PURPOSE: The role of consolidation radiotherapy in patients with primary mediastinal B-cell lymphoma (PMBCL) is controversial. METHODS: The IELSG37 trial, a randomized noninferiority study, aimed to assess whether irradiation can be omitted in patients with PMBCL with complete metabolic response (CMR) after induction immunochemotherapy. The primary end point was progression-free survival (PFS) at 30 months after random assignment. Patients with CMR were randomly assigned to observation or consolidation radiotherapy (30 Gy). With a noninferiority margin of 10% (assuming a 30-month PFS of 85% in both arms), a sample size of 540 patients was planned with 376 expected to be randomly assigned. RESULTS: The observed events were considerably lower than expected; therefore, primary end point analysis was conducted when ≥95% of patients were followed for ≥30 months. Of the 545 patients enrolled, 268 were in CMR after induction and were randomly assigned to observation (n = 132) or radiotherapy (n = 136). The 30-month PFS was 96.2% in the observation arm and 98.5% in the radiotherapy arm, with a stratified hazard ratio of 1.47 (95% CI, 0.34 to 6.28) and absolute risk difference of 0.68% (95% CI, -0.97 to 7.46). The 5-year overall survival (OS) was 99% in both arms. Nonrandomized patients were managed according to local policies. Radiotherapy was the only treatment in 86% of those with Deauville score (DS) 4 and in 57% of those with DS 5. The 5-year PFS and OS of patients with DS 4 (95.8% and 97.5%, respectively) were not significantly different from those of randomly assigned patients. Patients with DS5 had significantly poorer 5-year PFS and OS (60.3% and 74.6%, respectively). CONCLUSION: This study, the largest randomized trial of radiotherapy in PMBCL, demonstrated favorable outcomes in patients achieving CMR with no survival impairment for those omitting irradiation.

AO Brotzu Ospedale Oncologico Businco Unit of Hematology and Bone Marrow Transplant Cagliari Italy

AO Santa Croce e Carle Department of Medical Physics Cuneo Italy

AOU Careggi Department of Hematology Florence Italy

AOU Città della Salute e della Scienza Department of Hematology Turin Italy

AOU Città della Salute e della Scienza di Torino and CPO Piemonte Clinical Epidemiology Unit Turin Italy

ASST Grande Ospedale Metropolitano Niguarda Division of Hematology Milan Italy

ASST Spedali Civili Department of Haematology Brescia Italy

ASU Friuli Centrale Unit of Hematology Udine Italy

ASU Giuliano Isontina Ospedale Maggiore Trieste Italy

Azienda USL IRCCS of Reggio Emilia Department of Hematology Reggio Emilia Italy

Azienda USL IRCCS of Reggio Emilia Department of Nuclear Medicine Oncology and High Technology Reggio Emilia Italy

Centro di Riferimento Oncologico IRCCS Division of Medical Oncology and Immunerelated Tumors Aviano Italy

Charles University General Hospital Department of Medical Oncology Prague Czech Republic

Ente Ospedaliero Cantonale Imaging Institute of Southern Switzerland Lugano Switzerland

Ente Ospedaliero Cantonale Oncology Institute of Southern Switzerland Bellinzona Switzerland

Fondazione del Piemonte per l'Oncologia IRCCS Candiolo Cancer Institute Candiolo Italy

Fondazione IRCCS Policlinico San Matteo Division of Hematology Pavia Italy

Guy's and St Thomas' NHS Foundation Trust Guy's Cancer Center London United Kingdom

Humanitas University Department of Biomedical Sciences Pieve Emanuele Italy

Inselspital Bern University Hospital University of Bern Department of Radiation Oncology Bern Switzerland

Institute of Oncology Research Bellinzona Switzerland

IRCCS Humanitas Research Hospital Department of Medical Oncology and Hematology Rozzano Milan Italy

IRCCS Istituto Tumori Giovanni Paolo 2 Hematology Unit Bari Italy

IRCCS San Raffaele Scientific Institute Lymphoma Unit Milan Italy

King's College London and Guy's and St Thomas' PET Centre School of Biomedical Engineering and Imaging Sciences King's College London United Kingdom

Maria Sklodowska Curie National Research Institute of Oncology Department of Lymphoid Malignancies Warsaw Poland

National Cancer Institute Onco Hematology Department Kyiv Ukraine

Nicolaus Copernicus University Department of Diagnostics Imagining Torun Poland

Oslo University Hospital Department of Oncology Oslo Norway

Ospedale Santa Maria delle Croci Department of Hematology Ravenna Italy

Presidio Ospedaliero Pescara Unità Operativa Semplice Dipartimentale Centro Diagnosi e Terapia Linfomi Pescara Italy

Princess Margaret Cancer Centre Radiation Medicine Program Toronto Canada

Ruijin Hospital Affiliated to Shanghai Jiao Tong University School of Medicine Department of Hematology Shanghai China

Sapienza University Division of Hematology Department of Translational and Precision Medicine Rome Italy

Skane University Hospital Department of Oncology Lund University Lund Sweden

Università della Svizzera Italiana Faculty of Biomedical Sciences Lugano Switzerland

University College London Hospitals NHS Trust Department of Hematology London United Kingdom

University Health Network Princess Margaret Hospital University Medical Imaging Toronto University of Toronto Toronto Canada

University Hospital Brno Brno Czech Republic

University of Oslo KG Jebsen Center for B cell malignancies Oslo Norway

University of Pavia Department of Molecular Medicine Pavia Italy

University of Southampton Clinical Trials Unit School of Cancer Sciences Southampton United Kingdom

University of Turin Department of Oncology Turin Italy

University Vita Salute San Raffaele Milan Italy

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NCT01599559