Strategies of Spinal Tumors Involving the Vertebral Artery : Usefulness of Parent Artery Occlusion before Tumor Resection, and Indication of Balloon Test Occlusion before Parent Artery Occlusion 椎骨動脈を巻き込んだ腫瘍に対する治療戦略―腫瘍摘出前の親動脈閉塞術の有用性と血管閉塞テスト施行の選択について―