Mature T-cell leukemias including T-prolymphocytic leukemia, adult T-cell leukemia/lymphoma, and Sézary syndrome

Mature T-Cell leukemias: Challenges in Diagnosis

T-cell clones can frequently be identified in peripheral blood. It can be difficult to appreciate whether these are benign and transient or whether they signify a clonal disorder. We review factors that aid in understanding the relevance of T-cell clones. Conversely, obvious pathological T-cell clones can be detected in blood, but there is uncertainty in how to categorize this clonal T cell population, thus, we adopt a multidisciplinary review of the clinical features, diagnostic material and radiology before making the diagnosis. In this review we shall discuss some of these challenges faced when diagnosing mature T-cell leukemias.

CCR7 in Blood Cancers - Review of Its Pathophysiological Roles and the Potential as a Therapeutic Target

According to the classical paradigm, CCR7 is a homing chemokine receptor that grants normal lymphocytes access to secondary lymphoid tissues such as lymph nodes or spleen. As such, in most lymphoproliferative disorders, CCR7 expression correlates with nodal or spleen involvement. Nonetheless, recent evidence suggests that CCR7 is more than a facilitator of lymphatic spread of tumor cells. Here, we review published data to catalogue CCR7 expression across blood cancers and appraise which classical and novel roles are attributed to this receptor in the pathogenesis of specific hematologic neoplasms. We outline why novel therapeutic strategies targeting CCR7 might provide clinical benefits to patients with CCR7-positive hematopoietic tumors.

An unusual case of T-cell prolymphocytic leukemia mimicking a cutaneous vasculitis

T-cell prolymphocytic leukemia (T-PLL) is an aggressive post-thymic T-cell malignancy, which accounts for 2% of mature lymphocytic leukemias in adults. Though typically presenting with a brief history of B symptoms, hepatosplenomegaly, and marked lymphocytosis, erythematous or nodular skin rashes involving the trunk or limbs may be seen in 25% to 30% of patients, as well as a purpuric rash in a periorbital distribution. Cutaneous involvement typically presents in the context of patients with an established history of T-PLL, but it can less frequently present as an initial symptom heralding the diagnosis. An unusual case of T-PLL is described, presenting initially as palmoplantar ulcerated nodules with an initial biopsy suggestive of perniosis, followed by rapid progression of dark violaceous and bright red papules throughout the body after initiation of Obinutuzumab. The diagnosis of T-PLL was subsequently fully supported by the clinical, laboratory, cytologic, and immunophenotypic findings. This case highlights the importance of a multidisciplinary team approach to address such rare and atypical presentations.

TCR β chain-directed bispecific antibodies for the treatment of T cell cancers

Immunotherapies such as chimeric antigen receptor (CAR) T cells and bispecific antibodies redirect healthy T cells to kill cancer cells expressing the target antigen. The pan-B cell antigen-targeting immunotherapies have been remarkably successful in treating B cell malignancies. Such therapies also result in the near-complete loss of healthy B cells, but this depletion is well tolerated by patients. Although analogous targeting of pan-T cell markers could, in theory, help control T cell cancers, the concomitant healthy T cell depletion would result in severe and unacceptable immunosuppression. Thus, therapies directed against T cell cancers require more selective targeting. Here, we describe an approach to target T cell cancers through T cell receptor (TCR) antigens. Each T cell, normal or malignant, expresses a unique TCR β chain generated from 1 of 30 TCR β chain variable gene families (TRBV1 to TRBV30). We hypothesized that bispecific antibodies targeting a single TRBV family member expressed in malignant T cells could promote killing of these cancer cells, while preserving healthy T cells that express any of the other 29 possible TRBV family members. We addressed this hypothesis by demonstrating that bispecific antibodies targeting TRBV5-5 (α-V5) or TRBV12 (α-V12) specifically lyse relevant malignant T cell lines and patient-derived T cell leukemias in vitro. Treatment with these antibodies also resulted in major tumor regressions in mouse models of human T cell cancers. This approach provides an off-the-shelf, T cell cancer selective targeting approach that preserves enough healthy T cells to maintain cellular immunity.

Adult T-cell leukemia/lymphoma clinically confused with viral/drug skin eruptions and pathologically misinterpreted as mycosis fungoides/Sézary syndrome

Adult T-cell leukemia/lymphoma (ATLL) is a lymphoproliferative disorder of mature CD4-positive T-cell lymphoid cells associated with retrovirus human T-lymphotropic virus type-1 (HTLV-1) with a wide clinical and pathologic spectrum. We report a case of a 53-year-old African man who presented with fever and skin eruptions on the trunk composed of non-itchy erythematous reticulated macules and papules initially suspected for viral exanthem or drug rash. Skin punch biopsy showed a dermal T-cell lymphoid infiltrate with epidermotropism. The patient developed generalized lymphadenopathy and his peripheral blood showed lymphocytosis with atypical lymphocytes with convoluted nuclei. Our initial diagnosis was mycosis fungoides with Sézary syndrome. However, some clinical and histopathologic features were unusual. The acute onset, lack of previous skin lesions, the histomorphologic features of the dermal, nodal and peripheral blood lymphocytes and the geographic origin of the patient raised the suspicion of other T-cell lymphomas, particularly ATLL. This was confirmed by a positive anti-HTLV-1 serology. Our final diagnosis was acute variant ATLL. Different T-cell lymphomas can involve the skin with overlapping clinical, histomorphologic and immunohistochemical features. Some clinical and pathologic features should alarm dermatologists and pathologists to the possibility of ATLL particularly in patients from HTLV-1 endemic geographic areas.

Composite Chronic Lymphocytic Leukemia/Small Lymphocytic Lymphoma and T-Prolymphocytic Leukemia Presenting with Lymphocytosis, Skin Lesions, and Generalized Lymphadenopathy

Chronic lymphocytic leukemia (CLL) is the most common type of leukemia in Western countries with an incidence of 3-5 cases per 100,000 persons. Most patients follow an indolent clinical course with eventual progression and need for therapy. In contrast, T-prolymphocytic leukemia (T-PLL) is a rare type of T-cell leukemia with most patients having an aggressive clinical course and a dismal prognosis. Therapies are limited for T-PLL patients and there is a high relapse rate. Morphologically, the cells of CLL and T-PLL can show overlapping features. Here, we report the case of a 61-year-old man who presented with a clinically indolent CLL and T-PLL, initially diagnosed solely and followed as CLL, despite the presence of an associated but unrecognized aberrant T-cell population in blood. After 2 years, the T-PLL component became more apparent with cutaneous and hematologic manifestations and the diagnosis was confirmed by immunophenotypic and cytogenetic analysis. Fluorescence in situ hybridization demonstrated an ATM deletion in both CLL and T-PLL components. Retrospective testing demonstrated that composite CLL and T-PLL were both present in skin and lymph nodes as well as in blood and bone marrow since initial presentation. This case is also unique because it highlights that a subset of T-PLL patients can present with clinically indolent disease. The concomitant detection of ATM mutation in CLL and T-PLL components raises the possibility of a common pathogenic mechanism.

Comparison of karyotyping, TCL1 fluorescence in situ hybridisation and TCL1 immunohistochemistry in T cell prolymphocytic leukaemia

Aims

T cell prolymphocytic leukaemia (T-PLL) is defined as an aggressive T cell leukaemia composed of small to medium-sized lymphocytes with a mature T cell immunophenotype. Most of these cases are known to be associated with inv(14q11q32)/t(14;14)(q11;q32) or rarely t(X;14)(q28;q11). However, T-PLL can show variations in clinical presentation, morphology or immunophenotype that can make a diagnosis of T-PLL challenging. We aim to explore the value of ancillary testing in the diagnosis of T-PLL.

Methods

With this large cohort of 69 patients with T-PLL, we compared the diagnostic utility of conventional cytogenetics, TCL1 rearrangement by fluorescence in situ hybridisation (FISH) and TCL1 expression by immunohistochemistry (IHC).

Results

Conventional karyotyping was performed in all 69 patients and was abnormal in 44 (65%), showing 14q32 abnormalities in 31 (43%) and t(X;14) (MTCP) in 2 (3%). TCL1 rearrangement was assessed by FISH in 26 cases and was positive in 23 (85%). All cases with 14q32 abnormalities shown by karyotype were positive for TCL1 rearrangement by FISH, whereas 12/15 (80%) cases without 14q32 abnormalities were also positive. TCL1 overexpression by IHC was detected in 51/64 (81%), including 40/42 (95%) cases with TCL1/14q32 rearrangement, and 3 cases without, showing a concordance of 89%. TCL1 IHC was negative in both cases with t(X;14)(q28;q11).

Conclusions

Our study shows that TCL1 by IHC is a convenient test, positive in >80% T-PLL. Conventional cytogenetics is insensitive in the detection of 14q32/TCL1 rearrangements but provides more complete information of the chromosomal landscape of T-PLL. FISH for TCL1 rearrangement is very valuable in diagnostic challenging cases.

TCL-1-positive hematogones in a patient with T-cell prolymphocytic leukemia after therapy

T-prolymphocytic leukemia (T-PLL) is a rare mature T-cell neoplasm characterized by proliferation of prolymphocytes. Most cases involve the T-cell leukemia-1 (TCL1) gene at 14q11.2 resulting in overexpression of TCL-1, which is helpful for distinguishing T-PLL from other T-cell neoplasms. We report a patient with T-PLL whose leukemic cells were positive for TCL-1 by immunohistochemistry but with a normal karyotype. The patient had anti-CD52 antibody therapy for 12 weeks. In a follow-up bone marrow biopsy specimen, numerous TCL-1-positive cells were present, which raised the differential diagnosis of residual T-PLL. However, further immunophenotypic studies confirmed that these cells were hematogones. Therefore a diagnosis of recovering bone marrow was established. The patient underwent stem cell transplant and is now in complete remission. This case demonstrates that hematogones can express TCL-1, and this knowledge is very important for the differential diagnosis in the follow-up marrow of T-PLL patients.

Adult T-cell leukemia/lymphoma in South and Central America and the Caribbean: systematic search and review

Adult T-cell leukemia/lymphoma (ATL) is caused by the human T-cell lymphotropic virus type 1 (HTLV-1) which is endemic in countries of Caribbean and Central and South America. We performed a systematic search and review to identify publications on ATL in these countries to verify if this disease was getting recognition in these regions as well as the characteristics of the observed cases. The median age of 49.4 years was lower than that referred to in Japan. According to our findings in most Brazilian states and in some other countries, ATL is not being recognized and should be strongly considered in the differential diagnosis of T-cell leukemias/lymphomas. Failure to identify these cases may be due to the unsystematic realization of serology for HTLV-1 and phenotypic identification of non-Hodgkin lymphomas that may result from lack of resources. Detection of ATL cases has been more feasible with cooperation from foreign research centers. A huge effort should be made to improve the surveillance system for ATL diagnosis in most of the South- and Central-American and Caribbean countries, and this attitude should be embraced by public organs to support health professionals in this important task.

Adult T-cell leukemia/lymphoma treatment in Bahia, Brazil

Background

Adult T-cell leukemia/lymphoma is a peripheral disease associated with human T-cell lymphotropic virus type 1. Treatment is carried out according to clinical type with watchful waiting being recommended for less aggressive types. Aggressive adult T-cell leukemia/lymphoma is generally treated with chemotherapy and/or antivirals. The objective of this study was to correlate the survival of patients diagnosed in Bahia, Brazil, with the therapeutic approaches employed and to evaluate what issues existed in their treatment processes.

Methods

Eighty-three adult T-cell leukemia/lymphoma patients (26 smoldering, 23 chronic, 16 acute, 13 lymphoma and five primary cutaneous tumoral) with available data were included in this study.

Results

Complete response was achieved in seven smoldering patients with symptomatic treatment, in two with chronic disease using antivirals/chemotherapy, in one with acute disease using antivirals and in one lymphoma using the LSG15 regimen [vincristine, cyclophosphamide, doxorubicin, and prednisolone (VCAP); doxorubicin, ranimustine, and prednisolone (AMP); and vindesine, etoposide, carboplatin, and prednisolone (VECP)]. Smoldering patients who received symptomatic treatment presented longer survival. Favorable chronic patients treated with antivirals presented longer survival compared to the unfavorable subtype. However, for the acute form, first-line chemotherapy was better, albeit without significance, than antivirals. Only one of the patients with lymphoma and primary cutaneous tumors responded.

Conclusions

Watchful waiting associated with phototherapy represents the best option for smoldering adult T-cell leukemia/lymphoma with survival in Bahia being superior to that described in Japan. There was a trend of better results with zidovudine/interferon-alpha in favorable chronic disease. Excellent results were achieved in the lymphoma type treated with the LSG15 protocol. Patients are diagnosed late probably due to lack of knowledge of adult T-cell leukemia/lymphoma by primary healthcare doctors and a Brazilian treatment protocol needs to be established.

[T-cell prolymphocytic leukemia: potential diagnostic pitfalls]

BACKGROUND

Sezary syndrome (SS) presents clinically as erythroderma, which may be pigmented, and pruritic, associated with peripheral lymphadenopathies. Erythroderma may also occur in a broad range of reactive and malignant conditions including T-cell prolymphocytic leukemia (T-PLL). We report a case initially diagnosed as SS but ultimately diagnosed as T-PLL based upon skin involvement.

CASE REPORT

A 70-year-old man was referred by his hematologist for management of SS. Physical examination revealed lymphadenopathies and mild diffuse erythema without infiltration. His WBC count was elevated at 8.3 G/L. A peripheral blood smear showed Sezary-like cells. Flow cytometry of peripheral blood revealed prolymphocytic T-cells staining positively for CD2, CD3, CD4 and CD7. Cytogenetic studies showed chromosomal abnormalities in terms of number and structure with missing chromosomes 6 and13, as well as deletion of chromosome 17. Finally, a diagnosis of T-PLL was made. Pentostatin was initiated pending treatment with alemtuzumab, but the patient's overall condition deteriorated rapidly and he died 10 days later.

DISCUSSION

Diagnosis of LPLT is based upon a number of factors. In the case presented herein, the clinically atypical nature of the skin lesions prompted the dermatologist to review the diagnosis. The morphology of the circulating T-lymphocytes and their immunologic and phenotypic characteristics finally ruled out the diagnosis of Sezary syndrome, while their association with compatible cytogenetic anomalies enabled a diagnosis of prolymphocytic leukemia to be made instead.

CONCLUSION

Prolymphocytic leukemia involves complex differential diagnosis with regard to Sezary syndrome, posing potential pitfalls for hematologists and dermatologists.

The Diagnostic Value of Flow Cytometry Imunophenotyping in an Albanian Patient Population with a Preliminary Clinical Diagnosis of Chronic Lymphocytic Leukemia

Objective: Based on the flow cytometry multiparametric immunophenotyping methodology we studied some useful cell marker criteria needed for the practical differentiation of the chronic lymphocytic leukemia from other chronic limphoproliferative diseases with a leukemic component.Materials and Methods: The applied methodology is a four color flow cytometry multiparametric immunophenotyping technique using EDTA blood samples taken from 84 consecutive patients diagnosed with CLL through a preliminary clinical and white blood cell examination. The following fluorescent stained monoclonal antibodies were used: CD3, CD4, CD5, CD8, CD11c, CD19, CD20, CD23, CD25, FMC7 and kappa/lambda light chains.Results: From the 84 individuals tested, 2 out of them (2.4%) resulted with a abnormal T-cell population while 82 (97.6%) showed a pathological B cell line. 58 (69.1%) patients resulted with typical CLL markers (CD19+CD5+CD23+) while 5 (5.9%) of them presented a non typical chronic lymphocytic leukemia profile (CD19+CD5+CD23-). 19 (22.6%) out of patients displayed an abnormal CD19+CD5- B cell population. A statistically significant correlation was found between the clinical stage of CLL and the positivity for the CD38 marker (p=0.04).Conclusion: Flow cytometry immunophenotyping is a fundamental examination for the final diagnosis of chronic lymphocytic leukemia. The expression of CD38+ in CLL patients stands for a more advanced clinical stage.

Immunophenotypic characterization of T-cell prolymphocytic leukemia

OBJECTIVES

To review clinical data, cytogenetic findings, and flow cytometric analysis in 20 patients with T-cell prolymphocytic leukemia (T-PLL), a rare, aggressive, mature T-cell leukemia with poor prognosis and short survival.

METHODS

Using multiparameter flow cytometry with a large combination of antibodies, we summarize the immunophenotypic features of T-PLL, including unusual immunophenotypic variants, and illustrate immunophenotypic clues that may help distinguish this entity from other T-cell malignancies.

RESULTS

By flow cytometry, T-PLL is characterized by a postthymic mature T-cell immunophenotype with a variety of abnormalities that usually allow distinction from other mature T-cell leukemias.

CONCLUSIONS

Although definitive diagnosis of T-PLL requires a systemic approach with integration of clinical data, morphology, immunophenotype, cytogenetics/fluorescence in situ hybridization, and molecular features, our results indicate immunophenotyping by multiparameter flow cytometry greatly facilitates diagnosis and assists with subclassification of this mature T-cell leukemia.

Adult T-cell leukemia/lymphoma with an unusual CD1a positive phenotype

BACKGROUND

Acute T-cell leukemia lymphoma (ATLL) tumor cells generally express CD2/CD3/CD5, but lack CD7. These T cells are usually CD4+CD8- and strongly express CD25, although some variability in this basic pattern may be found. Here we report a case with a very unusual CD1a positive phenotype.

METHODS

Samples from peripheral blood, bone marrow aspirate, lymph node, and cerebrospinal fluid obtained from a 45-year-old male patient with a T-cell lymphoproliferative disorder were immunophenotyped by multiparametric flow cytometry. Analysis of HTLV-I genome integration in tumoral cells was performed by PCR.

RESULTS

Neoplastic T cells were cCD3, CD2/CD5/CD30/CD25, and CD1a positive, but CD3/CD7/CD4/CD8/CD34/CD10/TdT negative. Serology and integration of HTLV-I were positive.

CONCLUSION

To the best of our knowledge, CD1a expression has not been previously described in this entity. Its detection raised the differential diagnosis with acute T lymphoblastic leukemia. The rest of the phenotypic markers, the morphology of the neoplastic cells, and the demonstration of HTLV-I genome integration provided the final diagnosis.

EuroFlow antibody panels for standardized n-dimensional flow cytometric immunophenotyping of normal, reactive and malignant leukocytes

Most consensus leukemia & lymphoma antibody panels consist of lists of markers based on expert opinions, but they have not been validated. Here we present the validated EuroFlow 8-color antibody panels for immunophenotyping of hematological malignancies. The single-tube screening panels and multi-tube classification panels fit into the EuroFlow diagnostic algorithm with entries defined by clinical and laboratory parameters. The panels were constructed in 2-7 sequential design-evaluation-redesign rounds, using novel Infinicyt software tools for multivariate data analysis. Two groups of markers are combined in each 8-color tube: (i) backbone markers to identify distinct cell populations in a sample, and (ii) markers for characterization of specific cell populations. In multi-tube panels, the backbone markers were optimally placed at the same fluorochrome position in every tube, to provide identical multidimensional localization of the target cell population(s). The characterization markers were positioned according to the diagnostic utility of the combined markers. Each proposed antibody combination was tested against reference databases of normal and malignant cells from healthy subjects and WHO-based disease entities, respectively. The EuroFlow studies resulted in validated and flexible 8-color antibody panels for multidimensional identification and characterization of normal and aberrant cells, optimally suited for immunophenotypic screening and classification of hematological malignancies.

An algorithmic approach to the diagnosis of NK and T cell lymphomas

Lymphomas of natural killer (NK) and T cell lineages are uncommon disorders, although as a group they are more usually encountered in Asia compared to Western populations. In part due to their rarity, diagnosis and classification of T cell lymphomas often pose a challenge to clinicians and pathologists. Although there are morphological features that are characteristic of certain subtypes, correct classification of NK and T cell neoplasms relies heavily on the immunophenotype. With few exceptions, non-random genetic alterations such as translocations are less often seen in T cell neoplasms, adding to the diagnostic difficulty. Given these limitations, pathological diagnosis and classification of NK and T cell lymphomas are anything but straightforward. This paper attempts to present a practical algorithmic approach for the general pathologist who is confronted with these neoplasms.

A practical approach to the flow cytometric detection and diagnosis of T-cell lymphoproliferative disorders

The flow cytometric analysis of T-cell malignancies is difficult due to the heterogeneity of T-cells and the lack of convenient methods to detect T-cell clonality. Neoplastic T-cells are most often detected by their altered level of surface antigen expression, and detection requires an extensive knowledge of the phenotype of normal T-lymphocytes. This review focuses on the methods to distinguish malignant T-cells from their normal counterparts and the phenotypic features of the T-cell lymphoproliferative disorders.

Pathobiology of mature T-cell lymphomas

Mature T- and natural killer (NK)-cell neoplasms are relatively rare forms of leukemia/lymphoma. The diagnosis of these entities is often difficult, necessitating extensive immunophenotypic, molecular, and genetic testing. Despite the accumulating information on the pathobiology of these neoplasms, in many cases the prognosis remains poor. This article presents an updated view of the morphologic, immunophenotypic, genetic, and molecular characteristics of the mature T- and NK-cell neoplasms. For a better understanding of this complex topic, the development of normal T and NK cells is briefly discussed. The presentation of the characteristic features of the neoplasms in the 2008 World Health Organization classification of hematopoietic neoplasms includes advances in the understanding of the pathobiology of each diagnostic category.

Emerging drugs for rarer chronic lymphoid leukemias

BACKGROUND

Rarer indolent lymphoid leukemias include well defined mature B-cell and T-cell neoplasm with widely varying natural history and specific morphological, immunophenotypic and molecular characteristics. Among these are prolymphocytic leukemia (PLL), hairy cell leukemia (HCL) and its variants, large granular lymphocyte leukemia (LGLL) and adult T-cell leukemia/lymphoma (ATLL).

OBJECTIVE

To present current therapies and emerging drugs potentially useful in the treatment of rarer chronic lymphoid leukemias.

METHODS

After searching MEDLINE, PubMed and the Current Contents database, and conference proceedings from the previous 3 years of the American Society of Hematology (ASH), the European Society of Hematology (EHA) and the American Society of Clinical Oncology (ASCO) were searched manually; articles written in English and additional relevant publications were then selected.

RESULTS/CONCLUSION

New drugs including monoclonal antibodies (mAbs), new purine analogs, small molecules targeting specific molecular targets and other agents are included. Future research should focus on the novel therapeutic strategies based on the molecular pathogenic mechanisms and the development of new targeted therapies for each distinct chronic lymphoid leukemia.

CNS T-cell lymphoma: an under-recognized entity?

The incidence of CNS lymphoma has increased significantly in the past 30 years, primarily in the elderly and immunocompromised. While T-cell lymphomas comprise 15-20% of systemic lymphomas, they comprise less than 4% of primary CNS lymphomas, suggesting that they may be under-recognized compared to their systemic counterparts. To investigate this, we studied brain biopsies from three patients who were diagnosed with T-cell lymphoma confined to the brain. They had enhancing lesions by MRI, arising in the cerebellum and brainstem in one and temporal lobe in two. We compared these to biopsies from three patients who had reactive lymphoid infiltrates and who had clinical signs/symptoms and radiographic findings that were indistinguishable from the lymphoma group. Biopsies from both the lymphoma group and reactive group showed considerable cytomorphologic heterogeneity. Although one lymphoma case contained large atypical cells, the other two contained small, mature lymphocytes within a heterogeneous infiltrate of neoplastic and reactive inflammatory cells. Surface marker aberrancies were present in two lymphoma cases, but this alone could not reliably diagnose T-cell lymphoma. The proliferation index was not useful for differentiating lymphoma from reactive infiltrates. In five of the six cases the diagnosis was most influenced by clonality studies for T-cell receptor-gamma gene rearrangements. We conclude that because of the high degree of overlap in cytomorphologic and immunophenotypic features between T-cell lymphoma and reactive infiltrates, T-cell lymphoma may not be recognized unless studies for T-cell receptor gene rearrangements are performed for CNS lesions composed of a polymorphous but predominantly T-cell infiltrate.

Flow cytometric immunophenotyping for hematologic neoplasms

Flow cytometric immunophenotyping remains an indispensable tool for the diagnosis, classification, staging, and monitoring of hematologic neoplasms. The last 10 years have seen advances in flow cytometry instrumentation and availability of an expanded range of antibodies and fluorochromes that have improved our ability to identify different normal cell populations and recognize phenotypic aberrancies, even when present in a small proportion of the cells analyzed. Phenotypically abnormal populations have been documented in many hematologic neoplasms, including lymphoma, chronic lymphoid leukemias, plasma cell neoplasms, acute leukemia, paroxysmal nocturnal hemoglobinuria, mast cell disease, myelodysplastic syndromes, and myeloproliferative disorders. The past decade has also seen refinement of the criteria used to identify distinct disease entities with widespread adoption of the 2001 World Health Organization (WHO) classification. This classification endorses a multiparametric approach to diagnosis and outlines the morphologic, immunophenotypic, and genotypic features characteristic of each disease entity. When should flow cytometric immunophenotyping be applied? The recent Bethesda International Consensus Conference on flow cytometric immunophenotypic analysis of hematolymphoid neoplasms made recommendations on the medical indications for flow cytometric testing. This review discusses how flow cytometric testing is currently applied in these clinical situations and how the information obtained can be used to direct other testing.