Grade 2: follicles have 6 to 15 centroblasts per hpf. Grade 3: follicles have >15 centroblasts per hpf. Grade 3A: Grade 3 in which the follicles contain predominantly centrocytes. Grade 3B: Grade 3 in which the follicles consist almost entirely of centroblasts.Grades 1 and 2 are regarded as low grade FL; Grade 3A is usually also regarded as low grade FL although some studies have regarded it as high grade FL; and Grade 3B is regarded as a highly aggressive FL in the t-FL category.In addition to grade 3B disease, histologic examinations may reveal other evidence of t-FL such as histologic findings consistent with FL and diffuse large cell lymphoma in the same tissue (referred to as composite lymphomas) or in separate tissues (referred to as (discordant lymphomas) or histologic findings similar to those found in Burkitt lymphoma, precursor B-cell lymphoblastic leukemia, plasmablastic lymphoma, the high grade subtype of B-cell lymphoma, Hodgkin lymphoma of the B-cell type, chronic lymphocytic leukemia/small cell lymphocytic lymphoma, or histiocytic sarcoma. Other findings indicating the presence of this transformation include rapid growth in size of lymph nodes, recently acquired or new B symptoms, recent development of FL lesions in non-nodal tissue, rapid rises in serum lactate dehydrogenase levels, and the presence of high levels of serum calcium. Differential diagnosis
FL may be confused with marginal zone B-cell lymphoma, mantle cell lymphoma, and the small lymphocytic lymphoma variant of chronic lymphocytic leukemia.
However, a subset of PGTFL cases had lesions that were localized to the duodenum and other parts of the small intestine usually without involving other parts of the GI tract or tissues outside of the GI tract. This contrasts with the other cases of PGTFL which were systemic diseases involving a wide range of GI tract and non-GI tract tissues. Consequently, the World Health Organization (2017) removed the localized disease from the primary gastrointestinal tract follicular lymphoma category, reclassified it as a distinct disease entity, and termed it duodenal-type follicular lymphoma. DFL is most often an asymptomatic disease that is diagnosed on endoscopic examination of the GI tract conducted for other reasons. Less commonly, it presents with vague abdominal symptoms. In one review of former studies, the lesions in 85% of primary duodenal follicular lymphoma were located not only in the duodenum but also other sites in the intestine (i.e. jejunum and/or ileum), with rare cases having lesions in the rectum or cecum PDF is an indolent disease that may spontaneously remit and relapse but only rarely progresses to a more aggressive form. A watch-and-wait strategy has been a generally recommended method for the initial treatment of the disease. Primary gastrointestinal tract follicular lymphoma
PGTFL is a follicular lymphoma (which as currently defined excludes cases of duodenal-type follicular lymphoma) that has a prominent component of GI tract involvement. The disease may present with signs and symptoms typical of the common type of  follicular lymphoma.