Department of Radiation Oncology, The Netherlands Cancer Institute, Plesmanlaan 121, 1066 CX, Amsterdam, The Netherlands.
Radiat Oncol. 2022 Jun 21;17(1):110. doi: 10.1186/s13014-022-02079-2.
To determine PTV margins for intrafraction motion in MRI-guided online adaptive radiotherapy for rectal cancer and the potential benefit of performing a 2nd adaptation prior to irradiation.
Thirty patients with rectal cancer received radiotherapy on a 1.5 T MR-Linac. On T2-weighted images for adaptation (MRI), verification prior to (MRI) and after irradiation (MRI) of 5 treatment fractions per patient, the primary tumor GTV (GTV) and mesorectum CTV (CTV) were delineated. The structures on MRI were expanded to corresponding PTVs. We determined the required expansion margins such that on average over 5 fractions, 98% of CTV and 95% of GTV on MRI was covered in 90% of the patients. Furthermore, we studied the benefit of an additional adaptation, just prior to irradiation, by evaluating the coverage between the structures on MRI and MRI A threshold to assess the need for a secondary adaptation was determined by considering the overlap between MRI and MRI RESULTS: PTV margins for intrafraction motion without 2nd adaptation were 6.4 mm in the anterior direction and 4.0 mm in all other directions for CTV and 5.0 mm isotropically for GTV. A 2nd adaptation, applied for all fractions where the motion between MRI and MRI exceeded 1 mm (36% of the fractions) would result in a reduction of the PTV margin to 3.2 mm/2.0 mm. For PTV a margin reduction to 3.5 mm is feasible when a 2nd adaptation is performed in fractions where the motion exceeded 4 mm (17% of the fractions).
We studied the potential benefit of intrafraction motion monitoring and a 2nd adaptation to reduce PTV margins in online adaptive MRIgRT in rectal cancer. Performing 2nd adaptations immediately after online replanning when motion exceeded 1 mm and 4 mm for CTV and GTV respectively, could result in a 30-50% margin reduction with limited reduction of dose to the bowel.
确定直肠癌 MRI 引导在线自适应放疗中分次内运动的 PTV 边界，以及在照射前进行第 2 次自适应的潜在获益。
30 例直肠癌患者在 1.5T MR 直线加速器上接受放疗。在每位患者的 5 个治疗分次中，对自适应 MRI、照射前 MRI 和照射后 MRI 进行分析，勾画原发肿瘤 GTV（GTV）和直肠系膜CTV（CTV）。在 MRI 上对结构进行扩展，以得到相应的 PTV。我们确定了所需的扩展边界，以便在 5 个分次的平均情况下，98%的 CTV 和 95%的 GTV 在 90%的患者中得到覆盖。此外，我们通过评估 MRI 上的结构与 MRI A 之间的覆盖情况，研究了在照射前进行额外自适应的获益。通过考虑 MRI 和 MRI 之间的重叠，确定了进行二次自适应的阈值。
不进行第 2 次自适应时，CTV 的分次内运动的 PTV 边界在前方向为 6.4mm，在所有其他方向为 4.0mm；GTV 的各向同性 PTV 边界为 5.0mm。对于所有运动超过 1mm（占分次的 36%）的分次进行第 2 次自适应，会将 PTV 边界减少到 3.2mm/2.0mm。当在运动超过 4mm（占分次的 17%）的分次中进行第 2 次自适应时，PTV 边界可以减少到 3.5mm。
我们研究了在直肠癌 MRI 引导在线自适应放疗中，通过分次内运动监测和第 2 次自适应来降低 PTV 边界的潜在获益。当 CTV 和 GTV 的运动分别超过 1mm 和 4mm 时，在线重新计划后立即进行第 2 次自适应，CTV 和 GTV 的 PTV 边界可以减少 30%-50%，同时对肠道的剂量限制减少。
