13.1.1.2: Stronium isotopes treat bone cancer pain

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The element strontium (Sr) is right underneath calcium (Ca) on the periodic table, which means that they tend to have similar chemical behavior and Sr can easily bind to bones in the same places Ca would. This can be useful—Sr is used in Sensodyne toothpaste to minimize tooth sensitivity and ⁸⁹Sr (isotopic mass = 88.907 g/mol) is used to treat pain in bone cancer patients. The radioactive ⁹⁰Sr (isotopic mass = 89.908 g/mol) released by the Chernobyl disaster, however, is extremely dangerous for this same reason.

Use the following information about the 3 naturally occurring isotopes of Sr to calculate its average atomic mass. Does your calculation match that on the periodic table? Why are we calculating the average atomic mass using only these three isotopes when we mentioned above that ⁸⁹Sr and ⁹⁰Sr exist?

Isotope	Isotopic abundance (%)	Isotopic mass (g/mol)
⁸⁶Sr	?	85.90926
⁸⁷Sr	7.00%	86.90888
⁸⁸Sr	82.58%	87.90561

I would like to know how many moles of ⁸⁹Sr are administered to a cancer patient who weighs 16 kg. A radiation dose of 1.5 MBq/kg of body mass using ⁸⁹Sr will decrease pain in 80% of bone cancer patients without significant toxicity. Note that Bq are a unit of radiation and MBq is “mega” Bq. ⁸⁹Sr is delivered as a drug called Metastron^TM that delivers 37 MBq/mL. Each mL of Metastron contains 15 mg of the drug, which is 55% ⁸⁹Sr by mass.

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