Surface structure and chemical durability of P⁺-implanted Y₂O₃-Al₂O₃-SiO₂ glass for radiotherapy of cancer

Yttrium-containing glass microspheres are already used clinically for radiotherapy of cancers, since the microspheres are activated to β-emitters by neutron bombardment and have a high chemical durability. The microspheres can irradiate cancer directly when they are injected into the tumor without irradiating neighboring normal tissues. Radioactive ⁹⁰Y has, however, too short a half-life time. Phosphorus is also activated to a β-emitter by neutron bombardment and has a little longer half-life. In the present study, phosphorus (P⁺) ions were implanted into Y₂O₃-Al₂O₃-SiO₂ glass plates at 200 keV with various doses in order to examine the possibility for enhancing the radiotherapeutic effect of the glass. No phosphorus was present at the surface for doses below 1×10¹⁷ cm^-2, whereas phosphorus atoms were distributed up to the glass surface and a portion of them was oxidized at the surface for doses above 5×10¹⁷ cm^-2. The glass plates implanted with doses of 5×10¹⁶ and 1×10¹⁷ cm^-2 hardly released any Si, P, Al and Y into hot water at 95 °C even after soaking for 7 days, whereas appreciable amounts of elements implanted with doses above 5×10¹⁷ cm^-2 were released from the surface into the hot water. It is considered that no release of the elements from the glass plates implanted with doses below 1×10¹⁷ cm^-2 is attributed to the lack of presence of phosphorus at the glass surface. On the contrary, hygroscopic phosphorus oxide at the glass surface causes an appreciable release of the elements from the glass plates implanted with doses above 5×10¹⁷ cm^-2. The results indicate that a Y₂O₃-Al₂O₃-SiO₂ glass containing a large amount of phosphorus can be obtained without losing its high chemical durability when P⁺ ions are implanted at 200 keV with a dose up to 1×10¹⁷ cm^-2.