Technological and Nuclear Institute, Estrada Nacional No 10, 2686-953 Sacavém, Portugal.
Phys Med Biol. 2011 Nov 7;56(21):6919-34. doi: 10.1088/0031-9155/56/21/010. Epub 2011 Oct 7.
The radiation-induced posterior subcapsular cataract has long been generally accepted to be a deterministic effect that does not occur at doses below a threshold of at least 2 Gy. Recent epidemiological studies indicate that the threshold for cataract induction may be much lower or that there may be no threshold at all. A thorough study of this subject requires more accurate dose estimates for the eye lens than those available in ICRP Publication 74. Eye lens absorbed dose per unit fluence conversion coefficients for electron irradiation were calculated using a geometrical model of the eye that takes into account different cell populations of the lens epithelium, together with the MCNPX Monte Carlo radiation transport code package. For the cell population most sensitive to ionizing radiation-the germinative cells-absorbed dose per unit fluence conversion coefficients were determined that are up to a factor of 4.8 higher than the mean eye lens absorbed dose conversion coefficients for electron energies below 2 MeV. Comparison of the results with previously published values for a slightly different eye model showed generally good agreement for all electron energies. Finally, the influence of individual anatomical variability was quantified by positioning the lens at various depths below the cornea. A depth difference of 2 mm between the shallowest and the deepest location of the germinative zone can lead to a difference between the resulting absorbed doses of up to nearly a factor of 5000 for electron energy of 0.7 MeV.
辐射诱导的后囊下白内障长期以来被普遍认为是一种确定性效应，在至少 2 Gy 的剂量以下不会发生。最近的流行病学研究表明，白内障诱发的阈值可能低得多，或者根本没有阈值。要彻底研究这个问题，就需要比 ICRP 出版物 74 中提供的更准确的晶状体剂量估计。使用考虑到晶状体上皮不同细胞群体的眼睛几何模型，并结合 MCNPX 蒙特卡罗辐射传输代码包，计算了电子辐照时单位注量的晶状体吸收剂量转换系数。对于对电离辐射最敏感的细胞群体——生殖细胞——确定了单位注量的吸收剂量转换系数，对于低于 2 MeV 的电子能量，其值比平均晶状体吸收剂量转换系数高 4.8 倍。将结果与之前发表的略有不同的眼睛模型的值进行比较，对于所有电子能量，结果通常都非常吻合。最后，通过将晶状体放置在角膜下方的不同深度来量化个体解剖结构变化的影响。在生殖层的最浅和最深位置之间存在 2 毫米的深度差异，对于 0.7 MeV 的电子能量，会导致吸收剂量的差异高达近 5000 倍。
