—  SOCIETY FOR HEMATOPATHOLOGY   —

Mediastinal Lymphomas: Practical and Theoretical Issues


Elaine S. Jaffe
National Cancer Institute
Bethesda, MD


Introduction
Mediastinal lymphomas arise in either mediastinal lymph nodes or the thymus gland. Thymic lymphomas are unique in many respects, as they reflect the function of the thymus gland as an organ involved in T-cell generation and differentiation. [1] Precursor T-lymphoblastic lymphoma/leukemia (T-LBL) presents as a mediastinal mass in 50-80% of cases, and the immunophenotype of the neoplastic cells reflects the stages of cortical thymocyte differentiation. [2] There are also rare reports of natural killer (NK)-cell tumors with a immature phenotype arising in the thymus gland, and the fetal thymus is one site of NK-cell development. [1] B-cell lymphomas of the thymus gland are relatively rare. The most common of these is primary mediastinal large B-cell lymphoma (PMLBCL), of proposed origin from specialized thymic B-cells found in the medullary perivascular space. [3, 4] Classical Hodgkin lymphoma, nodular sclerosis type, (NSCHL) also arises in the thymus gland, and is genotypically of B-cell origin, although B-cell markers may be absent. Lymphomas of mucosa-associated lymphoid tissue (MALT )-type may arise in the thymus gland, as well as in other mucosal or epithelial sites, and reflect the intimate functional relationship between epithelial and lymphoid components in the thymus gland. [5] A functionally related lesion is the multilocular thymic cyst seen in autoimmune disease and HIV-infection. [6, 7, 8] Lymphomas involving the mediastinal lymph nodes reflect to some extent the spectrum of systemic nodal lymphomas. However, because of its inaccessibility as a biopsy site, the primary diagnosis of lymphoma is uncommonly made in mediastinal lymph nodes.

Myeloid neoplasms rarely have primary presentations in the mediastinum, and may represent a pitfall in the differential diagnosis of lymphoblastic lymphoma and PMLBCL. A recently described entity, precursor T-lymphoblastic lymphoma with eosinophilia and t(8;13) typically presents with a mediastinal tumor with the immunophenotype of T-LBL, but is associated with development of acute myeloid leukemia in the bone marrow [9] . Acute myeloid leukemias , often with megakaryoblastic differentiation may develop in the mediastinum and bone marrow in association with non-seminomatous germ cell tumors with an i(12)p [10, 11, 12, 13, 14, 15, 16, 17, 18] .

Histiocytic and dendritic cell tumors are rare tumors that occasionally may present in mediastinal lymph nodes and the thymus gland. However, as with myeloid neoplasms, most histiocytic neoplasms presenting in the mediastinum are related to teratomatous germ cell tumors, indicative of the capacity of germ cell neoplasms to differentiate along many cell lines. [16, 17, 18, 19]

Incidence and Clinical Features
Precursor T-cell and NK-cell neoplasms are for the most part diseases of children and young adults, with an increased male: female ratio. Mediastinal large B-cell lymphoma and nodular sclerosis Hodgkin's lymphoma share many epidemiological features, including prevalence in young adult females, and propensity to present with localized disease. This observation, plus the fact that synchronous and metachronous instances of mediastinal large B-cell lymphoma and nodular sclerosis Hodgkin's lymphoma may be encountered, has suggested that these neoplasms may share a common cell of origin. In addition, there are rare gray zone lymphomas with features intermediate between both entities.

With the exception of the relatively rare MALT- type lymphomas, most mediastinal lymphomas and hematopoietic neoplasms are clinically aggressive. The treatment approach is based on the primary diagnosis, and does not differ for the same disease presenting in other anatomic sites. [20] For cases of nodular sclerosis Hodgkin's lymphoma presenting with massive mediastinal disease, with a mediastinal mass in excess of one-third of the chest diameter, combined modality therapy employing both radiation and chemotherapy may be used.

Precursor T-lymphoblastic lymphoma/leukemia
Most T-LBL are cytologically indistinguishable from their B-cell counterparts. Deep nuclear indentations or convolutions may or may not be present. The cells have finely distributed chromatin, inconspicuous nucleoli, and sparse, pale cytoplasm. This is a disease of adolescents and young adults, with an increased male-to-female ratio. Fifty to 80 percent of patients present with an anterior mediastinal mass, usually with involvement of the thymus gland [20] . A pleural effusion is common, and in contrast to the effusions associated with classical Hodgkin's lymphoma, usually contains neoplastic cells. Bone marrow involvement is common, and progression to a leukemic picture will occur in the absence of effective therapy. This tumor also has a high frequency of involvement of the central nervous system.

In lymph nodes T-LBL has a diffuse leukemic pattern of infiltration. There is very little stromal reaction, and the cells diffusely infiltrate the lymph node parenchyma. Streaming of cells in the medullary cords may be prominent, especially around vascular structures. Some residual follicles may be present, but ultimately architectural effacement is the rule. A starry sky pattern is seen in approximately one-third of cases. Mitotic figures are readily observed.

T-LBL presenting as lymphoma usually has a more mature immunophenotype, than those cases presenting purely as leukemia [21] . Neoplastic cells are positive for terminal transferase (TDT), and express CD3, CD5, CD5, and CD7 in the majority of cases. Cytoplasmic CD3 is present prior to the acquisition of surface CD3 [22] . The cells may lack CD4 and CD8, may be double positive for CD4 and CD8, or express only one of these antigens. Although CD79a generally is indicative of a B-cell neoplasm, CD79a expression has been reported in some T-LBL [23] .

Extranodal Marginal Zone B-cell Lymphomas (MALT lymphomas) of the Thymus Gland
Thymic MALT lymphomas are strongly associated with autoimmune disease, especially Sjogren's syndrome [5] . They occur more often in females than males (M: F = 1:3). They often have a cystic component, which is common to many neoplasms involving the thymus gland. Most of the patients present with localized disease. There may be involvement of regional lymph nodes. Involvement of distant extranodal MALT sites is seen in approximately 20% of cases.

Histologically, the lesions resemble MALT lymphomas in other sites [24] . Prominent lymphoepithelial lesions are observed in the thymic epithelium lining the cystic spaces. The atypical lymphoid cells may also infiltrate the Hassall's corpuscles. As in other MALT lymphomas, reactive lymphoid follicles and numerous plasma cells may be present.

The differential diagnosis includes multilocular thymic cyst and lymphoid hyperplasia of the thymus gland [6, 7, 8] . In both of the above lesions monocytoid cells are not prominent, and molecular studies will reveal a polyclonal rather than a monoclonal B-cell population.

Primary Mediastinal Large B-cell Lymphoma (PMLBCL)
PMLBCL is thought to originate from a malignant thymic B-cell [4] . Histologically it is composed of sheets of large B-cells, often with clear cytoplasm [25] . Sclerosis is often present, and may be extensive. PMLBCL is characterized by a female predominance, young age at onset, frequent involvement of thymus, anterior mediastinum and supraclavicular lymph nodes. Patients may present with a superior vena cava syndrome, due to invasion and compression of local structures.

The cells express CD20 and CD79a. CD30 is expressed in a high proportion of cases, and the cells typically lack both Ig and HLA Class I and Class II molecules [26, 27] . Recent studies have identified expression of CD23, a marker that is also characteristic of the asteroid cells of the thymus gland [28] . Asteroid cells have dendritic cytoplasm and are CD20-positive [29, 30] , and the recent identification of CD23 in PMLBCL lends support to the idea that this cell is the precursor of PMLBCL.

PMLBCLs do not characteristically contain the BCL2 or BCL6 rearrangements harbored by other DLBCLs [31] . However, two genes, MAL (encodes a cell surface protein / lipid raft component) and FIG1 (an IL-4 responsive gene) have been identified as frequently expressed in PMLBCL [32] ; however, they are not expressed in all PMLBCLs and FIG1 is also expressed in some other DLBCLs [33]

A diagnostic challenge arises from the fact that other types of diffuse large B-cell lymphoma (DLBCL) can present with mediastinal disease, involving mediastinal lymph nodes either primarily or secondarily. Distinguishing among these entities can be challenging based on current diagnostic criteria. The prognosis and response to treatment associated with PMLBCL has been somewhat controversial – perhaps in part due to the difficulty in accurately separating PMLBCL from other DLBCL. One study reported an overall 82% survival at 3 yrs with combined chemotherapy and radiotherapy, notably better than other DLBCL [34]. However, other studies reported a poorer response to therapy, similar or worse than other DLBCLs [35]. Immunohistochemical studies have been imprecise in separating PMLBCL from other DLBCL, since both CD10 and BCL-6 have been reported in some cases, and CD30 and other markers lack specificity [36]. Notably, a recent study utilizing gene expression profiling was able to distinguish PMLBCL from other DLBCL, even those presenting with mediastinal disease [37]. Using this approach PMLBCL has a favorable prognosis when appropriately treated. Moreover, both Rosenwald et al. and Savage et al. found similarities in the gene expression profile between PMLBCL and CHL-NS [37, 38] .

Earlier studies had shown that PMLBCL was characterized by chromosomal gains at 9p24, associated with amplification of JAK2 and REL [26, 39, 40] . In keeping with the amplification of JAK2 at 9p, Savage et al. also verified the protein expression of Stat1 and Traf1 by immunohistochemistry on paraffin-embedded sections from cases of PMLBCL, DLCBL, and NS-CHL. Consistent with the data obtained via gene expression profiling, they found Traf1 and Stat 1 proteins to be expressed in PMLBCLs and NS-CHL, but not in other DLBCL. These data suggest that pathways utilizing Stat1 and Traf1 are commonly activated in both PMLBCL and in NS-CHL.

The recent gene expression studies also suggested activation of the NF-kB pathway in PMLBCL. Activation of the NF- k Bpathway also has been reported in classical Hodgkin's lymphoma, and can enhance survival of malignant cells [41, 42] . Activation of this pathway in both PMLBCL and NS-CHL may constitute a shared mechanism of blocking apoptosis, and lends further support to common pathogenetic mechanisms in both tumors.

Classical Hodgkin's Lymphoma, Nodular Sclerosis Subtype
CHL-NS is the most common subtype of HL, accounting for approximately 75% of cases in the United States [20] . This is the only subtype without a male predominance (M :F ratio approximately 1:1). It tends to occur in young adults, usually under the age of 50 years. Anterior mediastinal involvement is exceedingly common, with subsequent involvement of cervical and supraclavicular lymph nodes, upper abdominal lymph nodes, and spleen. Most patients present with stage II disease. Bulky mediastinal masses may occur and are a poor prognostic sign. The disease also may extend directly into the adjacent lung.

The diagnosis of CHL-NS requires the presence of

  1. A nodular growth pattern,

  2. Broad bands of fibrosis

  3. A characteristic variant of the Reed-Sternberg (RS) cell known as the lacunar cell [43]
The lacunar cell has abundant clear cytoplasm with a sharply demarcated cell membrane. In formalin-fixed tissue a characteristic artifact often occurs; the cytoplasm of the cell retracts, leaving a clear space or lacunus. The lacunar cell may be mononuclear, hyperlobated, or multinucleated. The nucleoli of lacunar cells generally are smaller than those seen in classic RS cells. In the cellular phase of CHL-NS, tissue sections show a nodular growth pattern with lacunar cells but with absent or minimal fibrous bands. This finding represents a phase in the development of CHL-NS, and is not associated with unique clinical features. A syncytial variant of CHL-NS has been described, in which prominent aggregates of lacunar cells are seen, often with frequent eosinophils [44] .

The British National Lymphoma Investigation (BNLI) proposed a grading system for CHL-NS, based on the frequency of malignant cells, as well as other factors including necrosis [45] . In Grade 1 lesions, at least 75% of the nodules contain scattered RS cells, whereas in Grade 2 lesions, at least 25% of the nodules contain numerous malignant cells which sheet out, often surrounding areas of necrosis. BNLI Grade 2 lesions, corresponding to the previous designation of lymphocyte-depleted CHL-NS [46] , are associated with a more aggressive clinical course, although not all studies have supported the importance of histological grading in CHL-NS [47] .

Classical Hodgkin's lymphoma has a characteristic immunophenotype. CD30 is positive in close to 100% of cases, while CD15 is expressed in 75-85% of cases. CD45 is typically negative, but due to the admixture of numerous reactive cells, the reactivity of the neoplastic cells may be difficult to determine. CD20 expression is variable, but when present is usually weak and positive in few than 100% of cells [48] . Other B-cell markers, such as CD79a are typically negative, whereas PAX-5 is positive [49, 50] . The absence of CD15 and the expression of B-cell markers has been suggested to be an adverse prognostic factor in some series [51] . Epstein Barr sequences are typically negative in CHL-NS, and more often positive in mixed cellularity CHL.

The neoplastic cells are rosetted by CD3 T-cells, and expression of T-cell associated antigens has been reported sporadically [52, 53] . In tissue sections, a positive reaction product sometimes surrounds the surface membrane of Hodgkin-Reed Sternberg (H-RS) cells when sections are stained for CD3, CD2, and other T-cell associated antigens. In some cases this reaction product appears extrinsic to the cell membrane, as though it were adsorbed on the cell surface. It may result from remnants of T-cell membranes adherent to the H-RS cells. It also has been shown that activated T-cells may transfer T-cell antigens to other cell types by intercellular antigen transfer [54] . This process involves CD2 and CD58(LFA3), the ligand for the E-rosette receptor, CD2. Notably, H-RS cells express CD58, the antigen responsible for the T-cell rosetting phenomenon [55] . This pattern of immunoreactivity may lead to suspicion of a diagnosis of T-cell lymphoma. However, molecular studies usually fail to show evidence of clonal T-cell receptor gene rearrangement.

A B-cell lineage has been ascribed to the neoplastic cells of CHL through the use of microdissection of H-RS cells and molecular analysis [56] . However, the cells fail to function as normal B-cells, and do not synthesize Ig. A number of mechanisms have been postulated to play a role, including crippling mutations of the Ig genes, and failure of expression of the transcription factors OCT-2 and Bob.1 [57, 58, 59] .

Mediastinal Gray Zone Lymphoma
Gray zone lymphoma is defined as a lymphoma with indeterminate histological and immunophenotypic features. A variety of gray zone lymphomas have been described, and include the interface between CHL and non-Hodgkin's lymphoma, and the interface between nodular lymphocyte predominant Hodgkin's lymphoma and T-cell/ histiocyte-rich large B-cell lymphoma. The existence of gray zone lymphomas implies a biological relatedness between the entities, and not simply superficial histological similarities. One of the more common gray zone lymphomas, which also may present as a composite lymphoma, involves mediastinal (thymic) large B-cell lymphoma (PMLBCL) and CHL-NS. [61, 62, 63] CHL-NS and PMLBCL share a number of common clinical features. They both show a female predominance, present in young adults -- albeit at a slightly older age in PMLBCL --, and involve the anterior mediastinum, thymus gland and supraclavicular lymph nodes [25, 64] .

In gray zone lymphoma cases the histological and immunophenotypic features are transitional between CHL-NS and PMLBCL. Clusters of cells akin to lacunar cells or even classical RS-cells may be seen in a background resembling PMLBCL. In some cases the histology is composite, with some areas resembling CHL-NS and other areas showing sheets of large B-cells characteristic of PMLBCL. The inflammatory background and pattern of sclerosis usually corresponds to the appearance of the neoplastic cells, resembling either CHL-NS or PMLBCL, respectively. Immunophenotypically, the features also are intermediate. Scattered RS-like cells will be CD30-positive, but CD15-positivity is more inconsistent. Interestingly, the CD30 antigen is often expressed in PMLBCL [26] . The majority of the infiltrating cells are usually CD20-positive, sometimes with variable staining intensity. Both CHL-NS and PMLBCL are negative for immunoglobulin expression, so studies of immunoglobulin are non-informative [27, 65] .

In a recent study, we identified asynchrony between the morphology and immunophenotype in a series of mediastinal gray zone lymphomas [66] . If the morphological appearance suggested PMLBCL, the immunophenotype was often more typical of CHL, and vice versa. These cases are problematic for pathologists and clinicians because there is uncertainty in how to treat these patients. At the NCI, a treatment approach using dose-adjusted EPOCH, with rituximab for patients with CD20-expressing tumors, has proved useful.

PMLBCL has been reported following CHL, but in contrast to most DLBCL and other high grade B-cell lymphomas that typically present 10 years or longer after the diagnosis of CHL, PMLBCL presents early, frequently within one year [61] . This close association suggests a different pathogenesis for secondary PMLBCL, compared to the late occurring aggressive B-cell lymphomas, and raises the likely possibility that these cases are clonally related to the CHL. [3] CHL-NS and PMLBCL also share a number of molecular characteristics, such as REL amplification and gains on chromosomal 9, suggesting molecular overlap as well [39, 67] . Recent studies have identified the MAL gene as aberrantly expressed in PMLBCL, and likewise MAL has been detected in some cases of mediastinal CHL-NS, and grey zone lymphomas (unpublished data) [32, 33] .

References

  1. Spits H, Blom B, Jaleco AC, et al. Early stages in the development of human T, natural killer and thymic dendritic cells. Immunol Rev. 1998;165:75-86.

  2. Bernard A, Boumsell L, Reinherz EL, et al. Cell surface characterization of malignant T cells from lymphoblastic lymphoma using monoclonal antibodies: evidence for phenotypic differences between malignant T cells from patients with acute lymphoblastic leukemia and lymphoblastic lymphoma. Blood. 1981;57:1105-1110.

  3. Addis B, Isaacson P. Large cell lymphoma of the mediastinum: a B-cell tumor of probable thymic origin. Histopathology. 1986;10:379-390.

  4. Isaacson P, Norton A, Addis B. The human thymus contains a novel population of B-lymphocytes. Lancet. 1987;ii:1488-1490.

  5. Inagaki H, Chan JK, Ng JW, et al. Primary thymic extranodal marginal-zone B-cell lymphoma of mucosa-associated lymphoid tissue type exhibits distinctive clinicopathological and molecular features. Am J Pathol. 2002;160:1435-1443.

  6. Suster S, Rosai J. Multilocular thymic cyst: an acquired reactive process. Study of 18 cases. Am J Surg Pathol. 1991;15:388-398.

  7. Kontny HU, Sleasman JW, Kingma DW, et al. Multilocular thymic cysts in children with human immunodeficiency virus infection: clinical and pathologic aspects. J Pediatr. 1997;131:264-270.

  8. Mishalani SH, Lones MA, Said JW. Multilocular thymic cyst. A novel thymic lesion associated with human immunodeficiency virus infection. Arch Pathol Lab Med. 1995;119:467-470.

  9. Xiao S, Nalabolu SR, Aster JC, et al. FGFR1 is fused with a novel zinc-finger gene, ZNF198, in the t(8;13) leukaemia/lymphoma syndrome. Nat Genet. 1998;18:84-87.

  10. DeMent SH, Eggleston JC, Spivak JL. Association between mediastinal germ cell tumors and hematologic malignancies. Report of two cases and review of the literature. Am J Surg Pathol. 1985;9:23-30.

  11. Nichols CR, Hoffman R, Einhorn LH, et al. Hematologic malignancies associated with primary mediastinal germ-cell tumors. Ann Intern Med. 1985;102:603-609.

  12. Chaganti RS, Ladanyi M, Samaniego F, et al. Leukemic differentiation of a mediastinal germ cell tumor. Genes Chromosomes Cancer. 1989;1:83-87.

  13. Domingo A, Romagosa V, Callis M, et al. Mediastinal germ cell tumor and acute megakaryoblastic leukemia. Ann Intern Med. 1989;111:539.

  14. Ladanyi M, Samaniego F, Reuter VE, et al. Cytogenetic and immunohistochemical evidence for the germ cell origin of a subset of acute leukemias associated with mediastinal germ cell tumors. J Natl Cancer Inst. 1990;82:221-227.

  15. Nichols CR, Roth BJ, Heerema N, et al. Hematologic neoplasia associated with primary mediastinal germ-cell tumors. New England Journal of Medicine. 1990;322:1425-1429.

  16. Berruti A, Paze E, Fara E, et al. Acute myeloblastic leukemia associated with mediastinal nonseminomatous germ cell tumors. Report on two cases. Tumori. 1995;81:299-301.

  17. Saito A, Watanabe K, Kusakabe T, et al. Mediastinal mature teratoma with coexistence of angiosarcoma, granulocytic sarcoma and a hematopoietic region in the tumor: a rare case of association between hematological malignancy and mediastinal germ cell tumor. Pathol Int. 1998;48:749-753.

  18. Govender D, Pillay SV. Mediastinal immature teratoma with yolk sac tumor and myelomonocytic leukemia associated with Klinefelter's syndrome. Int J Surg Pathol. 2002;10:157-162.

  19. Koo CH, Reifel J, Kogut N, et al. True histiocytic malignancy associated with a malignant teratoma in a patient with 46XY gonadal dysgenesis. Am J Surg Pathol. 1992;16:175-183.

  20. Jaffe ES, Harris NL, Stein H, et al. Pathology and Genetics of Tumours of Haematopoietic and Lymphoid Tissues. Lyon, France: IARC Press; 2001.

  21. Gouttefangeas C, Bensussan A, Boumsell L. Study of the CD3-associated T-cell receptors reveals further differences between T-cell acute lymphoblastic lymphoma and leukemia. Blood. 1990;74:931-934.

  22. Pittaluga S, Uppenkamp M, Cossman J. Development of T3/T cell receptor gene expression in human pre-T neoplasms. Blood. 1987;69:1062-1067.

  23. Pilozzi E, Pulford K, Jones M, et al. Co-expression of CD79a (JCB117) and CD3 by lymphoblastic lymphoma. J Pathol. 1998;186:140-143.

  24. McCluggage WG, McManus K, Qureshi R, et al. Low-grade B-cell lymphoma of mucosa-associated lymphoid tissue (MALT) of thymus. Hum Pathol. 2000;31:255-259.

  25. Moller P, Moldenhauer G, Momburg F, et al. Mediastinal lymphoma of clear cell type is a tumor corresponding to terminal steps of B cell differentiation. Blood. 1987;69:1087-1095.

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

  27. Kanavaros P, Gaulard P, Charlotte F, et al. Discordant expression of immunoglobulin and its associated molecule mb-1/CD79a is frequently found in mediastinal large B-cell lymphomas. Am J Pathol. 1995;146:735-741.

  28. Calaminici M, Piper K, Lee AM, et al. CD23 expression in mediastinal large B-cell lymphomas. Histopathology. 2004;45:619-624.

  29. Taubenberger JK, Jaffe ES, Medeiros LJ. Thymoma with abundant L26-positive 'asteroid' cells. A case report with an analysis of normal thymus and thymoma specimens. Arch Pathol Lab Med. 1991;115:1254-1257.

  30. Fend F, Nachbaur D, Oberwasserlechner F, et al. Phenotype and topography of human thymic B cells. An immunohistologic study. Virchows Arch B Cell Pathol Incl Mol Pathol. 1991;60:381-388.

  31. Tsang P, Cesarman E, Chadburn A, et al. Molecular characterization of primary mediastinal B cell lymphoma. Am J Pathol. 1996;148:2017-2025.

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

  33. 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. 2002;15:1172-1180.

  34. Zinzani PL, Martelli M, Bendandi M, et al. Primary mediastinal large B-cell lymphoma with sclerosis: a clinical study of 89 patients treated with MACOP-B chemotherapy and radiation therapy. Haematologica. 2001;86:187-191.

  35. 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. 1999;17:784-790.

  36. 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. 2001;25:1277-1282.

  37. 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. 2003;198:851-862.

  38. 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. 2003;102:3871-3879.

  39. 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. 1996;87:1571-1578.

  40. Barth TF, Leithauser F, Joos S, et al. Mediastinal (thymic) large B-cell lymphoma: where do we stand? Lancet Oncol. 2002;3:229-234.

  41. Jungnickel B, Staratschek-Jox A, Brauninger A, et al. Clonal deleterious mutations in the IkappaBalpha gene in the malignant cells in Hodgkin's lymphoma. J Exp Med. 2000;191:395-402.

  42. Kuppers R, Schwering I, Brauninger A, et al. Biology of Hodgkin's lymphoma. Ann Oncol. 2002;13 Suppl 1:11-18.

  43. Lukes R, Butler J, Hicks E. Natural history of Hodgkin's disease as related to its patholgical picture. Cancer. 1966;19:317-344.

  44. Strickler J, Michie S, Warnke R, et al. The "syncytial variant" of nodular sclerosing Hodgkin's disease. Am J Surg Pathol. 1986;10:470-477.

  45. MacLennan K, Bennett M, Tu A, et al. Relationship of histopathologic features to survival and relapse in nodular sclerosing Hodgkin's disease. Cancer. 1989;64:1686-1693.

  46. Kant JA, Hubbard SM, Longo DL, et al. The pathologic and clinical heterogeneity of lymphocyte-depleted Hodgkin's disease. J Clin Oncol. 1986;4:284-294.

  47. Ferry JA, Linggood RM, Convery KM, et al. Hodgkin disease, nodular sclerosis type. Implications of histologic subclassification. Cancer. 1993;71:457-463.

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

  49. Foss HD, Reusch R, Demel G, et al. Frequent expression of the B-cell-specific activator protein in Reed-Sternberg cells of classical Hodgkin's disease provides further evidence for its B-cell origin. Blood. 1999;94:3108-3113.

  50. Korkolopoulou P, Cordell J, Jones M, et al. The expression of the B-cell marker mb-1 (CD79a) in Hodgkin's disease. Histopathology. 1994;24:511-515.

  51. von Wasielewski R, Mengel M, Fischer R, et al. Classical Hodgkin's disease. Clinical impact of the immunophenotype. Am J Pathol. 1997;151:1123-1130.

  52. Falini B, Stein H, Pileri S, et al. Expression of lymphoid-associated antigens on Hodgkin's and Reed-Sternberg cells of Hodgkin's disease. An immunocytochemical study on lymph node cytospins using monoclonal antibodies. Histopathology. 1987;11:1229-1242.

  53. Kadin ME, Muramoto L, Said J. Expression of T-cell antigens on Reed-Sternberg cells in a subset of patients with nodular sclerosing and mixed cellularity Hodgkin's disease. Am J Pathol. 1988;130:345-353.

  54. Rabinowitz R, Pokroy R, Yu Y, et al. Activated human T-cells bestow T-cell antigens to non-T-cells by intercellular antigen transfer. Hum Immunol. 1998;59:331-342.

  55. Sanders ME, Makgoba MW, Sussman EH, et al. Molecular pathways of adhesion in spontaneous rosetting of T-lymphocytes to the Hodgkin's cell line L428. Cancer Res. 1988;48:37-40.

  56. Kuppers R, Rajewsky K, Zhao M, et al. Hodgkin's disease: Hodgkin and Reed Sternberg cells picked from histological sections show clonal immunoglobulin gene rearrangements and appear to be derived from B cells at various stages of development. Proc Nat Acad Sci USA. 1994;91:1092-1096.

  57. Kanzler H, Kuppers R, Hansmann ML, et al. Hodgkin and Reed-Sternberg cells in Hodgkin's disease represent the outgrowth of a dominant tumor clone derived from (crippled) germinal center B cells. J Exp Med. 1996;184:1495-1505.

  58. Marafioti T, Hummel M, Foss HD, et al. Hodgkin and reed-sternberg cells represent an expansion of a single clone originating from a germinal center B-cell with functional immunoglobulin gene rearrangements but defective immunoglobulin transcription. Blood. 2000;95:1443-1450.

  59. Saez AI, Artiga MJ, Sanchez-Beato M, et al. Analysis of octamer-binding transcription factors Oct2 and Oct1 and their coactivator BOB.1/OBF.1 in lymphomas. Mod Pathol. 2002;15:211-220.

  60. Gonzalez CL, Medeiros LJ, Jaffe ES. Composite lymphoma. A clinicopathologic analysis of nine patients with Hodgkin's disease and B-cell non-Hodgkin's lymphoma. Am J Clin Pathol. 1991;96:81-89.

  61. Zarate OA, Medeiros LJ, Longo DL, et al. Non-Hodgkin's lymphomas arising in patients successfully treated for Hodgkin's disease. A clinical, histologic, and immunophenotypic study of 14 cases. Am J Surg Pathol. 1992;16:885-895.

  62. Perrone T, Frizzera G, Rosai J. Mediastinal diffuse large-cell lymphoma with sclerosis. A clinicopathologic study of 60 cases. Am J Surg Pathol. 1986;10:176-191.

  63. Rudiger T, Jaffe ES, Delsol G, et al. Workshop report on Hodgkin's disease and related diseases ('grey zone' lymphoma). Ann Oncol. 1998;9 Suppl 5:S31-38.

  64. Lamarre L, Jacobson J, Aisenberg A, et al. Primary large cell lymphoma of the mediastinum. Am J Surg Pathol. 1989;13:730-739.

  65. 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. 2003;162:243-253.

  66. Traverse-Glehen A, Pittaluga S, Vivero A, et al. Mediastinal grey zone lymphoma: The "missing link" between classical Hodgkin's lymphoma and mediastinal large B cell lymphoma. Mod Pathol. 2004;17:274A.

  67. Barth TF, Martin-Subero JI, Joos S, et al. Gains of 2p involving the REL locus correlate with nuclear c-Rel protein accumulation in neoplastic cells of classical Hodgkin lymphoma. Blood. 2003;101:3681-3686.