Du Weiliang
Department of Radiation Physics, The University of Texas MD Anderson Cancer Center, Houston, Texas, USA.
J Appl Clin Med Phys. 2025 Apr;26(4):e70000. doi: 10.1002/acm2.70000. Epub 2025 Feb 4.
This study aimed to evaluate the impact of collimator angle, ball bearing (BB) phantom position, and field size on the accuracy of Winston-Lutz (WL) test-derived radiation isocenters.
WL tests were performed on four TrueBeam linear accelerators. Fifty-six images (eight gantry angles multiplied by seven collimator angles) were acquired for each WL test. Images with different sets of collimator angles were used to compute the radiation isocenters. The resulting radiation isocenters were correlated with the collimator angles. Then, the BB position and radiation field size were varied for the subsequent WL tests. The calculated BB shifts were compared with the known shifts, and the radiation isocenters were compared between different field sizes.
The use of a single collimator angle led to errors of as much as 0.4 mm in the calculated radiation isocenters. Systematic differences were observed between the radiation isocenters derived with collimator angle 0° and those derived with 90° and/or 270°. A commonly used opposing collimator angle pair, 90° and 270°, resulted in a vertical 0.1 mm offset of the radiation isocenters toward the ceiling. Oblique opposite or mixed collimator angles yielded radiation isocenter errors less than 0.1 mm. The BB shifts derived from WL tests were less than 0.1 mm from the known shifts. The radiation isocenters varied by less than 0.1 mm between field sizes ranging from 2 × 2 cm to 20 × 20 cm.
Oblique opposing collimator angle pairs should be considered to minimize errors in localizing radiation isocenters. Uncertainty in BB positioning could be eliminated if the BB is used as a static reference point in space. The field size had no significant effect on the radiation isocenters. With careful design of WL test parameters and image processing, it is possible to achieve a precision of 0.1 mm in localizing radiation isocenters using WL tests.
本研究旨在评估准直器角度、球模（BB）位置和射野大小对温斯顿-卢茨（WL）测试得出的放射等中心精度的影响。
在四台TrueBeam直线加速器上进行WL测试。每次WL测试采集56张图像（八个机架角度乘以七个准直器角度）。使用不同准直器角度组合的图像来计算放射等中心。将得出的放射等中心与准直器角度进行相关性分析。然后，在后续的WL测试中改变BB位置和放射野大小。将计算出的BB位移与已知位移进行比较，并比较不同射野大小之间的放射等中心。
使用单一准直器角度会导致计算出的放射等中心误差高达0.4毫米。在准直器角度为0°得出的放射等中心与90°和/或270°得出的放射等中心之间观察到系统差异。常用的相对准直器角度对90°和270°会导致放射等中心垂直向上偏移0.1毫米。斜向相对或混合准直器角度产生的放射等中心误差小于0.1毫米。WL测试得出 的BB位移与已知位移相差小于0.1毫米。在射野大小从2×2厘米到20×20厘米之间，放射等中心的变化小于0.1毫米。
应考虑使用斜向相对的准直器角度对，以尽量减少放射等中心定位的误差。如果将BB用作空间中的静态参考点，可以消除BB定位的不确定性。射野大小对放射等中心没有显著影响。通过精心设计WL测试参数和图像处理，使用WL测试定位放射等中心可以达到0.1毫米的精度。
