Background: Evolution of indolent to aggressive lymphoma has been described in dogs but is difficult to\ndistinguish from the de novo development of a second, clonally distinct lymphoma. Differentiation of these\nscenarios can be aided by next generation sequencing (NGS)-based assessment of clonality of lymphocyte antigen\nreceptor genes.\nCase presentation: An 8-year-old male intact Mastiff presented with generalized lymphadenomegaly was\ndiagnosed with nodal T zone lymphoma (TZL) based on cytology, histopathology, immunohistochemistry and flow\ncytometry. Thirteen months later, the dog re-presented with progressive lymphadenomegaly, and based on\ncytology and flow cytometry, a large B cell lymphoma (LBCL) was diagnosed. Sequencing-based clonality testing\nconfirmed the de novo development of a LBCL and the persistence of a TZL.\nConclusions: The occurrence of two distinct lymphoid neoplasms should be considered if patient features and\ntumor cytomorphology or immunophenotype differ among sequential samples. Sequencing-based clonality testing\nmay provide conclusive evidence of two concurrent and distinct clonal lymphocyte populations, termed most\nappropriately â??composite lymphomaâ?.
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