Mijnheer B J, Chin L M
Joint Center for Radiation Therapy, Harvard Medical School, Boston, Massachusetts 02115.
Med Phys. 1989 Jan-Feb;16(1):119-22. doi: 10.1118/1.596398.
Exposure rates were adjusted at the National Institute of Standards and Technology (NIST) on January 1, 1986 to take into account more recent values for some physical parameters, mainly in electron stopping power ratios. Exposure calibration factors for 60Co gamma rays Nx will therefore be lowered by 1.1%. Consequently, absorbed dose determinations in high-energy photon beams will be reduced by the same amount if the values for these physical parameters remain unchanged in the American Association of Physicists in Medicine (AAPM) protocol. If the same data base as used at NIST is applied in the AAPM protocol, then Ngas/Nx values, water-air stopping power ratios, and Pwall values will be different. The overall change in absorbed dose determinations using a consistent set of data will be a reduction of 0.8% for 60Co gamma rays and 1.5% for a 20-MV x-ray beam compared to the values before January 1, 1986. Since the net effect is small when different sets of data are applied, the new NIST exposure calibration factors may be used in combination with the AAPM protocol without significant error.
1986年1月1日，美国国家标准与技术研究院（NIST）对照射率进行了调整，以考虑一些物理参数的最新值，主要是电子阻止本领比。因此，60Coγ射线的照射校准因子Nx将降低1.1%。如果这些物理参数在美国医学物理学家协会（AAPM）的协议中保持不变，那么高能光子束中的吸收剂量测定值将降低相同的量。如果在AAPM协议中应用与NIST相同的数据库，那么Ngas/Nx值、水-空气阻止本领比和Pwall值将有所不同。与1986年1月1日之前的值相比，使用一组一致数据进行吸收剂量测定的总体变化是，60Coγ射线降低0.8%，20MV X射线束降低1.5%。由于应用不同数据集时的净效应较小，新的NIST照射校准因子可与AAPM协议结合使用，而不会产生显著误差。
