[Cancer therapy using unsealed radioisotopes-the present and future]

Iodine-131 (I-131) has been used for the ablation of residual thyroid remnants and cancer cells in well-differentiated thyroid cancers. It has also been used for metastatic well-differentiated thyroid cancers, especially lung and bone metastases. For small lung metastases, I-131 treatment has curative potential, particularly in young patients. Suppression of the thyroid stimulating hormone is also important for prolonging the survival of thyroid cancer patients. Strontium-89 (Sr-89) chloride has a mechanism similar to calcium, and it has been used for the treatment of bone metastases, especially osteoblastic metastases. It has been reported to have analgesic effects in an average of 76% of cases, and it is more effective if used in the early bone metastatic phase. Ibritumomab tiuxetan (Zevalin) is an anti-CD20 mouse monoclonal antibody labeled with Yttrium-90 (Y-90). It is used for treatment-resistant low grade or follicular B-cell non-Hodgkin's lymphomas and mantle lymphomas. Recently, radium-223 (Ra-223) has been used for bone metastases from castration resistant prostate cancers, and in a phase III trial, it has been proven to prolong survival of these patients. Cancer therapy using unsealed radioisotopes has been thought to be promising because it exhibits more targeted therapy than external beam irradiation. Therefore, if many more ideal targeting agents are developed in the future, this treatment might be used more commonly. As many agents such as I-131, Sr-89, and Ra-223 are available for treating bone metastasis, the combined use of other treatments such as high precision radiotherapy, bisphosphonates, hormonal agents, and molecular targeted agents should be investigated.