We describe a case involving a 75-year-old woman presenting with a femur fracture 10 years after radiation therapy for metastatic breast cancer, which developed in the right femur. The lesion showed complete response with bone healing following radiation therapy; however, the patient sustained a femur fracture ten years later. Histological examination of the specimens obtained from the lesion revealed features of radiation osteonecrosis, but there was no histological evidence of tumor. To our knowledge, there has been no reported case of pathological fracture ten years after radiation therapy from radiation osteonecrosis rather than progression of the metastatic lesion. Late complications of radiation therapy should be considered with care, even when metastatic lesions demonstrate complete response to treatment.