Cheung Joel Y C, Ng Ben K P, Yu K N
Brain Centre, Canossa Hospital, Hong Kong.
Med Phys. 2004 Oct;31(10):2787-91. doi: 10.1118/1.1797531.
Three photon interaction processes, namely, the photoelectric effect, Compton effect, and pair production, can occur when materials with high atomic numbers are irradiated by the high- and low-energy bremsstrahlung photons from a linear accelerator. A dose enhancement, due to the photoelectric effect and pair production, near targets with platinum implants (with a high atomic number) in radiosurgery cannot be predicted by the XKnife radiosurgery treatment planning system. In the present work, Monte Carlo simulations using PRESTA EGS4 were employed to investigate the resulting dose enhancements from 4, 6, and 10 MV energies commonly used in the stereotactic radiosurgery system. Dose enhancements from 32% to 68% were observed close to the platinum implant for the above energies when using a 12.5 mm collimator. Comparatively higher dose enhancements were observed when using smaller collimators. It was found that this dose enhancement increased with beam energy but decreased as beam size increased.
当高原子序数的材料受到来自直线加速器的高能和低能轫致辐射光子照射时，会发生三种光子相互作用过程，即光电效应、康普顿效应和电子对产生。在放射外科手术中，由于光电效应和电子对产生，靠近植入铂（原子序数高）的靶区会出现剂量增强，而XKnife放射外科治疗计划系统无法预测这种情况。在本研究中，使用PRESTA EGS4进行蒙特卡罗模拟，以研究立体定向放射外科系统中常用的4、6和10兆电子伏能量所产生的剂量增强。当使用12.5毫米准直器时，在上述能量下，靠近铂植入物处观察到剂量增强为32%至68%。使用较小准直器时观察到相对较高的剂量增强。研究发现，这种剂量增强随束流能量增加而增加，但随束流尺寸增大而减小。
