Follicular T cell lymphoma C86.-

Follicular T-cell lymphomas are very rare lymphomas with a rather aggressive clinical course and strong association with the germinal center setting. Follicular T-cell lymphoma was previously assigned to PTCL-NOS as the "follicular variant" (Swerdlow SH et al.2017). The lymphoma entity shows a characteristic morphological and immunophenotypic picture and was first described by de Leval in 2001 (de Leval L et al. 2001) as "PTCL with follicular involvement and CD4+/BCL6+ phenotype".

FTCL account for <1% of all T-cell lymphomas. M>w.

FTC affect people in middle age to the elderly (33 to 88 years - Huang Y et al.2009).

The clinicopathologic aspects of peripheral T-cell lymphomas with a follicular growth pattern have been poorly described. The lymphomas involve the lymph nodes. Less commonly, there is involvement of the skin and bone marrow. The clinical presentation is similar to that of AITL with usually advanced disease stage, generalized lymphadenopathy, splenomegaly, B symptoms and. Skin lesions are found in a good 1/3 of all patients. These are not "MF-typical". Non-scaly, firm, red papules and plaques are found (Huang Y et al. 2009).

As with AITL, blood changes such as hypergammaglobulinemia, eosinophilia, or a positive Coombs test can be found.

Initially, biopsies may show clinical and histopathological features of angioimmunoblastic T-cell lymphoma. FTCL show partial or complete lymph node infiltration with nodular/follicular proliferation of medium-sized monomorphic lymphoid cells.

Differentially, differentiation from reactive lymph node changes and some B-cell lymphomas (follicular lymphoma, marginal zone lymphoma, or nodular lymphocyte-predominant Hodgkin's lymphoma) can be difficult without immunohistochemical differentiation (Huang Y et al.2009). However, the vascular and FD cell proliferates typical of AITL and the inflammatory background are absent.

Immunophenotypically, the neoplastic cells express pan T cell antigens (CD2, CD3 and CD5) with frequent deficiency of CD7. They show a CD4+ TFH phenotype with expression of numerous TFH cell markers such as PD-1, CXCL13, BCL6, CD10 and ICOS. Interspersed CD20-positive immunoblasts are often EBV-positive. They may also show a Hodgkin- or Reed- Sternberg-like morphology and even the phenotype of a classical Hodgkin lymphoma (CD30+, CD15+, PAX5+, EBV+/-), making the lymphocyte-rich classical Hodgkin lymphoma an important differential diagnosis here as well.

Molecular biological: Characteristic are t(5;9)(q33;q22)-ITK-SYK translocation is considered a specific genetic aberration and is detectable in about 20% of all FTCL (Streubel B et al. 2006).

The most important differential diagnoses are Hodgkin's lymphoma and angioimmunoblastic T-cell lymphoma (Huang Y et al.2009).

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