Steel G G, Peacock J H
Radiotherapy Research Unit, Institute of Cancer Research, Sutton, Surrey, UK.
Radiother Oncol. 1989 May;15(1):63-72. doi: 10.1016/0167-8140(89)90119-9.
There is now good evidence that the radiosensitivity of human tumour cells varies form one tumour type to another, and that the steepness of the initial part of the cell survival curve correlates with clinical radioresponsiveness. Studies at low dose rate allow differences between tumour cells to be seen more clearly. Current mathematical models of radiation cell killing include two components: a linear (i.e. exponential "alpha-component") and a bending component ("beta-component"). Repair of radiation damage affects only the beta-component. Among the 17 human tumour cell lines that we have studied, the average surviving fraction at 2 Gy due to the alpha-component is 0.44 and that due to the beta-component is 0.88. The beta-effect at 2 Gy appears to be similar in radiosensitive and radioresistant tumours; thus among radiosensitive tumours where the survival due to the alpha-component is below 0.3 the beta-effect makes a very small contribution to overall radiosensitivity in the low-dose region. Steep cell survival curves may appear straight but still be consistent with a modest beta-value. The overall effect of many small fractions will be to amplify the dominance of the alpha-component. The beta-effect is then unimportant because repair will be almost complete. Repair inhibitors may however change this situation, reducing recovery, increasing the beta-effect, and thereby increasing sensitivity. But in the absence of such inhibitors the differences between radiosensitive and radioresistant tumours must be looked for not in repair capacity but in the nature of the alpha-component. The most radiocurable tumours do not have low beta-values; they have higher alpha-values than the less curable tumours.
现在有充分证据表明，人类肿瘤细胞的放射敏感性因肿瘤类型而异，并且细胞存活曲线初始部分的斜率与临床放射反应性相关。低剂量率研究能更清楚地观察到肿瘤细胞之间的差异。目前辐射细胞杀伤的数学模型包括两个部分：线性部分（即指数“α部分”）和弯曲部分（“β部分”）。辐射损伤的修复仅影响β部分。在我们研究的17种人类肿瘤细胞系中，2 Gy时α部分导致的平均存活分数为0.44，β部分导致的平均存活分数为0.88。2 Gy时的β效应在放射敏感和放射抗性肿瘤中似乎相似；因此，在α部分导致的存活分数低于0.3的放射敏感肿瘤中，β效应在低剂量区域对总体放射敏感性的贡献非常小。陡峭的细胞存活曲线可能看起来是直线，但仍可能与适度的β值一致。许多小剂量分割的总体效果将是放大α部分的主导作用。此时β效应不重要，因为修复几乎是完全的。然而，修复抑制剂可能会改变这种情况，减少修复，增加β效应，从而增加敏感性。但在没有此类抑制剂的情况下，放射敏感和放射抗性肿瘤之间的差异不应从修复能力中寻找，而应从α部分的性质中寻找。最可放射治愈的肿瘤并非具有低β值；它们的α值比较难治愈的肿瘤更高。
