Leung P M, Johns H E
Med Phys. 1977 Sep-Oct;4(5):441-4. doi: 10.1118/1.594314.
It has been shown that for Cobalt-60 units capable of producing large fields (up to 40 X 40 cm2)at 80 cm, the dose distribution in the buildup region is quite different from that observed with small fields. First, the maximum dose occurs at 1-2 mm instead of 5 mm; and second, the maximum does in some extreme cases is 15%-20% higher than the dose at 5 mm. Thus, the dose is decreased by more than 15%-20% by 3 mm of tissue, suggesting severe electron contamination. In this report, it is shown that this contamination can be best removed by a filter of thickness 0.4 g/cm2 of medium atomic number, placed some 25 cm from the source inside the collimator. For existing units, positioning the filter inside the collimator is not practical. Acceptable results, however, can be obtained by placing the filter just below the collimator.
已经表明，对于能够在80厘米处产生大射野（高达40×40平方厘米）的钴 - 60治疗机，其剂量建成区的剂量分布与小射野观察到的情况有很大不同。首先，最大剂量出现在1 - 2毫米处而非5毫米处；其次，在某些极端情况下，最大剂量比5毫米处的剂量高15% - 20%。因此，3毫米组织可使剂量降低超过15% - 20%，这表明存在严重的电子污染。在本报告中表明，这种污染可以通过一块厚度为0.4克/平方厘米的中等原子序数滤过板最佳地去除，该滤过板放置在准直器内距源约25厘米处。对于现有的治疗机，将滤过板放置在准直器内不切实际。然而，通过将滤过板放置在准直器下方，可以获得可接受的结果。
