Answer: Small lymphocytic lymphoma

Histology: The overall lymph node architecture is effaced. At low power, there are vague areas of nodularity characterized by paler regions. However, well-defined nodules as seen in follicular lymphomas are lacking. At higher magnification, the cells in these paler regions appear round and uniform. These cells are minimally enlarged with more prominent nucleoli than the surrounding dark small mature round lymphocytes. The cells within these pale islands appear to have somewhat more cytoplasm leading to a greater degree of separation compared to the densely packed lymphocytes in the background. These small pale areas are known as pseudofollicular growth centers and are diagnostic of small lymphocytic lymphoma.

Discussion: The follicles in a follicular small cleaved lymphoma are much more well developed than pseudofollicular growth centers. Furthermore, the cells within a follicular small cleaved lymphoma exhibit a greater degree of nuclear irregularity. Small lymphocytic lymphoma coexpresses B and T cell markers, where the cells would be positive for CD20 (B cell marker) as well as CD5 (T cell marker) and/or CD43 (T cell marker). Rather than forming small pale nodules scattered throughout the lesion, large cell lymphoma appears as a distinct irregular nodule of large cells arising within a background of small mature lymphocytes. In large cell lymphoma, the large cells are much more atypical, mitotically active, and sheet-like in contrast to the cells in pseudofollicular growth centers. Mantle lymphoma lacks pseudofollicular growth centers and shows nuclear irregularity intermediate between follicular small cleaved lymphoma and a small lymphocytic lymphoma. Both mantle cell lymphoma and small lymphocytic lymphoma coexpress B and T cell markers, however, only mantle cell lymphomas express cyclin D1.