Oligometastatic Mesothelioma Treated with Ablative Radiotherapy (OMAR): A Multicenter Study

Background/Objectives: This multicenter retrospective study aims to evaluate the role of Ablative Radiotherapy (RT) in patients with unresectable pleural mesothelioma (PM) who experienced radiological progression after at least one line of chemotherapy, with a maximum involvement of three pleural or extrapleural sites. Methods: Adult patients (≥18 years) with PM treated with stereotactic radiotherapy between 2011 and 2022, limited to a maximum of three pleural or extrapleural sites, were included in the analysis. Ablative RT was required to be administered with radical intent. Endpoints were time to further systemic therapy (TFST), local control (LC), progression-free survival (PFS), overall survival (OS), and acute and late radiotherapy-related toxicity. Results: A total of 56 patients were identified from six Italian and one Swiss radiotherapy center. Treatment was generally well tolerated. Ten patients experienced grade 1 or 2 acute toxicity, while four patients reported persistent chest pain, with one case reaching grade 3 as late toxicity. The median TFST was 18.6 months, with TFST rates of 61.7% and 46.4% at 12 and 24 months, respectively. The median OS was 37.63 months, with 1- and 2-year OS rates of 85.2% and 65.6%. Local control was favorable (79% at 1 year), but most patients experienced disease recurrence outside the SABR treatment volume. The median disease progression-free survival (DPFS) was 8.17 months, with 1- and 2-year DPFS rates of 36% and 19%, respectively. Smoking history correlated with OS and DPFS in univariate analysis, while statistical significance for OS was maintained in multivariate analysis. Additionally, nodal status and PTV volume were associated with OS. Conclusion: SABR is a safe and effective approach for the treatment of oligorecurrent/oligoprogressive PM. The time to further systemic therapy was extended up to 18 months. At two years, 10% of patients remained disease-free, and more than half were alive at three years, suggesting a potentially indolent biological behavior in oligometastatic PM.