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SBRT is an important modality for the radical treatment of malignant tumors in the lungs. Treatment with highly ablative doses in central or ultracentral tumors has been demonstrated to result in high rates of toxicity. MRI-guidance during treatment with daily plan adaptation may aid accurate delineation of OARs and target volumes in proximity of the mediastinum with daily plan adaptation allowing for safer treatment. In this study, we report the first clinical experiences on the safety and feasibility of SBRT of (ultra)central lung tumors on an 1.5 T MR-linac.

Both patients with primary NSCLC and patients with an oligoprogressive metastatic lung nodule or mediastinal lymph node were eligible for treatment on the Unity MR-linac (Elekta AB, Stockholm, SE) if their PTV was within 2 cm of the mediastinum. Pre-treatment imaging with 4D-CT and MRI was performed in treatment position and a pre-treatment offline IMRT plan was created in Monaco v5.40.01. Patients were treated to a stereotactic dose of 60 Gy in 8 or 12 fractions. An in-house developed T2-weighted 3D sequence acquired during free breathing was used for online delineation and treatment planning. For each fraction, contours of ITV and OARs were propagated using deformable image registration. A radiation oncologist modified the contours of ITV and relevant OARs and a new IMRT plan was created.

Ten patients were treated and completed all of their planned 104 fractions on the MR-linac. The median duration of treatment was 41 minutes (range 32-70 minutes) with a median beam-on time of 8.9 minutes (Figure 1). No grade ≥3 acute toxicity was observed. In 2 patients, a significant decrease in ITV size of 58% and 37% was observed due to tumor shrinkage during treatment (Figure 2). In the other patients, the majority (81%) of online ITVs were within ±15% of the fraction 1 volume. Compared to the offline treatment plan, ITV coverage of the online plan was similar in 52%, better in 34% and worse in 14% of fractions. Reduced coverage was associated with preferentially meeting OAR constraints.

Figure 1. Pie-chart demonstrating the different steps and respective average durations of the MR-guided online workflow.

Figure 2. Online MRI and dose distribution of the 1st (left) and 12th (right) fraction of a patient with a mediastinal lymph node metastasis of a SCLC in whom a 58% decrease in ITV was observed during treatment. The ITV (green) and OARs (esophagus (red), trachea (blue), spinal cord (purple)) within a 3 cm ring (yellow) around the PTV (green) are modified during treatment by a radiation oncologist.

We demonstrate the safety and feasibility of SBRT of (ultra)central lung tumors on an 1.5 T MR-Linac. All patients completed all treatment fractions on the MR-linac with improved ITV coverage in 34% of fractions through daily plan adaptation. In 2 patients, a significant decrease in ITV size was observed during treatment, illustrating the additional benefits of MRI-guided radiotherapy.