BACKGROUND: Evolution of indolent to aggressive lymphoma has been described in dogs but is difficult to distinguish from the de novo development of a second, clonally distinct lymphoma. Differentiation of these scenarios can be aided by next generation sequencing (NGS)-based assessment of clonality of lymphocyte antigen receptor genes. CASE PRESENTATION: An 8-year-old male intact Mastiff presented with generalized lymphadenomegaly was diagnosed with nodal T zone lymphoma (TZL) based on cytology, histopathology, immunohistochemistry and flow cytometry. Thirteen months later, the dog re-presented with progressive lymphadenomegaly, and based on cytology and flow cytometry, a large B cell lymphoma (LBCL) was diagnosed. Sequencing-based clonality testing confirmed the de novo development of a LBCL and the persistence of a TZL. CONCLUSIONS: The occurrence of two distinct lymphoid neoplasms should be considered if patient features and tumor cytomorphology or immunophenotype differ among sequential samples. Sequencing-based clonality testing may provide conclusive evidence of two concurrent and distinct clonal lymphocyte populations, termed most appropriately "composite lymphoma".

Department of Internal Medicine 2 University Hospital Schleswig Holstein Kiel Germany Central European Institute of Technology Masaryk University Brno Czech Republic

Departments of Biomedical Sciences University of Guelph Guelph ON N1G 2W1 Canada

Departments of Clinical Studies University of Guelph Guelph ON N1G 2W1 Canada

Departments of Pathobiology University of Guelph Guelph ON N1G 2W1 Canada

Departments of Pathobiology University of Guelph Guelph ON N1G 2W1 Canada Present address Department of Pathology Microbiology Immunology University of California Davis USA

Departments of Pathobiology University of Guelph Guelph ON N1G 2W1 Canada Present address Idexx Laboratories 1345 Denison St Markham ON L3R 5V2 Canada

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