Incidence of preclinical manifestations of mantle cell lymphoma and mantle cell lymphoma in situ in reactive lymphoid tissues

Indolent clonal lymphoid disorders

Indolent clonal lymphoid disorders are not recognized as lymphomas as they generally need no systemic treatment, and depending on the lesion, need only limited clinical follow-up. These lesions are usually incidentally diagnosed during the work up for other disease. The recognition of indolent clonal lymphoid disorders is important to avoid misdiagnosis as lymphoma and unnecessary treatment. Notwithstanding, some indolent disorders, especially B-cell disorders, may give important morbidity that is not related to disease burden but related to auto-immune disease which may need treatment. Further, some of these lesions may, at various rates, ultimately progress to lymphoma. As such, the indolent clonal lymphoid disorders also give an insight into the earliest stages of clonal lymphoid disease that may increase our understanding of lymphoma, although much needs yet to be elucidated. In this article both B- and T-cell indolent clonal lymphoid disorders are reviewed. Not included in this review are lymphoid lesions that may be mistaken for lymphoma, but are not clonal, such as indolent T-lymphoblastic proliferation or marginal zone hyperplasia with immunoglobulin light chain restriction. Further, an emphasis has been given to clonal lymphoid lesions and therefore indolent plasma cell lesions have not been included. Also excluded is indolent lymphoma that may not need treatment but nonetheless requires more regular follow up. One may rightfully argue that there may be a gray zone between what constitutes an indolent clonal lymphoid disorder and an indolent lymphoma. This discussion is reflected in the different terminology used for some entities between editions of the WHO classification and between the Fifth Edition of the WHO Classification and the International Consensus Classification (ICC). The former has been used as a selection basis for this review, but cross-reference has been made to the ICC nomenclature when that differs as well as to the earlier Revised Fourth Edition of the WHO Classification (WHO-r4). For this reason, indolent T-cell lymphoma of the gastrointestinal tract (ICC: indolent clonal T-cell lymphoproliferative disorder of the gastrointestinal tract) is not included in this review.

Fifth Edition of the World Health Classification of Tumors of the Hematopoietic and Lymphoid Tissues: B-cell Neoplasms

We review B-cell neoplasms in the 5th edition of the World Health Organization classification of hematolymphoid tumors (WHO-HEM5). The revised classification is based on a multidisciplinary approach including input from pathologists, clinicians, and other experts. The WHO-HEM5 follows a hierarchical structure allowing the use of family (class)-level definitions when defining diagnostic criteria are partially met or a complete investigational workup is not possible. Disease types and subtypes have expanded compared with the WHO revised 4th edition (WHO-HEM4R), mainly because of the expansion in genomic knowledge of these diseases. In this review, we focus on highlighting changes and updates in the classification of B-cell lymphomas, providing a comparison with WHO-HEM4R, and offering guidance on how the new classification can be applied to the diagnosis of B-cell lymphomas in routine practice.

Chronic lymphocytic leukaemia/small lymphocytic lymphoma and mantle cell lymphoma: from early lesions to transformation

The International Clinical Advisory Committee reviewed advances in our understanding of the clinicopathologic and biologic features of chronic lymphocytic leukaemia/small lymphocytic lymphoma, B-cell prolymphocytic leukaemia, and mantle cell lymphoma since the revised 4th edition of the WHO Classification of Tumours of the Haematopoietic and Lymphoid Tissues. Discussions amongst pathologists, clinicians, and molecular geneticists around these diseases focussed on incorporating new knowledge into the next classification system. In this manuscript, we review these disease entities and incorporate results of these deliberations, including advances in our understanding of early lesions and transformation.

B Lymphoproliferative Neoplasms of Uncertain Biological Significance: Report from the IV Workshop of the Italian Group of Hematopathology and Review of the Literature

Lymphoproliferative neoplasms of uncertain biological significance are increasingly encountered due to widespread usage of immunophenotypic and molecular techniques. Considering that clearer biological criteria and patient management have been established for B-cell lymphoproliferative diseases of undetermined significance occurring in the peripheral blood, many issues are still obscure for early lesions detected in lymphoid tissues. Regardless that some categories of lymphoproliferative neoplasms of uncertain biological significance have been recognized by the 4th edition of the WHO, other anecdotal early lymphoproliferative lesions still remain fully undefined. Some early lesions frequently originate from the germinal center, including atypical germinal centers BCL2-negative, an early pattern of large B-cell lymphoma with IRF4 rearrangement, and “in situ” high-grade B lymphomas. Moreover, other early lymphoproliferative lesions arise outside the germinal center and include those developing within the setting of monocytoid B-cell hyperplasia, but they also can be directly or indirectly associated with chronic inflammations. This review aims to summarize the concepts discussed during the IV Workshop organized by the Italian Group of Hematopathology, focus on the state-of-the-art on B-cell lymphoproliferative neoplasms of uncertain biological significance, and offer operative insights to pathologists and clinicians in routine diagnostics.

Multifocal in situ mantle cell neoplasia of the ileocecal region: a case report with simultaneous nodal and extranodal involvement

In situ mantle cell neoplasia (ISMCN) is a rare entity of disputed clinical significance. We report an additional case, unusual by its presentation in the large intestine and its multifocal involvement of several nodal and extranodal sites. The diagnosis was made in a 46-year-old male patient from a surgical specimen resected for cecal adenocarcinoma. Gross examination showed multiple small polypoid lesions surrounding the ileocecal valve, corresponding to lymphoid aggregates with hyperplastic follicles. Numerous cyclin D1/SOX11+ lymphoid cells, harboring the t(11;14)(q13;q32) translocation, were present in the inner layers of mantle zones. The same lesions were found in the ileum, the appendix, and the regional lymph nodes. The final diagnosis was multifocal ISMCN of the ileocecal region, with both nodal and extra-nodal involvement. A simple surveillance was decided. Our observation expands the clinical spectrum of the disease and underlines the necessity to closely examine even normal-appearing reactive lymphoid tissues.

An overlapping case of in situ mantle cell neoplasia and leukemic non-nodal mantle cell lymphoma

In situ mantle cell neoplasia (isMCN) and leukemic non-nodal mantle cell lymphoma (nnMCL) are classified as an indolent subtype of mantle cell lymphoma (MCL). The tumor cells of isMCN are restricted to the inner layer of the lymphoid tissue mantle zone, exhibiting an in situ pattern histologically. On the other hand, nnMCL is distributed in the peripheral blood, bone marrow and sometimes the spleen, but lymphadenopathy or systemic organ involvement is rare. We report a case of isMCN in a submandibular lymph node resected from a 65-year-old Japanese male. The tumor cells were positive for cyclin D1 (CCND1) and SOX11 expression, and were restricted to the mantle zone area of the lymph node. However, tumor cells were also detected in the stomach mucosa, bone marrow tissue and peripheral blood, suggesting nnMCL. isMCN and nnMCL may have a partly overlapping disease spectrum, although the correlation between these two subtypes has not been well described. This present case demonstrated characteristics overlapping between isMCN and nnMCL.

Is Limited-Stage Mantle Cell Lymphoma Curable and How Is It Best Managed?

Limited-stage (stage I-II) mantle cell lymphoma (MCL) is rarely encountered. There is no standard approach to treatment and available data to guide management decisions mainly are retrospective studies. A thorough staging evaluation, including positron emission tomography/computed tomography, bone marrow biopsy, and gastrointestinal evaluation, should be completed because disseminated disease is common. Radiation therapy is effective for local control, and, although prolonged remission can be achieved, distant relapses are common and there are insufficient data to say that patients can be cured using this treatment. This article reviews literature pertaining to management of patients with limited-stage MCL and discusses approach to treatment.

Early Events in Lymphoid Neoplasms and Indolent Follicular Lymphomas

Technical advances in diagnostic modalities have led to the characterization of indolent lymphoid disorders similar to the in situ lesions described in epithelial malignancies. These early and indolent lymphoid lesions share clinicopathologic characteristics with well-characterized lymphoid malignancies such as chronic lymphocytic leukemia and follicular lymphoma. The in situ lesions have an indolent clinical course with only a minor subset shown to progress to frank malignancies. In addition to the in situ lesions, new indolent lymphoproliferative disorders have been recently characterized. Diagnosis and characterization of these indolent lesions is necessary to prevent overtreatment with aggressive therapeutic regimens.

Mantle Cell Lymphoma With Mantle Zone Growth Pattern

OBJECTIVES

To characterize the clinical and pathologic features of mantle cell lymphoma with mantle zone growth pattern (MCL-MZGP).

METHODS

The clinicopathologic data from 35 cases of MCL-MZGP obtained in 12 centers were analyzed.

RESULTS

The patients with MCL-MZGP typically sought treatment at high clinical stages (81%). Intriguingly, 40% (14/35) of cases were incidentally noted. The lymph nodes with MCL-MZGP showed preserved architecture and expanded mantles containing lymphoma cells with classic or small cell cytology. MCL-MZGP was positive for BCL2 (96%, bright), CD5 (82%, moderate), cyclin D1 (100%), and SOX11 (89%). Clinically, our study revealed no significant difference in the overall survival between patients managed with observation alone and those who received chemotherapy.

CONCLUSIONS

MCL-MZGP was often incidentally identified and resembled reactive mantles. Therefore, recognition of this unusual morphology emphasizes the utility of cyclin D1 immunostain in the cases with suspicious morphology. However, the clinical significance of these findings is still unclear.

SOX11, a key oncogenic factor in mantle cell lymphoma

PURPOSE OF REVIEW

SOX11 has emerged as a key transcription factor in the pathogenesis of mantle cell lymphoma (MCL) whereas it is not expressed in normal B cells or virtually in any other mature B-cell neoplasm. This review will examine the role of SOX11 as a biomarker in MCL, the new information on its transcriptional targets, and the mechanisms regulating its expression in MCL.

RECENT FINDINGS

SOX11 is highly expressed in conventional MCL, including cyclin D1-negative cases, but it is not expressed in the indolent leukemic nonnodal MCL subtype. These two MCL subtypes also differ in their cell-of-origin, IGHV mutational status and genomic instability. SOX11 promotes tumor growth of MCL cells in vivo and regulates a broad transcriptional program that includes B-cell differentiation pathways and tumor-microenvironment interactions, among others. The mechanisms upregulating SOX11 in MCL are not well understood but are mediated in part by the three-dimensional reconfiguration of the DNA, bringing together a distant enhancer region and the SOX11 promoter.

SUMMARY

SOX11 is a relevant element in the pathogenesis of MCL and has been instrumental to identify two distinct clinicobiological subtypes of this tumor. Further studies should clarify the mechanisms mediating its oncogenic potential and leading to its intriguing expression in these tumors.

[Clonal evolution of early lymphoproliferations]

The identification and molecular characterisation of premalignant precursor lesions of lymphomas, such as monoclonal gammopathy of unknown significance (MGUS) and the so-called in situ lymphoproliferations, has made significant progress in the recent years. The in situ follicular neoplasia (ISFN), the best-characterised entity, is by definition not identifiable by morphology and represents a t(14;18)+ precursor lesion of follicular lymphoma with characteristic immunophenotype, low potential for progression, and already identifiable secondary genetic alterations. The use of high-throughput genetic techniques on microdissected tissues has generated novel insights into clonal evolution and biological progression of early lesions and documented that an isolated genetic analysis is insufficient to understand the complexity of proliferations.

Smoldering mantle cell lymphoma

Background

Mantle cell lymphoma (MCL) is an aggressive disease, with poor prognosis and a limited survival. However, some patients with indolent MCL can survive beyond 7~10 years. These patients remain largely asymptomatic and can be in observation for a long time without any treatment. The process of “wait and watch” leaves these patients with the potential risk of evolution to classic, aggressive MCL. On the other hand, early treatment for these patients may not impact overall survival but rather affects the quality of life. Therefore, it is essential to clearly identify this type of indolent MCL at the time of diagnosis.

Results

Reported findings of indolent presentation of MCL include: lack of B symptoms, normal serum lactic dehydrogenase (LDH) and β2-microglobulin levels (β2M), low MCL-International Prognostic Index (MIPI) score, maximum tumor diameter less than 3 cm, spleen size < 20 cm, positron emission tomography/computerized tomography with the Standard Uptake Value max <6, Ki-67 less than 30%, with some particular immunophenotype, such as CD5 and CD38 negative, markedly increased CD23 positive lymphocytes proportions, high expression of CD200, kappa light chain restriction, without C-myc, TP53 and NOTCH1/2 mutations, non-blastoid/pleomorphic histology, and no tumor growth on reevaluation every 2~3 months (followed for at least 6 months). Imaging evaluation may only be performed in the presence of disease-related symptoms or organ involvement. Meanwhile, if novel nodal or extranodal lesion is found, biopsy is mandatory to exclude lymphoma.

Common clinopathological forms of indolent presentations include monoclonal B lymphocytosis with t (11; 14); “indolent leukemic” presentation of MCL with involvement of peripheral blood, bone marrow involvement, splenomegaly, and minimal lymphadenopathies and in situ lymphoma (often found in lymph nodes removed for other reasons, and in gastrointestinal biopsies).

Conclusions

Considering these distinct indolent clinical presentations with particular features in cytology and gene mutational status, we propose to include these MCL clinical presentations under the umbrella of “Smoldering Mantle Cell Lymphoma”.

Early stages in the ontogeny of small B-cell lymphomas: genetics and microenvironment

In this review, we focus on the mechanisms underlying lymphomagenesis in chronic lymphocytic leukaemia, follicular lymphoma, mantle cell lymphoma and splenic marginal zone lymphoma. The cells of origin of these small B-cell lymphomas are distinct, as are the characteristic chromosomal lesions and clinical courses. One shared feature is retention of expression of surface immunoglobulin. Analysis of this critical receptor reveals the point of differentiation reached by the cell of origin. Additionally, the sequence patterns of the immunoglobulin-variable domains can indicate a role for stimulants of the B-cell receptor before, during and after malignant transformation. The pathways driven via the B-cell receptor are now being targeted by specific kinase inhibitors with exciting clinical effects. To consider routes to pathogenesis, potentially offering earlier intervention, or to identify causative factors, genetic tools are being used to track pretransformation events and the early phases in lymphomagenesis. These methods are revealing that chromosomal changes are only one of the many steps involved, and that the influence of surrounding cells, probably multiple and variable according to tissue location, is required, both to establish tumours and to maintain growth and survival. Similarly, the influence of the tumour microenvironment may protect malignant cells from eradication by treatment, and the resulting minimal residual disease will eventually give rise to relapse. The common and different features of the four lymphomas will be summarized to show how normal B lymphocytes can be subverted to generate tumours, how these tumours evolve and how their weaknesses can be attacked by targeted therapies.

Mantle Cell Lymphoma: Contemporary Diagnostic and Treatment Perspectives in the Age of Personalized Medicine

Mantle cell lymphoma is a clinically heterogeneous disease occurring within a heterogeneous patient population, highlighting a need for personalized therapy to ensure optimal outcomes. It is therefore critical to understand the benefits and risks associated with both intensive and deintensified approaches. In the following review we provide a therapeutic roadmap to strategically guide treatment for newly diagnosed and relapsed/refractory patients highlighting pivotal and recently published results involving known and novel therapies.

The Pathology of Reactive Lymphadenopathies: A Discussion of Common Reactive Patterns and Their Malignant Mimics

Context.—Distinguishing between a reactive and a neoplastic lymphoid proliferation is a clinically significant task frequently performed by the surgical pathologist in routine practice.

Objectives.—To highlight common situations in lymph node pathology where reactive changes and lymphoma may be misdiagnosed.

Data Sources.—Data sources are peer-reviewed journal articles, textbooks, and clinical experience.

Conclusions.—This review aims to refresh and enhance the surgical pathologist's awareness of the shared and distinguishing features of select reactive and neoplastic lymphoproliferations, which in turn will allow the surgical pathologist to make more accurate diagnoses and avoid the pitfalls of misdiagnosis. This will be done by describing a selection of commonly encountered reactive histologic changes observed in lymph nodes, present the lymphomas with which they share overlapping features, outline the features that distinguish them, and describe an approach to making an accurate diagnosis and avoiding a misdiagnosis in each scenario.

[Classification of malignant lymphomas. Current situation]

BACKGROUND

The World Health Organization (WHO) is planning an update of the WHO classification of malignant lymphomas.

OBJECTIVE

To present new findings concerning the diagnostics and subclassification of malignant lymphomas.

MATERIAL AND METHODS

A selective literature search ( http://www.ncbi.nlm.nih.gov ) was carried out and combined with the practical experiences of the authors in clinicopathological diagnostics.

RESULTS

In recent years an increasing number of early lesions of malignant lymphomas have been described but the potential malignancy of these lesions is at least for some entities still uncertain. Newly defined entities have been described within the group of T-cell lymphomas and prognostic subgroups have been identified in the heterogeneous group of diffuse large B-cell lymphomas. Detection of mutations facilitates the differential diagnostics of morphologically similar diseases and can be an important component of the diagnostics.

CONCLUSION

Recent scientific insights are being included more and more into the diagnostics of lymphomas. The update of the WHO classification is a consequence of these developments.

Early lymphoid lesions: conceptual, diagnostic and clinical challenges

There are no "benign lymphomas", a fact due to the nature of lymphoid cells to circulate and home as part of their normal function. Thus, benign clonal expansions of lymphocytes are only rarely recognized when localized. Recent studies have identified a number of lymphoid proliferations that lie at the interface between benign and malignant. Some of these are clonal proliferations that carry many of the molecular hallmarks of their malignant counterparts, such as BCL2/IGH and CCND1/IGH translocations associated with the in situ forms of follicular lymphoma and mantle cell lymphoma, respectively. There are other clonal B-cell proliferations with low risk of progression; these include the pediatric variants of follicular lymphoma and marginal zone lymphoma. Historically, early or incipient forms of T/NK-cell neoplasia also have been identified, such as lymphomatoid papulosis and refractory celiac disease. More recently an indolent form of T-cell lymphoproliferative disease affecting the gastrointestinal tract has been described. Usually, CD8(+), the clonal cells are confined to the mucosa. The clinical course is chronic, but non-progressive. NK-cell enteropathy is a clinically similar condition, composed of cytologically atypical NK-cells that may involve the stomach, small bowel or colon. Breast implant-associated anaplastic large cell lymphoma is a cytologically alarming lesion that is self-limited if confined to the seroma cavity. Atypical lymphoid proliferations that lie at the border of benign and malignant can serve as instructive models of lymphomagenesis. It is also critical that they be correctly diagnosed to avoid unnecessary and potentially harmful therapy.

Multifocal mantle cell lymphoma in situ in the setting of a composite lymphoma

OBJECTIVES

Mantle cell lymphoma in situ (MCLIS) consists of immunophenotypically defined but histologically inapparent neoplastic cells restricted to narrow mantle zones, without expansion or invasion beyond the mantle zone. We report a unique case of MCLIS associated with a much more manifest nodal marginal zone lymphoma (MZL) in an inguinal lymph node, porta hepatis lymph node, and bone marrow.

METHODS

Biopsies from all three locations were evaluated using standard H&E-stained sections, immunohistochemistry, flow cytometry, metaphase cytogenetics, and/or fluorescence in situ hybridization (FISH).

RESULTS

This case is unique for three reasons. First, the histologically covert mantle cell lymphoma was multifocal, detected in all three locations using one or more of flow cytometry, immunohistochemistry, cytogenetics, and FISH. Second, the MCLIS was always accompanied by a more histologically dominant MZL. Third, where evaluable, it did not grow in an appreciable mantle zone distribution, presumably due to destruction of the normal nodal architecture by the neoplastic MZL cells and the resulting absence of recognizable follicles and mantle zones.

CONCLUSIONS

This unique case provides new insight into the pathogenesis of MCLIS.

Pathology of B-cell lymphomas: diagnosis and biomarker discovery

The diagnosis of B-cell non-Hodgkin lymphomas has changed significantly over the past few decades as new immunophenotypic markers, molecular subtype classification schemes, and novel biomarkers have emerged. Meanwhile, there has been an increasing emphasis on individualizing treatment approaches in accordance with a biologic heterogeneity that has been uncovered within many of the individual B-cell lymphoma entities. The application of high-throughput genomic sequencing to B-cell lymphomas has yielded large amounts of valuable information. The data encompass discoveries essential to an understanding of pathogenesis, clonal or tumoral evolution, and identification of biomarkers that may be useful for prognostic or therapeutic considerations. The following review discusses several of the more common, primarily tissuebased B-cell lymphomas, with a focus on pathologic classification and certain phenotypic characteristics or genetic lesions that apply to refinement of diagnosis and therapy.

Clonally related composite follicular lymphoma and mantle cell lymphoma with clinicopathologic features and biological implications

Composite lymphoma with follicular lymphoma (FL) and mantle cell lymphoma (MCL) components is rare and can pose a substantial diagnostic challenge. We report two cases of composite lymphoma with FL and MCL components occurring in lymph nodes. Both cases showed near total effacement of the lymph node architecture by grade 1 FL (CD10+ and BCL2+) with accompanying in situ MCL component (CD5+ and cyclin D1+) surrounding neoplastic follicles. The diagnosis of composite FL and MCL was confirmed by detecting the t(14;18)(q32;q21) and t(11;14)(q13;q32) in the FL and MCL components, respectively. Immunoglobulin heavy chain fragment length analysis in both cases showed identical dominant monoclonal peaks in microdissected neoplastic lymphoid cells from FL and MCL components. These findings suggest a common clonal origin for the FL and MCL components in both cases.

Follicular lymphoma-like B cells of uncertain significance (in situ follicular lymphoma) may infrequently progress, but precedes follicular lymphoma, is associated with other overt lymphomas and mimics follicular lymphoma in flow cytometric studies

In situ follicular lymphoma, more recently known as follicular lymphoma-like B cells of uncertain/undetermined significance is well accepted. However, the morphological criteria have evolved since it was first described and data are limited and conflicting regarding its clinical implications and whether the extent of involvement predicts an association with overt lymphoma. It is also unknown how often it will be identified by flow cytometric studies and how often it precedes overt follicular lymphomas. A multiparameter study of 31 biopsies with follicular lymphoma-like B cells of uncertain significance and 4 'benign' lymph node biopsies that preceded an overt follicular lymphoma was, therefore, performed. Fifty-two percent of biopsies with follicular lymphoma-like B cells were associated with a prior or concurrent lymphoma but only 6% subsequently developed lymphoma (median follow up 26 months). Neither the number, proportion or density of BCL2(+) germinal centers were associated with overt follicular lymphoma/diffuse large B-cell lymphoma. Flow cytometric studies identified follicular lymphoma-like B cells in 8 of 15 evaluable cases. The proportion but not the absolute number of BCL2(+) germinal centers was associated with the likelihood of positive flow cytometric studies (P<0.01). All 4 'benign' biopsies that preceded an overt follicular lymphoma demonstrated follicular lymphoma-like B cells. Thus, although few patients with follicular lymphoma-like B cells of uncertain significance progress within the follow-up period, it at least precedes many follicular lymphomas. The extent of involvement does not predict the occurrence of prior or concurrent lymphomas. Flow cytometric studies demonstrating follicular lymphoma-like B cells must not be over-interpreted as they may only reflect follicular lymphoma-like B cells.

Indolent mantle cell lymphoma

Over the past decade, it has become increasingly clear that mantle cell lymphoma (MCL) is a more heterogeneous disease than originally recognized. Several groups have reported on a subgroup of patients with a less aggressive course than expected resulting in the term "indolent MCL". Unlike the recognized histologic variants, the definition of indolent mantle cell lymphoma is unclear, and patients with indolent MCL are often identified only after having undergone prolonged periods of observation. In this review, we will discuss clinical and biologic features and provide a framework for the approach in identifying patients with indolent MCL.

Monoclonal B cell lymphocytosis and "in situ" lymphoma

The finding of monoclonal B-cell populations not fulfilling criteria for a lymphoid malignancy has given great impulse to study mechanisms involved in lymphomagenesis and factors responsible for the transition from B-cell precursor states to overt lymphoproliferative disorders. Monoclonal B cell expansions were initially recognized in peripheral blood of otherwise healthy subjects (thus defined monoclonal B-cell lymphocytosis, MBL) and in most cases share the immunophenotypic profile of chronic lymphocytic leukemia (CLL). The clinical relevance of this phenomenon is different according to B-cell count: high-count MBL is considered a preneoplastic condition and progresses to CLL requiring treatment at a rate of 1-2% per year, while low-count MBL, though persisting over time, has not shown a clinical correlation with frank leukemia so far. MBL other than CLL-like represent a minority of cases and are ill-defined entities for which clinical and biological information is still scanty. In situ follicular lymphoma (FL) and mantle cell lymphoma (MCL) are characterized by the localization of atypical lymphoid cells, carrying t(14;18)(q32;q21) or t(11;14)(q13;q32), only in the germinal centers and mantle zones respectively, where their normal counterparts are localized. The localization of these cells indicates that germinal centers or mantle zones provide appropriate microenvironments for cells carrying these oncogenic alterations to survive or proliferate. The progression of these lesions to overt lymphomas occurs rarely and may require the accumulation of additional genetic events. Individuals with these lymphoid proliferations should be managed with caution.

[In situ lymphoma and other early stage malignant non-Hodgkin lymphomas]

The increasing use of immunohistochemical and molecular investigations of lymphatic tissues results in more frequent detection of early lymphoid proliferations. These show some but not all features of malignant lymphomas without fulfilling the diagnostic criteria for the diagnosis of lymphoid malignancy. In addition to well-known premalignant B-cell proliferations, such as monoclonal gammopathy of unknown significance (MGUS) and monoclonal B-cell lymphocytosis (MBL), so-called in situ lymphomas have recently been described with minimal infiltrates of clonal B-cells in morphologically reactive lymphoid tissues which show the phenotypic and genetic features of specific B-cell lymphoma subtypes and often show a characteristic topographical distribution. This article addresses a group of clonal lymphoproliferations with usually localized disease and excellent clinical prognosis, such as pediatric follicular lymphoma and nodal marginal zone lymphoma. Another group of early lesions not addressed in this review are virally induced lymphoproliferations which represent a grey zone between purely reactive lesions and malignant lymphomas and may pose significant diagnostic as well as clinical problems. In this review diagnostic criteria for early or in situ lesions and their distinction from partial infiltration by malignant lymphoma are described.