Stock material or miscellaneous articles – Coated or structually defined flake – particle – cell – strand,... – Rod – strand – filament or fiber
Reexamination Certificate
2002-03-04
2003-03-11
Edwards, N. (Department: 1774)
Stock material or miscellaneous articles
Coated or structually defined flake, particle, cell, strand,...
Rod, strand, filament or fiber
C428S372000
Reexamination Certificate
active
06531217
ABSTRACT:
The present invention relates to novel filaments formed from a polymer matrix and from thermoluminescent particles, especially for use as thermoluminescent dosimeters.
It also relates to a method for measuring, by thermoluminescence, the beta radiation doses delivered by an emitter to a target organ of a mammal.
The use of thermoluminescent dosimeters has already been described in internal radiotherapy in patent FR-B-1 470 914.
This patent describes especially polytetrafluoroethylene (PTFE) filaments about 1 mm in diameter, these being filled to about 8% with lithium fluoride microcrystals obtained by extrusion and able to be used in vivo.
Lithium fluoride forms part of the thermoluminescent mineral compounds called “phosphors”, among which mention may also be made of calcium fluoride CaF
2
or calcium sulfate CaSO
4
, to which special activators may possibly be added.
Implantable Dy:CaSO
4
mini-dosimeters (B. W. Wessels et al., The Journal of Medicine, Vol. 27, No. 8, August 1986, pp. 1308-1314) have in particular been manufactured by cutting disks of Teflon and CaSO
4
with a microtome.
The small dimensions (0.5×0.2×0.4 mm) of these osimeters make it possible to detect large dose gradients during the therapeutic use of beta-emitting radioelements.
Many subsequent studies have been carried out using these Dy:CaSO
4
mini-thermoluminescent dosimeters (mini-TLDs). Thus, S-E Strand et al., (Acta Oncologica, Vol. 32, No. 718, pp. 787-791, 1993) have demonstrated the phenomenon of Dy:CaSO
4
crystals dissolving in aqueous phase. This leads to a loss of signal in vivo (by 60% for immersion for 9 days at pH 6) making the luminescence measurements at the very least inaccurate. These authors propose covering the dosimeter with a thin layer of Teflon or with other suitable materials.
A. J. Demidecki et al., Med. Phys. 20:1079:1087, 1993 concluded that there was a great difficulty in using Dy:CaSO
4
mini-TLDs unless correction factors were applied.
Many subsequent studies have been carried out using these Dy:CaSO
4
mini-thermoluminescent dosimeters (mini-TLDs). Thus, S-E Strand et al., (Acta Oncologica, Vol. 32, No. 718, pp. 787-791, 1993) have demonstrated the phenomenon of Dy:CaSO
4
crystals dissolving in aqueous phase. This leads to a loss of signal in vivo (by 60% for immersion for 9 days at pH 6) making the luminescence measurements at the very least inaccurate. These authors propose covering the dosimeter with a thin layer of Teflon or with other suitable materials.
A. J. Demidecki et al., Med. Phys. 20:1079:1087, 1993 concluded that there was a great difficulty in using Dy:CaSO
4
mini-TLDs unless correction factors were applied.
However, mini-dosimeters have been used in vivo by M. H. Griffith et al., J. Nucl. Med. 1988, 29:1795-1809.
Nevertheless, it is desirable to provide novel mini-thermoluminescent dosimeters which do not have the abovementioned disadvantages, especially in the case of in vivo use.
Unexpectedly, the applicants have shown that improving the adhesion properties between the polymer material and the thermoluminescent particles helps to overcome the drawbacks described above.
It is an object of the present invention to provide novel filaments which can be used in particular to make mini-thermoluminescent dosimeters which:
exhibit excellent mechanical strength and excellent elasticity within the context of in vivo use;
provide effective protection of the TL crystals from an aqueous medium;
show a lack of exchange between content and container (dissolution, chemical reaction);
exhibit satisfactory biocompatibility with tissue;
allow the absorbed radiation dose from a few mGy to a few tens of Gy to be determined with improved accuracy;
have an advantageous manufacturing cost.
One or more of the objects of the invention are achieved by the solution that the patent application provides.
The invention therefore relates to a filament comprising thermoluminescent particles uniformly distributed in a polymer matrix, characterized in that the polymer matrix coating said thermoluminescent particles is a hydrophobic thermoplastic polymer having sufficient adhesion to the thermoluminescent particles to ensure cohesion of the filament and being such that a thermoluminescent response (signal) from the filament corresponding approximately to the absorbed radiation dose is obtained, after the filament has been brought into contact with a physiological medium.
The polymer is therefore such that, after the filament has been immersed in an irradiation physiological medium, the thermoluminescent response (signal) corresponds approximately to the absorbed radiation dose.
The term “polymer” is understood to mean within the context of the present description a homopolymer, a copolymer, a blend of homopolymers or a blend of copolymers.
The term “sufficient adhesion” is understood to mean that the polymer has a high surface energy, however not exceeding the surface energy of a hydrophilic polymer. PTFE, for example, known for its nonstick properties, has a very low surface energy (its surface energy is 21 mJ/m
2
at 20° C.). The surface energy of the polymer allowing the invention to be implemented advantageously lies between 25 and 50 mJ/m
2
at 20° C. and advantageously between 27 and 46 mJ/m
2
. The expression “corresponding approximately” is understood to mean that the response must be at least of the order of 90% with respect to the absorbed dose. That is to say, the response is not affected by prolonged residence of the filament in the physiological medium. In other words, the filament behaves equivalently in air and when it is immersed in a physiological medium. There is practically no loss of response due to immersion in a physiological medium.
The expression “good cohesion” is understood to mean that the filament has a high mechanical strength and retains its physical integrity in a physiological medium.
The thermoluminescent particles are formed from crystals which, after irradiation, have the property of releasing the absorbed energy in the form of light when they are subjected to a sufficiently high temperature. There is storage of the ionization phenomena. The amount of light emitted is then, within certain limits, a linear function of the dose absorbed by the crystal. They can therefore be used as dosimeters. All luminescent particles in powder form may be suitable.
There are a large number of thermoluminescent particles known to those skilled in the art.
Among thermoluminescent particles, mention may especially be made of crystals based on lithium fluoride (LiF), calcium fluoride (CaF
2
), these possibly being doped with magnesium, copper or phosphorus.
Among polymers which are suitable within the context of the present invention, it is preferred to use those which can be processed using well-known extrusion processes. These polymers must therefore have a melting point below the critical temperature of the thermoluminescent particles so as not to degrade the dosimetric characteristics of the TL material. However, it should be noted that some thermoluminescent materials have a sufficiently high critical temperature for this proviso to be unnecessary (the critical “temperature” is the limit above which the thermoluminescent material loses its properties of releasing the stored energy).
Among thermoplastic polymers meeting this requirement, mention may be made of nonhalogenated polyolefins, especially polyethylenes, polypropylenes or polyisobutylenes. Among polyethylenes, mention may be made of low-density polyethylenes (LDPEs) or high-density polyethylenes (HDPEs). Polypropylenes have a better mechanical strength than polyethylenes and have a higher melting point.
Among the other extrudable polymers, mention may be made of (co)polymers resulting from the polymerization of vinyl chloride (PVC), acrylic and methacrylic ester polymers, such as polymethyl methacrylate (PMMA), or other equivalent polymers, polystyrenes, polyethylene terephthalates, polyamides and ethylene-vinyl acetate copolymers.
Of course, other polymers able to be processed by extrusion are al
Bardies Manuel
Benoit Jean-Pierre
Denizot Benoit
Lisbona Albert
Martin Stéphane
Edwards N.
Foley & Lardner
Mainelab
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