Bea-Gilabert José, Baños-Capilla M Carmen, García-Martínez M Ángeles, López-Muñoz Enrique, Larrea-Rabassa Luis M
Medical Physics Department, Vithas Hospital Virgen del Consuelo, C/ Callosa d'En Sarrià 12, E-46007 Valencia, Spain.
Brainlab Sales GmbH, Olof Palme Strasse 9, D-81829 Munich, Germany.
J Radiosurg SBRT. 2019;6(1):45-54.
This study aims to estimate a realistic margin in stereotactic body radiotherapy (SBRT) through examining the determination uncertainties of gross tumour volume (GTV).
Three computed tomography (CT) scans were performed on each patient in different sessions as a treatment simulation. Registration of the different CT image sets was based on the fiducial marks from two stereotactic guides. GTV was defined in each one of them, as well as both the encompassing (UNI) and overlapping (INT) volumes. This protocol was altered following imaging guided radiotherapy (IGRT) implementation, so tumour displacements could be corrected for. The patient was scanned without repositioning solely considering tumour intrafraction variations. In addition, isocentre and dimension variations were obtained for each patient and cohort. A Monte Carlo code was developed to simulate tumour volume, considering them as ellipsoids in order to study their behaviour. Lastly, the equivalent radius ( ) was defined for each of these volumes, experimental and simulated, and both and values were derived by simple linear regression to the mean value .
The global margin can be defined as this systematic error plus an additional residual random uncertainty, with values = 3.4 mm for Body Frame, = 2.3 mm for BodyFIX and = 2.1 mm without repositioning. The experimental results obtained are in good agreement with simulated values, validating the use of the Monte Carlo code to calculate a margin formula.
Introducing IGRT is not enough to obtain a zero margin; consequently, the safety margin, dependent on tumour shape and size dispersion, can be evaluated using this formulation.
本研究旨在通过检查大体肿瘤体积（GTV）的确定不确定性，来估计立体定向体部放疗（SBRT）中的实际边界。
在不同疗程对每位患者进行三次计算机断层扫描（CT），作为治疗模拟。不同CT图像集的配准基于两个立体定向引导器的基准标记。在每个图像集中定义GTV，以及包含体积（UNI）和重叠体积（INT）。在实施影像引导放疗（IGRT）后改变了该方案，以便可以校正肿瘤位移。仅考虑肿瘤分次内变化，在不重新定位的情况下对患者进行扫描。此外，获得了每位患者和队列的等中心和尺寸变化。开发了一个蒙特卡罗代码来模拟肿瘤体积，将它们视为椭球体以研究其行为。最后，为每个实验和模拟体积定义等效半径（ ），并且通过对平均值的简单线性回归得出 和 值。
全局边界 可定义为该系统误差加上额外的残余随机不确定性，对于体架， 值为3.4毫米，对于BodyFIX为2.3毫米，不重新定位时为2.1毫米。获得的实验结果与模拟值高度一致，验证了使用蒙特卡罗代码来计算边界公式的有效性。
引入IGRT不足以获得零边界；因此，可以使用该公式评估取决于肿瘤形状和大小离散度的安全边界。
