McCullough E C, Gortney J, Blackwell C R
Division of Radiation Oncology, Mayo Clinic/Foundation, Rochester, Minnesota 55905.
Med Phys. 1988 Jul-Aug;15(4):621-3. doi: 10.1118/1.596216.
The depth dependence (up to 25 cm) of the in-phantom wedge transmission factor (WTF) has been determined for three medical linear accelerator x-ray beams with energies of 4, 6, and 10 MV containing 15 degrees-60 degrees (nominal) brass wedges. All measurements were made with a cylindrical ionization chamber in water, for a field size of 10 X 10 cm2 with a source-skin distance of 80 or 100 cm. We conclude that, for the accelerators studied, the WTF factor at depth is less than 2% different from that determined at dmax (for the nominal wedge angles and photon energies studied) unless the depth of interest is greater than 10 cm. Up to the maximum depth studied (25 cm) the relative wedge factor--that is, wedge factor at depth compared to that determined at dmax--was about equal to or less than 1.02 for the 15 degrees and 30 degrees wedges and any of the photon beam energies studied. For the seldom utilized combination of a nominal wedge angle in excess of 45 degrees with a depth greater than 10 cm, the WTF at depth can differ from the WTF determined at dmax, by up to 5%. Since the wedge transmission factor is reflective of relative percent dose data, our results also indicate that it is in error to use open field percent depth doses for certain combinations of wedge angle, photon energy, and depth.
已针对能量分别为4兆伏、6兆伏和10兆伏的三种医用直线加速器X射线束，确定了体模内楔形滤过器传输因子（WTF）随深度（最大25厘米）的变化情况，这些射线束包含15度至60度（标称）的黄铜楔形滤过器。所有测量均在水中使用圆柱形电离室进行，射野尺寸为10×10平方厘米，源皮距为80或100厘米。我们得出结论，对于所研究的加速器，在感兴趣深度处的WTF因子与在剂量最大值（dmax）处确定的值相差不到2%（对于所研究的标称楔形角度和光子能量），除非感兴趣深度大于10厘米。在所研究的最大深度（25厘米）范围内，对于15度和30度楔形滤过器以及任何所研究的光子束能量，相对楔形因子（即深度处的楔形因子与在dmax处确定的值相比）约等于或小于1.02。对于标称楔形角度超过45度且深度大于10厘米这种很少使用的组合，深度处的WTF可能与在dmax处确定的WTF相差高达5%。由于楔形滤过器传输因子反映了相对百分剂量数据，我们的结果还表明，对于楔形角度、光子能量和深度的某些组合，使用开放射野百分深度剂量是错误的。
