Beach J L, Mendiondo M S, Mendiondo O A
Med Phys. 1987 Jan-Feb;14(1):140-4. doi: 10.1118/1.596101.
The inclusion of air-filled spaces in treatment fields creates a potential dosimetric problem due to the loss of charged particle equilibrium near the air-tissue interface. We have used a simulated larynx phantom and a small buildup/extrapolation chamber to compare the magnitude and spatial extent of underdosing and overdosing at the distal surface for two linear accelerators (10- and 6-MV x-rays) and a cobalt-60 machine. Surface doses were compared to doses measured in a similar but homogeneous phantom to give observed/expected ratios (O/E), which were greater than 1.0 for large field sizes and less than 1.0 for small field sizes on all machines. The minimum field sizes which produce no surface underdosing for a simulated 2-cm-diam larynx are roughly 7 X 7 cm for 10-MV x-rays, 6 X 6 cm for 6-MV x-rays, and 5 X 5 cm for cobalt-60. In addition, the depth over which underdosing occurs is seen to increase with increasing energy.
由于在空气 - 组织界面附近带电粒子平衡的丧失，在治疗野中包含气腔会产生潜在的剂量学问题。我们使用了一个模拟喉模体和一个小型建成/外推电离室，来比较两台直线加速器（10兆伏和6兆伏X射线）以及一台钴 - 60治疗机在远端表面的剂量不足和剂量过量的程度及空间范围。将表面剂量与在类似但均匀的模体中测得的剂量进行比较，得出观察值/预期值比值（O/E），在所有机器上，大野尺寸时该比值大于1.0，小野尺寸时小于1.0。对于模拟的直径2厘米的喉部，10兆伏X射线产生无表面剂量不足的最小野尺寸约为7×7厘米，6兆伏X射线为6×6厘米，钴 - 60为5×5厘米。此外，剂量不足发生的深度随能量增加而增加。
