60Co γ선과 10MV X 선의 조사면 밖의 선량분포에 관한 연구

A Study on Dose Distribution outside Co-60γ Ray and 10 MV X Ray Fields

The peripheral dose, defined as the dose outside therapeutic photon fields, which is responsible for the functional damage of the critical organs, fetus, and radiation-induced carcinogenesis, has been investigated for Coγ ray and 10 MV Xray. It was measured by silicon diode controlled by semiautomated water phantom without any shielding or with lead plate of HVL thickness put horizontally or vertically to shield stray radiations. Authors could obtain following results. 1. The peripheral dose was larger than 0.7% of central axis maximum dose even at 20cm distance from field margin. That is clinically significant, so it should be redused. 2. Even for square fields of 10 MV Xray, radial peripheral dose distribution did not coincide with transverse distribution, because of the position for collimator jaw. 3. Between surface and dm, the peripheral dose distributions show a pattern of the dose distribution of electron beams and the maximum dose was approximately propotrional to the length of a side of square field. 4. The peripheral doses depended on radiation quality, field size, distance from field margin and depth in water. Distance from field margin was the most important factor. 5. Except for near surface, the peripheral dose from phantom was approximately equal to that from therapy unit. 6. To reduce the surface dose outside fields, therapist should shield stray radiations from therapy unit by lead plate of at least on HVL for 10 MV X-ray and by bolus equivalent to tissue of 0.5cm thickness for 60CO. 7. To reduce the dose at depth deeper than dm, it is desirable to shield stray radiations from therapy unit by lead.