Coexisting Follicular and Mantle Cell Lymphoma With Each Having an In Situ Component

A diagnosis of composite lymphoma is typically prompted by the observation of morphologic discordance. We present a case of a spleen revealing histologic features of follicular lymphoma, without any indication of a second lymphoma. Immunohistochemical stains supported this diagnosis and showed the follicular lymphoma to be BCL2-. However, these studies revealed 2 additional unexpected findings: cyclin D1+ mantle zone cells surrounding neoplastic and reactive follicles (indicative of in situ mantle cell lymphoma) and BCL2-bright, histologically nonneoplastic follicles (indicative of in situ follicular lymphoma). ImmunoFISH and microdissection and polymerase chain reaction analysis documented the clonal nature of the cyclin D1+ mantle zones and illustrated clonal independence from the follicular lymphoma. This case illustrates an uncommon and unusual composite follicular and mantle cell lymphoma, with the follicular lymphoma accompanied by an in situ component, whereas the only manifestation of the mantle cell lymphoma was in situ.