Strontium-89 vs strontium-90
The half-life of $\ce{^{89}Sr}$ is 50 days compared to a half-life of 29 years for $\ce{^{90}Sr}$. So a large enough dose of $\ce{^{90}Sr}$ to be useful would leave the patient radioactive for the rest of his/her life. 



Edited for use of $\ce{^{89}Sr}$. In a comment Karl pointed out that the isotope is used to treat bone cancer.