Amols H I, Zellmer D L
Med Phys. 1984 May-Jun;11(3):247-53. doi: 10.1118/1.595499.
Experimental techniques have been developed for obtaining microdosimetric spectra on a hospital-based linear accelerator. Teletherapy beams of 10 and 15 MeV bremsstrahlung x rays from a Varian Clinac-18 and Clinac-20, respectively, have been produced at ultralow dose rates (50-200 microGy/h) which enables direct measurements of lineal energy distributions with a conventional Rossi-type gas proportional counter. Extensive measurements have been made to insure that the dosimetric properties of these low dose rate beams are nearly identical to those produced under high dose rate clinical conditions. Analytical procedures have been developed to correct measured lineal energy spectra for pileup caused by the low duty factor of the linear accelerator. The lineal energy spectra of these megavoltage beams differ significantly from Co-60, with dose averaged lineal energies (yD) being 20%-30% lower than for Co-60. Although such differences may not be important at clinical doses, the theory of dual radiation action does predict a lower biological effectiveness for these beams at very low dose levels.
已经开发出实验技术，用于在医院的直线加速器上获取微剂量谱。分别在瓦里安Clinac - 18和Clinac - 20上产生了能量为10 MeV和15 MeV的轫致辐射X射线的远距离治疗束，其剂量率极低（50 - 200微戈瑞/小时），这使得能够使用传统的罗西型气体正比计数器直接测量线能量分布。已经进行了广泛的测量，以确保这些低剂量率束的剂量学特性与高剂量率临床条件下产生的束几乎相同。已经开发出分析程序，用于校正由直线加速器的低占空因数引起的堆积对测量的线能量谱的影响。这些兆伏级束的线能量谱与钴 - 60有显著差异，剂量平均线能量（yD）比钴 - 60低20% - 30%。尽管这种差异在临床剂量下可能并不重要，但双辐射作用理论确实预测在非常低的剂量水平下这些束的生物有效性较低。
