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The genetic hallmark of follicular lymphoma (FL) is the translocation t(14;18)(q32;q21), which is found in up to 90% of FL and results in constitutive expression of the anti-apoptotic protein Bcl2. The t(14;18)+ translocation is not sufficient to cause lymphoma, since t(14;18)+ B lymphocytes can also be found in the peripheral blood of around 50% of healthy adults. In situ follicular neoplasia (ISFN) is a lymphoma precursor lesion considered to be a tissue manifestation of these circulating t(14;18)+ B cells, which shows the genetic and immunophenotypic characteristics of FL, but is exclusively localized in the germinal centers (GC) of a morphologically reactive-appearing lymph node and by definition is only detectable by immunostaining for Bcl2. The true incidence of ISFN is unknown, but studies of unselected lymph nodes from adults without evidence of lymphoma revealed a prevalence of 2-3%. The risk of progression to manifest FL seems to be low. A similar proliferation of t(14;18)+ B cells with limited malignant potential is duodenal-type follicular lymphoma (DFL), now considered as a distinct variant of FL in the 2016 update of the World Health Organization classification. It shows overlapping characteristics with ISFN, including a low risk for progression to manifest FL even in the absence of specific treatment, and a lack of tumor mass formation. Here we present a case of clonally related DFL and ISFN simultaneously present in the duodenal mucosa and peripancreatic lymph nodes found by chance in a Whipple resection specimen, respectively, documenting the close biological relationship of these two precursor lesions. The case occurred in an 84-year-old man without a his-

Clonally related duodenal-type follicular lymphoma and in situ follicular neoplasia