• Title/Summary/Keyword: surface energy effects

Search Result 1,241, Processing Time 0.021 seconds

Dosimetric Characteristics of Dual Photon Energy Using Independent Collimator Jaws (고에너지 선형가속기의 Independent Collimator를 이용한 비대칭 방사선 조사시 방사선량 결정에 미치는 요인에 관한 연구)

  • Kim Jeung-kee;Choi Young-Min;Lee Hyung-Sik;Hur Won-Joo
    • Radiation Oncology Journal
    • /
    • v.14 no.3
    • /
    • pp.237-244
    • /
    • 1996
  • Purpose : The accurate dosimetry of independent collimator equipped for 6MV and 15MV X-ray beam was investigated to search for the optimal correction factor. Materials and Methods : The field size factors, beam quality and dose distribution were measured by using 6MV, 15MV X-ray Field size factors were measured from $3{\times}3cm^2$ to $35{\times}35cm^2$ by using 0.6cc ion chamber (NE 2571) at Dmax. Beam qualities were measured at different field sizes, off-axis distances and depths. Isodose distributions at different off-axis distance using $10\times10cm^2$ field were also investigated and compared with symmetric field. Result: 1) Relative field size factors was different along lateral distance with maximum changes in $3.1\%$ for 6MV and $5\%$ for 15MV. But the field size factors of asymmetric fields were identical to the modified central-axis values in symmetric field, which corrected by off-axis ratio at Dmax. 2) The HVL and PDD was decreased by increasing off-axis distance. PDD was also decreased by increasing depth For field size more than $5{\times}cm^2$ and depth less than 15cm, PDD of asymmetric field differs from that of symmetric one ($0.5\~2\%$ for 6MV and $0.4\~1.4\%$ for 15MV). 3) The measured isodose curves demonstrate divergence effects and reduced doses adjacent to the edge close to the flattening filter center was also observed. Conclusion . When asymmetric collimator is used, calculation of MU must be corrected with off-axis and PDD with a caution of underdose in central axis.

  • PDF