Scripps Proton Therapy Center, 9730 Summers Ridge Road, San Diego, CA 92121, USA.
Phys Med Biol. 2013 Dec 7;58(23):8477-91. doi: 10.1088/0031-9155/58/23/8477. Epub 2013 Nov 15.
The dose response for films exposed to clinical x-ray beams is not linear and a calibration curve based on absorbed dose can be used to account for this effect. However for proton dosimetry the dose response of films exhibits an additional dependence because of the variation of the linear energy transfer (LET) as the protons penetrate matter. In the present study, we hypothesized that the dose response for EBT2 films can be mathematically described as a bimolecular chemical reaction. Furthermore, we have shown that the LET effect can be incorporated in the dose-response curve. A set of EBT2 films was exposed to pristine 161.6 MeV proton beams. The films were exposed to doses ranging from 0.93 to 14.82 Gy at a depth of 2 cm in water. The procedure was repeated with one film exposed to a lower energy beam (85.6 MeV). We also computed the LET and dose to water in the sensitive layer of the films with a validated Monte Carlo system, taking into account the film construction (polyester, adhesive and sensitive layers). The bimolecular model was able to accurately fit the experimental data with a correlation factor of 0.9998, and the LET correction factor was determined and incorporated into the dose-response function. We also concluded that the film orientation is important when determining the LET correction factor because of the asymmetric construction of the film.
胶片的剂量响应对于临床 X 射线束来说是非线性的，可以使用基于吸收剂量的校准曲线来解释这种效应。然而，对于质子剂量测量，由于质子穿透物质时线性能量转移（LET）的变化，胶片的剂量响应会表现出额外的依赖性。在本研究中，我们假设 EBT2 胶片的剂量响应可以用双分子化学反应来进行数学描述。此外，我们已经表明，可以将 LET 效应纳入剂量响应曲线中。一组 EBT2 胶片被暴露在原始的 161.6 MeV 质子束下。胶片在水深 2 厘米处的剂量范围从 0.93 到 14.82 Gy 不等。用一束能量较低的光束（85.6 MeV）对其中一片胶片进行了重复曝光。我们还使用经过验证的蒙特卡罗系统计算了胶片敏感层中的 LET 和水剂量，考虑到了胶片的结构（聚酯、粘合剂和敏感层）。双分子模型能够以 0.9998 的相关系数准确拟合实验数据，并且确定了 LET 校正因子并将其纳入剂量响应函数中。我们还得出结论，由于胶片的不对称结构，在确定 LET 校正因子时，胶片的取向很重要。
