Anaplastic large-cell lymphomas of B-cell phenotype are anaplastic lymphoma kinase (ALK) negative and belong to the spectrum of diffuse large B-cell lymphomas

CD30+ Lymphoproliferative Disorders as Potential Candidates for CD30-Targeted Therapies

CONTEXT.—

In the early 1980s, a monoclonal antibody termed Ki-1 was developed against a cell line derived from a patient with Hodgkin lymphoma. This antibody detected a limited number of benign activated lymphocytes in lymphoid tissue, whereas in Hodgkin lymphoma it appeared to be nearly specific for Reed-Sternberg cells and their mononuclear variants. Subsequent studies showed that Ki-1 expression defined a new type of lymphoma that was later designated anaplastic large cell lymphoma with or without anaplastic large cell kinase expression/translocation. In the past 30 years, numerous new lymphoma entities have been defined, many of which are variably positive for CD30. Many virally transformed lymphoproliferative disorders are also frequently positive for CD30.

OBJECTIVE.—

To illustrate the broad spectrum of CD30+ hematologic malignancies and to provide an update of CD30-targeted therapies.

DATA SOURCES.—

Personal experiences and published works in PubMed.

CONCLUSIONS.—

Because of its low expression in normal tissue, CD30 was studied as a therapeutic target for many years. However, the first functional humanized antibody against CD30 was developed only about 10 years ago. Brentuximab vedotin is a humanized anti-CD30 antibody linked to a cytotoxin, and was approved by the US Food and Drug Administration in 2012 for treating refractory Hodgkin lymphoma and anaplastic large cell lymphoma. Since then, the list of Food and Drug Administration-approved CD30-targeted hematologic malignancies has grown. Recently, the therapies using tumor antigen-specific chimeric antigen receptor T cells targeting CD30 have incited a great deal of enthusiasm and are studied in clinical trials.

Primary splenic anaplastic variant of diffuse large B-cell lymphoma: a case report

Background

Primary splenic lymphoma represents a rare entity that constitutes less than 1% of non-Hodgkin lymphomas, and less than 2% of all lymphomas. Diffuse large B-cell lymphoma (DLBCL) is the most common histological subtype of primary splenic lymphomas. DLBCL encompasses a heterogeneous entity with distinct morphological variants. The anaplastic variant of DLBCL was first defined in the 2017 World Health Organization classification as a rare histological subtype that constitutes less than 3.4% of DLBCL cases.

Case presentation

A 65-year-old Syrian man presented to our hospital with constant dull localized left upper quadrant abdominal pain for about 20 days accompanied by general weakness, loss of appetite, and rapid weight loss. Clinical examination revealed isolated splenomegaly and left upper abdominal tenderness. Following physical, laboratory, and radiologic examinations, the patient underwent splenectomy. Interestingly, pathological and immunohistochemical examinations of the resected spleen confirmed the diagnosis of a primary anaplastic variant of DLBCL.

Conclusions

Herein, we aimed to present an unusual combination of a rare splenic neoplasm and a unique lymphoma subtype. Furthermore, we aimed to highlight the difficulties in differential diagnosis and the importance of histological and immunohistochemical examinations with clinical correlation.

Anaplastic diffuse large B-cell lymphoma cytologically diagnosed in pleural effusion: Report of two cases

Diffuse large B-cell lymphoma (DLBCL) has some rare special subtypes. Anaplastic variant has the immunophenotypic and genotypic features of DLBCL, and is characterized by the presence of large bizarre hallmark-like and Reed-Sternberg-Hodgkin-like cells. Lymphoblastic lymphoma and DLBCL, non-otherwise specified are the most common lymphoma effusions in adults. Lymphomas in effusion fluids usually show a homogenous uniform population of either small, medium or large cells depending on lymphoma types. Anaplastic pleomorphic lymphoid cells in pleural effusions are not common cytologic findings. Effusions due to lymphomas with large bizarre anaplastic cells, such as anaplastic large cell lymphoma and Hodgkin lymphomas, are rare. Presence of anaplastic cells in effusion fluid is diagnostically challenging. They may be confused with nonlymphoid malignancies. Certain cytomorphologic features are helpful clues. Ancillary immunocytochemistry is a useful confirmatory tool in doubtful cases. We report two cases of anaplastic variant-DLBCL (ADLBCL) cytologically diagnosed in elderly female patients with bilateral pleural effusions. Cytologic examination of effusion fluids showed predominantly medium-large mononuclear uniform cells intermixed with frequent large pleomorphic mononuclear and binucleated cells, and bizarre multinucleated giant cells. Background showed karyorrhexis and absent mixed inflammatory infiltrates. A panel of immunocytochemistry markers in cell block sections confirmed B-cell phenotype of the neoplastic and anaplastic cells. Lymph node biopsy supplemented with immunohistochemistry confirmed the cytologic diagnosis. Even though the presence of anaplastic cells may be confused with other malignancies, cytopathologists should consider ADLBCL in the differential diagnosis of lymphomas with large pleomorphic cells, look for certain cytomorphologic clues and implement an appropriate immunocytochemistry panel.

Hematolymphoid Neoplasms Rarely Mimic Undifferentiated Pleomorphic Sarcoma of Soft Tissue

CONTEXT.—

Undifferentiated pleomorphic sarcoma (UPS) of soft tissue is defined as a sarcoma with no recognizable line of differentiation. During the past few decades, advances in ancillary studies and review of prior UPS diagnoses have narrowed the category of UPS by excluding more-specific malignancies. However, few of those studies have specifically targeted pleomorphic hematolymphoid neoplasms.

OBJECTIVE.—

To determine what fraction of UPS cases are misclassified pleomorphic hematolymphoid neoplasms, such as anaplastic large cell lymphoma, diffuse large B-cell lymphoma, histiocytic sarcoma (HS), myeloid sarcoma, and follicular dendritic cell sarcoma.

DESIGN.—

Sixty-one UPS cases were screened by tissue microarray and an immunostain panel with subsequent analysis on whole block sections for suspicious cases.

RESULTS.—

Five of 61 tumors (8%) were suggestive of HS based on the screening panel and were further evaluated with additional immunostains (PU.1, CD45, CD163) using whole sections. The 5 candidate HS cases were only focally positive for at most one stain with most staining in smaller, less-pleomorphic cells. Ultimately, no UPS met criteria for anaplastic large cell lymphoma, diffuse large B-cell lymphoma, myeloid sarcoma, follicular dendritic cell sarcoma, or HS.

CONCLUSIONS.—

Our results suggest that a UPS of somatic soft tissue is unlikely to represent a misclassified hematopoietic malignancy. Exclusion of HS is most challenging, but immunostaining for PU.1, a nuclear transcription factor, may be easier to interpret in this context.

Anaplastic variant of diffuse large B-cell lymphoma: Reappraisal as a nodal disease with sinusoidal involvement

Anaplastic variant (av) of diffuse large B-cell lymphoma (DLBCL) is morphologically defined in the 2017 World Health Organization classification, but still an enigmatic disease in its clinicopathologic distinctiveness, posing the differential diagnostic problem from gray zone lymphoma (GZL) and classic Hodgkin lymphoma (cHL). Thirty-one cases previously diagnosed as avDLBCL were reassessed. Of these, 27 (87%) and 4 (13%) were node-based and extranodal diseases, respectively. They were further reclassified into nodal avDLBCL (n = 18), nodal CD30+ DLBCL with T-cell/histiocyte-rich large B-cell lymphoma-like features (CD30+ DLBCL-THRLBCL) (n = 6), GZL with features intermediate between DLBCL and cHL (n = 3) and CD30+ extranodal DLBCL, NOS (n = 4). The nodal avDLBCL cases had a sheet-like proliferation of large cells and/or Hodgkin/Reed-Sternberg (HRS)-like cells in 12 (67%) notably with a sinusoidal pattern in 16 (89%). They showed an expression of CD20 and/or CD79a in all and CD30 in 15 of 18. All of them were negative for PD-L1 on tumor cells, although HRS-like cells showed negativity or partial loss of other B-cell markers to varying degrees. The present study highlighted the distinctiveness of the nodal avDLBCL with sinusoidal pattern, but without neoplastic PD-L1 expression, which provide refined diagnostic criteria for a more precise pathologic and clinical characterization of this disease.

Anaplastic large cell lymphoma in human immunodeficiency virus-infected people and solid organ transplant recipients

Human immunodeficiency virus (HIV)-infected people and solid organ transplant recipients have elevated risk of anaplastic large cell lymphoma (ALCL). Little is known regarding ALCL risk factors in immunosuppressed populations. We used data from US cancer registries linked to HIV registries (1996-2016) and to the national transplant registry (1992-2017). ALCL risk in HIV-infected people and transplant recipients relative to the general population was calculated as a standardized incidence ratio (SIR). ALCL risk factors were evaluated using Poisson regression. We identified 121 incident ALCL cases in the HIV (n = 86) and transplant (n = 35) populations. We reviewed pathology reports for 45 cases and most (86·7%) were confirmed as ALCL. Epstein-Barr virus tested positive in 1/8 (12·5%) cases. Compared to the general population, ALCL risk was strongly elevated among HIV-infected people [SIR 5·43; 95% confidence interval (CI) 4·27-6·81] and transplant recipients (5·96; 4·03-8·49). Among HIV-infected people, ALCL incidence was strongly related to CD4 count [adjusted incidence rate ratio (aIRR) 0·15 for ≥500 vs. <200 cells/μl; P trend < 0·001]. Among transplant recipients, risk was highest within the first year (aIRR 6·82) and 10+ years post-transplant (5·99). In conclusion, ALCL risk is strongly increased in these immunosuppressed populations but may be unrelated to EBV infection based on limited reports.

Approach to nodal-based T-cell lymphomas

Peripheral T-cell lymphomas (PTCLs) represent a heterogeneous group of uncommon malignancies derived from mature T cells and usually characterised by an aggressive clinical course. Their clinical presentation, localisation and pattern of dissemination are highly variable, but the majority of cases present as nodal diseases. The recently revised classification of lymphomas has incorporated many new molecular genetic data derived from gene expression profiling and next generation sequencing studies, which refine the definition and diagnostic criteria of several entities. Nevertheless, the distinction of PTCL from various reactive conditions, and the diagnosis of PTCL subtypes remains notably challenging. Here, an updated summary of the clinicopathological and molecular features of the most common nodal-based PTCLs (angioimmunoblastic T-cell lymphoma and other nodal lymphomas derived from follicular T helper cells, anaplastic large cell lymphomas and peripheral T-cell lymphoma, not otherwise specified) is presented. Practical recommendations in the diagnostic approach to nodal T-cell lymphoproliferations are presented, including indications for the appropriate use and interpretation of ancillary studies. Finally, we discuss commonly encountered diagnostic problems, including pitfalls and mimics in the differential diagnosis with various reactive conditions, and the criteria that allow proper identification of distinct PTCL entities.

Anaplastic variant of diffuse large B-cell lymphoma with hallmark cell appearance: Two cases highlighting a broad diversity in the diagnostics

The anaplastic variant of diffuse large B-cell lymphoma (A-DLBCL) is morphologically defined but remains an enigmatic disease in its clinicopathologic distinctiveness. Here, we report two cases involving Japanese women aged 59 years, both with A-DLBCL with the hallmark cell appearance and both indistinguishable from common and giant cell-rich patterns, respectively, of anaplastic lymphoma kinase (ALK)-positive anaplastic large cell lymphoma. Case 1 was immunohistochemically positive for CD20, CD79a and OCT-2 but not for the other pan-B-cell markers, CD30 and ALK. Case 2 showed CD20 and CD30 positivity for 50% and 20% of tumor cells in addition to strong expression of p53 and MYC. Both were positive for fascin without Epstein-Barr virus association. Our cases provide additional support for the earlier reports that A-DLBCL exhibits clinicopathologic features distinct from ordinal diffuse large B-cell lymphoma (DLBCL), and documented its broader morphologic diversity than previously recognized. They also shed light on the unique feature of absent expression of pan-B-cell markers except for CD20 and CD79a, suggesting that A-DLBCL may biologically mimic a gray zone or intermediate lymphoma between DLBCL and classic Hodgkin lymphoma.

Transformation of CLL to ALCL: the role of clonality studies in diagnostic molecular haematopathology

Clonality studies greatly assist in the diagnosis of challenging haematopathology cases. These robust and standardised tests aid the detection of clonal lymphoid populations and may assist in lymphocyte subtyping. In this case report, a gentleman presented with a high-grade transformation of a B cell neoplasm which histologically and immunophenotypically mimicked a T cell anaplastic large-cell lymphoma. With the aid of T cell and B cell receptor clonality studies, it was demonstrated that this tumour was in fact of B cell lineage. This report exemplifies the role of these increasingly used and relatively new molecular tests in unusual and difficult lymphoma presentations and highlights potential pitfalls in the interpretation of their results.

How our practice of histopathology, especially tumour pathology has changed in the last two decades: reflections from a major referral center in Pakistan

Continued advances in the field of histo-pathology (and cyto-pathology) over the past two decades have resulted in dramatic changes in the manner in which these disciplines are now practiced. This is especially true in the setting of a large university hospital where the role of pathologists as clinicians (diagnosticians), undergraduate and postgraduate educators, and researchers has evolved considerably. The world around us has changed significantly during this period bringing about a considerable change in our lifestyles and the way we live. This is the world of the internet and the world-wide web, the world of Google and Wikipedia, of Youtube and Facebook where anyone can obtain any information one desires at the push of a button. The practice of histo (and cyto) pathology has also evolved in line with these changes. For those practicing this discipline in a poor, developing country these changes have been breathtaking. This is an attempt to document these changes as experienced by histo (and cyto) pathologists practicing in the biggest center for Histopathology in Pakistan, a developing country in South Asia with a large (180 million) and ever growing population. The Section of Histopathology, Department of Pathology and Microbiology at the Aga Khan University Hospital (AKUH) in Karachi, Pakistan's largest city has since its inception in the mid-1980s transformed the way histopathology is practiced in Pakistan by incorporating modern methods and rescuing histopathology in Pakistan from the primitive and outdated groove in which it was stuck for decades. It set histopathology in Pakistan firmly on the path of modernity and change which are essential for better patient management and care through accurate and complete diagnosis and more recently prognostic and predictive information as well.

A Rare Case of Anaplastic Variant of Diffuse Large B-Cell Lymphoma Presenting as a Lung Primary

Primary pulmonary lymphoma is an uncommon neoplastic disorder representing approximately 0.5% to 1% of primary pulmonary malignancies. The vast majority are of low-grade, mucosa-associated lymphoid tissue type. Primary diffuse large B-cell lymphoma of the lung is rare, though cases of the centroblastic and immunoblastic variants have been described. We present herein an interesting case of an 80-year-old man who presented with both respiratory and constitutional symptoms and was found to have a 4.5 cm left hilar mass with bilateral hilar and mediastinal lymphadenopathy on imaging. Endobronchial biopsy revealed an aggressive large cell lymphoma, with scattered large, bizarre-shaped nuclei resembling Reed–Sternberg cells, positive for CD20, PAX5, CD30, and MUM-1, consistent with an anaplastic variant of diffuse large B-cell lymphoma. Imaging showed no evidence of extrathoracic disease. Standard treatment with cyclophosphamide/vincristine/prednisone and rituximab resulted in significant clinical and radiological response and the patient remains in remission 21 months later. To the best of our knowledge, this modified Ann Arbor stage II2E primary pulmonary lymphoma, is the first description in the English literature of anaplastic variant of diffuse large B-cell lymphoma presenting as a lung primary.

CD30-positive anaplastic variant diffuse large B cell lymphoma: a rare case presented with cutaneous involvement

A 67-year-old man suffered from a left cervical lymph node swelling and tenderness. Biopsy of the cervical lymph node showed pleomorphic large cells containing large atypical nuclei. Immunohistochemical stains of these cells were positive for CD30, but negative for CD3 and CD20. After the biopsy, his left cervical skin was ulcerated. Biopsy of the left cervical skin was performed. Large pleomorphic cells with constricted nuclei and Reed-Sternberg-like cells existed in the dermis and epidermis. Immunohistochemical stains of the former cells were positive for CD30, CD45 and PAX5, but negative for CD3, CD10, CD20, CD15, Bcl-2, EBER ISH, EMA and ALK. He was diagnosed with diffuse large B cell lymphoma, anaplastic variant. He achieved complete remission with CHOP chemotherapy. CD30-positive DLBCL, anaplastic variant is a rare B cell lymphoma. Most of the patients presented with primary nodal disease, and skin involvement of lymphoma is very rare. This is the report of a rare case of CD30-positive DLBCL, anaplastic variant, with both nodal and skin lesions.

Activation-induced cytidine deaminase expression in diffuse large B-cell lymphoma with a paracortical growth pattern: a lymphoma of possible interfollicular large B-cell origin

CONTEXT

Activation-induced cytidine deaminase, necessary for immunoglobulin somatic hypermutation and class switch recombination, is usually expressed within the follicular dendritic network but is also expressed in a population of interfollicular large B cells outside the germinal center.

OBJECTIVE

To report 7 cases of diffuse large B-cell lymphoma with a distinct paracortical distribution. Expression of activation-induced cytidine deaminase, previously described in interfollicular large B cells, was evaluated.

DESIGN

A panel of immunohistochemical markers, including double staining for activation-induced cytidine deaminase and CD20, was used to illustrate the cases. Molecular studies were performed by polymerase chain reaction in the paraffin-embedded tissue for t(14;18) chromosomal translocation and immunoglobulin heavy chain and T-cell receptor rearrangements.

RESULTS

Patients included 3 males and 4 females ranging in age from 11 to 59 years (mean, 39 years). All specimens were lymph nodes (4 from the groin, 2 from the neck, and 1 from the axilla). Malignant lymphocytes were positive for CD20 and negative for CD5 and CD10. Staining for CD30, CD43, and BCL-2 was variable. The malignant cells showed at least focal staining with activation-induced cytidine deaminase. All cases were found to be monoclonal by immunoglobulin heavy-chain gene rearrangement or showed light-chain restriction. None of the tested cases showed t(14;18).

CONCLUSIONS

Diffuse large B-cell lymphoma with a paracortical distribution is unusual and may be a distinct morphologic variant. More study is necessary to determine the stage of B-cell development and the cell of origin of these tumors. However, activation-induced cytidine deaminase expression suggests they may arise from a putative interfollicular large B cell.

Diffuse aggressive B-cell lymphomas

Diffuse aggressive B-cell lymphomas comprise a relatively common and increasingly diverse group of neoplasms. Newer modalities including gene expression profiling and an increasing panel of immunohistochemical markers have contributed to greater accuracy in defining these entities. Attention is paid not only to the neoplastic cells but also to the cellular and stromal milieu in which they proliferate. These distinctions may have therapeutic implications as well, with improved outcome related to newer and sometimes targeted therapies. At the same time there is increasing understanding of the overlap, which occurs in the grey zone between diffuse large B-cell lymphoma and Burkitt lymphoma as well as between diffuse large B-cell lymphoma and Hodgkin lymphoma. This review aims to provide practical insights in the correct identification and differential diagnosis of these lymphomas, with emphasis on the changes that have occurred with the publication of the 2008 World Health Organization updated classification.

Anaplastic lymphoma kinase: role in cancer pathogenesis and small-molecule inhibitor development for therapy

Anaplastic lymphoma kinase (ALK), a receptor tyrosine kinase in the insulin receptor superfamily, was initially identified in constitutively activated oncogenic fusion forms - the most common being nucleophosmin-ALK - in anaplastic large-cell lymphomas, and subsequent studies have identified ALK fusions in diffuse large B-cell lymphomas, systemic histiocytosis, inflammatory myofibroblastic tumors, esophageal squamous cell carcinomas and non-small-cell lung carcinomas. More recently, genomic DNA amplification and protein overexpression, as well as activating point mutations, of ALK have been described in neuroblastomas. In addition to those cancers for which a causative role for aberrant ALK activity is well validated, more circumstantial links implicate the full-length, normal ALK receptor in the genesis of other malignancies - including glioblastoma and breast cancer - via a mechanism of receptor activation involving autocrine and/or paracrine growth loops with the reported ALK ligands, pleiotrophin and midkine. This review summarizes normal ALK biology, the confirmed and putative roles of ALK in the development of human cancers and efforts to target ALK using small-molecule kinase inhibitors.

Flow Cytometric Immunophenotyping of Anaplastic Large Cell Lymphoma

Context.—Anaplastic large cell lymphoma (ALCL) is usually diagnosed by histologic and immunohistochemical analysis. However, fine-needle aspiration is becoming a popular alternative to lymph node biopsy, and flow-cytometric immunophenotyping is often used to analyze fine-needle aspiration specimens.

Objective.—To review our experience using flow-cytometric immunophenotyping to assess cases of ALCL and to evaluate the diagnostic utility of this technique.

Design.—Each case of ALCL was assessed by flow cytometry with 3-color or 4-color antibody panels, and data were reanalyzed by cluster analysis using Paint-a-Gate for cases with retrievable flow cytometry data files. Anaplastic lymphoma kinase (ALK) was assessed by using immunohistochemistry.

Results.—Twenty-three ALCL cases were analyzed by flow cytometry. In 4 cases, neoplastic cells could not be identified. In the remaining 19 cases (11 ALK+, 8 ALK−), all were positive for CD30 and CD45. Anaplastic large cell lymphoma cells were large and usually CD45 bright, with many or most cells falling in the region of monocytes on the CD45/side scatter plot. The frequencies of T-cell antigen expression in ALK+ cases were CD2, 67%; CD7, 60%; CD3, 45%; CD4, 33%; CD5, 14%; and CD8, 14%. In ALK− cases, the frequencies of the T-cell antigen expression were CD2, 100%; CD3, 50%; CD4, 40%; CD7, 40%; CD5, 25%; and CD8, 20%.

Conclusions.—Flow cytometry can be used to immunophenotype ALCL cases. Neoplastic cells may be few, and they may fall outside of the lymphocyte gate. Cluster analysis using software like Paint-A-Gate is often helpful because it allows for flexible, sequential gating strategies to identify and characterize the neoplastic cells.

Development of anaplastic lymphoma kinase (ALK) small-molecule inhibitors for cancer therapy

Anaplastic lymphoma kinase (ALK) is a receptor tyrosine kinase (RTK) involved in the genesis of several human cancers; indeed, ALK was initially identified in constitutively activated and oncogenic fusion forms--the most common being nucleophosmin (NPM)-ALK--in a non-Hodgkin's lymphoma (NHL) known as anaplastic large-cell lymphoma (ALCL) and subsequent studies identified ALK fusions in the human sarcomas called inflammatory myofibroblastic tumors (IMTs). In addition, two recent reports have suggested that the ALK fusion, TPM4-ALK, may be involved in the genesis of a subset of esophageal squamous cell carcinomas. While the cause-effect relationship between ALK fusions and malignancies such as ALCL and IMT is very well established, more circumstantial links implicate the involvement of the full-length, normal ALK receptor in the genesis of additional malignancies including glioblastoma, neuroblastoma, breast cancer, and others; in these instances, ALK is believed to foster tumorigenesis following activation by autocrine and/or paracrine growth loops involving the reported ALK ligands, pleiotrophin (PTN) and midkine (MK). There are no currently available ALK small-molecule inhibitors approved for clinical cancer therapy; however, recognition of the variety of malignancies in which ALK may play a causative role has recently begun to prompt developmental efforts in this area. This review provides a succinct summary of normal ALK biology, the confirmed and putative roles of ALK fusions and the full-length ALK receptor in the development of human cancers, and efforts to target ALK using small-molecule kinase inhibitors.

Prevalence of bone marrow involvement in systemic anaplastic large cell lymphoma: are immunohistochemical studies necessary?

The frequency of bone marrow involvement in anaplastic large cell lymphoma has been reported with great variation. A prior study found that anaplastic large cell lymphoma involvement of bone marrow was often not evident on routine stains and advocated using immunohistochemical studies. We evaluated 70 bone marrow biopsies from 41 patients with anaplastic large cell lymphoma and found 10 morphologically involved cases (14% of all biopsies, 22% of all patients). In most cases (9/10 biopsies), the involvement of the bone marrow by anaplastic large cell lymphoma was massive and, thus, was evident on the hematoxylin and eosin section. In only 1 biopsy (1% of all biopsies, 2% of all patients), the involvement was minimal and more difficult to detect. To determine if the hematoxylin and eosin evaluation missed bone marrow involvement, we used a panel of antibodies including CD30, ALK-1, epithelial membrane antigen, and granzyme. Only the 10 morphologically involved cases showed anaplastic large cell lymphoma cells with distinct CD30 expression. Other stains highlighted only a subset of the CD30-positive cases. Clinical follow-up was available for 30 patients and shows correlation of marrow involvement with lower overall survival (P = .033). Overall, marrow involvement in anaplastic large cell lymphoma was relatively uncommon, and when present, it was identified on hematoxylin and eosin sections.

Application of Immunohistochemistry in the Diagnosis of Non-Hodgkin and Hodgkin Lymphoma

Context.—Beginning with the immunologic classifications of Lukes and Collins and Kiel and culminating in the Revised European-American Lymphoma and World Health Organization classifications, the diagnosis of lymphoid tumors relies heavily on the determination of cell lineage, maturation, and function, based on antigen expression in addition to morphology and clinical features. Technologic advances in immunology, antibody production, genetic analysis, cloning, and the identification of new genes and proteins by microarray and proteomics have provided pathologists with many antibodies to use in routine diagnosis.

Objective.—To provide guidance to the practicing pathologist in the appropriate selection of an antibody panel for the diagnosis of lymphoma based on morphology and relevant clinical data and to avoid pitfalls in the interpretation of immunohistochemical data. Attention is given to some of the newer antibodies, particularly against transcription factors, that are diagnostically and prognostically useful.

Data Sources.—The information presented in this article is based on review of the literature using the OVID database (Ovid MEDLINE 1950 to present with daily update) and 20 years of experience in diagnostic hematopathology.

Conclusions.—Immunophenotyping is required for the diagnosis and classification of lymphoid malignancies. Many paraffin-reactive antibodies are available to the pathologist but most are not specific. To avoid diagnostic pitfalls, interpretation of marker studies must be based on a panel and knowledge of a particular antigen's expression in normal, reactive, and neoplastic conditions.

Primary gastric Hodgkin's lymphoma expressing a B-Cell profile including Oct-2 and Bob-1 proteins

Classic Hodgkin's lymphoma (cHL) most often involves lymph nodes, and gastric involvement is rare. Hodgkin's and Reed-Sternberg (H-RS) cells in cHL are known to often lack expression of several B-lineage markers, such as CD20, CD79a, Oct-2, and Bob-1. We present an extremely rare case of mixed-cellularity cHL in the stomach in which expression of these B-cells was detected immunohistochemically. The patient was an 83-year-old Japanese woman who developed a sensation of abdominal fullness and appetite loss. Endoscopic and abdominal computed tomography examinations revealed a gastric ulcer lesion and swelling of para-aortic lymph nodes, respectively. A subtotal gastrectomy was performed, and the histopathologic diagnosis was established as a typical cHL compatible with stomach origin. The patient underwent postoperative chemotherapy of 3 cycles of ABVD (doxorubicin, bleomycin, vinblastine, and dacarbazine) and has since been in complete remission. Immunohistochemically, the H-RS cells in the cHL were positive not only for CD30 but also for CD20, CD79a, Oct-2, and Bob-1, whereas they were negative for CD3, CD15, CD45, EMA, and ALK1. Our patient may have had an intermediate cHL disease overlapping that of non-Hodgkin's peripheral B-cell lymphoma, possibly reflecting derivation from germinal-center B-cells.

Primary mediastinal anaplastic alk-1-positive large-cell lymphoma of T/NK-cell type expressing CD20

We describe an unusual case of ALK-1-positive primary mediastinal lymphoma with the morphology of an anaplastic large-cell lymphoma (ALCL) of T/NK cell type but expressing CD20. This tumour had T/NK morphology and immunophenotype, as demonstrated by its expression of CD30, EMA, ALK-1, CD7 and TiA-1 and the lack of expression of B-cell markers other than CD20. The significance of such a co-expression of a B cell-associated antigen in a case of ALCL of T/NK cell type is discussed.

ALK-Positive Anaplastic Large Cell Lymphoma Mimicking Nodular Sclerosis Hodgkin's Lymphoma

Anaplastic large cell lymphoma (ALCL) and Hodgkin lymphoma (HL) are recognized as biologically distinct entities. However, occasionally, these two entities may share some morphologic features responsible for diagnostic difficulties. In the last 10 years, we have collected 380 cases of ALK-positive ALCL of which 10 cases were originally diagnosed as nodular sclerosis classic HL (NSHL) on conventional histopathological examination. After immunostaining, these cases proved to be ALK-positive ALCL mimicking HL (so-called Hodgkin-like ALCL). These cases account for 2.6% of our cases of ALK-positive ALCL (10 of 380 cases). Median age was 11 years (3-92 years) with a female predominance (male/female ratio, 3:7). Characteristically, these lesions showed thick nodular fibrosing bands highly suggestive of NSHL. Neoplastic cells were scarce in 6 cases, whereas in the 4 remaining cases, sheets of tumor cells were also present. A perivascular and a sinusoidal growth pattern was observed in various degrees in all cases. Few binucleated Reed-Sternberg-like cells were present in every case in a background of small lymphocytes. Inflammatory cells (ie, granulocytes, eosinophils, and histiocytes) were rare. Neoplastic cells were positive for CD30 (10 of 10 cases), ALK protein (10 of 10 cases), epithelial membrane antigen (EMA) (9 of 9 cases), CD43 (6 of 9 cases), and perforin (8 of 8 cases), but negative for CD15 (10 of 10 cases), CD20 (10 of 10 cases), Pax5/BSAP (6 of 6 cases), and EBV (8 of 8 cases). In addition, in 7 cases, neoplastic cells were of T-phenotype, whereas the 3 remaining cases were considered to be of null/undetermined phenotype. Although rare, Hodgkin-like ALCL may mimic NSHL, and it is advisable to include EMA in the first line panel and to ask for ALK staining in EMA-positive, CD15-negative lesions with morphologic features suggestive of NSHL.

Differential expression of cyclin D3 in ALK+ and ALK− anaplastic large cell lymphoma

As defined in the World Health Organization classification, anaplastic large cell lymphoma (ALCL) is a distinct type of non-Hodgkin lymphoma of T/null cell lineage, a subset of which is associated with translocations involving 2p23 resulting in expression of anaplastic lymphoma kinase (ALK). The most common translocation, the t(2;5)(p23;q35), results in expression of nucleophosmin (NPM)-ALK. NPM-ALK has been shown to activate signal transducer and activator of transcription (STAT) 3, a transcriptional regulator of cyclin D3. In this study, we assessed cyclin D3 expression in 2 ALK+ ALCL cell lines (Karpas 299 and SU-DHL1) and 1 ALK- ALCL cell line (Mac2A) by Western blot analysis. We also assessed cyclin D3 expression in 52 ALCL tumors (32 ALK+, 20 ALK-) by immunohistochemistry using tissue microarrays. These results were compared with phosphorylated (activated) STAT3 (pSTAT3) expression. Both ALK+ ALCL cell lines, but not the ALK- ALCL cell line, expressed cyclin D3 and pSTAT3. Cyclin D3 was expressed in 25 (78%) of 32 ALK+ ALCL tumors and in 4 (20%) of 20 ALK- ALCL tumors (P < .001, Fisher exact test ). In ALK+ ALCL tumors, the mean percentage of cyclin D3-positive tumor cells was 40.6% compared with 5.1% in ALK- ALCL tumors (P < .001, Mann-Whitney U test). The percentages of cyclin D3-positive and pSTAT3-positive tumor cells were positively correlated (Spearman R = 0.35, P = .036). We conclude that cyclin D3 is differentially expressed in ALK+ and ALK- ALCL and that high expression levels of cyclin D3 in ALK+ ALCL may be attributable to STAT3 activation.

Extra copies of chromosome 2 are a recurring aberration in ALK-negative lymphomas with anaplastic morphology

The purpose of this study was to evaluate fluorescence in situ hybridization abnormalities of the 2p23 anaplastic lymphoma kinase (ALK) gene loci in lymphomas with anaplastic morphology. We studied 24 anaplastic large cell lymphomas (ALCL) classified by World Health Organization criteria [17 primary nodal/systemic (10 ALK+, 7 ALK-), seven primary cutaneous], and 17 additional non-Hodgkin's lymphomas [one ALK+ B-lineage lymphoma, 14 ALK- diffuse large B-cell lymphomas (seven anaplastic variants, five nonanaplastic, two secondary CD30+), two follicular lymphomas]. ALK- lymphomas with anaplastic morphology showed extra nonrearranged anaplastic lymphoma kinase gene loci (P=0.004) due to trisomy 2 irrespective of the following factors: B or T/null phenotype (P=0.315), diagnostic categories of systemic or cutaneous ALCL or the above-mentioned B-cell lymphomas (P=0.131), and CD30 positivity by immunohistochemistry (P=1.000). Trisomy 2 was absent in all ALK+ lymphomas (P=0.009), which showed rearranged ALK gene loci (P<0.001). Whether trisomy 2 is a primary or secondary event that leads to ALK- lymphomas cannot be determined from this study. Its presence in secondary B-cell lymphomas suggests that trisomy 2 may be a secondary cytogenetic aberration in lymphomas in general. Further investigation of this finding is necessary to further our understanding of the heterogeneous group of ALK- lymphomas.

ALK-positive diffuse large B-cell lymphoma of the stomach associated with a clathrin-ALK rearrangement

ALK-positive diffuse large B-cell lymphoma is a rare, recently characterized lymphoma subtype that shows granular cytoplasmic ALK expression. This report describes a primary gastric ALK-positive B-lineage lymphoma in which a clathrin (CLTC)-ALK fusion was identified by RT-PCR and direct sequencing of the breakpoint. This confirmed the presence of t(2;17)(p23;q23) involving the CLTC gene and is only the 4th report of such a translocation in this lymphoma subtype and the first to describe this tumor within the stomach. As in previous reports, immunophenotyping showed the malignant cell to be a terminally differentiated B-lineage cell characterized by the absence of B-cell antigens and expression of antigens associated with plasma cell differentiation. This case confirms the existence of such a lymphoma subtype arising in extranodal locations and underscores the importance of detailed immunophenotyping and specialized molecular genetic investigations in confirming the diagnosis.

Suppressor of cytokine signaling 3 expression in anaplastic large cell lymphoma

Using a cDNA microarray, we found that suppressor of cytokine signaling 3 (SOCS3) is highly expressed in anaplastic lymphoma kinase (ALK)+ anaplastic large cell lymphoma (ALCL) cell lines. As SOCS3 is induced by activated signal transducer and activator of transcription 3 (STAT3), and ALK activates STAT3, we hypothesized that SOCS3 may play a role in ALK+ ALCL pathogenesis via the Janus kinase 3 (JAK3)-STAT3 pathway. Using ALCL cell lines, we show by coimmunoprecipitation experiments that SOCS3 physically binds with JAK3 in vitro, and that JAK3 inhibition by WHI-P154 downregulates SOCS3 expression. Western blot analysis confirmed expression of SOCS3 and also showed coexpression of phosphorylated (activated) STAT3 (pSTAT3). Direct sequencing of the SOCS3 gene showed no mutations or alternative splicing. In ALCL tumors that were assessed by immunohistochemistry, nine of 12 (75%) ALK+ tumors were SOCS3 positive and eight (67%) coexpressed pSTAT3. In comparison, 18 of 25 (72%) ALK-- tumors were SOCS3 positive and seven (28%) coexpressed pSTAT3. These results show that SOCS3 is overexpressed in ALCL, attributable to JAK3-STAT3 activation and likely related to ALK in ALK+ tumors. However, SOCS3 is also expressed in tumors that lack STAT3 and ALK suggesting alternative mechanisms of upregulation.

Anaplastic lymphoma kinase proteins in growth control and cancer

The normal functions of full-length anaplastic lymphoma kinase (ALK) remain to be completely elucidated. Although considered to be important in neural development, recent studies in Drosophila also highlight a role for ALK in gut muscle differentiation. Indeed, the Drosophila model offers a future arena for the study of ALK, its ligands and signalling cascades. The discovery of activated fusion forms of the ALK tyrosine kinase in anaplastic large cell lymphoma (ALCL) has dramatically improved our understanding of the pathogenesis of these lymphomas and enhanced the pathological diagnosis of this subtype of non-Hodgkin's lymphoma (NHL). Likewise, the realisation that a high percentage of inflammatory myofibroblastic tumours express activated-ALK fusion proteins has clarified the causation of these mesenchymal neoplasms and provided for their easier discrimination from other mesenchymal-derived inflammatory myofibroblastic tumour (IMT) mimics. Recent reports of ALK expression in a range of carcinoma-derived cell lines together with its apparent role as a receptor for PTN and MK, both of which have been implicated in tumourigenesis, raise the possibility that ALK-mediated signalling could play a role in the development and/or progression of a number of common solid tumours. The therapeutic targeting of ALK may prove to have efficacy in the treatment of many of these neoplasms.

A case of a diffuse large B-cell lymphoma of plasmablastic type associated with the t(2;5)(p23;q35) chromosome translocation

Anaplastic large cell lymphomas are associated with the t(2;5)(p23;q35) chromosome translocation in 40% to 60% of cases, leading to a new chimeric gene NPM-ALK. NPM-ALK positive lymphomas are generally reported to be of either T cell or null phenotype. In this report, we describe a diffuse large B-cell lymphoma associated with the classic t(2;5) translocation and both nuclear and cytoplasmic expression of ALK. The tumor consisted of medium-sized to large immunoblasts and plasmablasts that on immunohistology were negative for CD30, CD20, and CD79a but showed monotypic cytoplasmic expression of lambda light chains. Clonality analysis confirmed B-cell lineage of the tumor cells. The t(2;5)(p23;q35) chromosome translocation was demonstrated as part of a complex karyotypic alteration by classic banding and spectral karyotyping (SKY) analyses. Reverse transcription polymerase chain reaction confirmed rearrangement of NPM and ALK genes. This case exemplifies that the t(2;5) can, albeit rarely, occur in large B-cell lymphomas and is not entirely limited to anaplastic large cell lymphomas of T or null cell phenotypes.

BCL-2 family proteins in peripheral T-cell lymphomas: correlation with tumour apoptosis and proliferation

The present study investigated expression levels of the anti-apoptotic proteins BCL-2, BCL-XL and MCL-1 and the pro-apoptotic proteins BAX and BCL-XS in a series of 112 peripheral T-cell lymphomas (PTCLs) classified according to the WHO classification. Using immunohistochemical methods and a 10% cut-off, each protein was detected in a subset of PTCLs: BCL-2 in 46%, BCL-XS in 49%, BAX in 57%, BCL-XL in 57%, and MCL-1 in 65%. The mean percentage of positive cells for these proteins varied significantly among the PTCL types. Only two types of PTCL, ALK-positive anaplastic large cell lymphoma (ALCL) and enteropathy-type T-cell lymphoma, had a distinctive pattern of expression; all were BCL-2-negative and MCL-1-positive. The mean percentage of BAX-positive and BCL-XS-positive tumour cells was higher in ALK-positive ALCL than in ALK-negative ALCL or other types of PTCL (p = 0.06 and p = 0.01, respectively, Kruskal-Wallis test). MCL-1 was detected significantly more frequently (p = 0.01, chi-square test) and at higher levels (p = 0.0001, Kruskal-Wallis test) in ALK-positive ALCL and ALK-negative ALCL than in other PTCL types. The apoptotic rate, evaluated by the TUNEL assay, correlated inversely with BCL-2 expression (p = 0.035). The proliferation index, assessed by the MIB-1 antibody, correlated with expression levels of MCL-1 (R = 0.42, p = 0.003), BCL-2 (R = 0.32, p = 0.027), BAX (R = 0.33, p = 0.014), and BCL-XL (R = 0.34, p = 0.015) (Spearman rank). In conclusion, BCL-2 family proteins are expressed by a subset of PTCLs and their levels correlate with some histological types, apoptotic rate, and proliferation index. Expression of these proteins may explain the poor response of many types of PTCL to standard chemotherapy. These proteins may also provide novel targets for experimental therapy.

Neutrophil-rich anaplastic large cell lymphoma presenting in the skin

A neutrophil-rich anaplastic large cell lymphoma (ALCL) presented in the skin of a 47-year-old Japanese woman. The solitary cutaneous lesion was an eroded, 10-mm, dome-shaped nodule involving the skin of her left upper arm. Histologically, it showed a proliferation of pleomorphic, anaplastic, large tumor cells with nuclei of various shapes, including embryo-shaped, Reed-Sternberg cell-like binucleated, and wreath-shaped multiple nuclei, in the dermis and subcutaneous tissues. There was an admixture of neutrophils ranging from about 30% to more than 50% per field. Immunophenotypically, the neoplastic cells were positive for CD30, CD4, leukocyte common antigen, anaplastic lymphoma kinase-1, epithelial membrane antigen, and granzyme B. She had no peripheral neutrophilia. The lesson was excised, and the site on the left upper arm was irradiated. Six and a half months after diagnosis, however, swelling of a left axillary lymph node appeared; it also showed a proliferation of anaplastic large tumor cells admixed with numerous neutrophils ranging from about 25% to more than 60% per field. Southern blot analysis of T-cell receptor gene rearrangement revealed a clonal band. The patient was treated with six courses of cyclophosphamide, doxorubicin, vincristine, and prednisolone chemotherapy with complete remission. Seventeen cases of neutrophil-rich ALCL arising in the skin, lymph node, muscle, testis, and skull bone were reviewed. This form may possibly be induced by trauma or irritation of conventional ALCL, although the true mechanism for the infiltration of neutrophils is still unclear.

Neutrophil-rich Ki-1-positive anaplastic large cell lymphoma: a study by fine-needle aspiration biopsy

Fine-needle aspiration biopsy (FNAB) is an accurate, cost-effective method of evaluating lymphomas. The neutrophil-rich variant of anaplastic large cell lymphoma (NR-ALCL) is a rare non-Hodgkin lymphoma. To our knowledge, we present thefirst study of NR-ALCL by FNAB cytology. Histologic confirmation was available for both patients. Both cases were positive for Ki-1 (CD-30) and were either T-cell or null-cell phenotype. FNAB specimens were highly cellular with a single-cell pattern composed of pleomorphic tumor cells, "hallmark" tumor cells, and a background rich in neutrophils that occasionally obscured tumor cells. Diagnosis on FNAB is difficult owing to the rarity of this tumor, its resemblance to Hodgkin lymphoma and other non-Hodgkin lymphomas that express CD30, its similarity to an infectious process, and its occasional confusion with metastatic carcinoma and melanoma. Reproducible cytologic features usually are present, and the diagnosis can be made conclusively by FNAB in conjunction with ancillary studies.

Proteins encoded by genes involved in chromosomal alterations in lymphoma and leukemia: clinical value of their detection by immunocytochemistry

Acquired chromosomal anomalies (most commonly translocations) in lymphoma and leukemia usually result in either activation of a quiescent gene (by means of immunoglobulin or T-cell-receptor promotors) and expression of an intact protein product, or creation of a fusion gene encoding a chimeric protein. This review summarizes current immunocytochemical studies of these 2 categories of oncogenic protein, with emphasis on the clinical relevance of their detection in diagnostic samples. Among the quiescent genes activated by rearrangement, expression of cyclin D1 (due to rearrangement of the CCND1 [BCL-1] gene) is a near-specific marker of t(11;14) in mantle cell lymphoma; BCL-2 expression distinguishes follicular lymphoma cells from their nonneoplastic counterparts in reactive germinal centers and appears to be an independent prognostic marker in diffuse large cell lymphoma; and TAL-1 (SCL) expression identifies T-cell acute lymphoblastic neoplasms in which this gene is activated. The protein products of other genes activated by chromosomal rearrangement have a role as markers of either lineage (eg, PAX-5 [B-cell-specific activator protein] for B cells, including B-lymphoblastic neoplasms), or maturation stage (eg, BCL-6 for germinal-center and activated B cells and MUM-1/IRF4 for plasma cells). Currently, no hybrid protein encoded by fusion genes is reliably detectable by antibodies recognizing unique junctional epitopes (ie, epitopes absent from the wild-type constituent proteins). Nevertheless, staining for promyelocytic leukemia (PML) protein will detect acute PML with t(15;17) because the microspeckled nuclear labeling pattern for PML-RARalpha is highly distinctive. Similarly, antibodies to the anaplastic lymphoma kinase (ALK) tyrosine kinase are valuable (because wild-type ALK is not found in normal lymphoid tissue) in detecting neoplasms (CD30-positive large T-cell lymphomas) with t(2;5) or its variants. Thus, immunocytochemical detection of the products of many rearranged genes in lymphoma and leukemia can be clinically informative and provide information on cellular and subcellular protein expression that cannot be inferred from studies based on messenger RNA.

An immunophenotypic and molecular study of primary large B-cell lymphoma of bone

Primary non-Hodgkin's lymphomas of bone (PNHLB) is a rare form of extranodal lymphoma. Many studies have reported the clinical, radiologic, and histopathologic characteristics of PNHLB; however, their molecular features have not been well studied. In this report, we present the immunophenotypic and molecular characteristics of 20 primary large B-cell lymphoma (PLBCL) of bone from 20 adults. Most demonstrated centroblastic morphology, with the majority exhibiting nuclear multilobation. One case (5%) demonstrated anaplastic features with strong CD30 expression but was ALK-1 negative. BCL-6 expression was seen in 6 of 20 cases, and strong p53 protein expression was seen in 11 of 20 (55%) cases. The majority of cases analyzed (13/18 = 72%) demonstrated a clonal B-cell process by IgH gene rearrangement studies. Of the five cases that did not demonstrate a clonal population, two expressed BCL-6 protein. No cases demonstrated a bcl-2/JH rearrangement, but BCL-2 protein expression was seen in 11 of 20 (55%) cases. In summary, primary lymphoma of bone is largely a non-Hodgkin's lymphoma of large B-cell type. Our studies demonstrate that p53 and BCL-2 expression may play a role in the pathogenesis of PLCBL of bone. In addition, a subset of the cases are of putative germinal center B-cell origin based on the expression of BCL-6 protein and may be genetically distinct from follicle center lymphomas. The results provide evidence for molecular heterogeneity within primary large B-cell lymphomas of bone.

Lymphoid neoplasms

The majority of B cell lymphomas, but only a minority of T cell lymphomas, are characterized by recurring chromosome translocations. Many involve the immunoglobulin or T cell receptor loci with various partner chromosomes and lead to abnormal proto-oncogene expression. Other recurring translocations result in the production of a novel fusion protein. The detection of translocations is of particular value in diagnosis and in the detection of minimal residual disease. Aneuploidy and deletion of specific chromosome regions are common secondary chromosomal events which are rarely specific to a particular type of lymphoma but provide valuable prognostic information. Analysis by G banding, 24-colour FISH and CGH provides global genomic information; however, more specifically directed investigations utilizing locus-specific FISH probes, PCR techniques or monoclonal antibodies may be more appropriate to answer particular questions regarding diagnosis and prognosis. The known molecular consequences of abnormalities and the appropriate methods of detection are discussed for each subtype of lymphoma.

Differential expression of BCL-2 family proteins in ALK-positive and ALK-negative anaplastic large cell lymphoma of T/null-cell lineage

Anaplastic large-cell lymphoma (ALCL) of T- or null-cell lineage, as defined in the revised European-American lymphoma classification, includes a subset of tumors that carry the t(2;5)(p23;q35) resulting in overexpression of anaplastic lymphoma kinase (ALK). Patients with ALK+ ALCL are reported to have a better prognosis than patients with ALK- ALCL. Because the mechanisms for this survival difference are unknown, we investigated the hypothesis that apoptotic pathways may be involved. We therefore assessed expression levels of the anti-apoptotic proteins BCL-2 and BCL-XL and the pro-apoptotic proteins BAX and BCL-XS in T/null-cell ALCL using immunohistochemical methods and correlated the findings with ALK expression and apoptotic rate (AR), the latter assessed by a modified Tdt-mediated dUTP nick-end labeling assay. ALK was detected in 21 of 66 (31.8%) ALCLs. BCL-2 was not detected in 21 ALK+ ALCLs but was present in 26 of 45 (57.8%) ALK- ALCLs (P < 0.0001). ALK+ and ALK- ALCLs also showed significant differences in expression of BCL-XL, BAX, and BCL-XS. ALK+ tumors less commonly had a high level of BCL-XL (1 of 17 versus 14 of 35, P = 0.01), and more commonly had high levels of BAX (13 of 18 versus 15 of 36, P = 0.05), and BCL-XS (11 of 16 versus 12 of 31, P = 0.05) compared with ALK- tumors. ALK+ tumors also had a higher mean AR than ALK- tumors (3.4% versus 1.1%, P = 0.0002). Differential expression of BCL-2 family proteins may be responsible for the higher AR observed in ALK+ ALCL and provides a possible biological explanation for the better prognosis reported for patients with ALK+ ALCL.

Anaplastic lymphoma kinase proteins and malignancy

The anaplastic lymphoma kinase (ALK) gene fuses to the nucleophosmin (NPM) gene as a result of a (2;5) translocation associated with a subtype of human lymphoma (initially designated anaplastic large cell lymphoma [ALCL] or Ki-1/CD30-positive lymphoma). The immunocytochemical detection of NPM-ALK (and proteins encoded by other ALK fusion genes) has allowed the definition of a tumor entity, "ALK-positive lymphoma" (which shows only partial overlap with pathologists' diagnosis of ALCL), to be defined and is invaluable in distinguishing this disease from ALK-negative large cell lymphomas. Eight variant ALK fusion proteins have been identified. Some are expressed only in ALCL, some are found only in the nonhematopoietic neoplasm inflammatory myofibroblastic tumor (IMT), and some are present in both types of malignancy. The ALK gene is silent in adult tissues except for restricted sites within the nervous system (consequently, patients with ALK-positive lymphoma produce antibodies to the ALK protein) but is expressed in some neuroblastomas and rhabdomyosarcomas. Biochemical studies suggest an anti-apoptotic function of NPM-ALK, and this may contribute to oncogenesis. Although ALK-positive lymphomas have a generally good prognosis, new therapeutic regimens are still needed for patients whose disease is refractory to conventional treatment.