Attix F H, Lopez F, Owolabi S, Paliwal B R
Med Phys. 1983 May-Jun;10(3):301-6. doi: 10.1118/1.595305.
All radiotherapy photon beams are accompanied to some extent by secondary electrons which originate in interactions with source hardware, collimator, shadow tray, and/or the air through which the beam passes. Skin sparing, the shape of the dose buildup curve, and the depth of the dose maximum are all influenced by this electron "contamination." The present study of a 60Co source employs a flat ion chamber to measure dose buildup curves in polystyrene at source distances of 72 to 200 cm, with an open beam or a filter of Lucite, Cu, Pb-loaded acrylic, or Ba- or Pb-loaded nonbrowning glass placed 57 cm from the source, using 5 X 5, 20 X 20, and 35 X 35-cm2 beams as defined at 80 cm SSD. The effect of electron generation in the air was studied by placing a He-gas-filled plastic bag in the beam. A value of about 12% is estimated for the lowest relative dose obtainable with a polystyrene phantom in a "clear" 60Co gamma-ray beam of 1-cm diameter.
所有放疗光子束在一定程度上都伴随着次级电子，这些次级电子源自与源硬件、准直器、遮挡托盘以及/或者光束穿过的空气的相互作用。皮肤 sparing、剂量累积曲线的形状以及剂量最大值的深度均受这种电子“污染”的影响。本项关于钴 - 60 源的研究采用扁平电离室，在源距离为 72 至 200 厘米的聚苯乙烯中测量剂量累积曲线，使用开放光束或者在距离源 57 厘米处放置璐彩特、铜、含铅丙烯酸或含钡或含铅的不泛黄玻璃滤过器，使用在 80 厘米源皮距（SSD）下定义的 5×5、20×20 和 35×35 平方厘米的光束。通过在光束中放置一个充氦气的塑料袋来研究空气中电子产生的影响。对于直径为 1 厘米的“纯净”钴 - 60γ射线束，用聚苯乙烯模体可获得的最低相对剂量估计值约为 12%。
