Dosimetry and Therapeutic Ratios for Rhenium-186 HEDP

Rhenium-186 (Re-186) is a β-emitting radionuclide. Emitted β-particles have ranges up to 4.5？mm in tissue, capable of delivering high doses to skeletal regions of high Re-186 concentrations while sparing adjacent radiosensitive regions and thus making the irradiation well tolerated for the patient. Along with the β-emissions, γ-rays are emitted having an adequate energy for imaging during therapy and biodistribution assessment for patient-specific dosimetry calculations. The relatively short physical half-life combined with the β-emissions allows the delivery of relatively high activity rate for a short period of time in areas of concentration. This study is a short review concerning the palliative treatment of skeletal metastases using 186Re-HEDP. After presenting the dominant ways of 186Re production, special emphasis is given to dosimetry issues while the effect of palliation therapy can be evaluated through the comparison of the absorbed dose in metastatic lesion relatively to the normal bone region. Accurate dose estimation is required taking into account the anatomic individual difference of each patient. For this purpose a patient specific dosimetric model considering metastatic lesions as spherical nodules is introduced. In order to quantify in a representative way the results of palliation treatment, the concept of therapeutic ratios is analyzed. 1. Production of Re-186 Re-186 can be produced either at a nuclear reactor or at a particle accelerator (cyclotron). The first method utilizes neutron capture of enriched Re-185 [1, 2]. Although the thermal and epithermal cross-sections for neutron capture are high (106 and 1632 barns, resp.), the specific activity of Re-186 required for antibody labeling is hardly achieved using low neutron flux reactors. However, it is marginally sufficient for preparation of phosphonates for palliative treatment of bone pain from osseous metastases. Re-186 when produced at a nuclear reactor is in carrier-added form. To prepare the target material, rhenium metal is dissolved in either concentrated nitric acid or hydrogenperoxide and then it reacts with aluminum chloride to yield the aluminum perrhenate solution . The solution is irradiated in neutron flux and dissolved in sterile water. The chemical recovery exceeds 90% in the chemical form of . Because very few of reactors with higher neutrons fluxes are operating in the world, methods of enhancing the specific activity of Re-186 by other means are desirable. In spite of the advantages of isotope production at reactors in terms of quantity and unit cost, cyclotrons