Division Medical Radiation Physics, Department of Radiation Oncology, Medical University of Vienna/AKH Wien, Vienna, Austria.
MedAustron Ion Therapy Centre, Wiener Neustadt, Austria.
Med Phys. 2022 Sep;49(9):6150-6160. doi: 10.1002/mp.15828. Epub 2022 Jul 14.
Radiochromic films are versatile 2D dosimeters with high-resolution and near tissue equivalence. To assure high precision and accuracy, a time-consuming calibration process is required. To improve the time efficiency, a novel calibration method utilizing the ratio of the same dose profile measured at different monitor units (MUs) is introduced and tested in a proton and photon beam.
The calibration procedure employs the dose ratio of film measurements of the same relative profile for different absolute dose values. Hence, the ratio of the dose is constant at any point of the profile, but the ratio of the net optical densities is not constant. The key idea of the method is to optimize the calibration function until the ratio of the calculated doses is constant. The proposed method was tested in the dose range between 0.25-12 and 1-6 Gy in a proton and photon beam, respectively. A radial symmetric profile and a rectangular profile were created, both having a central plateau region of about 3 cm diameter and a dose falloff of about 1.5 cm at larger distances. The dose falloff region was used as input for the optimization method and the central plateau region served as dose reference points. Only the plateau region of the highest dose entered the optimization as an additional objective. The measured data were randomly split into differently sized training and test sets. The optimization was repeated 1000 times with random start value initialization using the same start values for the standard and the gradient method. Finally, a proton plan with four dose levels was created, which were separated spatially, to test the possibility of a full calibration within a single measurement.
Parameter estimation was possible with as low as one dose ratio used for optimization in both the photon and the proton case, yet exhibiting a high sensitivity on the dose level. The root mean squared deviation (RMSD) of the dose was less than 1% when the dose ratio was in the order of 20, whereas the median RMSD of all optimizations was 1.7%. Using four dose levels for optimization resulted in a median RMSD of 1% when randomly selecting the dose levels. Having at least one dose ratio of about 20 included in the optimization considerably improved the RMSD of the calibration function. Using six or eight dose levels reduced the sensitivity on the dose level selection and the median RMSD was 0.8%. A full calibration was possible in a single measurement having four dose levels in one plan but spatially separated.
The number of measurements required to obtain an EBT3 film calibration function could be reduced using the proposed dose ratio method while maintaining the same accuracy as with the standard method.
放射色胶片是一种具有高分辨率和近似组织等效性的通用 2D 剂量计。为了确保高精度和准确性，需要进行耗时的校准过程。为了提高时间效率，引入了一种新的校准方法，利用在不同监测器单位 (MU) 测量的相同剂量分布的比值，并在质子和光子束中进行了测试。
校准过程采用相同相对分布的胶片测量的剂量比来测量不同的绝对剂量值。因此，在分布的任何点，剂量比都是恒定的，但净光密度比不是恒定的。该方法的关键思想是优化校准函数，直到计算出的剂量比保持恒定。该方法在质子和光子束中分别在 0.25-12 和 1-6 Gy 的剂量范围内进行了测试。创建了一个径向对称的分布和一个矩形分布，两者都有一个约 3 厘米直径的中央平台区域和一个在较大距离处约 1.5 厘米的剂量下降区域。剂量下降区域被用作优化方法的输入，而中央平台区域则作为剂量参考点。只有最高剂量的平台区域作为附加目标进入优化。测量数据随机分为不同大小的训练集和测试集。使用相同的起始值对标准和梯度方法进行了 1000 次重复优化，随机初始化起始值。最后，创建了一个具有四个剂量水平的质子计划，这些计划在空间上是分开的，以测试在单次测量中进行完全校准的可能性。
在光子和质子情况下，使用优化中使用的一个剂量比就可以进行参数估计，但对剂量水平的灵敏度很高。当剂量比在 20 左右时，剂量的均方根偏差 (RMSD) 小于 1%，而所有优化的中位数 RMSD 为 1.7%。当随机选择剂量水平时，使用四个剂量水平进行优化会导致中位数 RMSD 为 1%。在优化中包含至少一个约 20 的剂量比可以显著提高校准函数的 RMSD。使用六个或八个剂量水平会降低对剂量水平选择的敏感性，中位数 RMSD 为 0.8%。在一个计划中具有四个剂量水平但在空间上分开的情况下，可以在单次测量中完成完全校准。
使用提出的剂量比方法可以减少获得 EBT3 胶片校准函数所需的测量次数，同时保持与标准方法相同的准确性。
