Answer: Follicular dendritic cell sarcoma

Histology: Histologic description: This is a cellular neoplasm associated with geographic areas of necrosis. There is a dense lymphoplasmacytic infiltrate which is in fact the predominant component of the lesion; however, there are underlying neoplastic cells which are spindled with elongated nuclei, pink cytoplasm, and indistinct cell borders. There are additional areas of collagenization, particularly associated with blood vessels. The lymphoid infiltrate is predominantly composed of CD3-positive T-cells, with a sprinkling of CD20-positive B-cells. The spindle cells are immunoreactive for CD21 and CD35 (not shown). The spindle cells also label intensely for the EBER early transcript of Epstein Barr virus by in situ hybridization, which confirms the diagnosis.

Discussion: Differential diagnosis: Until recently, many liver lesions of this morphology had been classified as inflammatory pseudotumors. More recent studies including the use of follicular dendritic cell markers and in situ hybridization for EBV has allowed the delineation of these distinctive follicular dendritic cell tumors. A subset of inflammatory pseudotumors/inflammatory myofibroblastic tumors will label for ALK-1, but should not label for EBER, CD21 or CD35. While the lymphoid infiltrate is dense within the lesion, it is composed of small lymphocytes that lack significant cytologic atypia. Also, one would not expect a low-grade lymphoma to have such a dense follicular dendritic cell component. The current lesion is analogous to a thymoma in that it is a neoplasm of non-lymphoid cells that attracts non-neoplastic lymphocytes to it. In thymoma, neoplastic cells are keratin positive, and the lymphoid component would have an immature T-cell phenotype (TDT, MIC2 positive).

Normal follicular dendritic cells function to attract and stimulate germinal center B lymphocytes. Follicular dendritic cell tumors recapitulate this function role by attracting non-neoplastic lymphocytes to them. Tumor cells are typically oval to spindle shaped, have eosinophilic cytoplasm, and demonstrate sheet-like, fascicular, whorling, or storiform patterns. Most cases involve the lymph nodes of the head and neck or mediastinum, but about 30% are extranodal. The liver, tonsil, and abdomen are common extranodal sites of disease. In general, neoplasms of follicular dendritic cells behave as low grade sarcomas: approximately 50% recur but only 25% metastasize. The presence of necrosis (as in the current case) and high mitotic rate suggest the likelihood of aggressive behavior. Follicular dendritic cell tumors of the liver and spleen appear to be different from other types of follicular dendritic cell tumors; they have a dense lymphoid infiltrate which characteristically obscures the neoplastic dendritic cells, are more frequent in females, and are positive for Epstein Barr virus.