November 2009
Written by Peter M. Banks, M.D., Director of Hematopathology, Carolinas Medical Center, Charlotte, North Carolina.

Submitting Physician
Eric Skipper, M.D. , Sanger Heart and Vascular Institute, Charlotte, North Carolina.

Diagnosis
Primary mediastinal large B-cell lymphoma, arising in thymus.

Clinical History
This 45 year old male from the Middle East complained of chest pain and was found to have a 3 cm mass in the left superior thymus which, over a period of 3 months, was noted to grow slightly and to become more PET-avid. The entire thymus was resected in continuity.

Microscopy
Sections of thymus showed normal adult glandular tissue without evidence of lymphoid hyperplasia (see Figure 1).

The tumor consisted of a diffuse proliferation of lymphoid cells ranging in size from medium to large with round or oval nuclei and abundant pale cytoplasm. There was relatively little fibrosis in this case (see Figure 2).

Ancillary Studies
Flow cytometry detected an abnormal population of B-cells positive for CD19 and CD20 but negative for surface immunoglobulin, CD5 and CD10. CD 45 was faintly expressed.

Paraffin section immunohistochemical preparations showed the tumor cells to be negative for pan-cytokeratin cocktail, weakly positive for CD45, and strongly positive for CD20. The cells were variably positive for CD30, negative for CD10, but positive for bcl-2 protein (see Figure 3).

Discussion
Primary B-cell lymphoma of the mediastinum is presumed to arise from the thymus, and in this case, the issue is decided with demonstrated localization of the process within a whole resected thymic gland.

Although morphologically in this case the neoplasm resembles conventional large B-cell lymphoma arising within lymph nodes and other sites, its immunophenotype is distinct, lacking surface immunoglobulin and with only faint CD45 expression and variable CD30 expression. In this regard, there is some slight resemblance to the neoplastic cells of classical Hodgkin lymphoma. This similarity has been demonstrated as well by gene expression profiling (1).

In some cases, there is even microscopic resemblance to classical Hodgkin lymphoma, as in the April 2002 Case of the Month. In the extreme situation of overlap in both appearance and immunophenotype, the overlap term "gray zone" lymphoma may be invoked to represent a process which cannot be definitely considered either Hodgkin or non-Hodgkin lymphoma (see the August 2005 Case of the Month). Many cases of primary mediastinal large B-cell lymphoma show abundant clear cytoplasm and extensive fibrosis, however these characteristic features are not present in this case.

Clinically, stage of disease is particularly important for predicting survival (2).

References

1. Rosenwald A, Wright G, Leroy K, Yu X, Gaulard P, Gascoyne RD, et al.
   Molecular diagnosis of primary mediastinal B cell lymphoma identifies a clinically favorable subgroup of diffuse large B cell lymphoma related to Hodgkin lymphoma. J Exp Med. 2003; 198: 851-862.
2. van Besien K, Kelta M, Bahaguna P.
   Primary mediastinal B-cell lymphoma: a review of pathology and management. J Clin Oncol. 2001; 19: 1855-1864.

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