Aldrovandi Leon G, Dessein Matthias E T, Pearson Shelley M, Bulling Shelley M
Radiation Treatment, Wellington Blood and Cancer Centre, Newtown, Wellington, New Zealand.
Varian Medical Systems, Oncology House, Crawley, UK.
J Appl Clin Med Phys. 2025 Jun;26(6):e70079. doi: 10.1002/acm2.70079. Epub 2025 Mar 19.
Due to the tight curvature in their design, ring applicators are usually associated with large positioning errors. The standard practice to correct for these deviations based on global offsets may not be sufficient to comply with the recommended tolerance. In this work, we investigate two methods for applicator reconstruction that implement position-dependent source offset corrections.
Measurements were performed using the Varian Interstitial PEEK Ring 60° and a Varian BRAVOS afterloader. Source positioning was characterized by means of autoradiographs acquired for three different loading patterns and three Ir sources over a period of 5 months. Additionally, the actual source path was determined by means of a series of planar kV images for different dummy cable positions. The first position-dependent correction method consists of locally modifying the radius of the reconstructed source path according to the measured offsets. The second method, recommended by Varian, simulates a bidirectional movement of the source during applicator reconstruction to compensate for positioning errors.
Autoradiographs showed a quasi-linear increase of the dwell position offsets, with a negligible error at the tip and a value close to 3 mm at the end of the ring. This result, consistent with a circular wire movement with an effective radius 0.5 mm larger than the nominal value, was in agreement with the observations from the kV images. After implementation of the position-dependent correction methods, residual positioning errors for the two methods resulted in a mean value (±1 SD) of 0.0 (±0.3) mm, and a range of [-0.7 mm, 0.7 mm].
The two tested methods for applicator reconstruction with position-dependent source offset corrections were able to successfully correct the positioning errors. The method recommended by the manufacturer had the additional advantages of a more straightforward implementation and the potential for use in other applicator types.
由于环形施源器设计中的曲率较紧，通常会存在较大的定位误差。基于全局偏移量校正这些偏差的标准做法可能不足以符合推荐的公差要求。在本研究中，我们研究了两种用于施源器重建的方法，这些方法实现了与位置相关的源偏移校正。
使用瓦里安间质聚醚醚酮（PEEK）60°环形施源器和瓦里安BRAVOS后装治疗机进行测量。通过在5个月的时间内针对三种不同的加载模式和三个铱源获取的放射自显影片来表征源定位。此外，通过针对不同虚拟电缆位置的一系列平面千伏图像确定实际源路径。第一种与位置相关的校正方法包括根据测量的偏移量局部修改重建源路径的半径。第二种方法是瓦里安推荐的，在施源器重建期间模拟源的双向移动以补偿定位误差。
放射自显影片显示驻留位置偏移量呈准线性增加，尖端处误差可忽略不计，环末端的值接近3毫米。该结果与有效半径比标称值大0.5毫米的圆形导线移动一致，与千伏图像的观察结果相符。实施与位置相关的校正方法后，两种方法的残余定位误差的平均值（±1标准差）为0.0（±0.3）毫米，范围为[-0.7毫米，0.7毫米]。
两种经过测试的用于施源器重建且带有与位置相关的源偏移校正的方法能够成功校正定位误差。制造商推荐的方法具有实施更直接以及有可能用于其他施源器类型的额外优点。
