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Patients with skeletal metastases from hormone-refractory prostate cancer have shown variable responses to high-activity therapy with ¹⁸⁶Re-HEDP and peripheral stem cell support. In this paper, we report on the use of a novel technique to compare sequential planar images acquired post-¹⁸⁶Re-HEDP therapy administration with pretherapy diagnostic ^99mTc-MDP scans, to evaluate the turnover of the radiopharmaceutical in normal and abnormal bone. It was found that the activity in normal (i.e., disease-free) segments of the spine demonstrates a faster effective decay than that of the metastases, with the latter showing only physical decay. This study showed, for the first time, a detailed correlation in the behavior of the ^99mTc-MDP and ¹⁸⁶Re-HEDP images, encouraging the possibility of using the pretherapy 99mTc-MDP scan for estimations of absorbed doses to be delivered by prescribed activities of ¹⁸⁶Re-HEDP.