Reduction of Electron Contamination Using a Filter for 6MV Photon Beam

6MV 광자선에서 전자오염 감소에 관한 연구

  • Lee, Choul-Soo (Department of Radiation Oncology Kosin Medical College and Medical Center) ;
  • Yoo, Myung-Jin (Department of Radiation Oncology Kosin Medical College and Medical Center) ;
  • Yum, Ha-Yong (Department of Radiation Oncology Kosin Medical College and Medical Center)
  • 이철수 (고신의료원 방사선 종양학과) ;
  • 유명진 (고신의료원 방사선 종양학과) ;
  • 염하용 (고신의료원 방사선 종양학과)
  • Published : 1997.06.01

Abstract

Purpose : Secondary electrons generated by interaction between Primary X-rar beam and block tray in megavoltage irradiation, result in excess soft radiation dose to the surface layer To reduce the surface dose from the electron contamination, electron filters were attached under the tray when a customized block was used. Materials and Methods : Cu, Al or Cu/Al combined Plate with different thickness was used as a filter and the surface dose reduction was measured for each case. The measurement to find optimal filter was performed with $10m\times10cm$ field size and 78.5cm source to surface distance. The measurement points are positioned with 2mm intervals from surface to maximum build-up point. To acquire the effect of field size dependence on optimal electron filter, the measurement was performed from $4cm\times4cm\;to\;25cm\times25cm$ field sizes. Results : The surface dose was slowly increased by increasing irradiation field but rapidly increased beyond $15cm\times15cm$ field size. Al plate was found to be inadequate filter because of the failure to have surface dose kept lowering than the dose of deep area. Cu 0.5mm plate and Cu/Al=0.28mm/1.5mm combined plate were found to be optimal filters. By using these 2 filters, the absorbed dose to the surface layer was effectively reduced by $5.5\%,\;11.3\%,\;and\;22.3\%$ for the field size $4cm\times4cm,\;10m\times10cm,\;and\;25cm\times25cm$, respectively. Conclusion : The surface dose attributable to electron contamination had a dependence on field size. The electron contamination was increased when tray was used. Specially the electron contamination in the surface layer was greater when the larger field was used. 0.5mm Cu Plate and Cu/Al=0.28mm/15mm combined plates were selected as optimal electron filters. When the optimal electron filter was attached under the tray, excessive surface dose was decreased effectively The effect of these electron filters was better when a larger field was used.

목적 : 광자선과 블록을 얻는 트레이와의 상호작용에 의해 생성된 2차 전자는 피하층에 과도 한 선량증가의 원인이 된다. 이런 전자오염으로 인한 표면선량을 감소시키기 위해 트레이 아래 전자필터를 부착하는 것이 유용하다. 대상 및 방법 :두께가 다른 Cu판, Al판 그리고 Cu/Al 복합판을 사용하여 전자오염에 의한 표면선량을 감소시키는 효과를 측정하였다. 최적의 필터를 찾기 위해서 $10m\times10cm$ 조사야와 SSD 78.5cm의 기하학적 모형으로 표면에서부터 최대선량 지점까지 2mm 간격으로 흡수선량을 측정하였다. 최적필터의 조사야 의존성을 구하기 위해서 $4cm\times4cm$에서부터 $25cm\times25cm$까지 조사면적을 변화시켜가며 흡수선량을 측정하였다 결과 : 표면선량은 조사면적을 증가시키면 서서히 증가하였고 $15cm\times15cm$ 조사면적 이상에 서는 표면선량이 급속히 증가하였다. Al판을 사용했을 경우 표면선량이 최대선량 지점의 선량보다 낮게 유지되지 않으므로 Al판은 전자필터로는 부적합하였으며 0.5mm Cu판과 Cu/Al=0.28mm/1.5mm 복합판이 최적 전자필터로 결정되었다. 두가지 최적필터의 경우 $4cm\times4cm$ 조사면에서는 표면선량을 $5.5\%$(tray 사용할때 표면선량에서 filter 사용할때 표면선량을 뺀 것) 낮출 수 있었고 $10cm\times10cm$에서는 $11.3\%,\;25cm\times25cm$에서는 $21.3\%$로 낮출 수 있었다. 결론 : 전자오염에 기인한 표면선량은 조사면적에 의존하였고 트레이를 설치했을때 표면의 전자오염은 많았고 특히 조사면적이 클 때 표면의 전자오염은 더욱 증가하였다 표면 및 build-up영역의 선량 특성을 측정한 결과 0.5mm Cu판과 Cu/Al=0.28mm/1.5mm 복합판이 최적 필터로 결정되었다. 트레이 밑에 최적 필터를 부착하면 표면선량을 유효하게 감소시킬 수 있었고 필터의 효과는 조사면적이 클 때 더욱 좋았다.

Keywords