Highly radioactive miniaturized ceremic strontium 90...

Details Highly radioactive miniaturized ceremic strontium 90... Highly radioactive miniaturized ceremic strontium 90...

2000-07-10

2003-09-02

Padmanabhan, Sreeni (Department: 1617)

Drug, bio-affecting and body treating compositions

Radionuclide or intended radionuclide containing; adjuvant...

C424S001610, C423S249000, C264S000500

Reexamination Certificate

active

06613303

ABSTRACT:

The present invention relates to highly radioactive, miniaturized, cylindrical strontium 90 titanate, strontium 90 zirconate, and strontium 90 silicate radiation sources having an activity exceeding 25 mCi/mm
3
, preferably ≧30 mCi/mm
3
, and a diameter less than 0.7 mm, preferably less than 0.4 mm. Another subject of this invention is a method for the production of these extremely small, but highly radioactive radiation sources.
In the following description, strontium 90 titanate is also referred to as
90
SrTiO
3
, strontium 90 zirconate as
90
SrZrO
3
, and strontium 90 silicate as
90
SrSiO
3
.
With respect to medical applications, the importance of miniaturization of the radioactive radiation sources is steadily increasing. For example, in tumour therapy and intravascular brachytherapy, i.e. the exposure of the inner wall of blood vessels to radiation, inserted miniaturized sources are used.
There are essentially two known methods of producing miniaturized radiation sources of the strontium 90 isotope. In the production of tabular radiation sources, a mixed precipitation of Ag
2
CO
3
/
90
SrCo
3
/TiO
2
with subsequent malleabilization of the precipitate is used wherein the emerging silver cake is brought into the desired shape. Regarding the production of miniaturized, cylindrically shaped strontium 90 sources, it is known to soak a pre-formed carrier body consisting of titanium dioxide with a
90
Sr(NO
3
)
2
solution, to dry and then to anneal it at a temperature exceeding 1,000° C. In this process, insoluble strontium 90 titanate (
90
SrTiO
3
) is generated. These radiation sources are characterized by having an activity of only 5 to 7 mCi per mm
3
. This activity and the resulting dose rate, however, are not sufficient, for instance, for the aforementioned medical applications. There still remains a need of having possibly small but highly radioactive strontium 90 radiation sources.
Therefore, it was an object of this invention to provide a manufacturing method for producing highly radioactive and very small strontium 90 radiation sources using a possibly automated or partly automated technique. In order to reach even very small blood vessels in medical applications, the diameter of the radiation source should be less than 0.6 mm.
The object of this invention is solved by a method for producing ceramic strontium 90 radiation sources wherein the aqueous solution of a strontium 90 salt is united with a titanium, zirconium, and/or silicon compound being in a dissolved state and the solution of one or more ammonium salts of carbonic acid and/or a low-molecular organic acid, the solvent is expelled from the mixture, the residue is calcinated and, after adding auxiliary agents, is transformed into a plastic state, the plastic matter is microextruded, the emerging thread is exposed to a sintering process and finally cut at the desired lengths so that miniaturized radiation sources are obtained which can be encapsulated in a manner known per se if necessary. It is also possible, of course, to first cut and then sinter the strontium 90 mass thread obtained. Below, the
90
SrTiO
3
,
90
SrSiO
3
, and
90
SrZrO
3
bodies produced according to the present invention are also referred to as “radioactive ceramics”.
The manufacturing method according to the present invention by which the radioactive ceramics is produced by microextrusion is advantageous in comparison with the conventional soaking technique in which pre-fabricated inactive ceramic carriers are soaked with the strontium 90 solution in that radiation sources having a higher Sr 90 portion (in the case of
90
SrTiO
3
, the density is ≧4 g/cm
3
) can be produced. The method according to the present invention may (in part) be automated and remote-controlled. No grinding processes, no screening, no filtration processes, no spraying operations, and, except cutting, no finishing processes are necessary. The cylindrical sources are not manufactured as individual cylinders but as string (thread) which is cut in the raw or sintered state.
The initial compounds for the manufacturing method according to the invention are commercially available. For instance, strontium 90 nitrate having a concentration of 0.2 g solid matter/ml, which is commercially available and contains portions of barium nitrate and minor iron impurities, can be used as strontium 90 salt. The strontium 90 salt used may also be the salt of a low-molecular organic acid such as for instance
90
Sr formiate or
90
Sr acetate.
Although the present invention allows the use of water-soluble salts such as chlorides as titanium, zirconium, or silicon compounds, alcoholates are preferred. Mixtures of titanium, zirconium, and silicon alcoholates may also be used here so that mixed ceramics are generated, for instance comprising
90
SrSiO
3
and
90
SrTiO
3
, or
90
SrSiO
3
and
90
SrZrO
3
. The embodiment using either a titanium or a zirconium or a silicon alcoholate is preferred, however. According to the present invention, ethylates, propylates, butylates, the corresponding iso-compounds, or the corresponding mixed alcoholates are used as preferred alcoholates. Special preference is given to tetra-isopropyl-orthotitanate (TiPOT) in the production of
90
SrTiO
3
ceramics. For producing
90
SrSiO
3
ceramics, tetraethoxysilane (TEOS) is particularly preferred. For producing
90
SrZrO
3
ceramics, zirconium (IV) propylate is particularly preferred. The alcoholates are, according to the present invention, preferably used in an anhydrous alcoholic solution.
As an ammonium salt, all those compounds may be used the anion of which is thermally separable or thermally decomposable and which form a hardly soluble compound with strontium, such as carbonate or oxalate. The ammonium may also be present in a substituted form as an organic ammonium compound. Ammonium compounds soluble in alcohol such as ammonium oxalate which may be used in a solution together with the silicon, titanium, and zirconium alcoholates are also suitable. In a preferred embodiment, (NH
4
)
2
CO
3
is used.
According to the present invention, the mol ratio of
90
Sr:Me:NH
4
is 0.85-1:0.95-1.05:1.7-2, preferably 0.93:1:1.86, wherein Me means Ti, Zr, and/or Si.
The initial solutions described above are mixed by starting with the
90
Sr solution and homogenized, preferably by stirring. Thereafter, the solvent is mostly expelled and the residue calcinated, preferably at 650-1,000° C. wherein the duration period at this temperature is approximately one hour. The preferred calcination temperature ranges between 800-830° C., in particular preferably at 820-830° C.
The expulsion of the solvent may be accomplished by evaporation and/or sublimation.
Afterwards a plasticator is mixed into the calcinated mass. A number of recipes of plasticators for oxide ceramics are known which usually include organic auxiliary substances such as a solvent, a bonding agent, a softener, a lubricant, and a dispersion agent. One substance may also fulfil the function of several components.
An aqueous plasticator comprising a cellulosic derivative of a medium mol mass, a polysaccharide, a polyol, e.g. glycerol, and a polyelectrolyte has proved to be advantageous for the plastication of the strontium 90 mass according to the present invention. These auxiliary agents are added to the calcinated powder after cooling in a quantity ranging between 6 and 18 percent by weight in relation to the weight of the powder. Apart from these auxiliary agents being per se usual with respect to plastication, according to the present invention a silicon, titanium, and/or zirconium alcoholate in a quantity between 0.5 to 2 percent by weight is added to the calcinated powder during the plastication process. The alcoholates used may be the same as mentioned above in connection with the production of the initial mixture. In case of the use of TiPOT, the mass ratio of cellulosic derivative:polysaccharide:polyol:polyelectrolyte:TiPOT is 7-9:3.5-4.5:6-8:0.8-1.2:15-24, preferably 8:4:7:1:19. In case of the use of TEOS, the mass ratio of cellulosic derivative

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