Abstract:
An implantable radiation therapy device includes a biocompatible radiotranslucent outer capsule containing a radiation shielding element and a radioactive isotope at least partially shielded by the shielding element. When the device is at or below body temperature, radiation is prevented or limited from being transmitted through the outer capsule by the shielding element. When non-ambient energy is applied to the device, the shielding element and radioactive isotope are reconfigured such that an increased level of radiation is transmitted through the outer capsule and emitted by the device.

Description:
This application is a continuation-in-part of U.S. Ser. No. 09/200,698, filed Nov. 27, 1998, now U.S. Pat. No. 6,066,083 which is hereby incorporated by reference herein in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to implantable radiation therapy devices. More particularly, the invention relates to improved radiation therapy and brachytherapy devices, also known as radioactive therapeutic seeds, for the treatment of oncological and other medical conditions. 
     2. State of the Art 
     Radioactive seed therapy is a well known and well accepted medical procedure for the treatment of various oncological and other medical conditions. Seed therapy, also known as interstitial brachytherapy typically involves the implantation of one to one hundred relatively small capsules (seeds) into or around a treatment site. The capsules contain a radioactive isotope which irradiates the treatment site at close range without adversely affecting other parts of the body. Brachytherapy has been used successfully in the treatment of various types of cancers such as prostate cancer. It has also been used to prevent the growth or regrowth of tissues in the treatment of various occlusive diseases such as arteriosclerosis and arthrosclerosis subsequent to balloon angioplasty. 
     Radioactive therapeutic seeds are carefully designed to possess several important qualities. First, in the case of prostatic interstitial brachytherapy they should be relatively small, approximately 0.025 inch in diameter and approximately 0.16 inch long so that they may be implanted into the prostate gland using minimally invasive instruments and techniques. However, it should be appreciated by those skilled in the art that implantable radioactive sources come in all shapes and sizes. Second, the radioactive isotope must be enclosed in a biocompatible protective package since the seeds are typically not removed and will remain in the body for many years. Third, each seed preferably includes a radiopaque (e.g. high Z material) marker so that it can be located at the treatment site with the aid of fluoroscopy. 
     The state of the art of radioactive therapeutic seeds is substantially disclosed in seven U.S. Patents: U.S. Pat. No. 5,713,828 to Coniglione for “Hollow-Tube Brachytherapy Device”, U.S. Pat. No. 5,405,309 to Carden, Jr. for “X-Ray Emitting Interstitial Implants”, U.S. Pat. No. 4,891,165 to Suthanthiran for “Device and Method for Encapsulating Radioactive Materials” and U.S. Pat. No. 4,784,116 to Russell, Jr. et al. for “Capsule for Interstitial Implants”, U.S. Pat. No. 4,702,228 to Russell, Jr. et al. for “X-Ray Emitting Interstitial Implants”, U.S. Pat. No. 4,323,055 to Kubiatowicz for “Radioactive Iodine Seed”, and U.S. Pat. No. 3,351,049 to Lawrence for “Therapeutic Metal Seed Containing within a Radioactive Isotope Disposed on a Carrier and Method of Manufacture”, which are each incorporated by reference herein in their entireties. In addition, the art has been significantly advanced in co-owned U.S. Ser. Nos. 09/133,072, 09/133,081, and 09/133,082, which are hereby incorporated by reference herein in their entireties. 
     The Lawrence patent, which issued in 1967, describes many of the essential features of radioactive therapeutic seeds. Lawrence describes radioactive isotopes (I-125, Pd-103, Cs-131, Xe-133, and Yt-169) which emit low energy X-rays and which have relatively short half-lives. When implanted at a treatment site, these isotopes provides sufficient radiotherapy without posing a radiation danger to the medical practitioner(s), people in the vicinity of the patient, or other parts of the patient&#39;s body. Lawrence further describes a protective capsule which contains the isotope and prevents it from migrating throughout the body where it might interfere with healthy tissue. The capsule is cylindrical and made of low atomic number biocompatible materials such as stainless steel or titanium which do not absorb X-rays. The isotope is coated on a rod shaped carrier made of similar X-ray transparent (e.g. low Z) material and is placed inside the capsule cylinder which is then closed. The other patents each provide some improvement over the original Lawrence design. 
     Despite the fact that radioactive therapeutic seeds have been in use for over thirty years and despite the several significant improvements made in the seeds, many concerns still exist regarding the use of the seeds. One problem is that prior to and during implantation of the therapeutic seeds, the physician must handle the radioactive seeds, and therefore take precautions to limit his or her exposure. The precautions may include the use of lead lined clothing and limiting the time for completing any one procedure. However, such clothing is generally heavy and tiring to wear, and limiting procedure time may not be in the best interest of the patient. 
     In addition, it is difficult to store radioactive therapeutic seeds, as special radiation shielding materials must be used in the container storing the seeds. 
     Moreover, there may be situations in which it is desirable to increase the level of radiation emitted by a seed after implantation, or keep the level of radiation at a certain level despite the natural decay of the radioactive source over a more prolonged period of time. For example, it may be desirable to provide a first dosage of radiation for a period of time and then, based upon a later diagnosis, increase the dosage for a second period of time. With the present radioactive implants of the art this can only be done through a subsequent invasive procedure of implanting additional seeds, as radioactive elements decrease their radiation output according to their respective half-life. 
     None of the art addresses any manner of providing an “inactive” seed which can later, e.g., after implantation, be activated to emit radiation. Likewise, none of the art addresses otherwise increasing the amount of radiation emitted by the seed after the seed is implanted in the patient, or maintaining a level of radiation over a longer period of time than the half-life of the radioactive isotope in the implant would otherwise permit. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the invention to provide radioactive therapeutic seeds which have means for controllably altering the radiation transmitted through the seed capsule. 
     It is also an object of the invention to provide radioactive therapeutic seeds which are controllably activated to “turn on” the seeds to cause radiation to be emitted therefrom or to increase the radiation emitted therefrom. 
     In accord with these objects which will be discussed in detail below, the radioactive brachytherapy seeds of the present invention generally include an outer capsule containing a radioactive material, and a substantially radiopaque shield which in a first (pre-implantation) configuration substantially obstructs radiation emitted by the radioactive material. One or both of the radioactive material and the shield are controllably movable relative to the other into a second (post-implantation) configuration such that the radioactive material is at least partially unobstructed by the shield. As a result, the level of radiation emitted by the seed is increased. For purposes herein, “radiopaque” refers to the property of having a relatively “high Z” value, and the terms “radiopaque” and “high Z” are used interchangeably herein. 
     Various embodiments of the radioactive material and the radiopaque shield are provided. In a first embodiment, a low melt temperature low Z material, e.g., wax, includes radioactive particles suspended therein. The low Z material is preferably substantially provided entirely within a high Z casing. The low Z material, with radioactive particles therein, may be heated and forced to flow, by pressurized fluid or mechanical means, through an opening in the high Z casing to at least partially surround the high Z casing and substantially cause the seed to emit radiation. In a second embodiment, an elastic or heat shrinkable casing is stretched over a radioactive material and a high Z material is deposited on the casing. When the radioactive material is heated to a melted state, the force of the casing on the radioactive material moves the radioactive material out of the casing, the casing collapses, and the radioactive material surrounds the high Z material on the casing to initiate or increase radiation emission from the seed. In a third embodiment, a flowable radioactive material is retained within a radiopaque casing by a removable barrier. The barrier may be removed by melting (e.g., a wax stopper barrier), breaking, or by a valve mechanism, and a pressurizing agent then forces the flowable radioactive material to surround the radiopaque casing. In a fourth embodiment, a first member is provided with regions upon which a radioactive isotope is deposited. The first member is disposed within a second member which includes one or more substantially radiopaque regions through which transmission of radiation is limited and one more substantially radiotransparent regions through which the radiation may be transmitted. In a first configuration, the radiopaque regions are positioned over the radioactive isotope regions. The first member may be controlled to move relative to the second member, e.g., by heat, vibration, or inertia, into a second configuration wherein the radiotransparent regions are positioned over the isotope and substantially permit the emission of radiation by the seed. In a fifth embodiment, a radiopaque shape memory alloy coil element is provided over an elongate element having an isotope deposited on a portion thereof. The rings of the coil are in a naturally compressed state over the portion of the elongate element on which an isotope is provided to prevent transmission of radiation through the rings of the coil and out of the outer capsule. The coil is trained to expand when heated and expose the portion of the elongate element provided with the isotope. In a sixth embodiment, a plurality of radiopaque shape memory alloy elements are provided, with each element having a portion on which an isotope is deposited. The portions provided with the isotope are initially oriented inward such that they do not emit radiation through the outer capsule. The elements are trained such that when they are heated, the elements change shape (or otherwise move) to substantially expose the portions provided with the isotope and thereby substantially initiate emission of radiation. 
     It will be appreciated that in embodiments utilizing heat to “activate” the seed, the heat may be provided by hot water, microwave technology, or other radiating means provided at or near (e.g., from adjacent to a few feet away) the seed implant site. Additional means for substantially “activating” or at least increasing seed radioactivity may also be used. 
     It will be further appreciated that the ability to control the amount of radiation emitted by the seed enables the physician to “turn on” the seed or at least increase the radiation emitted by the seed when desired; i.e., upon the application of non-ambient energy, preferably of a predetermined amount. In addition, the seeds may be relatively safely handled without cumbersome precautions prior to activation. 
     Additional objects and advantages of the invention will become apparent to those skilled in the art upon reference to the detailed description taken in conjunction with the provided figures. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a section view of a first embodiment of an at least partially activatable brachytherapy seed in an “inactive” configuration; 
     FIG. 1A is a section view of an alternate first embodiment of an at least partially activatable brachytherapy seed in an “inactive” configuration; 
     FIG. 2 is a section view of the first embodiment of an at least partially activatable brachytherapy seed in an “activated” configuration; 
     FIGS. 3 and 4 are section views of a second embodiment of an at least partially activatable brachytherapy seed in “inactive” and “active” seed configurations, respectively; 
     FIGS. 5 and 6 are section views of a third embodiment of an at least partially activatable brachytherapy seed in “inactive” and “active” seed configurations, respectively; 
     FIGS. 7 and 8 are section views of a fourth embodiment of an at least partially activatable brachytherapy seed in “inactive” and “active” seed configurations, respectively; 
     FIGS. 9 and 10 are section views of a fifth embodiment of an at least partially activatable brachytherapy seed in “inactive” and “active” seed configurations, respectively; 
     FIGS. 11 and 12 are section views of a sixth embodiment of an at least partially activatable brachytherapy seed in “inactive” and “active” seed configurations, respectively; 
     FIGS. 13 through 15 are cross section views of the seventh embodiment of “inactive”, “transitional” and “activated” seed configurations, respectively; and 
     FIGS. 16 through 19 are cross section views of an eighth embodiment of an at least partially activatable brachytherapy seed in “substantially inactive”, “first transitional”, “second transitional”, and “activated” seed configurations, respectively. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Referring now to FIG. 1, a radiation therapy seed  10  according to the invention is shown. The seed  10  includes an inner capsule  12 , preferably made from a radiopaque material, such as lead, provided within a biocompatible outer capsule  14 , preferably made from titanium, aluminum, stainless steel, or another substantially radiotranslucent material. Alternatively, referring to FIG. 1A, the inner capsule may be made from a radiotranslucent material and its exterior surface  25   a  may be coated or other provided with, e.g., as a sleeve, a radiopaque material  24   a.  Furthermore, while not preferred, the radiopaque material may be provided to the interior surface  27   a  of the inner capsule  12   a  (either by deposition thereon or an internal sleeve provided thereagainst). The outer capsule  14  is sealed closed about the inner capsule  12  according to any method known in the art, including the methods disclosed in previously incorporated U.S. Ser. No. 09/133,081. For treatment of the prostate, the outer capsule preferably has a diameter of less than 0.10 inches, and more typically a diameter of less than 0.050 inches, and preferably has a length of less than 0.50 inches, and more typically a length of less than 0.16 inches. 
     The inner capsule  12  includes first and second ends  16 ,  18 , and respective first and second openings  20 ,  22  at the respective ends. The inner capsule  12  is preferably coaxially held within the outer capsule  14  at the first and second ends  16 ,  18  of the inner capsule  12 , such that a preferably uniform space  28  is provided between the inner and outer capsules. 
     At the first end  16 , the inner capsule  12  is at least partially filled with a meltable solid radioactive material  30 . The radioactive material is preferably a low temperature melting, low Z carrier in which particles  31  provided with a radioactive isotope  33  are suspended. For the carrier, a low melting point is preferably characterized by under 160° F., and more preferably under 140° F. but over 105° F., such that at room temperature and body temperature, the seed is inactive as the radioactive material is substantially contained within the radiopaque inner capsule  12 . Wax is a preferred carrier, although other carriers such as certain metals and polymers may be used. Exemplar isotopes include I-125, Pd-103, Cs-131, Xe-133, and Yt-169, which emit low energy X-rays and which a have relatively short half-life. 
     A piston  32  is provided in the inner capsule  12  and, upon the liquefaction of the radiopaque material  30 , is capable of moving, e.g., by sliding, along a length of the inner capsule. A spring element  34  is provided between the second end  18  of the inner capsule  12  and the piston  32 , forcing the piston against the radiopaque material. 
     Turning now to FIG. 2, when it is desired to increase or initiate radiation emission by the seed, that is, “activate” the seed, the seed may be “activated” by applying heat which causes the radioactive material  30  to melt. The heat may be applied, for example, by hot water provided in the urethra (for seeds implanted to treat prostatic conditions), by microwave radiation, or by other types of radiation. The spring element  34  provides force against the piston  32  which, in turn, forces the radioactive material  30  out of the first openings  20  and into the space  28  between the inner and outer capsules  12 ,  14 . The second openings  22  permit gas trapped between the inner and outer capsules  12 ,  14  to be moved into the inner capsule  12  as the radioactive material  30  flows and surrounds the radiopaque inner capsule  12 . It will also be appreciated that second openings  22  are not required if the space  28  is evacuated during manufacture. Once the radioactive material has surrounded the inner capsule, the capsule is substantially “activated”. 
     In a variation of the above, it will be appreciated that some radioactive particles  31  or the isotope  33  may be initially provided outside the inner capsule (on the exterior surface of inner capsule, interior surface of outer capsule, or within space  28 ), such that movement of the radioactive material  30  out of the inner capsule operates to increase, rather than activate, radiation emission by the seed  10 . 
     Referring now to FIG. 3, according to a second embodiment of the invention, substantially similar to the first embodiment, the radiation therapy seed  110  includes a radiopaque inner capsule (or inner cylinder)  112  provided within a radiotransparent outer capsule  114 . The inner capsule  112  includes first and second ends  116 ,  118 , and one or more openings  120  at the first end. A solid, low temperature melting, radioactive material  130  is provided within the inner capsule  112 . 
     A piston  132  is provided in the inner capsule  112  against the radioactive material  130 , and a pressurized fluid (liquid or gas)  134  is provided between the piston  132  and the second end  118  of the inner capsule urging the piston toward the first end  116 . 
     Turning now to FIG. 4, the seed  110  may be “activated” by applying heat energy which causes the radioactive material  130  to melt. The pressurized fluid  134  then moves the piston  132  away from the second end  118 , and the piston  132  moves the melted radioactive material  130  through the first openings  120  in the inner capsule into the space  128  between the inner capsule  112  and the outer capsule  114 . Flow of the radioactive material  130  such that the radioactive material surrounds the inner capsule  112  is thereby facilitated. 
     Referring now to FIG. 5, according to a third embodiment of the invention, the radiation therapy seed  210  includes a capsule  214  having therein a rod  230  formed from a low melting point radioactive material which is provided with an elastic cover  244 , e.g., latex, stretched thereover. Alternatively, the cover may be made from a heat shrinkable material. The cover  244  is provided with a radiopaque coating  226  thereon. The rod  230  and cover  244  preferably substantially fill the interior  246  of the capsule  214 . As such, radiation emission is limited to the ends  248  of the rod. 
     Turning now to FIG. 6, when the capsule  214  is heated, the rod  230  liquefies and the cover  244  collapses inward to force the radioactive material out from within the cover. The radioactive material  230  thereby surrounds the collapsed cover  244 , with radiopaque material  226  deposited thereon, and increases the radioactive emission by the seed  210 . 
     Referring now to FIG. 7, according to a fourth embodiment of the invention, the radiation therapy seed  310  includes an inner capsule  312  provided within an outer capsule  314 . The inner capsule  312  includes first and second ends  316 ,  318 . The first end  316  includes openings  320 . A high Z material  326  is deposited on a surface  324  of the inner capsule  312 . Alternatively, the inner capsule is made from a high Z material. The inner capsule is preferably coaxially held within the outer capsule, and preferably a vacuum is provided therebetween. 
     The inner capsule  312  is partially filled with a radioactive material  330  which is liquid at body temperature, e.g., a dissolved radioactive compound. The inner capsule is also provided with a pressurized fluid (gas or liquid)  334 . A piston  332  separates the radioactive material  330  and the pressurized fluid  334 . The liquid material  330  is contained within the inner capsule by a wax plug  346  or the like, which is substantially solid at body temperature and which blocks the passage of the liquid radioactive material  330  through the openings  320  at the first end  316  of the inner capsule  312 . 
     Turning now to FIG. 8, when the seed  310  is heated, the plug  346  is melted and the pressurized fluid  334  forces the melted plug  346  and radioactive material  330  to exit the openings  320  at the first end  316  of the inner capsule  312  and surround the inner capsule and high Z material  326  thereof such that radiation may be emitted by the seed. 
     It will be appreciated that as an alternative to a wax plug  346  or the like, a frangible disc or valve may be utilized to retain the liquid radioactive material. The disc or valve may be operated via heat or mechanical means to controllably permit the radioactive material to flow out of the inner capsule. 
     Referring now to FIG. 9, according to a fifth embodiment of the invention, the radiation therapy seed  410  includes an inner capsule  412  provided within an outer capsule  414 . The inner capsule  412  is preferably held substantially coaxial within the outer capsule by a gas permeable tube  448 , e.g., a mesh or perforate tube formed of a low Z metal or plastic. The inner capsule  412  is comprised of first and second preferably substantially tubular components  450 ,  452 , each having a closed end  454 ,  456 , respectively, and an open end  458 ,  460 , respectively. The open end  458  of the first component  450  is sized to receive therein at least the open end  460  and a portion of the second component  452 . The first and second components  450 ,  452  together thereby form a “closed” inner capsule  412 . At least one of the first and second components is provided with a hole  462  which is blocked by the other of the first and second components when the inner capsule is in the “closed” configuration. A gas  434  is provided in the closed inner capsule  412 . 
     The first component and second components  450 ,  452  are made from a substantially low Z material. The second component  452  is provided with a plurality of preferably circumferential bands  464  of a radioactive material, while the first component  450  is provided with a plurality of preferably circumferential bands  466  of a high Z material. The first and second components are fit and aligned together such that along the length of the inner capsule  412  a series of bands in which the radioactive material  464  is covered by the high Z material  466  are provided. The bands  466  of high Z material substantially block the transmission of radiation at the isotope bands  464 . 
     Turning now to FIG. 10, when the seed  410  is heated, the gas  434  within the inner capsule  412  increases in pressure and forces the second component axially away from the first component such that the volume of the inner capsule increases. As the first and second components  450 ,  452  move axially apart, the hole  462  becomes exposed which equalizes the pressure between the interior of the inner capsule  412  and the interior of the outer capsule  414 , terminating the axial movement. The hole  462  is preferably positioned such that movement is terminated with the high Z bands  466  of the first component  450  substantially alternating with the radioactive isotope bands  464  of the second component  452 , such that the seed is activated for radiation emission. 
     It will be appreciated that the other means may be used to move the first and second components  450 ,  452  relative to each other. For example, a one-way inertial system or an electromagnetic system may be used. In addition, it will be appreciated that the inner capsule  412  may be configured such that the high Z bands  466  initially only partially block the radioactive isotope bands  464 ; i.e., that the seed  410  may be activated from a first partially activate state to a second state with increased radioactive emission. 
     Referring now to FIG. 11, according to a sixth embodiment of the invention, a radiation therapy seed  610  includes an inner wire  612  provided with a circumferential band  676  of radioactive isotope material. A close wound shape memory spring coil  678  is positioned centrally over the inner wire  612  over the band  676  of radioactive material. The shape memory coil  678  is preferably made from a relatively high Z material, e.g., Nitinol, and is trained to expand when subject to a predetermined amount of heat. Second and third spring coils  680 ,  682  are positioned on either side of the shape memory coil  678  to maintain the high Z coil  687  at the desired location. Washers  684  may be positioned between each of the coils  678 ,  680 ,  682  to maintain the separation of the coils; i.e., to prevent the coils from entangling and to better axially direct their spring forces. The wire  612  and coils  678 ,  680 ,  682  are provided in an outer capsule  614 . 
     Turning now to FIG. 12, when the seed  610  is subject to a predetermined amount of heat, the shape memory coil  678  expands to substantially expose the isotope band  676  and to thereby activate the seed. 
     Referring now to FIG. 13, according to a seventh embodiment of the invention, a radiation therapy seed  710  includes a relatively radiotranslucent capsule  714  provided with preferably six rods  786  oriented longitudinally in the capsule  714 . The rods  786  are made from a shape memory material which preferably is substantially radiopaque, e.g., a nickel titanium alloy. Each end of each rod is provided with a twisted portion  787 . In addition, the ends of the rods are secured, e.g., by glue  789  or weld, in the outer capsule  714 . When the rods are subject to heat energy, the rods are adapted to untwist at their respective twisted portions  787  about their respective axes. The rods  786  are each provided with a longitudinal stripe  788  (preferably extending about 60° to 120° about the circumference of the rods) of a radioactive isotope along a portion of their length, and preferably oriented in the capsule  714  such that the stripe  788  of each is directed radially inward toward the center C of the capsule with the high Z material of the rod substantially preventing or limiting transmission of radiation therethrough. 
     Turning now to FIG. 14, when subject to heat energy, the shape memory rods  786  within the seed  710  twist (or rotate) along their axes. The rods  786  are preferably oriented such that adjacent rods rotate in opposite directions. Turning now to FIG. 15, the rods  786  are trained to rotate preferably 180° about their respective axes. As a result, the isotope stripe  788  along each of the rods  786  is eventually directed radially outward to activate radiation emission by the seed. 
     It will be appreciated that the rods  786  are not required to be substantially radiopaque and that alternatively, or additionally, the rods may be circumferentially deposited with a relatively high Z material along their length at least diametrically opposite the longitudinal stripes of radioactive isotopes, and preferably at all locations on the rods other than on the stripes  788 . Furthermore, it will be appreciated that fewer than six or more than six rods may be provided in the capsule. Moreover, a central rod may also be used to maintain the rods in the desired spaced apart configuration; i.e., such that the rods together form a generally circular cross section. 
     Referring now to FIG. 16, according to an eighth embodiment of the invention, a radiation therapy seed  810  includes a relatively radiotranslucent capsule  814  provided with preferably three elongate shape memory strips  890  positioned lengthwise in the capsule  814 . It will be appreciated that two or four or more strips  890  may also be used. The strips are preferably made from Nitinol and are also preferably coated with a high Z material  891 , e.g., gold or a heavy metal, on one side (an initially outer side), and with a radioactive isotope  892  on the side opposite the high Z material (an initially inner side). The strips  890  are preferably positioned in the capsule at 120° relative separation. The configuration of the strips  890  and the high Z material on the outer side of the strips substantially limits radiation emission by the seed, as radiation is emitted only from between the ends of the strips, at  896 . 
     The shape memory strips  890  are trained to bend. As shown in FIGS. 17 through 19, when heat is applied to the seed, the strips  890  fold into their bent configuration such that eventually the radioactive material  892  of the strips  890  is located substantially on an exterior surface of the strips, while the high Z material is located on an interior side of the strips to further activate the seed. The strips  890  may be coupled to the capsule  814  by posts (not shown) to maintain their relative positions during bending. 
     There have been described and illustrated herein several embodiments of an activatable radioactive therapeutic seed. While particular embodiments of the invention have been described, it is not intended that the invention be limited thereto, as it is intended that the invention be as broad in scope as the art will allow and that the specification be read likewise. For example, those skilled in the art will appreciate that certain features of one embodiment may be combined with features of another embodiment to provide yet additional embodiments. Also, while hot water is disclosed as a heat source for “activating” many of the embodiments of the “activatable” seeds, it will be appreciated that microwave technology or other forms of radiated energy transmitted from a distance or provided at or near the seed implant site may also be used to generate sufficient heat. In addition, while a particular preferred temperature range for melting the radioisotope carrier is disclosed, it will be appreciated that a carrier may be used which melts at any temperature at or between body temperature, i.e., approximately 98° F., and an upper temperature which will not cause severe damage to body tissue if applied for a very short period of time, i.e., approximately 212° F. Thus, for example, seeds which are intended to be activated at body temperature are preferably stored at room temperature or kept refrigerated prior to use, but may not be handled by the practitioner without substantial activation. Furthermore, it will be appreciated that other types of energy can be used to trigger partial or complete seed “activation”. For example, mechanical, electromagnetic, and piezoelectric energy can also be used. In addition, while particular dimensions have been disclosed for the seeds, it will be appreciated that other dimensions may be likewise be used depending on the particular application of the seed; i.e., its locus of implantation. Also, it will be appreciated that the terms “radiotransparent”, “radiotranslucent”, “radiolucent”, and “low Z” are intended to have the same meaning for purpose of the prior description and in the construction of the claims which follow. In addition, the above “activatable” embodiments in conjunction with the “deactivatable” embodiments of the previously incorporated parent case, provide a complete system in which the radiation transmission of a brachytherapy seed can be controllably altered. It will therefore be appreciated by those skilled in the art that yet other modifications could be made to the provided invention without deviating from its spirit and scope as so claimed.