Département de Physique, de Génie Physique et d'Optique et Centre de Recherche sur le cancer, Université Laval, Québec, Québec G1V 0A6, Canada, and Département de Radio-Oncologie and CRCHU de Quebec, CHU de Québec, 11 Co^te du Palais, Québec, Québec G1R 2J6, Canada.
Med Phys. 2013 Oct;40(10):101703. doi: 10.1118/1.4819937.
3D dosimetry is recognized as an ideal for patient-specific quality assurance (QA) of highly conformal radiotherapy treatments. However, existing 3D dosimeters are not straightforward to implement in the clinic, as their read-out procedure is often tedious and their accuracy, precision, and∕or sample size exhibit limitations. The purpose of this work is to develop a 3D dosimeter based on the concept of tomodosimetry inside concentric cylindrical planes using long scintillating fibers for the QA of modern radiotherapy techniques such as intensity-modulated radiation therapy (IMRT) or intensity-modulated arc therapy (IMAT).
Using a model-based simulation, scintillating fibers were modeled on three concentric cylindrical planes of radii 2.5, 5.0, and 7.5 cm, inside a 10 cm radius water-equivalent cylinder phantom. The phantom was set to rotate around its central axis, made parallel to the linac gantry axis of rotation. Light acquisitions were simulated using the calculated dose from the treatment planning software and reconstructed in each cylindrical plane at a resolution of 1 mm(2) using a total-variation minimization iterative reconstruction algorithm. The 3D dose was then interpolated from the reconstructed cylindrical plane doses at a resolution of 1 mm(3). Different scintillating fiber patterns were compared by varying the angle of each fiber in its cylindrical plane and introducing a light-tight cut in each fiber. The precision of the reconstructed cylindrical dose distribution was evaluated using a Poisson modeling of the acquired light signals and the accuracy of the interpolated 3D dose was evaluated using an IMRT clinical plan for a prostate case.
Straight scintillating fiber patterns with light-tight cuts were the most accurate in cylindrical dose reconstruction, showing less than 0.5 mm distance-to-agreement in dose gradients and a mean local dose difference of less than 0.2% in the high dose region for a 10 × 10 cm(2) field. The precision attained with this fiber configuration was less than 0.9% in the high dose, low gradient region of an IMRT segment for light acquisitions of 0.1 MU over a 360 degree rotation of the cylinder phantom. 3D dose interpolation for the IMRT clinical plan yielded an overall dose difference with the reference input of less than 1%, except in high dose gradients.
Using long scintillating fibers inside rotating, concentric cylindrical planes, the authors demonstrate that their tomodosimetry method has the potential for high resolution, precise, and accurate 3D dosimetry. Moreover, because of its water-equivalence and rotational symmetry, this design should find interesting application for both treatment QA and machine commissioning.
3D 剂量测定被认为是高度适形放射治疗的患者特异性质量保证（QA）的理想方法。然而，现有的 3D 剂量计在临床中实施并不简单，因为其读出过程通常很繁琐，并且其准确性、精密度和/或样本量存在局限性。本工作的目的是开发一种基于同心圆柱平面内的调强剂量测定概念的 3D 剂量计，使用长闪烁光纤来对现代放射治疗技术（如调强放射治疗（IMRT）或调强弧形治疗（IMAT））进行 QA。
使用基于模型的模拟，在半径为 2.5、5.0 和 7.5 cm 的三个同心圆柱平面内对闪烁光纤进行建模，在 10 cm 半径的水等效圆柱体模体中。将模体设置为绕其中心轴旋转，使其与直线加速器旋转机架轴平行。使用治疗计划软件计算的剂量模拟光采集，并使用全变差最小化迭代重建算法在每个圆柱平面上以 1mm²的分辨率进行重建。然后，在 1mm³的分辨率下从重建的圆柱平面剂量插值 3D 剂量。通过改变每个纤维在其圆柱平面中的角度并在每个纤维中引入不透光切口来比较不同的闪烁纤维模式。使用采集光信号的泊松建模评估重建的圆柱剂量分布的精度，并使用前列腺病例的 IMRT 临床计划评估插值 3D 剂量的精度。
具有不透光切口的直闪烁纤维模式在圆柱剂量重建中最准确，在剂量梯度处的距离差异小于 0.5mm，在高剂量区域的平均局部剂量差异小于 0.2%，对于 10×10cm²的射野。对于圆柱体模体 360 度旋转时采集 0.1MU 的光，在 IMRT 段的高剂量、低梯度区域，这种纤维结构的精度小于 0.9%。对于 IMRT 临床计划的 3D 剂量插值，与参考输入的总剂量差异小于 1%，除了在高剂量梯度处。
使用长闪烁光纤在旋转的同心圆柱平面内，作者证明了他们的调强剂量测定方法具有高分辨率、精确和准确的 3D 剂量测定的潜力。此外，由于其水等效性和旋转对称性，这种设计应该对治疗 QA 和机器调试都有有趣的应用。
