Morphologic and immunohistochemical features of FL with predominantly diffuse growth pattern. (A) A diffuse infiltration pattern prevails with only few intermingled atypical follicles. In the neoplastic follicles, centrocytic cells dominate (B), whereas in the diffuse areas tumor cells present with rounder nuclear contours (C). Tumor cells express CD20 (D), and reactive T cells (CD3) are prominent in diffuse areas (E). (F) The proliferative activity (Ki-67) is accentuated in the atypical follicular structures and decreased in the diffuse infiltrates. CD10 is expressed in atypical follicles and diffusely infiltrated areas in the majority of cases (G), as is CD23 (H). (I) BCL6 expression can be observed in all atypical follicles and in the nuclei of some interspersed B cells in the diffuse infiltrates. CD21 highlights FDCs in the atypical follicle. No CD21 staining is observed in the diffuse infiltrate (J). BCL2 expression is variable between cases but also within different areas of the same case (K,L). Magnification: panels A, D-G, I,J: ×200; panels B,C,H,I (inset), K,L: ×400.