Résumé
Stereotactic body radiotherapy (SBRT) is increasingly used for the treatment of oligometastatic disease. However, SBRT is often difficult to achieve because of the proximity of mobile organs at risk (OAR). Magnetic resonance imaging-guided radiotherapy (MRgRT) is an innovative technique that enables daily dosimetric adaptation and monitoring of the target during irradiation. We evaluated the impact of MR-guided radiotherapy in patients with thoracic metastases. Methods: the primary endpoint was the assessment of acute and late toxicities. Secondary endpoints were overall survival (OS), progression-free survival (PFS), local control rate (LC), impact of adaptive treatment on target volume coverage and OAR sparing.Results: in this monocentric analysis, we reviewed 24 metastatic lesions from 21 consecutive patients treated with MRgRT for a thoracic metastasis from October 20th 2019 to January 15th 2024. The most common primary cancers were prostate (33.3%), colorectal (19%), and breast cancer (14.3%). The majority of patients were oligometastatic (57.1%), mainly in the context of oligorecurrence situation (90.5%). The main indications were irradiation of mediastinal lymph nodes (29.2%) and lung metastases (25%). The median dose was 38 Gy (30-60 Gy), most commonly 36 Gy in 6 fractions. The PTVopt coverage was significantly improved for adapted plans compared to predicted plans (coverage of PTV V95% and V100% increase of 7.8% and 8.8% respectively, p< 0.01), as was PTV coverage (coverage of PTV V95% increase of 3.0%, p<0.01). The adapted plan also significantly reduced Dmean for the heart, without compromising other dosimetric OAR measures. The treatment was well tolerated with no grade > 2 toxicities. With a median follow-up of 8.9 months (95% CI: 4.1-18.6), 11 patients had metastatic relapse (52.4%), and 4 patients died (19.1%). The 1-year overall survival (OS), progression-free survival (PFS), and local control (LC) rates were 81.3%, 39.1%, and 90.9%, respectively. Two local relapses appeared inside the field.Conclusion: MRgRT treatment seems effective for delivering ablative radiotherapy to thoracic metastases, allowing good targeting of volumes with limited toxicity thanks to a better saving from OAR. A longer follow-up is needed to assess late toxicities and to expand clinical outcomes.