Mediastinal (thymic) large B-cell lymphoma, Med-DLBCL

Definition

This is a subtype of diffuse large B-cell lymphoma arising in the mediastinum, with a distinct immunophenotype. The origin of this lymphoma is uncertain, but follicle centre cell, thymic and monocytoid B-cell derivations have all been suggested. The positivity for CD23 argues for an origin from activated dendritic thymic B cells³.

Epidemiology

It is relatively uncommon, accounting for 2.4% of all non-Hodgkin's lymphomas. It most commonly occurs in young adults (median 35 years), with a strong female predominance.

Clinical features

Patients usually present with an (anterior) mediastinal mass, which results in SVC obstruction in 25-30% of patients². There is a tendency for spread to unusual sites (brain, liver, ovaries, kidneys, adrenals and intestines).

Histopathology

The morphology is variable, but sclerosis, cells with relatively abundant clear or acidophilic cytoplasm, and multilobated nuclei are common features. The fibrosis is compartmentalizing. In most cases, the cells have abundant pale cytoplasm. Small numbers of benign lymphocytes and eosinophils may be present. Thymic remnants may be seen, recognition being facilitated by immunohistochemistry.

Rarely there is an association with Hodgkin lymphoma, nodular sclerosis, so called "composite lymphoma".

Immunohistochemistry (with nodal and extranodal DLBCL lymphoma for comparison)

	mediastinal large B-cell lymphoma	nodal DLBCL	extranodal DLBCL
CD3	negative2, 0/243	0/523	0/483
CD5	negative, 0/243	5/52	2/483
CD10	6/191, 2/243	20/52	12/483
CD11a	often positive
CD19	positive
CD20	positive, 24/243	52/523	47/483
CD21	usually negative
CD23	often positive, 17/243	8/523	4/483
CD30	often present but weak, 20/24 (often focal)3	14/523	7/483
CD45	positive
CD79a	positive
SIg	6/191
bcl-2	positive2, 15/243	28/523	21/483
bcl-6	19/19 (nuclear)1, 14/243	43/523	21/483

Differential diagnosis

• Positivty for CD23 may help to differentiated mediastinal large B-cell lymphomas from other DLBCL. CD30 is more often expressed in mediastinal large B-cell lymphomas, while CD10 is less often espressed.

• sclerosing mediastinitis

• Hodgkin's disease, nodular sclerosing, especially if small lymphocytes and eosinophils are present. Positivity for CD45 helps to exclude HL. There is probably an overlap between mediastinal large B-cell lymphoma and nodular sclerosing classical Hodgkin lymphoma, sometimes referred to as "mediastinal gray zone lymphoma".

• clear-cell thymoma

• seminoma / dysgerminoma

• In association with mediastinal large B-cell lymphoma, thymic remnants may form clusters, mimicking carcinoma

Prognosis

Mediastinal large B-cell lymphoma often responds well to aggressive therapy². Prognosis depends on the stage of the disease, but 80% of patients achive complete remission and 60% are alive at 5 years²..

References

World Health Organization Classification of Tumours, Tumours of the haematopoietic and lymphoid tissues, IARC Press 2001.

¹de Leval, L., Ferry, J. A., Falini, B., Shipp, M., Harris, N. L. Expression of bcl-6 and CD10 in primary mediastinal large B-cell lymphoma: evidence for derivation from germinal center B cells? Am J Surg Path 2001;25:1277-1282.

²Pileri, S.A., Dirnhofer, S., Went, P., Ascani, S., Sabattini, E., Marafioti, T., Tzankov, A., Leoncini, L., Falini, B. and Zinzani, P.L. Diffuse large B-cell lymphoma: one or more entities? Present controversies and possible tools for its subclassification. Histopathology 2002;41:482-509.

³Calaminici, M., K. Piper, et al. (2004). "CD23 expression in mediastinal large B-cell lymphomas." Histopathology 45(6): 619-24.

This page last revised 17.12.2004.