Purpose: To investigate the influence of changes in α/β ratio (range 1.5–3 Gy) on iso-effective doses, with varying treatment time, in spinal cord and central nervous system tissues with comparable radio-sensitivity. It is important to establish if an α/β ratio of 2 Gy, the accepted norm for neuro-oncology iso-effect estimations, can be used.

Methods: The rat spinal cord irradiation data of Pop et al. provided ED₅₀ values for radiation myelopathy for treatment times that varied from minutes to ∼6 days. Analysis using biphasic repair kinetics, allowing for variable dose-rates, provided the best fit with repair half-times of 0.19 and 2.16 hr, each providing ∼50% of overall repair; with an α/β ratio 2.47 Gy (CI 1.5–3.95 Gy). Using the above data set, graphical methods were used to investigate changes in the repair parameters for differing fixed α/β ratios between 1.5 and 3.0 Gy. Two different intermittent dose delivery equations were used to evaluate the implications in a radiosurgery setting.

Results: Changes in the α/β ratio (1.5–3.0 Gy) have a minor effect on equivalent doses for radiation myelopathy for treatment durations of a few hours. Changing the α/β value from 2 Gy to 2.47 Gy, modified equivalent single doses by < 1% when overall treatment times ranged from 0.1 to 5.0 hr. Significant changes were only found for treatment times longer than 5–10 hr. These two α/β ratios were also compared in a practical radiosurgery situation, using two different models for estimating BED, again there was no significant loss of accuracy.

Conclusions: It is reasonable to use an α/β ratio of 2 Gy for CNS tissue, with the same repair half-times as published in the original publication by Pop et al., in situations where the assessment of the BED in radiosurgery is used with other form of radiotherapy. In radiosurgery, the variation in BED with treatment duration (for a fixed physical dose) is very similar, but absolute BED values depend on the α/β value. In radiosurgery, clinical recommendations obtained using BED calculations using the originally proposed α/β ratio of 2.47 Gy are still appropriate. For calculations involving a combination of radiosurgery and other modalities, such as fractionated radiotherapy, it would be appropriate in all cases to apply a value of 2 Gy, the accepted norm in neuro-oncology, without significant loss of accuracy in the radio-surgical component. This may have important applications in retreatment situations.

目的：研究在具有类似放射敏感性的脊髓和中枢神经系统组织中，随着治疗时间的变化，α/β比（1.5-3 Gy范围）的变化对等效剂量的影响。重要的是要确定是否可以使用2 Gy的α/β比（神经肿瘤等效应估计的公认标准）。

方法： Pop等人的大鼠脊髓照射数据。提供了放射性脊髓病的ED ₅₀值，治疗时间从数分钟到约6天不等。使用双相修复动力学进行分析，并考虑到不同的剂量率，以0.19和2.16 hr的修复半衰期提供最佳的拟合，每次修复可提供约50％的整体修复；α/β比为2.47 Gy（CI 1.5–3.95 Gy）。使用以上数据集，使用图形方法研究修复参数在1.5和3.0 Gy之间的不同固定α/β比的变化。使用两个不同的间歇剂量输送方程式来评估放射手术环境中的影响。

结果： α/β比值（1.5-3.0 Gy）的变化对放射性脊髓病的等效剂量影响很小，持续时间为数小时。将α/β值从2 Gy更改为2.47 Gy，当总治疗时间在0.1到5.0小时之间时，等效单剂量修改<1％。仅在治疗时间超过5-10小时的情况下才发现明显变化。在实际的放射外科手术情况下，还使用两种不同的模型估算BED，比较了这两个α/β比率，再次没有显着的准确性损失。

结论：对于中枢神经系统组织，使用2 Gy的α/β比率是合理的，并且在与放射线外科中的BED评估结合使用的情况下，修复时间与Pop等人在原始出版物中发表的修复时间相同。放射疗法的形式。在放射外科中，BED随治疗持续时间（对于固定的物理剂量）的变化非常相似，但是绝对BED值取决于α/β值。在放射外科中，使用BED计算并使用最初建议的2.47 Gy的α/β比获得的临床建议仍然合适。对于涉及放射外科手术和其他方式（例如分段放射疗法）相结合的计算，在所有情况下都应应用神经肿瘤学公认的2 Gy值，而不会显着降低放射外科成分的准确性。