Samarium is labeled to phosphonates, for example, EDTMP, taking advantage of the bone-seeking properties of the bisphosphonates. It is produced by neutron irradiation of 153Sm. 153Sm is a gamma (with a principal photopeak of 103 keV) and beta emitter (Emax beta = 0.81 MeV for the main emission). Its convenient half-life of 1.95 days makes it useful for treating patients with widespread metastases and compromised bone marrow function (28). It has a biodistribution profile and uptake characteristics in the metastatic bone lesions [ratio of lesion to normal bone can be as high as 4:1 (29)]. As with other bone-seeking radiopharmaceuticals, the bone marrow is at risk for myelotoxicity (the calculated upper limit for marrow dose is 7 rad/mCi). Along with strontium chloride, it is one of the two radiopharmaceuticals approved by the FDA for palliation of bone pain from metastatic disease. Its shorter half-life is particularly attractive to prevent severe myelotoxicity when used in patients who have been heavily pretreated with radiation and or chemotherapy.

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