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Background Spine is the most common site of bone metastases in patients with cancer.Conventional extal beam radiotherapy lacks precision to allow delivery of large fraction radiation but simultaneously limit the dose to spinal cord.The purpose of this study was to evaluate the safety and efficacy of CyberKnife(R) radiation therapy for spinal metastases.Methods Seventy-three lesions in 62 patients treated with CyberKnife radiotherapy from September 2006 to June 2010 for spinal metastases were retrospectively reviewed.Thirteen tumors in 12 patients had received prior radiation.Patients were followed clinically and radiographically for at least 12 months or until death.In all patients,the spinal cord and thecal sac were contoured for dose-volume constraints,and maximum doses to 0.1,0.5,1,2,and 5-ml volumes were analyzed.Results Using the CyberKnife System,20-48 Gy in one to five fractions for unirradiated patients,and 21-38 Gy in one to five fractions for the previously irradiatied patients,were delivered.Median 2-Gy normalized Biological Equvaient Dose (nBED) of unirradiated targets and irradiated targets were 49.6 Gy10/2 (range,31.25-74.8 Gy10/2) and 46.9 Gy10/2 (range,29.8-66 Gy10/2),respectively.With a median follow-up of 9.4 months (range,2.5-45 months),twenty-nine patients (46.7％) were alive,whereas the others died of progressive disease.Fifty-six patients (93.3％) reported complete or partial reduction of pain after CyberKnife radiotherapy at one-month follow-up,17 patients (28.3％) reported some degree of pain relief after first fraction of the treatment course.Two patients experienced local recurrence at fifth and ninth months postradiotherapy.Median maximum nBED for spinal cord and thecal sac of naive targets were 68.6 Gy2/2 (range,8.3-154.5Gy2/2) and 83.5 Gy2/2 (range,10.5-180.5 Gy2/2),respectively.Median maximum nBED for spinal cord and thecal sac for the re-irradiated targets were 58.6 Gy2/2 (range,17.7-140 Gy2/2) and 70.5 Gy2/2 (range,21.7-141.3 Gy2/2),respectively.No patient developed radiation related myelopathy during the follow-up period.Conclusion Cyberknife radiotherapy is clinically effective and safe for spinal metastases,even in previously irradiated patients.