Journal of the ICRU Vol 4 No 2 (2004) Report 72 Oxford University Press
DOI: 10.1093/jicru/ndh026
2
SOURCES AND THEIR APPLICATION IN BRACHYTHERAPY
2.1
GENERAL 90
90
In this section, the beta-ray sources ( Sr---- Y, Ru----106Rh, 90Y, 32P) and low-energy photon sources (125I, 103Pd) that are widely and routinely used for brachytherapy applications are described in detail, including a discussion of their typical applications. A few radionuclides that have the potential for future use are also described briefly. Basic physical data and more details of the sources are given in Appendix A, and some clinical data on ophthalmic and intravascular applications are given in Appendix B. Dosimetric reference data for the sources are presented in Section 6.
106
2.2 2.2.1
BETA-RAY SOURCES Strontium-90/yttrium-90
Strontium-90 (90Sr) is an abundant fission product with a maximum beta-particle kinetic energy, Emax, of 546 keV and a mean kinetic energy, hEbi, of 196 keV. It decays to yttrium-90 (90Y) with a half-life of 29.12 y. The more energetic, short-lived daughter, 90 Y (Emax ¼ 2.28 MeV and hEbi ¼ 933 keV) decays with a half-life of 64.1 h. Since the daughter has such a short half-life, its emissions are in equilibrium with its parent. The radionuclide 90Sr is chemically isolated from other fission products, and an equilibrium mixture of 90Sr----90Y is supplied to source manufacturers in aqueous solution. Planar sources of 90Sr----90Y have been used for the post-operative treatment of pterygia since the 1950s (Cooper, 1978; Lentino et al., 1959; Pajonk et al., 1997). Such 90Sr----90Y sources typically consist of 2 GBq of radioactive material incorporated onto a metal base, usually silver (Sinclair and Trott, 1956) or into a ceramic matrix (Atlantic Research Corporation, 1966). The sources are typically encapsulated in stainless steel or silver, with a thin window (of the order of 100 mg cm2) over the source to allow transmission principally of the 90Y component of the spectrum. The sources are typically 10----12 mm in diameter (Soares, 1995; see Table A.2, Appendix A), mounted on the end of an 15-cm metal shaft, with an 10-cm diameter plastic shield
that is also mounted on the shaft for operator safety. A typical source of this type is shown in Fig. 2.1. Field shaping is sometimes done by applying apertures to the source surface (e.g., Castroviejo masks; see Castroviejo, 1956). Such apertures allow the therapist to limit the treatment area when needed. The treatment consists of manual application of the source to the surface of the area where the pterygium was removed. Typical treatment doses are 10 Gy, with treatment times typically 20 y (Gunthoff et al., 1986; Lommatzsch, 1974, 1983, 1986). This type of applicator has been marketed by Isocommerz GmbH since the early 1960s (at present, BEBIG GmbH in Berlin, Germany) and for some years by Amersham International. All commercially available eye plaques of 106 Ru----106Rh are formed as a part of a spherical shell. A thin film of 106Ru----106Rh is encapsulated between two layers of 99.99 % high-purity silver; the nuclide is electrolytically deposited on a 0.2-mm thick silver shell, bonded to a 0.7-mm thick backing, and is sealed on the concave exit side with a 0.1-mm thick silver window (see Fig. 2.2). This window stops all of the low-energy (Emax ¼ 39 keV) beta particles from 106Ru. The rear surface of the applicator absorbs 95 % of the beta radiation. The plaques are spherical shells with a radius of curvature, R (see Fig. 2.2), from 12 to 14-mm, and with the diameter of the circle limiting the spherical cap, D, from 10 to 25 mm. The available forms are illustrated in Fig. 2.3, and further data on practical sources are given in Appendix A (Table A.2). Some plaques have a cut-out to prevent direct irradiation of critical organs (optic nerve and iris). The plaques have rounded edges, a homogeneous lead seam, and special eyelets, so that they can be sutured to the sclera. The applicators also contain radioactivity from other radioisotopes, such as 103Ru----103Rh, 137 Cs, and europium-154 (154Eu). However, the contamination by these radionuclides is
DOI: 10.1093/jicru/ndh026
2
SOURCES AND THEIR APPLICATION IN BRACHYTHERAPY
2.1
GENERAL 90
90
In this section, the beta-ray sources ( Sr---- Y, Ru----106Rh, 90Y, 32P) and low-energy photon sources (125I, 103Pd) that are widely and routinely used for brachytherapy applications are described in detail, including a discussion of their typical applications. A few radionuclides that have the potential for future use are also described briefly. Basic physical data and more details of the sources are given in Appendix A, and some clinical data on ophthalmic and intravascular applications are given in Appendix B. Dosimetric reference data for the sources are presented in Section 6.
106
2.2 2.2.1
BETA-RAY SOURCES Strontium-90/yttrium-90
Strontium-90 (90Sr) is an abundant fission product with a maximum beta-particle kinetic energy, Emax, of 546 keV and a mean kinetic energy, hEbi, of 196 keV. It decays to yttrium-90 (90Y) with a half-life of 29.12 y. The more energetic, short-lived daughter, 90 Y (Emax ¼ 2.28 MeV and hEbi ¼ 933 keV) decays with a half-life of 64.1 h. Since the daughter has such a short half-life, its emissions are in equilibrium with its parent. The radionuclide 90Sr is chemically isolated from other fission products, and an equilibrium mixture of 90Sr----90Y is supplied to source manufacturers in aqueous solution. Planar sources of 90Sr----90Y have been used for the post-operative treatment of pterygia since the 1950s (Cooper, 1978; Lentino et al., 1959; Pajonk et al., 1997). Such 90Sr----90Y sources typically consist of 2 GBq of radioactive material incorporated onto a metal base, usually silver (Sinclair and Trott, 1956) or into a ceramic matrix (Atlantic Research Corporation, 1966). The sources are typically encapsulated in stainless steel or silver, with a thin window (of the order of 100 mg cm2) over the source to allow transmission principally of the 90Y component of the spectrum. The sources are typically 10----12 mm in diameter (Soares, 1995; see Table A.2, Appendix A), mounted on the end of an 15-cm metal shaft, with an 10-cm diameter plastic shield
that is also mounted on the shaft for operator safety. A typical source of this type is shown in Fig. 2.1. Field shaping is sometimes done by applying apertures to the source surface (e.g., Castroviejo masks; see Castroviejo, 1956). Such apertures allow the therapist to limit the treatment area when needed. The treatment consists of manual application of the source to the surface of the area where the pterygium was removed. Typical treatment doses are 10 Gy, with treatment times typically 20 y (Gunthoff et al., 1986; Lommatzsch, 1974, 1983, 1986). This type of applicator has been marketed by Isocommerz GmbH since the early 1960s (at present, BEBIG GmbH in Berlin, Germany) and for some years by Amersham International. All commercially available eye plaques of 106 Ru----106Rh are formed as a part of a spherical shell. A thin film of 106Ru----106Rh is encapsulated between two layers of 99.99 % high-purity silver; the nuclide is electrolytically deposited on a 0.2-mm thick silver shell, bonded to a 0.7-mm thick backing, and is sealed on the concave exit side with a 0.1-mm thick silver window (see Fig. 2.2). This window stops all of the low-energy (Emax ¼ 39 keV) beta particles from 106Ru. The rear surface of the applicator absorbs 95 % of the beta radiation. The plaques are spherical shells with a radius of curvature, R (see Fig. 2.2), from 12 to 14-mm, and with the diameter of the circle limiting the spherical cap, D, from 10 to 25 mm. The available forms are illustrated in Fig. 2.3, and further data on practical sources are given in Appendix A (Table A.2). Some plaques have a cut-out to prevent direct irradiation of critical organs (optic nerve and iris). The plaques have rounded edges, a homogeneous lead seam, and special eyelets, so that they can be sutured to the sclera. The applicators also contain radioactivity from other radioisotopes, such as 103Ru----103Rh, 137 Cs, and europium-154 (154Eu). However, the contamination by these radionuclides is
