Strontium-90 is not good news. It emits ionizing radiation, which means you want to keep it out of our body. Unfortunately, your body has other ideas. The body’s own mechanisms welcome this isotope in, and store it away.
Strontium isn’t the only calcium mimic. Strontium-89 has been used to treat cancer spread to bones, but has largely been replaced by Samarium-153 which is less toxic. These are most commonly used in prostate cancer, but Radium-223 has superseded both in that disease as it is more effective and probably safer.