Strontium-90 is an isotope of strontium which decays via beta emission to ytrrium-90. Yttrium-90 also decays through beta emission to a stable daughter product. Strontium-90 is typically used in ophthalmic applicators for the treatment of pterygium, although may also be used for endovascular brachytherapy.
90Sr has a half life of 28.8 years, and decays by beta emission (of maximum energy 546 keV) to 90Y. 90Y, also described below, has a short half life of 64 hours with a beta emission of 2.24 MeV. Ytrrium exists in a secular equilibrium with 90Sr and is the main source of therapeutic electrons.
Source Construction and Handling
As a β emitter, 90Sr sources are manufactured as a plaque - usually a silver disc coated in 90Sr. The disc is placed in a cavity within a larger disc (made from lead), and a very thin metal 'window' covers its external surface. This active surface should be directed away from staff and patients, and only applied to the area to be treated. Handling the source on the non-active surfaces, which are shielded, poses minimal health risk. Bremmstrahlung production from electron interactions does occur and leads to x-ray generation; therefore the source should not be handled unless strictly necessary.
The thin metal window which prevents leakage of 90Sr requires sterilisation between use; this should be done gently with an antiseptic solution as the metal window is very fragile.
Manufacture, supply and disposal
90Sr is a fission product arising from fission of 235U, 238U or other heavy nuclei. Secular equilibrium with the daughter product 90Y occurs within several half lives and beyond this their activities are equal. It may be contaminated by 89Sr, which has a much shorter half life of 50.5 days. Due to the long half life of 90Sr, if disposal is required procedures similar to 137Cs should be followed.
(Based on the LNHB Strontium 90 and Yttrium 90 data)
- 12: Brachytherapy