
     The size of the targets  600  may be adjusted as appropriate for their intended use (e.g., radiography targets). For instance, a target  600  may have a length of about 3 mm and a diameter of about 0.5 mm. It should be understood that the size of the holes  202  and/or the thickness of the target plates  102  may be adjusted as needed to accommodate the targets  600 . The targets  600  are strategically loaded in the appropriate holes  202  based on various factors (including the characteristics of each target material, known flux conditions of a reactor core, the desired activity of the resulting targets, etc.) so as to attain targets  600  having relatively uniform activity. 
     As shown in  FIG. 6 , the targets may be radially arranged such that more targets are grouped together in the outer holes  202  than the inner holes  202 . For instance, each of the outermost holes  202  are illustrated as containing seven targets  600 , while each of the innermost holes are illustrated as containing one target  600 . However, it should be understood that each hole  202  does not need to be occupied with a target  600 , and the placement of a target  600  as well as the number of targets  600  in a hole  202  may vary depending on various factors, including the characteristics of the target material, known flux conditions of a reactor core, the desired activity of the resulting target, etc. 
     Because the outer holes  202  will be closer to the flux when the target holding device  100  is placed in a reactor core, a greater number of targets  600  may be placed in each of the outer holes  202 , thereby resulting in more equal activity amongst the targets  600  in the outer holes  202 . On the other hand, fewer targets  600  may be placed in each of the inner holes  202  to offset the fact that these targets  600  will be farther from the flux, thereby allowing the targets  600  in the inner holes  202  to attain activity levels comparable to the targets  600  in the outer holes  202 . Thus, the number of targets  600  in each hole  202  may be increased so as to decrease the resulting activity of each target in the hole  202 . Conversely, the number of targets  600  in each hole  202  may be decreased so as to increase the resulting activity of each target in the hole  202 . 
     It should be understood that  FIG. 6  assumes that all the targets  600  are formed of the same isotope to simplify the radial target placement illustration (although the targets  600  may be formed of different isotopes). Different isotopes may have different characteristics, including different neutron absorption rates and different decay rates. These characteristics will affect the overall placement as well as the grouping of the targets  600  when different isotopes are involved in the production process. For instance, if the targets  600  in the outermost holes  202  are formed of different isotopes having higher self-shielding properties than the targets  600  in the inner holes  202 , then fewer such targets  600  may be needed in each of the outermost holes  202  to create the desired self-shielding effect. 
     In another example, iridium (Ir) and gold (Au) seeds were loaded in a target plate  102  having holes  202  corresponding to the coordinate system illustrated in  FIG. 5 . Iridium has a much higher neutron absorption rate, but gold has a higher decay rate and initially has higher activities. A single iridium seed was loaded in a hole  202  corresponding to Q  1 , R 5 , H 5 , while two gold seeds were loaded in a hole  202  corresponding to Q  1 , R 4 , H 4 . Based only on the radial placement and the number of seeds per hole, it would seem that the single iridium seed in the outermost ring would have the highest activity after irradiation. However, because of gold&#39;s high decay rate, the two gold seeds actually had the higher activities of 57.38 μCi and 58.61 μCi, respectively, compared to the 49.75 μCi for the iridium seed. Thus, characteristics of the target material (e.g., neutron absorption rate, decay rate, etc.) should be taken into account when deciding where to place and/or how to group the targets so as to attain more uniform activities. 
     The targets  600  may also be arranged based on cross-section, wherein cross-section (σ) is the probability that an interaction will occur and is measured in barns. For instance, targets  600  formed of materials having lower cross-sections will have a lower probability that an interaction will occur compared to targets  600  formed of materials having higher cross-sections. As a result, targets  600  formed of materials having lower cross-sections may be arranged in holes  202  that will be in closer proximity to the flux during irradiation. With regard to  FIG. 6 , such lower cross-section targets  600  may be placed in the outer holes  202  of the target plate  102 . 
       FIG. 7  is a cross-sectional view of a loaded target holding device, taken along its longitudinal axis, according to an embodiment of the invention. In addition to the determination of where to place a target  600  in a target plate  102 , there is also the consideration of which target plate  102  of the target holding device  100  to place the target  600 . As shown in  FIG. 7 , the targets  600  may be axially arranged such that more targets  600  are grouped together in an axial portion of the target holding device  100  that is subjected to higher flux during irradiation in a reactor core.  FIG. 7  illustrates an example where the mid-axial portion of the target holding device  100  is subjected to higher flux during irradiation in a reactor core. Furthermore, the targets  600  may be arranged so as to be more concentrated on a particular side of the target holding device  100  that will be subjected to a higher flux during irradiation. 
     It should be understood that when a plurality of targets  600  of different materials are to be placed in the target holding device  100  for irradiation, the individual characteristics (e.g., neutron absorption rate) of each target  600  will be considered in conjunction with external factors (e.g., known flux conditions of the reactor core) when determining the proper arrangement within the target holding device  100 . For instance, not only is the proper target plate  102  and hole  202  determined for a target  600  but also whether grouping is appropriate, and if so, the target(s)  600  that should be grouped together so as to attain targets  600  in the target holding device  100  having relative uniform activity. 
       FIG. 8  is a perspective view of a target holder assembly according to an embodiment of the invention. Referring to  FIG. 8 , the target holder assembly  800  includes a target holding device  100  connected to a cable  802 . The cable  802  may be formed of any material having sufficient rigidity to facilitate the introduction of the target holding device  100  into a reactor core, sufficient strength to facilitate the retrieval of the target holding device  100  from the reactor core, and sufficient flexibility to maneuver the target holding device  100  through piping turns. For instance, the cable  802  may be a braided steel cable or a flexible electrical conduit cable. To assist with the introduction of the target holding device  100  into a reactor core, the cable  802  may be marked at a predefined length, wherein the predefined length corresponds to a distance from a reference point, to a predetermined location within the reactor core. 
     After the target holding device  100  has been irradiated in the reactor core, a predetermined period of time may be allowed to pass before disassembling the target holding device  100  and collecting the targets  600 . This waiting period may be beneficial by permitting any impurities in the target holding device  100  (as well as the targets  600  themselves) to sufficiently decay, thereby reducing or preventing the risk of harmful radiation exposure to personnel. 
     While a number of example embodiments have been disclosed herein, it should be understood that other variations may be possible. Such variations are not to be regarded as a departure from the spirit and scope of the present disclosure, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.