Cutaneous Follicle Center Lymphoma : Differential

Comments:
Primary cutaneous follicle-center lymphoma (PCFCL): The differential diagnosis includes B-cell lymphoid hyperplasia (BCLH; aka pseudolymphoma), primary cutaneous marginal zone lymphomas (PCMZL), systemic follicular lymphomas (FL) with secondary cutaneous involvement, and primary cutaneous diffuse large B-cell lymphoma, leg type (PCDLBCL, LT).

PCFCL with diffuse growth pattern consists of sheets of large centrocytes and is easily distinguished from reactive follicular hyperplasia. However, PCFCL with follicular pattern can closely mimic BCLH and it may be difficult to distinguish between the two entities on morphology alone.

Features favoring PCFCL:

• cutaneous lesions localized to head or trunk
• ill-defined follicles with reduced or absent mantle zones
• no differentiation of germinal centers into light and dark zones
• composed of monomorphic population of medium to large centrocytes
• few tingible body macrophages
• low proliferation fraction with Ki-67 stain
• variable number of centroblasts not in confluent sheets, admixed with T cells
• CD20+, CD79a+, PAX5+, IgM-, IgD-, IRF4/MUM1-, FOXP1-, MYC-
• BCL6+, BCL2- (or weakly +), CD10+ (+ in follicular and neg in diffuse areas)
• clusters of CD10+ and/or BCL6+ cells outside the follicles in the interfollicular areas
• CD21/CD35 showing follicular dendritic cell meshwork
• gene expression profile of germinal center B-cell (GCB)-type DLBCL
• no translocations involving BCL6, MYC, and IGH