Wright A E, Gager L D
Med Phys. 1977 Nov-Dec;4(6):499-502. doi: 10.1118/1.594349.
Initial calibration of a linear accelerator requires physics instruments to measure accurately central axis depth-dose and off-axis data, both in and out of the beam. These data for an 8- MeV unit were first measured using film, a Farmer 0.6-cm3 ion chamber, a 0.3-cm3 ion chamber, and a 0.1-cm3 silicon diode. Both small probes and film gave a high response compared to the Farmer probe, which has a uniform energy response. Measurements with the diode interfaced to an X-Y recorder required only a fraction of the time required with the chambers, minimizing error due to change in machine output, and permitted resolution of isodose lines in the penumbra. However, corrections required at points in depth due to nonuniform energy response of the unshielded diode were laborious. Construction of a partially shielded diode which duplicates the response of the Farmer probe eliminated the necessity for corrections, permitting rapid accumulation of a wide range of depth-dose and off-axis data.
直线加速器的初始校准需要物理仪器精确测量束流中心轴深度剂量和离轴数据，包括束流内和束流外的数据。对于一台8兆电子伏特的设备，这些数据首先使用胶片、一个0.6立方厘米的 Farmer电离室、一个0.3立方厘米的电离室和一个0.1立方厘米的硅二极管进行测量。与具有均匀能量响应的Farmer探头相比，小型探头和胶片都有较高的响应。将二极管连接到X - Y记录仪进行测量所需的时间仅为使用电离室所需时间的一小部分，最大限度地减少了由于机器输出变化而产生的误差，并能分辨半影区的等剂量线。然而，由于未屏蔽二极管的能量响应不均匀，在深度点进行校正很繁琐。构建一个部分屏蔽的二极管，使其具有与Farmer探头相同的响应，消除了校正的必要性，从而能够快速积累广泛的深度剂量和离轴数据。
