Department of Radiation Oncology, Brigham and Women's Hospital and Dana-Farber Cancer Institute, Harvard Medical School, Boston, Massachusetts, USA.
J Appl Clin Med Phys. 2023 Jul;24(7):e13965. doi: 10.1002/acm2.13965. Epub 2023 Mar 16.
The role of biliary stents in image-guided localization for pancreatic cancer has been inconclusive. To date, stent accuracy has been largely evaluated against implanted fiducials on cone beam computed tomography. We aim to use magnetic resonance (MR) soft tissue as a direct reference to examine the geometric and dosimetric impacts of stent-based localization on the newly available MR linear accelerator.
Thirty pancreatic cancer patients (132 fractions) treated on our MR linear accelerator were identified to have a biliary stent. In our standard adaptive workflow, patients were set up to the target using soft tissue for image registration and structures were re-contoured on daily MR images. The original plan was then projected on treatment anatomy and dose predicted, followed by plan re-optimization and treatment delivery. These online predicted plans were soft tissue-based and served as reference plans. Retrospective image registration to the stent was performed offline to simulate stent-based localization and the magnitude of shifts was taken as the geometric accuracy of stent localization. New predicted plans were generated based on stent-alignment for dosimetric comparison.
Shifts were within 3 mm for 90% of the cases (mean = 1.5 mm); however, larger shifts up to 7.2 mm were observed. Average PTV coverage dropped by 1.1% with a maximum drop of 26.8%. The mean increase in V35Gy was 0.15, 0.05, 0.02, and 0.02 cc for duodenum, stomach, small bowel and large bowel, respectively. Stent alignment was significantly worse for all metrics except for small bowel (p = 0.07).
Overall discrepancy between stent- and soft tissue-alignment was modest; however, large discrepancies were observed for select cases. While PTV coverage loss may be compensated for by using a larger margin, the increase in dose to gastrointestinal organs at risk may limit the role of biliary stents in image-guided localization.
在影像引导定位中，胆道支架在胰腺癌中的作用尚无定论。迄今为止，支架的准确性主要是通过在锥形束 CT 上植入基准点来评估的。我们旨在使用磁共振（MR）软组织作为直接参考，检查基于支架的定位对新的 MR 直线加速器的几何和剂量学影响。
在我们的 MR 直线加速器上治疗的 30 例胰腺癌患者（132 个分次）被确定有胆道支架。在我们的标准自适应工作流程中，患者使用软组织进行图像配准以设定目标，并且每天在 MR 图像上重新勾画结构。然后将原始计划投影到治疗解剖结构上并预测剂量，接着进行计划重新优化和治疗实施。这些在线预测计划是基于软组织的，作为参考计划。对支架进行回顾性图像配准以模拟基于支架的定位，并将位移幅度作为支架定位的几何精度。基于支架对准生成新的预测计划进行剂量学比较。
90%的病例的位移在 3mm 以内（平均=1.5mm）；然而，也观察到了最大 7.2mm 的较大位移。PTV 覆盖率平均下降 1.1%，最大下降 26.8%。十二指肠、胃、小肠和大肠的 V35Gy 平均增加分别为 0.15、0.05、0.02 和 0.02cc。除了小肠（p=0.07），所有指标的支架对位都明显差于软组织对位。
支架和软组织对位之间的总体差异较小；然而，对于某些病例，差异较大。虽然可以通过使用更大的边缘来补偿 PTV 覆盖率的损失，但增加胃肠道风险器官的剂量可能会限制胆道支架在影像引导定位中的作用。
