Online adaptive MR-guided stereotactic radiotherapy for unresectable malignancies in the upper abdomen using a 1.5T MR-linac

Abstract

Purpose: The introduction of online adaptive MR-guided radiotherapy enables stereotactic body radiation therapy (SBRT) of upper abdominal tumors. The aim of this study was to evaluate the feasibility of MR-guided SBRT on a 1.5T MR-linac in a first group of patients with unresectable malignancies in the upper abdomen. Method: Patients with unresectable, pathology-proven upper abdominal malignancies who were treated at the UMC Utrecht (April 2019-December 2020) were identified in the prospective ‘Multi-OutcoMe EvaluatioN of radiation Therapy Using the MR-linac’ (MOMENTUM) study. Feasibility of treatment was arbitrarily defined as an on-table time of ≤60 minutes for >75% of delivered fractions and successful completion of >95% of fractions as scheduled. Treatment-related toxicity and tumor stability on CT-imaging were assessed. Results: Twenty-five consecutive patients with a median follow-up time of 7 (range 1 to 20) months were treated with 35 Gray (n=4) and 40 Gray (n=21) in five fractions over two weeks. For all fractions, contours were adapted based on the daily anatomy of the tumor and organs-at-risk and delivered within a median on-table time of 47 minutes/fraction (range 30-74). In 98/117 fractions (84%), adapted treatment was completed within one hour. All patients received the scheduled irradiation dose as planned. No acute grade 3 toxicity or higher was reported. Tumor stability is shown in figure 1. Conclusion: Online adaptive MR-guided SBRT on a 1.5T MR-linac is feasible in patients with unresectable malignancies in the upper abdomen. Dose escalation studies, followed by comparative studies, are needed to determine the optimal radiation dose needed.