Servicio de Radiofísica y Protección Radiológica, Complejo Asistencial de León, Altos de Nava s/n, E-24071 León, España.
Med Phys. 2011 May;38(5):2374-81. doi: 10.1118/1.3581377.
Asymmetric collimators are currently available in most of linear accelerators. They involve a lot of clinical improvements, such as the monoisocentric beam split technique that is more and more used in many external radiotherapy treatments. The tolerance established for each independent jaw positioning is 1 mm. Within this tolerance, a gap or overlap of the collimators up to 2 mm can occur in the half beams matching region, causing dose heterogeneities up to 40%. In order to solve this dosimetric problem, we propose an accurate jaw calibration method based on the Monte Carlo modeling of linac photon beams.
Simulating different jaw misalignments, the dose distribution occurring in the matching region for each particular configuration is precisely known, so we can relate the misalignment of the jaws with the maximum heterogeneity produced. From experimental measurements using film dosimetry, and taking into account Monte Carlo results, we obtain the actual misalignment of each jaw. By direct inspection of the readings of the potentiometers that control the position of the jaws, high precision correction can be performed, adjusting the obtained misalignments.
In the linac studied, the dose heterogeneity in the junction performed with X jaws (those farther from the source), and 6 MV photon beam was initially over 12%, although each jaw was within the tolerance in position. After jaw calibration, the heterogeneity was reduced to below 3%.
With this method, we are able to reduce the positioning accuracy to 0.2 mm. Consequently, the dose distribution in the junction of abutted fields is highly smoothed, achieving the maximum dose heterogeneity to be less than 3%.
目前大多数线性加速器都配备了不对称准直器。它们带来了许多临床改进，例如越来越多的外照射治疗中使用的单等中心射束分割技术。为每个独立准直器定位设定的公差为 1 毫米。在这个公差范围内，半射束匹配区域的准直器可能会出现 2 毫米的间隙或重叠，导致剂量不均匀度高达 40%。为了解决这个剂量学问题，我们提出了一种基于直线加速器光子束蒙特卡罗建模的精确准直器校准方法。
通过模拟不同的准直器错位，我们精确地了解了每个特定配置在匹配区域中出现的剂量分布，因此可以将准直器的错位与产生的最大不均匀性联系起来。通过使用胶片剂量测定进行实验测量，并考虑蒙特卡罗结果，我们获得了每个准直器的实际错位。通过直接检查控制准直器位置的电位计读数，可以进行高精度的校正，调整所获得的错位。
在研究的直线加速器中，使用 X 准直器（离源更远的那些）进行的交界处的剂量不均匀性最初超过 12%，尽管每个准直器的位置都在公差范围内。经过准直器校准后，不均匀性降低到 3%以下。
通过这种方法，我们能够将定位精度提高到 0.2 毫米。因此，相邻射野交界处的剂量分布得到了高度平滑，实现了最大剂量不均匀性小于 3%。
