Magnetic Resonance Guided Adaptive Stereotactic Body Radiotherapy for Lung Tumors in Ultracentral Location

Stereotactic body radiotherapy (SBRT) is a well-established local treatment method for early-stage NSCLC or lung metastases. However, clinicians are restricted in the utilization of sufficiently high radiation doses when the lung tumor is in ultracentral location next to radiosensitive organs-at-risk (OAR, e.g. the central airways or the esophagus). MR-guided SBRT can minimize the dose to these OAR by advanced techniques to correct for interfractional (daily plan adaptation) and intrafractional motion (respiratory gating, i.e. synchronization of beam delivery to the patient's breathing). Consequently, the MAGELLAN trial uses MR-guided SBRT to enable safe dose escalation to ultracentral lung tumors. The primary objective of this phase I dose escalation trial is to detect the maximum tolerated dose of MR-guided SBRT to ultracentral lung tumors with a dose limiting toxicity (DLT) rate = 35%. This dose should yield the optimum balance between acceptable treatment toxicity and good tumor control. The corresponding primary endpoint is the observation of the binary outcome dose-limiting toxicity (DLT) within 12 months from start of radiation. Secondary objectives include description of tumor control, patient survival and patient-reported outcomes, assessment of longitudinal cardiopulmonary function and dosimetry evaluations. Exploratory objectives include detection of early biomarkers of pulmonary toxicity and tumor response from multiparamtetric thoracic MRI examinations as well as peripheral blood samples before and early after treatment. A maximum of 38 adult patients may be accrued. Inclusion criteria encompass indication for pulmonary SBRT, ultracentral lung tumor location defined as overlap of the planning target volume (PTV) with the proximal bronchial tree or esophagus and a maximum tumor diameter ≤ 5cm. MR-guided SBRT is delivered on a 0.35T MR-Linac (6MV linear accelerator) with daily plan adaptation and gated dose delivery. Four dose levels may be employed:

• 0 (de-escalation): 10 x 5.0Gy
• 1 (start): 10 x 5.5Gy
• 2: 10 x 6.0Gy
• 3: 10 x 6.5Gy

Dose level 0 represents a de-escalation step which is deemed safe by retrospective clinical data. Dose level 3 confidently reaches a BED10 > 100 Gy necessary for adequate local tumor control in the lung according to hitherto data. Individual observation time for DLTs is 12 months. Dose escalation is performed in dose escalation cohorts consisting of three patients (N = 3) and the first cohort starts at dose level 1 (10 x 5.5 Gy). A time-to-event continual reassessment method (TITE-CRM) is used to recommend the dose level for the next cohort after a cumulative observation time of 18 months. TITE-CRM accounts for all available data at any given time to estimate DLT rates for each dose level and will recommend the dose level closest to the MTD. Admissible patients who present during the cumulative observation time are included as backup patients and treated on the dose level below the current recommendation. An escalation with overdose control (EWOC) approach is applied to prevent treatment with too toxic doses. According to the stopping criteria, patient accrual will be stopped after including 36 patients or 40 months after the first subject in date (recruitment of current dose escalation cohort may be completed), in case that estimation of the MTD is sufficiently certain or if all dose levels appear too toxic. After patient accrual has been stopped and all patients have completed their individual observation time, a final dose recommendaton will be performed which represents the MTD estimate. During follow-up, patients receive clinical assessments (3 monthly), thoracic CT- (3-monthly) and MR-imaging (after 3 months), one cardiology assessment (after 12 months), pulmonary function testing (12-monthly) and peripheral blood samples (last treatment day, after 3 months) for a total of 2 years after treatment.