Image Sciences Institute, University Medical Center Utrecht, Utrecht, The Netherlands.
Department of Radiotherapy, University Medical Center Utrecht, Utrecht, The Netherlands.
Med Phys. 2017 Oct;44(10):5051-5060. doi: 10.1002/mp.12505. Epub 2017 Sep 11.
An MR-only postimplant dosimetry workflow for low dose rate (LDR) brachytherapy could reduce patient burden, improve accuracy, and improve cost efficiency. However, localization of brachytherapy seeds on MRI scans remains a major challenge for this type of workflow. In this study, we propose and validate an MR-only seed localization method and identify remaining challenges.
The localization method was based on template matching of simulations of complex-valued imaging artifacts around metal brachytherapy seeds. The method was applied to MRI scans of 25 prostate cancer patients who underwent LDR brachytherapy and for whom postimplant dosimetry was performed after 4 weeks. The seed locations found with the MR-only method were validated against the seed locations found on CT. The circumstances in which detection errors were made were classified to gain an insight in the nature of the errors.
A total of 1490 of 1557 (96%) seeds were correctly detected, while 67 false-positive errors were made. The correctly detected seed locations had a high spatial accuracy with an average error of 0.8 mm compared with CT. A majority of the false positives occurred near other seeds. Most false negatives were found in either stranded configurations without spacers or near other seeds.
The low detection error rate and high localization accuracy obtained by the complex-valued template matching approach are promising for future clinical application of MR-only dosimetry. The most important remaining challenge is robustness with regard to configurations of multiple seeds in close vicinity, such as in strands of seeds without spacers. This issue could potentially be resolved by simulating specific configurations of multiple seeds or by constraining the treatment planning to avoid these configurations, which could make the proposed method competitive with CT-based seed localization.
对于低剂量率（LDR）近距离放射治疗，仅使用磁共振（MR）的后植入剂量学工作流程可以减轻患者负担、提高准确性并提高成本效益。然而，在这种工作流程中，金属近距离放射治疗种子在磁共振扫描上的定位仍然是一个主要挑战。在这项研究中，我们提出并验证了一种仅基于 MR 的种子定位方法，并确定了仍存在的挑战。
该定位方法基于围绕金属近距离放射治疗种子的复值成像伪影模拟的模板匹配。该方法应用于 25 例接受 LDR 近距离放射治疗且在 4 周后进行后植入剂量学的前列腺癌患者的 MRI 扫描。使用仅基于 MR 的方法找到的种子位置与 CT 上找到的种子位置进行了验证。对发生检测错误的情况进行分类，以深入了解错误的性质。
总共正确检测到 1557 个种子中的 1490 个（96%），而出现 67 个假阳性错误。与 CT 相比，正确检测到的种子位置具有较高的空间精度，平均误差为 0.8 毫米。大多数假阳性错误发生在其他种子附近。大多数假阴性错误发生在没有间隔物的缠结配置或靠近其他种子的地方。
基于复值模板匹配方法的低检测错误率和高定位准确性为未来仅基于 MR 的剂量学的临床应用提供了希望。仍然存在的最重要挑战是在近距离多个种子的配置下的稳健性，例如在没有间隔物的种子束中。通过模拟多个种子的特定配置或通过限制治疗计划以避免这些配置，可能会解决此问题，这将使所提出的方法具有竞争力与基于 CT 的种子定位。
