Ottawa Carleton Institute of Physics, Carleton University, Ottawa, Ontario K1S 5B6, Canada.
Med Phys. 2010 Feb;37(2):461-5. doi: 10.1118/1.3276735.
Plane-parallel chambers are recommended by dosimetry protocols for measurements in (especially low-energy) electron beams. In dosimetry protocols, the replacement correction factor P(repl) is assumed unity for "well-guarded" plane-parallel chambers in electron beams when the front face of the cavity is the effective point of measurement. There is experimental evidence that ion chambers which are not well-guarded (e.g., Markus) have nonunity P(repl) values. Monte Carlo simulations are employed in this study to investigate the replacement correction factors for plane-parallel chambers in electron beams.
Using previously established Monte Carlo calculation methods, the values of P(repl) are calculated with high statistical precision for the cavities of a variety of plane-parallel chambers in a water phantom irradiated by various electron beams. The dependences of the values of P(repl) on the beam quality, phantom depth, as well as the guard ring width are studied.
In the dose fall-off region for low-energy beams, the P(repl) values are very sensitive to depth. It is found that this is mainly due to the gradient effect, which originates from the fact that the effective point of measurement for many plane-parallel chambers should not be at the front face of the cavity but rather shifted toward the center of the cavity by a fraction of a millimeter. Using the front face of the cavity as the effective point of measurement, the calculated values of P(repl) at d(ref) are not unity for some well-guarded plane-parallel chambers. The calculated P(repl) values for the Roos chamber are close to 1 for all electron beams. The calculation results for the Markus chamber are in good agreement with the measured values.
The appropriate selection of the effective point of measurement for plane-parallel chambers in electron beams is an important issue. If the effective point of measurement is correctly accounted for, the P(repl) values would be almost independent of depth. Both the guard ring width and the ratio of the collecting volume diameter to the cavity thickness can influence the values of P(repl) For a diameter to thickness ratio of 5 (e.g., NACP02 chamber), the guard width has to be 6 mm for the chamber to be considered as well-guarded, i.e., have a P(repl) value of 1.00.
剂量学协议推荐在（特别是低能）电子束中使用平行平板室进行测量。在剂量学协议中，当腔的前表面是有效测量点时，对于“保护良好”的平行平板室，假设空腔替换校正因子 P(repl)为单位。有实验证据表明，未受良好保护的电离室（例如 Markus）的 P(repl)值不为单位。本研究采用蒙特卡罗模拟方法研究电子束中平行平板室的替换校正因子。
使用先前建立的蒙特卡罗计算方法，在水模中用各种电子束照射各种平行平板室的腔室，以高精度统计计算 P(repl)的值。研究了 P(repl)值随束质、体模深度以及保护环宽度的依赖性。
在低能束的剂量下降区域，P(repl)值对深度非常敏感。研究发现，这主要是由于梯度效应，该效应源于这样一个事实，即许多平行平板室的有效测量点不应在腔室的前表面，而是应该通过几毫米的分数向腔室的中心移动。如果将腔室的前表面用作有效测量点，则对于某些保护良好的平行平板室，在 d(ref)处计算出的 P(repl)值不为单位。对于所有电子束，Roos 室的计算出的 P(repl)值接近 1。对于 Markus 室，计算结果与测量值吻合良好。
在电子束中正确选择平行平板室的有效测量点是一个重要问题。如果正确考虑了有效测量点，P(repl)值将几乎与深度无关。保护环宽度和收集体积直径与腔室厚度的比值都可以影响 P(repl)值。对于直径与厚度比为 5（例如，NACP02 室）的情况，为了将腔室视为保护良好，即具有 P(repl)值为 1.00，保护环的宽度必须为 6 毫米。
