—  SHORT COURSE #26  —

Hematopathology Diagnoses Too Easy to Miss!

VI. Malignant Lymphomas that Mimic Other Lymphomas of a Different Type with Very Different Clinical Implications (continued)

Marsha Kinney, James Cook and Steven Swerdlow


Case 10

Diagnosis:
Primary mediastinal (thymic) large B-cell lymphoma

Clinical History:
The patient is a 39 year old woman who was injured in a fall from a ladder. A subsequent CT scan identified a mediastinal mass that focally invaded into lung parenchyma. A biopsy of the mediastinal mass and a wedge resection of the lung were performed.

Morphology:
The histologic sections demonstrated a diffuse and focally nodular proliferation of intermediate to large lymphoid cells with moderate amounts of pale to clear cytoplasm. There was accompanying fibrosis that focally produced nodules of tumor cells. Scattered cells were very large with irregular nuclear contours and more prominent nucleoli. Some of these latter cells resided within lacunar spaces or had a hyperchromatic, "mummified" appearance.

Immunophenotype:
Flow cytometric studies showed a predominance of T-cells, while the B-cells included a population with polytypic immunoglobulin light chain expression and another population (about 50% of the B-cells) that lacked surface immunoglobulin entirely. Immunohistochemical stains demonstrated the vast majority of the proliferation to consist of sheets of CD20 positive cells that coexpressed CD45 (LCA) and CD30. The largest atypical cells, including the lacunar forms, were positive for CD15.

Primary Mediastinal Large B-cell Lymphoma:
Primary mediastinal large B-cell lymphoma (PMLBCL) was first acknowledged as a formal entity in the REAL classification of 1994, [1] and is recognized in the current WHO classification as a subtype of diffuse large B-cell lymphoma with distinct clinical and pathologic features. [2] It has been proposed that PMLBCL is related to a unique subset of B-cells that normally reside within the thymus. [2, 3]

Clinically, these lymphomas will typically present as large anterior mediastinal masses, with symptoms usually related directly to impingement on mediastinal structures. Some cases may present with disseminated involvement, including extranodal sites such as kidney and brain. Patients with PMLBCL typically present in their 3rd to 5th decade of life, younger than patients with DLBCL overall, and there is a female predominance. [4, 5, 6, 7]

PMLBCL display several characteristic morphologic features which led to the first suggestions that these cases were a distinct pathologic entity. Histologically, these specimens consist of predominantly diffuse proliferations of intermediate to large lymphoid cells, often with moderate amounts of clear to pale cytoplasm. [6, 8] There is frequent sclerosis, which is usually mild to moderate in amount and is located interstitially throughout the proliferation. In some cases, there may be bands of dense sclerosis which focally separate the tumor mass into nodules. The cytologic features are somewhat variable, both between cases and within a case. Some PMLBCL will show scattered large to very large cells with irregular nuclear contours. These largest cells may be binucleate, and may be seen to reside within lacunar spaces, creating a Reed-Sternberg-like appearance. In some cases, these Reed-Sternberg-like cells may be prominent.

Flow cytometric studies show that most cases lack detectable surface immunoglobulin expression, although rearranged immunoglobulin heavy chain genes may be detected by PCR or Southern blot studies. [9, 10] Using flow cytometry or immunostains, most cases are clearly positive for B-cell markers including CD20 and CD79a. The B-cell transcription factors PAX5, PU.1, OCT2, and BOB1 are also positive. [10] Approximately one-third of cases have been reported to be positive for CD10 and 47-100% reported to be positive for BCL6 protein. Most cases also appear to be positive for BCL2 protein. [10, 11] Immunohistochemical studies have shown most cases to be positive for CD30, although the extent of positivity may vary from cases with weak and focal expression to those with strong, diffuse positivity. [10, 12] CD45 is typically strongly positive, and CD15 expression is characteristically absent, although exceptions to this rule clearly occur (see discussion of "grey zone" lymphomas below).

Expression of the MAL protein, typically associated with T-thymocytes and rarely seen in other B-cell lymphomas, has been documented in a majority of cases of PMLBCL. [13, 14] MAL protein is also expressed in an unusual subset of B-cells that normally resides within the thymus, leading to speculation that PMBCL represent a neoplastic counterpart of this differentiated B-cell subset. Unfortunately, antibodies to MAL protein are not commercially available at the present time.

Little information is available regarding classical cytogenetic studies of PMBCL. Using CGH studies and other molecular techniques, gains of chromosome 9p appear to be a common, recurring feature in PMBCL. [15, 16] Amplification of the REL gene, on chromosome 2p, also has been described in many cases of PMLBCL. [15] To date, however, neither of these potential molecular markers has been shown to be clinically useful for the recognition of PMBCL in routine practice.

PMBCL vs Classical Hodgkin Lymphoma:
As described above, cases of PMBCL may show clinical, morphologic, and phenotypic features that overlap with classical Hodgkin lymphoma. Like many cases of classical Hodgkin lymphoma, most PMLBCL will present as a large anterior mediastinal mass. Histologically, Reed-Sternberg-like cells may be seen in many PMBCL cases, and the characteristic fibrosis may focally create nodules of tumor that simulate the architectural background of nodular sclerosis Hodgkin lymphoma. The immunophenotype in PMLBCL, including CD30 expression and lack of surface immunoglobulin rearrangements, may also create a differential diagnosis with classical Hodgkin lymphoma. In this regard, it is very interesting that recent expression microarray studies have shown that PMLBCL cases show an expression profile that clearly distinguishes PMLBCL from the other forms of DLBCL, and that shows overlap with the expression profile of classical Hodgkin lymphoma. [17, 18, 19] These studies seem to support the long-standing suspicion that PMLBCL may be pathogenetically related to classical Hodgkin lymphoma.

Fortunately, in routine clinical practice, many cases of PMBCL can be separated from other cases of DLBCL and from classical Hodgkin lymphoma on the basis of the clinical, morphologic and phenotypic findings. In excisional specimens, cases of PMLBCL will display areas containing confluent sheets of intermediate to large B-cells, leading to an appropriate diagnosis even if Reed-Sternberg-like cells are present. Most problematic are needle biopsies, which may be partially crushed, and may reveal sclerosis and Reed-Sternberg-like cells that may lead to a misdiagnosis of classical Hodgkin lymphoma. Because classical Hodgkin lymphoma and PMLBCL are generally treated with different forms of therapy, misdiagnosis will have clinical implications.

A battery of immunohistochemical stains can be extremely useful to distinguish between PMBCL and classical Hodgkin lymphoma. CD20 and CD45 expression may be seen in cases of classical Hodgkin lymphoma, but such staining is typically weak and variable. [20, 21] In contrast, PMBCL is typically strongly and diffusely positive for both CD20 and CD45. [9, 10] Analysis of the B-cell transcription factors OCT2 and BOB1 is also very valuable. Almost all cases of Hodgkin lymphoma will be negative for either OCT2, BOB1, or, in more than two thirds of cases, both of these markers. [22, 23] PMBCL, on the other hand, should be positive for both of these markers.

Mediastinal "Grey Zone" Lymphomas:
While most cases of PMLBCL are clearly distinguishable from classical Hodgkin lymphoma, there are also clearly cases that span the findings in each of these categories. Such cases have been termed "grey zone" lymphomas, and they have been proposed to represent a "missing link" in the continuum between classical Hodgkin lymphoma and PMLBCL. [24, 25] These cases include those with morphologic features most suggestive of PMLBCL, but a phenotypic profile most consistent with classical Hodgkin lymphoma, and vice versa. In the 2008 revised WHO classification, these cases are recognized in a new category termed "B-cell lymphoma, unclassifiable, with features intermediate between DLBCL and classical Hodgkin lymphoma." [26]

The findings in case 10 include focal areas with sclerotic tumor nodules, a background that focally includes small lymphocytes and eosinophils, and Reed-Sternberg-like cells that express CD30 and CD15 and appear to lack CD45. This case may therefore represent part of the spectrum that some would term a "grey zone" lymphoma.

These uncommon cases represent a true diagnostic challenge for pathologists. In all such cases, an extensive battery of immunophenotypic studies should be performed. If, after extensive phenotyping, a clear diagnosis is not apparent, it may be appropriate to issue a diagnosis of "malignant lymphoma" or "grey zone lymphoma" with a comment describing the differential diagnosis. If only a needle biopsy is initially available, a larger excisional specimen should be requested, as a larger, more representative specimen may lead to a clear diagnosis. Whenever possible, however, the pathologist should attempt to favor either PMBCL or classical Hodgkin lymphoma based upon the preponderance of the available evidence – providing the clinician with as much guidance as possible in the subsequent choice of therapy (i.e. Hodgkin vs non-Hodgkin directed therapy).

References:
  1. Harris NL, Jaffe ES, Stein H, et al. A revised European-American classification of lymphoid neoplasms: a proposal from the International Lymphoma Study Group. Blood 84: 1361-1392, 1994.

  2. Banks PM, Warnke R, Mediastinal (thymic) Large B-cell Lymphoma. in Tumours of Haematopoietic and Lymphoid Tissues. Jaffe ES, Harris NL, Stein H, et al., [Eds.] IARC Press: Lyons. 2001, 175-176.

  3. Isaacson PG, Norton AJ, Addis BJ. The human thymus contains a novel population of B lymphocytes. Lancet 2: 1488-1491, 1987.

  4. Lichtenstein AK, Levine A, Taylor CR, et al. Primary mediastinal lymphoma in adults. Am J Med 68: 509-514, 1980.

  5. Abou-Elella AA, Weisenburger DD, Vose JM, et al. Primary mediastinal large B-cell lymphoma: a clinicopathologic study of 43 patients from the Nebraska Lymphoma Study Group. J Clin Oncol 17: 784-790, 1999.

  6. Davis RE, Dorfman RF, Warnke RA. Primary large-cell lymphoma of the thymus: a diffuse B-cell neoplasm presenting as primary mediastinal lymphoma. Hum Pathol 21: 1262-1268, 1990.

  7. Cazals-Hatem D, Lepage E, Brice P, et al. Primary mediastinal large B-cell lymphoma. A clinicopathologic study of 141 cases compared with 916 nonmediastinal large B-cell lymphomas, a GELA ("Groupe d'Etude des Lymphomes de l'Adulte") study. Am J Surg Pathol 20: 877-888, 1996.

  8. Paulli M, Strater J, Gianelli U, et al. Mediastinal B-cell lymphoma: a study of its histomorphologic spectrum based on 109 cases. Hum Pathol 30: 178-187, 1999.

  9. Lamarre L, Jacobson JO, Aisenberg AC, et al. Primary large cell lymphoma of the mediastinum. A histologic and immunophenotypic study of 29 cases. Am J Surg Pathol 13: 730-739, 1989.

  10. Pileri SA, Gaidano G, Zinzani PL, et al. Primary mediastinal B-cell lymphoma: high frequency of BCL-6 mutations and consistent expression of the transcription factors OCT-2, BOB.1, and PU.1 in the absence of immunoglobulins. Am J Pathol 162: 243-253, 2003.

  11. de Leval L, Ferry JA, Falini B, et al. Expression of bcl-6 and CD10 in primary mediastinal large B-cell lymphoma: evidence for derivation from germinal center B cells? Am J Surg Pathol 25: 1277-1282, 2001.

  12. Higgins JP, Warnke RA. CD30 expression is common in mediastinal large B-cell lymphoma. Am J Clin Pathol 112: 241-247, 1999.

  13. Copie-Bergman C, Gaulard P, Maouche-Chretien L, et al. The MAL gene is expressed in primary mediastinal large B-cell lymphoma. Blood 94: 3567-3575, 1999.

  14. Copie-Bergman C, Plonquet A, Alonso MA, et al. MAL expression in lymphoid cells: further evidence for MAL as a distinct molecular marker of primary mediastinal large B-cell lymphomas. Mod Pathol 15: 1172-1180, 2002.

  15. Joos S, Otano-Joos MI, Ziegler S, et al. Primary mediastinal (thymic) B-cell lymphoma is characterized by gains of chromosomal material including 9p and amplification of the REL gene. Blood 87: 1571-1578, 1996.

  16. Bentz M, Barth TF, Bruderlein S, et al. Gain of chromosome arm 9p is characteristic of primary mediastinal B-cell lymphoma (MBL): comprehensive molecular cytogenetic analysis and presentation of a novel MBL cell line. Genes Chromosomes Cancer 30: 393-401, 2001.

  17. Savage KJ, Monti S, Kutok JL, et al. The molecular signature of mediastinal large B-cell lymphoma differs from that of other diffuse large B-cell lymphomas and shares features with classical Hodgkin lymphoma. Blood 102: 3871-3879, 2003.

  18. Rosenwald A, Wright G, Leroy K, 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 198: 851-862, 2003.

  19. Bea S, Zettl A, Wright G, et al. Diffuse large B-cell lymphoma subgroups have distinct genetic profiles that influence tumor biology and improve gene-expression-based survival prediction. Blood 106: 3183-3190, 2005.

  20. Zukerberg LR, Collins AB, Ferry JA, et al. Coexpression of CD15 and CD20 by Reed-Sternberg cells in Hodgkin's disease. Am J Pathol 139: 475-483, 1991.

  21. Schmid C, Pan L, Diss T, et al. Expression of B-cell antigens by Hodgkin's and Reed-Sternberg cells. Am J Pathol 139: 701-707, 1991.

  22. Stein H, Marafioti T, Foss HD, et al. Down-regulation of BOB.1/OBF.1 and Oct2 in classical Hodgkin disease but not in lymphocyte predominant Hodgkin disease correlates with immunoglobulin transcription. Blood 97: 496-501, 2001.

  23. Re D, Muschen M, Ahmadi T, et al. Oct-2 and Bob-1 deficiency in Hodgkin and Reed Sternberg cells. Cancer Res 61: 2080-2084, 2001.

  24. Traverse-Glehen A, Pittaluga S, Gaulard P, et al. Mediastinal Gray Zone Lymphoma: The Missing Link Between Classic Hodgkin's Lymphoma and Mediastinal Large B-Cell Lymphoma. Am J Surg Pathol 29: 1411-1421, 2005.

  25. Poppema S, Kluiver JL, Atayar C, et al. Report: workshop on mediastinal grey zone lymphoma. Eur J Haematol Suppl: 45-52, 2005.

  26. Jaffe ES, Stein H, Swerdlow SH, et al., B-cell lymphoma, unclassifiable, with features intermediate between DLBCL and classical Hodgkin lymphoma. in WHO Classification of Tumours of Haematopoietic and Lymphoid Tissues, 4th Ed. Swerdlow SH, Campo E, Harris NL, et al., [Eds.] IARC Press: Lyons. 2008, 267-268.