[en] This paper summarizes radiation therapy in the treatment of bone metastases from breast cancer. Bone metastasis occurs in approximately 70% of breast cancer patients, and the goals of radiation therapy for bone metastasis are: palliation of pain, prevention and treatment of neuropathic symptoms, and prevention of pathologic fractures. The prognosis of bone metastasis from breast cancer is known to be better than that of bone metastasis from other solid tumors. Local-field radiation, hemibody (or wide-field) radiation, and systemic radionuclide treatment are the major methods of radiation therapy for pain palliation. Although many studies have shown that breast cancer is more responsive to radiation therapy for pain palliation than other solid tumors, some studies found no significant difference. Local-field radiation therapy, which includes multi-fraction irradiation and single-fraction irradiation, is currently the most generally used method of radiotherapy for pain palliation. Pain palliation has been reported to be achieved in approximately 80% to 90% of patients treated with local-field external beam irradiation. Three types of multi-fraction irradiation therapy are administered depending on the prognosis: high-dose fraction irradiation (36-50 Gy/12-25 Fr/2.4-5 wk), short-course irradiation (20-30 Gy/10-15 Fr/2-3 wk), and ultra-short-course irradiation (15-25 Gy/2-5 Fr/1 wk). The most common irradiation schedule is 30 Gy/10 Fr/2 wk. Although many reports indicate no significant difference in pain palliation according to the dose, the percentage of patients who show a complete cure is significantly higher in those treated with doses of 30 Gy or more, and thus the total irradiation dose should be at least 30 Gy. High-dose fraction irradiation is indicated for patients with an expected survival time of 6 months or more while short-course or single-fraction irradiation is indicated for those with an expected survival time of 3 months or more. Single-fraction irradiation, however, is not in commonly used in Japan, and it has been reported that there is no significant difference in pain palliation between single-fraction irradiation and multi-fraction irradiation. The effectiveness of prophylactic use of single-fraction irradiation to prevent fractures is unclear. Hemibody (or wide-field) radiation therapy is used to palliate pain from multiple bone metastases or as adjuvant therapy after local-field irradiation, but it has not been widely used in Japan because of adverse reactions. Further investigation is needed to determine the optimal method in terms of irradiation dose, timing of combination therapy, and reduction of adverse reactions. Systemic radionuclide treatment with 89SrCl is used for pain palliation and as adjuvant therapy after local-field irradiation. 89SrCl therapy is considered useful in improving the QOL of breast cancer patients with bone metastasis. Although phase III clinical trials of 89SrCl therapy have been completed in Japan, it will be several more years before clinical use of 89SrCl is approved. The mechanism of pain palliation by radiation therapy has not yet been clarified, however, it cannot be explained by the antitumor effect of irradiation alone. It is suspected that radiation therapy may suppress osteoclast function, the secretion of various cytokines causing pain stimuli, and the proliferation of tumor cells in bone. Clarification of the basic mechanism of pain palliation will be useful in determining the optimal irradiation schedule. Spinal cord compression occurs in about 5% of patients with malignant tumors, and it is particularly frequent in breast cancer, lung cancer, and prostate cancer. It usually requires surgery or radiation therapy. Neuropathic symptoms cannot be treated by radiation therapy alone. Accordingly, surgery is recommended for the purpose of diagnosis and treatment whenever spinal cord compression is suspected. Although steroid therapy is frequently combined with radiation therapy, the neurological effect of this combination is unknown. Early diagnosis and treatment a re therefore most important. The rate of metastases to the skull from breast cancer is high. Metastasis to the base of the skull causes damage to the cranial nerves and gives rise to a variety of clinical manifestations. Radiation therapy is considered to be effective in alleviating such symptoms. Pathologic fractures occur in 10% to 20% of patients with bone metastases, and they are particularly common in patients with osteolytic metastases. Postoperative high-dose multi-fraction irradiation and internal fixation should be considered for the treatment of impending fractures, while local-field irradiation should be used to achieve recalcification. When preparing to use radiation therapy to treat bone metastases from breast cancer it is important to consider the prognosis, to design a treatment schedule that imposes the least burden on patients, and to select the most appropriate method of irradiation according to the goal of treatment. (K.H.)