リニアックによるStereotactic Radiosurgery—位置決めとポジショニング

Stereotactic Radiosurgery Using a Linear Accelerator (LINAC):Simulation and positioning

I．はじめに

　Leksell^8）により開発されたGamma unitによる動静脈奇形や聴神経腫瘍などの頭蓋内病変に対する治療成績は非常に高く^7,13），近年わが国でもGamma unitを導入する施設が増加している．しかしGamma unitは極めて高額な機器であるため，広く普及することは困難であると考えられる．一方，現在放射線治療機器として広く普及しつつある医用直線加速器（リニアック）を用いた高エネルギーX線によるstereotactic radiosurgeryが考案され，1974年Larsson^6）により紹介された．現在までにリニアックを用いたradiosurgeryに関する多くの報告がある^3,7,12,16）．わが国でもその関心が高まり，多くの施設で照射方法などの治療技術が検討されている^15）．

　リニアックによるradiosurgeryを行うためには1）治療装置の精度，2）高エネルギーX線ナロー・ビームを得るためのコリメータの作製，3）高エネルギーX線ナロー・ビームの測定方法，4）高エネルギーX線ナロー・ビームが一点に焦点を結ぶような照射方法（リニアック本体と治療台の回転方法など），5）患者頭部の固定方法，6）病巣の位置決め方法などの多くの問題が検討されなければならない．

Stereotactic radiosurgery using a Gamma unit obtains good results for small intracranial diseases, arteriovenous malformation (AVM) and acoustic neuri-noma.

In stereotactic radiosurgery using a linear accelerator (LINAC), many fundamental problems are to be solved. 1) accuracy of a LINAC, 2) making the collima-tors for high energy X-ray narrow beams, 3) dosimetry for high energy X-ray narrow beams, 4) irradiation methods for stereotactic radiosurgery, 5) fixtation of a patient's head and 6) simulation of a target.

The usefulness of our method for simulation of a target and for positioning for radiosurgery was investi-gated. High energy X-ray narrow beams obtained with the collimators for narrow beams (field sizes: 9mmφ, 18mmφ and 27mmφ) satisfy clinical requirements for stereotactic radiosurgery, as indicated by dose profiles and isodose curves.

No dosimetry method for high energy X-ray narrow beams has been established yet. Of the main irradiation methods for stereotactic radiosurgery, the method using multiple non-coplanar converging arcs needs no drastic reconstruction for use with LINAC.

A patient's head was completely fixed by the stereotactic frame (Patil stereotaxic system or Leksell micro-stereotactic system).

Simulation of a target was performed under CT scan. On CT image, the center of a target was determined and the three-dimensional coodinate on the stereotactic head frame target was settled so that the target would be reached.

The three-dimensional coordinate for the target was coincided with the isocenter of a LINAC by the laser beams of three-directional pointers. Afterwards, the target position was finely adjusted, by using the target positioner manipulator system. Errors of rotation of the LINAC treatment table can be controled to within 5mm, and accurate positioning for stereotactic radiosurgery was able to be made in a short time.

To discern accurately the area of the irradiated field, the isodose curves constructed by calculation using computers are serviceable in clinical use. Using this method, stereotactic radiosurgery using a LINAC can be easily and safely performed within 2 hours.