Linacを用いたRadiosurgeryの技術の開発

Linac Based Radiosurgery:A technical report

I．はじめに

　Gamma unit（Gamma knife）は極めて良い線量分布が得られるので，近年多くの人の関心を集めている8）．この装置は頭の周りに201個のコバルト60の粒を配置して，頭の中の1点を集中的に照射するもので，これまでは，主として，動静脈奇形などの良性疾患の治療に用いられた．治療の際は，定位脳手術の際に用いられる固定枠を手術で頭に取り付け，一回で大量の照射をする．この方法の難点は，機械の価格が高いこと，Co−60を使う為，数年毎に線源を交換すること，放射線管理が面倒であること等である．われわれは，この難点を解決するために，linear accelerator（linac）を用いてgamma unitと同程度の線量分布が得られることを確認し，これを用いて多数に分割した照射で脳腫瘍を対象に，放射線治療を行う技術を開発した．以下にこの方法の概要を述べると共に，この方法をThomsonら^10）にならい，Stereo—tactic Multiple Arc Radiotherapy（SMART）とよぶことを提唱する．

A method for highly dose-localized irradiation using a linear accelerator (linac) for a brain tumor has been developed. The method requires a linac, a computed tomography (CT) system, a CT simulator, and a treat-ment planning system for radiotherapy, with which most major radiotherapy centers are equipped. To im-mobilize a patient during irradiation, a custom-made de-vice made of synthetic material which became flexible with heating was used. With the CT system and the CT simulator the target was identified and geometrical data for positioning the tumor at a point to which x-ray beams were directed (an isocenter of the linac) were obtained. By rotating a treatment couch it was made possible for the x-ray source to rotate around the isocenter on multiple planes. Dose distributnm obtained with this method was compared to that of the gamma unit and found comparable. Since the method requires no invasive procedure, it appears suitable for treatment, with fractionated irradiation, of malignant tumors.