Samarium-153 is an isotope of samarium.
It emits beta particles and gamma rays. (The therapeutic component is largely due to the beta particles, but the gamma rays make it easier to locate the distribution.)
It is used in samarium (153Sm) lexidronam.
It is treated by the body in a similar manner to calcium, and it localizes selectively to bone.
It is used in palliation of bone cancer.
In creative works
In the television crime drama Law and Order Criminal Intent, a murder victim is poisoned by ingesting three vials of samarium-153.