Sibata C H, Mota H C, Beddar A S, Higgins P D, Shin K H
MetroHealth Medical Center, Radiology Department, Cleveland, OH 44109.
Phys Med Biol. 1991 May;36(5):621-31. doi: 10.1088/0031-9155/36/5/005.
Correction is necessary to account for the detector size in clinical dosimetry of photon and electron beams. This correction is due to the absorbed dose gradient present in a finite-size detector. Further corrections are necessary when the detector and phantom materials are not the same. These corrections are due to the perturbation in the charged-particle fluence. Generally these corrections are applied to measurements along the central axis of the beam. Cross beam profile measurements, however, are not usually corrected for detector size. The ionization profile is also usually assumed to be equivalent to the absorbed dose profile. We have corrected the ionization chamber size effect by two approaches: extrapolation of measurements to zero detector size and deconvolution of measurements using a simple model for the detector response function. We have measured absorbed dose profiles to water using a small water-equivalent plastic scintillation detector. Film profile measurements were also studied. The ionization profile corrected for detector size and absorbed dose profile were not equal, probably due to loss of charged-particle equilibrium in the beam edges. For ionization chamber measurements, knowledge of the charged-particle spectrum is needed to convert ionization to absorbed dose to water. This is not necessary for relative absorbed dose measurements under charged-particle equilibrium. Film has been shown to be a straightforward and reliable method for cross beam profile measurements.
在光子和电子束的临床剂量测定中，考虑探测器尺寸进行校正很有必要。这种校正是由于有限尺寸探测器中存在吸收剂量梯度。当探测器和模体材料不同时，还需要进一步校正。这些校正是由于带电粒子注量的扰动。通常这些校正应用于沿射束中心轴的测量。然而，射束剖面测量通常不针对探测器尺寸进行校正。电离剖面通常也被假定等同于吸收剂量剖面。我们通过两种方法校正了电离室尺寸效应：将测量值外推至探测器尺寸为零以及使用探测器响应函数的简单模型对测量值进行反卷积。我们使用小型水等效塑料闪烁探测器测量了对水的吸收剂量剖面。还研究了胶片剖面测量。经探测器尺寸校正后的电离剖面与吸收剂量剖面并不相等，这可能是由于射束边缘带电粒子平衡的丧失。对于电离室测量，需要了解带电粒子能谱才能将电离转换为对水的吸收剂量。在带电粒子平衡条件下进行相对吸收剂量测量时则无需如此。胶片已被证明是进行射束剖面测量的一种直接且可靠的方法。
