Fig. 8.1
H&E morphology of primary mediastinal large B cell lymphoma. a+b Low power view showing solid as well as diffuse infiltrative tumor areas. b Higher magnification shows interstitial infiltration of fat cells by a tumor with clear cell morphology. At this magnification, one possible differential diagnosis would be mediastinal seminoma. c High power magnification shows large tumor cells with more basophilic cytoplasm growing in solid nests with some interstitial matrix. Some tumor cells share similarities with Hodgkin cells. d–f Typical dense fibrosis found in many cases. Some cases (f) show a slight inflammatory background, which is however quite different from the distinct and dense inflammatory background seen in Hodgkin lymphoma
Immunohistochemistry and Other Ancillary Studies
The key immunohistochemical findings are summarized in Table 8.1. PMBL shows diffuse expression of B-cell markers such as PAX5 (strong), OCT2, BOB.1, CD19, CD20, or CD79a, but surface immunoglobulins and light chains are not expressed. CD30 is present in >80% of cases, but usually weak and more focal than in Hodgkin lymphoma. Many tumors (>75%) express CD23 and p63. CD15 is negative (Fig. 8.2) [20]. Clonality assays show monoclonal immunoglobulin rearrangements with a post-germinal center phenotype (Ig-class switch, somatic hypermutation) [21].
Table 8.1
Informative immunohistochemical markers in the differential diagnosis of large B cell lymphomas in the mediastinum
PMBL | MGZL | NS-CHL | DLBCL | |
|---|---|---|---|---|
CD20 | ++ | +/++ | –/(+) | ++ |
PAX5 | ++ | ++ | (+) | ++ |
BOB.1 | ++ | ++ | –/(+) | ++ |
OCT2 | ++ | ++ | ∓ | ++ |
MAL | 60–70% | 30–40% | 20% | – |
CD30 | (+)/+ | ++ | ++Golgi pattern | ∓ |
CD15 | – | + | + (>75%) | – |
s-Immunoglobulins | – | – | – | ++ |
CD45 | ++ | ∓ | – | ++ |
HLA class 1 and 2 | – | ∓/++ | – | ++ |
LMP | – | ∓ | ∓(<25%) | – |
Fig. 8.2
Immunohistochemical features of primary mediastinal large B cell lymphoma. a Strong and diffuse expression of CD20, b Weak and focal expression of CD30, c moderate expression of CD23, e diffuse nuclear expression of OCT2, d nuclear expression of p63, e high proliferation index (ki67)
Differential Diagnosis and Pitfalls
Major differential diagnoses include mediastinal involvement of a nodal diffuse large B-cell lymphoma, mediastinal Hodgkin lymphoma, and mediastinal gray zone lymphoma. The distinction between these entities is important, since there are major differences in treatment and prognosis. Clinical information on presence of widespread extramediastinal dissemination should be obtained to rule out DLBCL. A background of inflammatory cells that is typical of Hodgkin lymphoma is absent in PMBL.
Pitfalls
Remnants of an infiltrated thymus (e.g., Hassall corpuscles) and residual keratin-positive epithelial cells can mimic a thymoma or thymic carcinoma. On the other hand, some thymic carcinomas, including rare variants of lymphoepithelioma-like carcinoma, can have clear cell features with or without a rich inflammatory background that may even contain eosinophils, and may show weak or only focal keratin expression. In mediastinal lymph nodes involved by PMBL, the infiltration pattern may be carcinoma-like with colonization of the marginal sinus, a feature that is also seen in anaplastic large T-cell lymphomas (ALCL). It is important to remember that both PMBL and many carcinomas (as well as thymomas) express p63. Mediastinal germ cell tumors (especially seminomas) can have a clear cell morphology, and embryonal carcinomas express CD30 and p63.
Suggested Strategy to Confirm PMBL in a Small Biopsy
If PMBL is suspected on morphology, the initial set of stainings should include CD5, CD15, CD20, CD30, CD23, p63, BOB.1/PAX5, LMP, ki67. Depending on the context, additional markers should include keratin (epithelial tumors), S100 (neurogenic tumors and melanoma), and SALL4 (germ cell tumors). Clonality assays are usually not required for diagnosis.
Take Home Messages
PMBL occurs predominantly in young females and is usually restricted to the mediastinum and supraclavicular lymph nodes.
Although PMBL, mediastinal Hodgkin lymphoma and mediastinal gray zone lymphomas are related diseases, their distinction is relevant due to differences in treatment and prognosis.
The morphology and immunophenotype of PMBL, in the appropriate clinical setting, are sufficiently specific to render a definite diagnosis in most cases and molecular studies are usually not required.
Rare cases with morphological and immunohistochemical features intermediate between PMBL and Hodgkin lymphoma fall into the “gray zone” category.
Mediastinal Hodgkin Lymphoma (NS-CHL)
Definition and Terminology
Mediastinal Hodgkin lymphoma is a monoclonal B-cell neoplasm arising in the thymus, the mediastinal lymph nodes, or both. Its histology is characterized by Reed–Sternberg cells and their variants in a rich inflammatory background [24]. For unknown reasons, virtually all mediastinal cases belong to the nodular sclerosis subtype (NS-CHL).
Epidemiology
NS-CHL is by far the most frequent mediastinal lymphoma and accounts for up to 70% of cases. Like primary mediastinal large B-cell lymphoma (PMBL), it is mainly a disease of young adults with a 2:1 female predominance. NS-CHL is most common among Caucasians in industrialized countries [25].
Etiology and Pathogenesis
NS-CHL is thought to arise from thymic germinal center B cells [26] that have escaped apoptosis. The major molecular pathways thought to contribute to the pathogenesis of NS-CHL (NFkB and JAK-STAT signaling, evasion from apoptosis, impaired MHC class II regulation) are shared with PMBL and both diseases are closely related on gene expression [14, 26]. The etiology of NS-CHL is unknown. Only a minority of cases (10–25%) is associated with EBV [27, 28].
Clinical Considerations and Prognosis
The clinical presentation of NS-CHL is very similar to PMBL. Most cases are in Ann-Arbor [29] stage II with a mediastinal mass (thymus and mediastinal lymph nodes) and supraclavicular or lower cervical lymph nodes. Infiltration of lung, pericardium or chest wall is possible. B symptoms (fatigue, fever, and night sweats) are often associated with higher clinical stage and increased risk of disseminated disease and are therefore considered risk factors [30]. With modern treatment, Hodgkin lymphoma is curable in at least 80% of patients [30].
Histomorphology
The diagnosis of NS-CHL requires the identification of multinucleated Reed–Sternberg (RS) cells or their variants (the mononuclear Hodgkin cells, the so-called “mummified” RS cells and lacunar cells). Detection of RS cells may sometimes require multiple sections. RS cells represent the neoplastic cell population in Hodgkin lymphoma and are typically accompanied by a rich background of mixed inflammatory cells including T cells, eosinophils, plasma cells, histiocytes, and granulocytes. In addition to the cellular component, there is variable fibrosis. Involvement of the thymus by NS-CHL often results in multilocular cystic changes (Fig. 8.3). The thymic epithelium is usually destroyed, but NS-CHL can rarely also evoke pseudoepitheliomatous hyperplasia.
Fig. 8.3
H&E morphology of mediastinal Hodgkin lymphoma. a Low power view showing nodular tumor infiltrates (corresponding to a nodular sclerosis pattern) with marked secondary cystic changes in the upper portion. Lower half of the image shows a well-preserved thymic remnant with some cholesterol granulomas next to it. b Higher magnification shows the cyst wall covered by a ciliated epithelium next to tumor infiltrates. c High power view showing multinuclear Reed–Sternberg cells and one mummified cell with condensed nuclear chromatin. d Numerous mummified cells next to several mononuclear Hodgkin- and multinuclear Reed–Sternberg cells. e Dense mixed inflammatory background with numerous eosinophils
Immunohistochemistry and Other Ancillary Studies
CD30 is the most important screening marker for NS-CHL and typically shows moderate to strong membranous staining with paranuclear accentuation of the Golgi area. CD15 is positive in at least 75% of cases and, if present, is a strong argument in favor of HL [20]. PAX5 is a very useful marker to confirm the B-cell lineage in the ≥80% of cases that are negative for CD20. Expression of PAX5 in RS and Hodgkin cells is typically weaker than in small reactive B cells and Non-Hodgkin lymphomas (including PMBL) (Fig. 8.4). In line with the concept that the regulation of the immunoglobulin machinery is disturbed in HL, the transcription factors BOB.1 and OCT2 are often negative [20]. In the inflammatory background, T cells usually outnumber B cells and T-cell markers such as CD3 or CD5 may show rosetting around the neoplastic RS cells.
Fig. 8.4
Immunohistochemical features of mediastinal Hodgkin lymphoma. a Keratin staining at low power magnification shows massive cystic changes with normal thymic remnants to the right and nodular tumor infiltrates with some residual epithelial structures to the left. b TdT staining at low power magnification of the same area shows strong demarcation of normal thymic cortex, with depletion of immature T cells in the areas colonized by the lymphoma. c Detail of (a) showing colonization and destruction of the epithelial thymic meshwork by the lymphoma. d CD30 staining showing incipient colonization of a thymic lobule by Hodgkin cells. e CD30 staining in an established tumor area shows strong membranous staining. In this image, the typical Golgi accentuation of the staining is not obvious. f moderate expression of CD15. g Nuclear Pax5 staining with characteristic attenuation in tumor cells compared to the surrounding non-neoplastic reactive B cells
Differential Diagnosis and Pitfalls
In addition to PMBL and mediastinal gray zone lymphoma (Table 8.1), the differential diagnosis also includes T cell/histiocyte-rich large B-cell lymphoma (THRLBCL) [31] and ALK+ anaplastic large T-cell lymphoma [32]. The tumor cells in THRLBCL may resemble Reed–Sternberg and Hodgkin cells, but are CD30 and CD15 negative. In addition, the inflammatory background in THRLBCL is mainly composed of T cells and macrophages without eosinophils and is distinctly different from Hodgkin lymphoma. Anaplastic large T-cell lymphoma (ALCL) with/without ALK expression can be a differential diagnosis [32], but the immunophenotype (CD30+, ALK±, CD4+ CD5+, perforin+, PAX5 negative, CD15 usually negative) is sufficiently different from Hodgkin lymphoma (CD30+, CD15+, PAX5+, CD4/CD5 negative, perforin negative) to allow for a safe distinction. Of note, 5% of otherwise classical Hodgkin (and ALK-negative) lymphomas show aberrant expression of T-cell antigens (usually CD2, CD3, or CD4) [33, 34], a feature considered an adverse prognostic factor [35].
Apart from lymphomas and soft tissue tumors (such as solitary fibrous tumors or inflammatory myofibroblastic tumors) or desmoplastic mesothelioma, the differential diagnosis also includes sclerosing mediastinitis with or without a granulomatous component. Sclerosing mediastinitis is not a single entity, but rather a group of infectious and noninfectious inflammatory conditions. The epidemiology is similar to Hodgkin lymphoma, since many patients are young females. The most important underlying conditions are histoplasmosis and mycobacteria, IgG4-related disease, sarcoidosis and pharmacologic substances such as methysergid. Suitable bacterial (such as PAS, Grocott, or fast-acid) and immunohistochemical stainings (IgG/IgG4) are therefore advisable if sclerosing mediastinitis is suspected.
Pitfalls
As illustrated in Fig. 8.3, NS-CHL with infiltration of the thymus frequently induces massive cystic changes with and without dense fibrosclerosis, features that are also seen in mediastinal seminoma. Since these secondary changes can be sharply separated from the neoplastic infiltrates, it is easy to miss the tumor on small biopsies. The acquired cysts can be lined by squamous nonkeratinizing, but also columnar or ciliated epithelium and may mimic various benign cystic lesions. The rare lymphoma-induced pseudoepitheliomatous hyperplasia can be mistaken for thymoma on small biopsies.
Suggested Strategy to Confirm NS-CHL in a Small Biopsy
Immunohistochemical stainings should include CD5, CD15, CD20, CD30, PAX5, and LMP. CD15 and LMP, if positive, have a high predictive value for NS-CHL [20]. A high level of suspicion is mandatory in cases with multiple cysts, epithelial hyperplasia, or granulomas.
Take Home Messages
NS-CHL is the most frequent mediastinal lymphoma with a predilection for young females (similar to PMBL)
Virtually all cases are of the nodular sclerosis subtype
NS-CHL can induce massive cystic change in the thymus which may obscure the underlying lymphoma
CD30, CD15, CD20, PAX5, and CD5 are the most relevant immunohistochemical markers required for the diagnosis
In cases with extensive sclerosis, the differential diagnosis also comprises non-neoplastic conditions such as sclerosing mediastinitis.
B-cell Lymphoma, Unclassifiable, with Features Intermediate Between Diffuse Large B-cell Lymphoma and Classical Hodgkin Lymphoma (Mediastinal Gray Zone Lymphoma)
Definition and Terminology
Mediastinal gray zone lymphoma (MGZL) is a B-cell lymphoma with clinical, morphological and immunohistochemical features that overlap with mediastinal Hodgkin lymphoma (NS-CHL) and primary mediastinal large B-cell lymphoma (PMBL). Some tumors simultaneously show areas corresponding to NS-CHL, PMBL and MGZL, others show heterogeneity e.g. between thymus and affected lymph nodes, suggesting that NS-CHL, MGZL, and PMBL represent related tumors within a disease spectrum.
Epidemiology
Etiology and Pathogenesis
The etiology of MGZL is unknown. MGZL shares many key molecular features with NS-CHL and PMBL, such as alterations at the 9p24.1 (the Jak2/PDL2 gene locus), amplifications at 2p16.1 (the REL/BCL11A gene locus), or rearrangements of the CIITA gene locus at 16p13.13 [22]. The epigenetic profile of MGZL shows both overlapping and divergent alterations in MGZL, NS-CHL, and PMBL, while MGZL and extramediastinal diffuse large B-cell lymphoma are distinctly different [37]. Only 5–10% of MGZL are associated with EBV infection [16].
Clinical Considerations and Prognosis
The clinical presentation is not different from NS-CHL or PMBL. Symptoms (dyspnea, pericardial and pleural effusions, etc.) are frequently related to a bulky anterior mediastinal mass. Genetic data suggest that tumors with features corresponding to MGZL can occur at other sites without mediastinal involvement [22]. In contrast to PMBL, extrathoracic parenchymal organs (such as kidney or CNS) are rarely involved as tumors progress [38].
Histomorphology
The majority of cases resembles NS-CHL, but with unusual features, such as sheeting out of Hodgkin and Reed–Sternberg (RS) cells. The inflammatory background is usually less pronounced than in NS-CHL. Most of the remaining cases resemble PMBL, but show numerous Hodgkin/RS or lacunar cells (Fig. 8.5) [16].
Fig. 8.5
H&E morphology and immunohistochemical features of mediastinal gray zone lymphoma. a Sheets of large polymorphic tumor cells with some resemblance towards Hodgkin and Reed–Sternberg cells. No inflammatory background. b In this example, there was subtotal loss of CD20 expression. c Strong and diffuse expression of CD30 with prominent Golgi pattern. d Strong expression of CD15, e strong, but variable nuclear expression of OCT2 (a similar pattern would be expected for PAX5). f Overlapping immunohistochemical features between primary mediastinal large B cell lymphoma (PMBL), mediastinal grey zone lymphoma (MGZL), and nodular sclerosis Hodgkin lymphoma (NS-CHL)
Immunohistochemistry and Other Ancillary Studies
Differential Diagnosis and Pitfalls
The differentiation of MGZL from NS-CHL and PMBL is important, since the behavior of MGZL is more aggressive and the outcome is worse. Apart from this distinction, the same differential diagnoses (including seminomas and carcinomas) apply.
Take Home Messages
Mediastinal gray zone lymphomas (MGZL) represent a spectrum of morphological and immunophenotypic features that are intermediate between classical Hodgkin lymphoma (NS-CHL) and primary mediastinal large B-cell lymphoma (PMBL).
In contrast to NS-CHL and PMBL, MGZL is more common in young men.
Sheeting out of Hodgkin and Reed–Sternberg cells and co-expression of CD20 and CD30 are typical histological and immunohistochemical findings.
Extranodal Marginal Zone Lymphoma (MALT Lymphoma) of the Thymus and Mediastinum
Definition and Terminology
Extranodal marginal zone lymphoma (MALT lymphoma) of the thymus is an indolent mature B-cell lymphoma composed of small monocytoid lymphocytes and plasma cells that infiltrate and destroy the thymic epithelium. IgA expression is a distinctive feature of most thymic MALT lymphomas.
Epidemiology
Etiology and Pathogenesis
Many patients have a history of long standing autoimmune disease, usually Sjögren syndrome (by far most frequent), rheumatoid arthritis, or systemic lupus erythematosus [39, 47, 48]. Rare cases develop in a micronodular thymoma [49], which should therefore always be checked for monoclonal B-cell populations. The chromosomal alterations found in thymic MALT lymphomas are similar to those in other autoimmunity-driven MALT lymphomas, such as thyroid or salivary gland lymphomas [48]. About 50% of cases show trisomy 3 (an alteration also found in many other MALT lymphomas), whereas translocations involving the MALT1 and IGH gene are absent [48]. There is evidence that silencing of tumor suppressor genes (such as p14/ARF, CDH1, and TIMP3) through methylation is involved in the pathogenesis of thymic MALT lymphoma.
Clinical Considerations and Prognosis
Most cases are incidental findings in elderly patients with long standing Sjögren syndrome. Some patients complain of chest pain or shortness of breath, which can either be due to the mediastinal mass or to infiltration of the lung [43]. Some patients show amyloid deposition [50]. Lymph nodes of the head and neck, axilla or mediastinum can be involved [51]. Most patients have increased IgA and restricted light chains in their serum. The prognosis of thymic MALT lymphoma is excellent. Patients with low-stage disease can be cured by surgical resection, patients with higher stage disease may require chemo- or radiotherapy.
Histomorphology
On macroscopy, many cases show prominent cystic change. At low-power magnification, there is effacement of the normal lobular structure of the thymus. The neoplastic lymphocytes are monotonous and have a moderate amount of pale cytoplasm that is more pronounced around the so-called lymphoepithelial lesions (i.e., infiltration and destruction of thymic epithelium by the lymphoma) and cysts. Plasma cells are seen intermingled or in aggregates and usually show immunohistochemical light chain restriction (Fig. 8.6a–c).
Fig. 8.6
H&E morphology and immunohistochemical features of extranodal marginal zone lymphoma of the thymus. a Low power view shows vaguely nodular infiltrates that efface the lobular structure of the thymus. b Higher magnification reveals monotonous lymphocytic infiltrates with pale cytoplasm. c Small monotonous lymphocytes surrounding residual Hassal corpuscles. Few scattered plasmacytes and extracellular deposition of homogenous eosinophilic material (eventually AL-amyloid). d Tumor cells but not the residual Hassal corpuscle show strong and diffuse expression of CD20. e Keratin staining highlights massive colonization and destruction of the thymic epithelial meshwork (lymphoepithelial lesions). f, g kappa and lambda staining showing unequivocal monotypic light chain restriction in many instances
Immunohistochemistry and Other Ancillary Studies
Tumor cells are positive for CD19, CD20, or CD79a and negative for CD5, CD10, or CD23. CD23 highlights effaced or colonized meshworks of follicular dendritic cells [49]. The plasma cells express CD138 and usually show light chain restriction. Strikingly, more than 70% of thymic MALT lymphomas express IgA, which discriminates them from non-thymic MALT lymphomas, which typically express IgM or IgG, and only rarely IgA [39, 52]. Cytokeratins are very helpful in highlighting the lymphoepithelial lesions (Fig. 8.6).
Differential Diagnosis and Pitfalls
Thymic follicular hyperplasia is a differential diagnostic consideration, since MALT lymphomas can also harbor reactive lymphoid follicles (Fig. 8.7a–c). Most patients with thymitis have myasthenia gravis (MG), which is a helpful clinical discriminator (of note, some MG patients may also have Sjögren disease [53]). In thymic follicular hyperplasia, there are no lymphoepithelial lesions and usually no cysts, and the B cells do not show light chain restriction. A much more challenging differential diagnosis is a recently described entity, the so-called thymic hyperplasia with lymphoepithelial sialadenitis-like features (TH-LESA) (Fig. 8.7g–j) [54]. The clinical setting is different, since men and women are equally affected and there is no association with autoimmune disease [54]. Low power magnification strikingly resembles MALT lymphoma with prominent cystic change and (often “incomplete” or equivocal) lymphoepithelial lesions. However, monocytoid B cells are absent, light chain expression is polytypic and molecular tests do not show monoclonal B-cell receptor rearrangements. Progression into MALT lymphoma has not been reported.
Fig. 8.7
Histological mimics of marginal zone lymphoma of the thymus. a–c Thymic lymphofollicular hyperplasia (thymitis) in a young female with myasthenia gravis. a Low power view showing so-called lymph node like transformation of a medullary area by numerous lymphoid follicles. b Detail of the same case showing reactive lymphoid follicle with germinal centre in the vicinity of several Hassal corpuscles. c Keratin stain (low power view) shows massive distention and condensation, but not destruction, of the medullary epithelial meshwork, while the thymic cortex seems uninvolved. d–f Micronodular thymoma with lymphoid stroma (MNT). Tumor grows in multiple small epithelial nodules with bland spindle cell morphology resembling type A thymoma, surrounded by a dense lymphoid stroma (d) that may contain numerous lymphoid follicles. Of note, some MNT have been shown to contain monoclonal B cell populations and rarely overt lymphomas including MALT lymphoma. e CD23 staining in this case shows regular meshwork of follicular dendritic cells with no evidence of colonization. f Low power view of keratin staining highlights multiple epithelial nodules surrounded by a dense lymphoid stroma that contains several lymphoid follicles. h–j So-called thymic hyperplasia with lymphoepithelial sialadenitis-like (LESA) features in a 45 years old male patient without autoimmune symptoms. h Low power view showing dense lymphoid infiltrates with effacement of thymic lobular structures and several cysts. At this magnification, the picture is indistinguishable from thymic MALT lymphoma. i Higher magnification shows condensed and seemingly colonized epithelial structures with a few Hassal corpuscles and surrounding lymphofollicular stroma. Again, the morphology is indistinguishable from thymic MALT lymphoma. However, the typical pale cytoplasm seen in most MALT lymphomas is absent. j Keratin stain reveals equivocal epithelial colonization by the lymphocytes. In this situation, B clonality assays are mandatory to safely rule out a lymphoma. At extended follow-up, the patient was well without evidence of disease
While rarely observed in the thymus, most other B-cell lymphomas that have been described either in the thymus or the mediastinal lymph nodes (bona fide primary mediastinal lymphomas) include follicular lymphoma [49], nodal marginal zone lymphoma, diffuse large B-cell lymphoma, and Burkitt lymphoma.
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