Samarium-153 is a beta emitting radionuclide with a half-life of approximately 48 hours. Like I-131, Sm-153 emits a beta particle that travels a very limited distance (millimeters) within tissue which localizes the therapeutic effect to those tissues that actively uptake the isotope. Bone tissue contains large amounts of the minerals calcium and phosphorus. Living bone actively metabolizes both calcium and phosphorus continuously extracting these minerals from the blood stream. Most primary or metastatic bone tumors metabolize calcium and phosphorus at even greater levels than normal bone. This physiology, that leads to bone tumors appearing "hot" on a bone scintigraphy, also leads to the preferential uptake of certain radiopharmaceuticals by bone tumors. Ethylenediamine-tetramethylene-phosphonic acid is a chemical with a high affinity for bone localization. Systemic radionuclide therapy of primary or metastatic bone tumors can be accomplished by combining the beta emitting properties of a radionuclide like 153Sm with the bone localizing properties of a chemical like EDTMP. The combination of the 2 creates the radiopharmaceutical 153Sm-EDTMP. The potential efficacy of systemic radionuclide therapy for primary or metastatic bone tumors may be confirmed by bone scintigraphy, utilizing another radiopharmaceutical with high bone affinity namely 99mTc-methylene diphosphonate (MDP). Bone scintigraphy should ideally be performed prior to considering systemic radionuclide therapy for primary or metastatic bone tumors.
Systemic radionuclide therapy with 153Sm-EDTMP involves a single IV injection of the radionuclide. No anesthesia or sedation is needed. Regulatory oversight requires 48-72 hour hospitalization following therapy for collection of waste and decay of the radionuclide remaining within the patient prior to the patient's discharge. Following discharge, limited close contact with the patient is also encouraged for 10 days following discharge to minimize exposure to others from the additional decay product of a gamma photon. Samarium-153 production requires a nuclear reactor and sources of 153Sm-EDTMP are limited. The short physical half-life of the radionuclide precludes storage of the material. Scheduling therapy with 153Sm-EDTMP requires consideration of the current production schedule for the radionuclide which is generally bi-monthly but can vary depending on reactor schedules and time of year. Hence it is usually necessary to schedule therapy with 153Sm-EDTMP a minimum of 2 weeks in advance.
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