The characteristics at diagnosis and clinical course of primary extranodal follicular lymphoma (EFL) have not been extensively described. The International Extranodal Lymphoma Study Group (IELSG) conducted an international retrospective survey aimed to describe the clinical features at diagnosis and the outcomes of FL cases with a clinically dominant extranodal component. The dataset included 605 pathologically reviewed cases from 19 different countries, and their outcomes were compared to those of nodal follicular lymphomas. The two most common presentation sites for EFL were the skin (n = 334) and the gastrointestinal tract (n = 72), with 22 cases having primary duodenal localization. These subsets exhibited unique features at diagnosis and significantly different overall survival (OS) patterns. After a median follow-up of 5.5 years, primary cutaneous lymphomas showed a superior outcome [10-year OS: 89% (95% CI, 83%-93%)], while primary gastrointestinal lymphomas had an intermediate outcome [10-year OS: 79% (95% CI, 59%-90%)]. Among the gastrointestinal lymphomas, primary duodenal lymphomas tended toward the best outcome [10-year OS: 95% (95% CI, 69%-99%)]. Other primary extranodal sites had inferior outcomes [10-year OS: 59% (95% CI, 48%-68%)], similar to primary nodal lymphomas [10-year OS: 57% (95% CI, 49%-64%)]. These findings support the identification of specific primary FL localizations as distinct entities with particular clinical and biological characteristics.

1st Faculty of Medicine Charles University General Hospital Prague Prague Czech Republic

AOU Maggiore della Carità Novara Italy

Azienda sanitaria locale Ematologia Ospedale degli Infermi Biella Italy

CHIMOMO Department Università degli Studi di Modena e Reggio Emilia Modena Italy

Department of Haemato Oncology St Bartholomew's Hospital London UK

Department of Hematology and Oncology University Hospital Brno Brno Czech Republic

Department of Hematology Hospital Clinic Univeritari Barcelona Spain

Department of Medicine Section of Hematology University of Verona Verona Italy

Department of Radiation Oncology Pete MacCallum Cancer Institute Melbourne Australia

Division of Hematology and Medical School University Hospital Centre Zagreb Zagreb Croatia

Division of Hematology Mayo Clinic Rochester SW USA

Faculty of Biomedical Sciences Institute of Oncology Research Università della Svizzera italiana Bellinzona Switzerland

Haematology Department A Tortora Hospital Pagani Italy

Hematology AUSL IRCCS of Reggio Emilia Reggio Emilia Italy

Hematology Department Athens Medical Center Psychion Branch Athens Greece

Hematology Department Instituto Nacional de Cancerologia Mexico City Mexico

Innovation Research and Teaching Service Hospital of Bolzano Lehrkrankenhaus der Paracelsus Medizinischen Privatuniversität Bolzano Bozen Italy

Istituto Cantonale di Patologia Ente Ospedaliero Cantonale Locarno Switzerland

Lymphoma Unit IRCCS Ospedale San Raffaele Milan Italy

Northwestern Medicine Hematology and Medical Oncology Chicago IL USA

Oncology Institute of Southern Switzerland Ente Ospedaliero Cantonale Bellinzona Switzerland

Pathology Unit IRCCS Ospedale San Raffaele Milan Italy

SCDU Ematologia Dipartimento di Medicina Traslazionale Unversità del Piemonte Orientale Novara Italy

SIB Swiss Institute of Bioinformatics Lausanne Switzerland

Universidad de Chile Hospital del Salvador Santiago Chile

University Health Network Princess Margaret Hospital Toronto Canada

University Hospital Hradec Králové Hradec Králové Czech Republic

University Hospital Olomouc Olomouc Czech Republic

University Vita Salute San Raffaele Milan Italy

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