Medical School · Year 2 · Hematology Oncology · includes a quiz and discussion video

Lecture 9: Chronic Leukemias

Unit 2.9: Hematology/Oncology

Learning Objectives

By the end of this lecture, students will be able to:

1. Describe the pathophysiology and diagnosis of CML
2. Explain the treatment and monitoring of CML
3. Describe the clinical features and staging of CLL
4. Explain the treatment approach for CLL
5. Describe hairy cell leukemia and other rare leukemias
6. Explain the differences between chronic and acute leukemias

Lecture Outline

I. Chronic vs Acute Leukemia

Chronic leukemias differ fundamentally from acute leukemias in that the malignant cells retain the capacity for maturation, resulting in accumulation of mature-appearing cells rather than the immature blasts that define acute disease. While acute leukemia is characterized by a maturation arrest with 20 percent or more blasts accumulating in the bone marrow, chronic leukemias maintain blast percentages below 20 percent and demonstrate differentiated cells that retain some functional capacity. The onset of chronic leukemia is typically indolent, often discovered incidentally on routine blood work, in contrast to the rapid and dramatic presentation of acute leukemias that present with severe marrow failure requiring urgent treatment. Without treatment, acute leukemia follows a fatal course within weeks to months, whereas chronic leukemias evolve over months to years, allowing for a more measured diagnostic and therapeutic approach.

The two principal types of chronic leukemia arise from distinct hematopoietic lineages and demonstrate markedly different clinical behaviors, diagnostic features, and treatment strategies. Chronic myeloid leukemia originates from a myeloid progenitor cell and is characterized by the Philadelphia chromosome, while chronic lymphocytic leukemia arises from a mature B lymphocyte and represents the most common adult leukemia in Western countries. Beyond these two major entities, several rarer chronic leukemias exist including hairy cell leukemia, T-cell prolymphocytic leukemia, and large granular lymphocyte leukemia, each with distinctive features that guide diagnosis and management. Understanding the distinctions between chronic and acute leukemias is essential because the treatment philosophies differ dramatically, with chronic leukemias often managed with oral targeted agents or observation rather than the intensive inpatient chemotherapy required for acute disease.

The clinical presentations of chronic myeloid leukemia and chronic lymphocytic leukemia reflect their different cellular origins and patterns of disease involvement. CML typically presents with very high white blood cell counts, often exceeding 100,000 per microliter, and prominent splenomegaly but uncommonly causes lymphadenopathy. CLL, by contrast, presents with elevated lymphocyte counts, common generalized lymphadenopathy, and variable splenomegaly. The median age at diagnosis differs substantially, with CML occurring at a median of 55 to 65 years while CLL predominantly affects the elderly with a median age of 72 years. These demographic and clinical differences provide initial clues that guide the diagnostic workup and distinguish between these entities at presentation.

Laboratory features further distinguish the two major chronic leukemias through characteristic findings on complete blood count and peripheral blood smear examination. CML demonstrates a markedly elevated white blood cell count with the entire spectrum of myeloid maturation represented in the peripheral blood differential, including myelocytes, metamyelocytes, bands, and segmented neutrophils, along with characteristic basophilia that is a hallmark of the disease. CLL shows elevated lymphocyte counts composed of mature-appearing small lymphocytes, and the peripheral smear reveals smudge cells, which are fragile CLL lymphocytes that rupture during the smear preparation process. These morphologic and laboratory distinctions allow experienced clinicians to suspect the correct diagnosis from initial blood work alone, though confirmatory testing with cytogenetics, molecular studies, and flow cytometry is required to establish the definitive diagnosis.

<image>Panel A: Side-by-side comparison of acute versus chronic leukemia showing blast predominance in acute and mature cells in chronic disease. Panel B: Peripheral blood smear showing CML with full myeloid spectrum and basophilia versus CLL with mature lymphocytes and smudge cells. Panel C: Clinical presentation differences with splenomegaly prominent in CML and lymphadenopathy in CLL. Panel D: Laboratory features table comparing WBC differential, characteristic cells, and diagnostic markers.</image>

II. Chronic Myeloid Leukemia - Pathophysiology

Chronic myeloid leukemia is defined by the Philadelphia chromosome, a reciprocal translocation between chromosomes 9 and 22 designated t(9;22)(q34;q11), which creates the BCR-ABL1 fusion gene encoding a constitutively active tyrosine kinase that drives uncontrolled myeloid proliferation. This translocation is present in approximately 95 percent of CML cases and serves as both the diagnostic hallmark and the therapeutic target of the disease. The BCR-ABL1 fusion protein functions as a permanently activated tyrosine kinase that phosphorylates multiple downstream signaling molecules, bypassing the normal regulatory mechanisms that control cell growth and survival. The discovery of this molecular driver and the subsequent development of targeted tyrosine kinase inhibitors represents one of the most significant advances in cancer therapy, transforming CML from a fatal disease with 3 to 5 year survival to one with near-normal life expectancy.

The BCR-ABL1 fusion protein activates multiple downstream signaling pathways that collectively promote proliferation and inhibit apoptosis of the malignant clone. The RAS/MAPK pathway activation drives cellular proliferation, while PI3K/AKT pathway stimulation promotes cell survival and resistance to programmed cell death. STAT5 pathway activation enhances transcription of genes that further support proliferative and anti-apoptotic programs. The combined effect of these signaling cascades is enhanced myeloid proliferation with decreased apoptosis, leading to progressive accumulation of myeloid cells at all stages of maturation in the bone marrow and peripheral blood, a process that can be effectively reversed by blocking the kinase activity of the BCR-ABL1 protein.

The natural history of CML follows a triphasic course progressing from chronic phase through accelerated phase to blast crisis, each defined by increasing blast percentages and decreasing responsiveness to therapy. Chronic phase is characterized by fewer than 10 percent blasts in the blood or bone marrow and represents the well-controlled stage of disease where most patients are diagnosed and respond favorably to tyrosine kinase inhibitor therapy. Accelerated phase is defined by 10 to 19 percent blasts and indicates progressive resistance to therapy with accumulation of additional genetic abnormalities. Blast crisis represents transformation to a disease resembling acute leukemia with 20 percent or more blasts, occurring as myeloid blast crisis in approximately 70 percent of cases and lymphoid blast crisis in 30 percent, and carries a dismal prognosis that is far more difficult to treat than chronic phase disease.

The epidemiology of CML reveals an incidence of 1 to 2 per 100,000 population with a median age at diagnosis of 55 to 65 years, though the disease can occur at any age. The only well-established risk factor for CML is radiation exposure, as demonstrated by increased incidence among atomic bomb survivors and recipients of radiation therapy. Before the introduction of tyrosine kinase inhibitor therapy, the median survival with CML was only 3 to 5 years, with inevitable progression through accelerated phase to blast crisis. The development of imatinib and subsequent generations of TKIs has revolutionized the prognosis of CML, with patients now achieving near-normal life expectancy when diagnosed in chronic phase and treated with appropriate targeted therapy, making CML one of the greatest success stories in the history of cancer treatment.

<image>Panel A: Diagram of t(9;22) translocation showing BCR and ABL gene fusion creating the Philadelphia chromosome. Panel B: BCR-ABL1 constitutive tyrosine kinase signaling through RAS/MAPK, PI3K/AKT, and STAT5 pathways promoting proliferation and survival. Panel C: Disease phase progression from chronic phase through accelerated phase to blast crisis with corresponding blast percentages. Panel D: Epidemiology graph showing survival improvement from 3-5 years pre-TKI to near-normal life expectancy with TKI therapy.</image>

III. CML - Clinical Presentation and Diagnosis

The clinical presentation of chronic myeloid leukemia is remarkably variable, with 40 to 50 percent of patients discovered incidentally through routine blood work showing an unexpectedly elevated white blood cell count. Among symptomatic patients, constitutional symptoms including fatigue, weight loss, and night sweats are common and reflect the hypermetabolic state driven by the expanded myeloid compartment. Splenomegaly is a frequent finding that may cause early satiety and left upper quadrant discomfort as the spleen enlarges from extramedullary hematopoiesis and infiltration by leukemic cells. Leukostasis, though rare in CML compared to acute leukemia, can occur when the white blood cell count reaches very high levels and requires urgent management to prevent pulmonary and neurologic complications.

Physical examination in CML characteristically reveals splenomegaly in 50 to 90 percent of patients, ranging from mild enlargement detected only on imaging to massive splenomegaly extending into the pelvis. Hepatomegaly is found in 10 to 20 percent of patients and typically accompanies more advanced or higher-burden disease. Unlike chronic lymphocytic leukemia, lymphadenopathy is uncommon in CML during chronic phase, and its presence should raise concern for disease progression. Bone tenderness, particularly over the sternum, may be elicited on examination and reflects the marked marrow expansion from the proliferating myeloid clone, though this finding is nonspecific and occurs in other conditions causing marrow hyperplasia.

Laboratory findings in CML are distinctive and often suggest the diagnosis before confirmatory molecular testing. The white blood cell count is typically markedly elevated, ranging from 25,000 to greater than 500,000 per microliter, with the peripheral blood differential demonstrating the full spectrum of myeloid maturation including a characteristic "myelocyte bulge" where myelocytes are disproportionately represented. Basophilia is a hallmark finding that helps distinguish CML from a leukemoid reaction or other causes of leukocytosis, and eosinophilia is also commonly observed. Platelet counts are often elevated, hemoglobin is normal or mildly decreased, and the leukocyte alkaline phosphatase (LAP) score is characteristically low in CML, in contrast to the elevated LAP seen in leukemoid reactions from infection or other causes.

The definitive diagnosis of CML requires demonstration of the Philadelphia chromosome or BCR-ABL1 fusion gene through cytogenetic or molecular testing. A complete blood count with peripheral smear provides the initial characteristic findings that suggest the diagnosis, while bone marrow aspiration and biopsy reveal a hypercellular marrow with expansion of the myeloid lineage and provide material for blast percentage enumeration essential for phase determination. Conventional cytogenetics on bone marrow demonstrates the t(9;22) Philadelphia chromosome, while fluorescence in situ hybridization (FISH) for BCR-ABL1 provides faster results and can be performed on peripheral blood specimens. Quantitative polymerase chain reaction for BCR-ABL1 transcript levels is essential not only for initial diagnosis but serves as the primary tool for monitoring treatment response and detecting molecular relapse throughout the course of therapy.

<image>Panel A: Peripheral blood smear demonstrating the myelocyte bulge with full spectrum of myeloid maturation from blasts to segmented neutrophils. Panel B: Characteristic basophilia and eosinophilia seen in CML differential count. Panel C: Karyotype showing the Philadelphia chromosome t(9;22) with FISH confirmation of BCR-ABL1 fusion. Panel D: Comparison of low LAP score in CML versus elevated LAP in leukemoid reaction.</image>

IV. CML - Treatment

The treatment of chronic myeloid leukemia has been revolutionized by tyrosine kinase inhibitors that directly target the BCR-ABL1 oncoprotein, and three generations of these agents are now available with increasingly potent and selective activity. First-generation imatinib was the breakthrough agent that transformed CML therapy and remains widely used due to its well-established efficacy, favorable long-term safety profile, and availability in generic formulation. Second-generation TKIs including dasatinib, nilotinib, and bosutinib are more potent inhibitors of BCR-ABL1 and achieve faster and deeper responses than imatinib, though each carries distinct side effect profiles with dasatinib causing pleural effusions, nilotinib associated with QT prolongation and cardiovascular risk, and bosutinib producing gastrointestinal side effects. Third-generation ponatinib is uniquely effective against the T315I gatekeeper mutation that confers resistance to all other TKIs, but carries a significant risk of arterial thrombosis that requires careful patient selection and monitoring.

Treatment response in CML is monitored through a hierarchy of response milestones that reflect progressively deeper suppression of the malignant clone. Complete hematologic response, defined as normalization of the complete blood count, is the first and most basic response level expected within the first few months of therapy. Complete cytogenetic response indicates 0 percent Philadelphia chromosome-positive metaphases on bone marrow examination, while major molecular response represents BCR-ABL1 transcript levels of 0.1 percent or less on the International Scale. Deep molecular response levels of MR4 and MR4.5 correspond to BCR-ABL1 levels of 0.01 percent or less and represent the degree of disease suppression that may eventually allow consideration of treatment discontinuation.

Monitoring of treatment response follows a structured timeline with specific molecular targets that must be achieved to confirm adequate disease control and identify treatment failure requiring intervention. At 3 months, the BCR-ABL1 transcript level should be 10 percent or less on the International Scale, representing an early response milestone that predicts long-term outcomes. By 6 months, BCR-ABL1 should decrease to 1 percent or less, and by 12 months, major molecular response with BCR-ABL1 of 0.1 percent or less should be achieved. Thereafter, monitoring continues every 3 to 6 months to confirm sustained response, and failure to meet these milestones or evidence of rising transcript levels should prompt ABL kinase domain mutation analysis to identify specific mutations that may guide selection of alternative TKIs.

Treatment-free remission represents a paradigm-shifting concept in CML management, offering selected patients the possibility of discontinuing TKI therapy while maintaining disease control through immune surveillance. Candidates for treatment-free remission must have been on TKI therapy for at least 3 years and must have sustained a deep molecular response of MR4 or better for at least 2 years, ensuring that the leukemic burden has been suppressed to a level where the immune system can maintain control. Approximately 40 to 60 percent of carefully selected patients who attempt treatment discontinuation successfully maintain remission off TKI therapy, representing a functional cure for a substantial proportion of patients. Those who attempt treatment-free remission require frequent PCR monitoring after discontinuation to detect molecular recurrence early, as most patients who relapse do so within the first 6 to 12 months and regain molecular response upon resumption of TKI therapy.

<image>Panel A: Mechanism of TKI action showing imatinib blocking ATP binding site of BCR-ABL1 kinase and preventing downstream signaling. Panel B: Response milestone timeline showing BCR-ABL1 targets at 3 months (≤10%), 6 months (≤1%), and 12 months (≤0.1% MMR). Panel C: Treatment-free remission criteria flowchart with TKI duration, sustained DMR requirements, and monitoring after discontinuation. Panel D: Comparison of first, second, and third generation TKIs with their specific indications and side effect profiles.</image>

V. Chronic Lymphocytic Leukemia - Overview

Chronic lymphocytic leukemia is defined as a clonal expansion of mature B lymphocytes and represents the most common adult leukemia in Western countries, with a median age at diagnosis of 72 years reflecting its predilection for the elderly population. The malignant cells demonstrate a characteristic immunophenotype with co-expression of the B-cell markers CD19 and CD23 along with the T-cell-associated marker CD5, and dim expression of CD20, a pattern that is virtually diagnostic of the disease. The pathogenesis of CLL involves a mature B lymphocyte as the cell of origin, with constitutively active B-cell receptor signaling driving survival and proliferation of the malignant clone. Importantly, the accumulation of CLL cells results more from defective apoptosis than from excessive proliferation, distinguishing CLL from many other malignancies and explaining the indolent clinical course characterized by gradual lymphocyte accumulation over years.

The tumor microenvironment plays a critical role in supporting CLL cell survival and proliferation through interactions between the malignant B cells and various accessory cells. T cells and nurse-like cells within lymph nodes and bone marrow provide survival signals through cell contact and cytokine secretion that protect CLL cells from spontaneous apoptosis. B-cell receptor signaling, which is constitutively active in CLL, transmits survival signals through downstream kinases including Bruton tyrosine kinase and PI3K, both of which have become important therapeutic targets. This dependence on microenvironmental signals explains why CLL cells rapidly undergo apoptosis when removed from supportive tissue environments and cultured in vitro, and why therapies targeting BCR signaling pathways are highly effective.

Genetic abnormalities detected by fluorescence in situ hybridization represent the most important prognostic factors in CLL and guide both monitoring intensity and treatment selection. Deletion of 13q as a sole abnormality is the most common genetic finding, occurring in approximately 50 percent of cases, and carries a favorable prognosis with prolonged survival. A normal karyotype or trisomy 12 each occur in approximately 15 percent of cases and confer intermediate prognosis. Deletion of 11q, found in approximately 10 percent of patients, is associated with adverse prognosis and often presents with bulky lymphadenopathy. Deletion of 17p or TP53 mutation, occurring in approximately 7 percent of cases, carries very adverse prognosis due to loss of the tumor suppressor that mediates response to DNA-damaging chemotherapy, making these patients resistant to standard chemoimmunotherapy approaches.

Beyond cytogenetic abnormalities, several additional molecular and laboratory features provide important prognostic information in CLL and increasingly guide treatment decisions. The immunoglobulin heavy chain variable region (IGHV) mutation status divides CLL into two biologically distinct subgroups, with mutated IGHV indicating a more indolent course and favorable prognosis and unmutated IGHV associated with a more aggressive disease trajectory. TP53 status, assessed by both FISH for deletion and sequencing for mutation, is critical because TP53-aberrant disease responds poorly to chemotherapy-based regimens and requires alternative targeted approaches. Beta-2-microglobulin reflects tumor burden and serves as a prognostic marker, while ZAP-70 and CD38 expression correlate with unmutated IGHV status and serve as surrogate markers for more aggressive biology, collectively enabling risk stratification that informs monitoring frequency and treatment timing.

<image>Panel A: Flow cytometry dot plots showing characteristic CLL immunophenotype with CD5+, CD19+, CD20 dim, and CD23+ expression pattern. Panel B: FISH images demonstrating common genetic abnormalities including del(13q), trisomy 12, del(11q), and del(17p) with prognostic categories. Panel C: Comparison of favorable versus adverse prognostic factors including IGHV mutation status, TP53 status, and beta-2-microglobulin levels. Panel D: Survival curves stratified by genetic risk groups.</image>

VI. CLL - Clinical Features and Staging

The clinical presentation of chronic lymphocytic leukemia is most commonly asymptomatic at the time of diagnosis, with the disease discovered incidentally through routine blood work revealing an unexpectedly elevated lymphocyte count. When symptoms are present, they may include B symptoms such as fever, night sweats, and unintentional weight loss exceeding 10 percent of body weight, which indicate more active disease. Generalized lymphadenopathy is a hallmark finding of CLL and may be the symptom that prompts initial evaluation, though the adenopathy is typically painless and slowly progressive. Recurrent infections resulting from hypogammaglobulinemia, a consequence of the dysfunctional immune system in CLL, are a significant source of morbidity, and autoimmune complications including autoimmune hemolytic anemia and immune thrombocytopenia may occur at any stage of the disease.

Physical examination in CLL typically reveals generalized, non-tender lymphadenopathy affecting cervical, axillary, and inguinal regions, with the lymph nodes being firm and mobile. Splenomegaly is variably present and when significant may cause early satiety or left upper quadrant discomfort. Hepatomegaly is less common than splenomegaly and typically indicates more advanced disease burden. Skin infiltration by CLL cells is rare but recognized, and the overall physical examination findings help guide clinical staging that determines prognosis and treatment approach.

The Rai staging system, predominantly used in the United States, classifies CLL into five stages based on the progressive accumulation of disease manifestations with corresponding median survival estimates. Stage 0 is defined by lymphocytosis only and carries the most favorable prognosis with median survival exceeding 10 years. Stage I adds lymphadenopathy to lymphocytosis with a median survival of 8 years, while stage II includes hepatomegaly or splenomegaly with a median survival of 6 years. Stages III and IV represent advanced disease defined by anemia with hemoglobin below 11 g/dL and thrombocytopenia with platelet count below 100,000 per microliter respectively, both carrying a median survival of approximately 2 years, though these historical survival figures have improved substantially with modern targeted therapies.

The Binet staging system, commonly used in Europe, provides a complementary classification based on the number of involved lymph node areas and the presence of cytopenias. Stage A is defined by fewer than 3 involved lymph node areas with normal hemoglobin and platelet counts, representing the most indolent presentation. Stage B includes 3 or more involved lymph node areas without significant cytopenias, indicating more widespread disease. Stage C is defined by the presence of anemia or thrombocytopenia regardless of the extent of nodal involvement, corresponding to the most advanced stage requiring treatment. Both staging systems serve the practical purpose of identifying patients who may be observed safely versus those requiring treatment initiation, though modern genetic and molecular markers have largely supplemented clinical staging in guiding therapeutic decisions.

<image>Panel A: Rai staging system diagram showing progression from stage 0 lymphocytosis through stages I-IV with corresponding median survival. Panel B: Binet staging comparison showing stages A, B, and C with lymph node area involvement and cytopenias. Panel C: Clinical photograph demonstrating generalized non-tender lymphadenopathy and splenomegaly. Panel D: Prognostic correlation graph linking stage to survival outcomes.</image>

VII. CLL - Diagnosis and Complications

The diagnosis of chronic lymphocytic leukemia requires demonstration of a sustained monoclonal B-cell lymphocytosis of 5,000 per microliter or more in the peripheral blood, with clonality confirmed by light chain restriction on flow cytometry. The characteristic immunophenotype showing CD5, CD19, and CD23 co-expression with dim CD20 essentially clinches the diagnosis in the appropriate clinical context and distinguishes CLL from other B-cell lymphoproliferative disorders. The peripheral blood smear demonstrates a hallmark finding of smudge cells, also called basket cells, which represent fragile CLL lymphocytes that rupture during the mechanical process of preparing the blood smear. Unlike many other hematologic malignancies, bone marrow biopsy is not routinely required for the diagnosis of CLL when the peripheral blood flow cytometry demonstrates the characteristic immunophenotype and adequate lymphocyte count, though it may be performed when clinical questions arise about cytopenias or disease transformation.

Smudge cells deserve particular emphasis as a morphologic feature closely associated with CLL and often the first clue to the diagnosis on routine blood smear review. These broken lymphocytes appear as amorphous nuclear material without intact cellular structure, resulting from the inherent fragility of CLL cells that lack the cytoskeletal proteins needed to withstand the shear forces of smear preparation. While smudge cells are highly characteristic of CLL, they are not entirely specific and can occasionally be seen in other conditions, though their abundance in the setting of lymphocytosis strongly suggests the diagnosis. The proportion of smudge cells has even been correlated with prognosis in some studies, with higher smudge cell percentages associated with more indolent disease.

Autoimmune complications represent a distinctive feature of CLL that may precede diagnosis, occur during the disease course, or develop in response to treatment. Autoimmune hemolytic anemia is the most common autoimmune complication, occurring in 10 to 25 percent of patients and presenting with fatigue, jaundice, and laboratory evidence of hemolysis including elevated lactate dehydrogenase, indirect bilirubin, reticulocytosis, and a positive direct antiglobulin test. Immune thrombocytopenic purpura occurs in 2 to 5 percent of patients with CLL and must be distinguished from thrombocytopenia due to marrow infiltration, as the treatment approach differs substantially. Pure red cell aplasia is a rare autoimmune complication causing severe anemia from selective destruction of erythroid precursors. Treatment of autoimmune cytopenias typically begins with corticosteroids, and CLL-directed therapy is initiated if the autoimmune complication is refractory to steroids or if the underlying CLL requires treatment for other indications.

Richter transformation represents one of the most feared complications of CLL, defined as transformation to an aggressive lymphoma that dramatically alters the clinical course and prognosis. The most common form of Richter transformation is conversion to diffuse large B-cell lymphoma (DLBCL), occurring in 2 to 10 percent of CLL patients over the course of their disease. Clinical suspicion for Richter transformation should arise when a patient experiences rapid lymph node growth, new or worsening B symptoms, or a rising lactate dehydrogenase level, all suggesting a change in disease biology. The prognosis following Richter transformation is poor, with median survival measured in months despite aggressive treatment with chemoimmunotherapy and consideration of allogeneic stem cell transplantation in eligible patients, underscoring the importance of recognizing this complication promptly.

<image>Panel A: Peripheral blood smear showing mature lymphocytes with characteristic smudge cells (basket cells) from fragile CLL cells rupturing during preparation. Panel B: Flow cytometry analysis demonstrating clonal B-cell population with light chain restriction confirming monoclonality. Panel C: Comparison of CLL versus Richter transformation showing rapid lymph node enlargement, rising LDH, and aggressive DLBCL histology. Panel D: Diagnostic algorithm for suspected CLL from lymphocytosis through flow cytometry to confirmed diagnosis.</image>

VIII. CLL - Treatment

The treatment approach for chronic lymphocytic leukemia is guided by the fundamental principle that early-stage, asymptomatic disease should be observed with a "watch and wait" strategy rather than treated immediately, as multiple studies have demonstrated no survival benefit from early intervention. Specific indications for initiating treatment include progressive symptoms such as fatigue severe enough to affect daily function, progressive disease with lymphocyte doubling time less than 6 months, cytopenias from marrow failure or autoimmune destruction, massive symptomatic organomegaly, and the presence of constitutional B symptoms. This selective treatment approach recognizes that many CLL patients will live for years or even decades with indolent disease that does not require therapy, and that unnecessary treatment exposes patients to toxicity without clinical benefit. The decision to observe rather than treat early-stage disease represents one of the most important management principles in CLL and requires clear communication with patients who may be anxious about their diagnosis.

First-line therapy selection in CLL has evolved dramatically with the introduction of targeted agents and is now guided by TP53 status, IGHV mutation status, and patient fitness. Patients with del(17p) or TP53 mutation are directed to BTK inhibitor therapy with ibrutinib or acalabrutinib, or venetoclax combined with obinutuzumab, because these patients respond poorly to chemotherapy-based approaches. Fit patients with mutated IGHV may receive FCR (fludarabine, cyclophosphamide, and rituximab), which can achieve prolonged treatment-free remissions in this favorable subgroup, or alternatively a BTK inhibitor. Fit patients with unmutated IGHV are best treated with a BTK inhibitor or venetoclax-based regimen due to the shorter duration of response expected with chemoimmunotherapy in this subgroup. Unfit patients who cannot tolerate intensive therapy have multiple effective options including BTK inhibitors, venetoclax with obinutuzumab, or the less intensive combination of chlorambucil with obinutuzumab.

Novel targeted agents have transformed CLL treatment by providing highly effective oral options that target critical survival pathways in the malignant B cells. Ibrutinib, the first BTK inhibitor approved for CLL, blocks Bruton tyrosine kinase in the B-cell receptor signaling pathway, and acalabrutinib and zanubrutinib represent more selective second-generation BTK inhibitors with potentially fewer off-target effects. Venetoclax is a BCL2 inhibitor that directly triggers apoptosis in CLL cells by displacing pro-apoptotic proteins from the BCL2 anti-apoptotic protein, and is notable for its fixed-duration treatment approach when combined with obinutuzumab. Obinutuzumab is a type II anti-CD20 monoclonal antibody that achieves enhanced direct cell death compared to rituximab, while idelalisib is a PI3K inhibitor that targets another node in the B-cell receptor signaling pathway, typically reserved for relapsed settings due to its toxicity profile.

BTK inhibitors deserve particular attention as the most widely prescribed class of agents in CLL, with unique clinical effects that reflect their mechanism of action on B-cell receptor signaling. The mechanism of action involves blocking B-cell receptor signaling at Bruton tyrosine kinase, which disrupts the survival signals that CLL cells receive in the lymph node microenvironment, causing cells to exit protective tissue compartments. This explains the characteristic clinical pattern of lymph node shrinkage accompanied by a transient lymphocytosis as CLL cells are mobilized from lymph nodes into the peripheral blood, a phenomenon that should not be misinterpreted as disease progression. BTK inhibitors are administered continuously until disease progression or intolerance, and important side effects include bleeding risk from platelet dysfunction, atrial fibrillation from off-target kinase inhibition, and increased infection susceptibility, all requiring ongoing monitoring throughout treatment.

<image>Panel A: Treatment algorithm stratified by del(17p)/TP53 status, IGHV mutation, and patient fitness directing therapy selection. Panel B: BTK inhibitor mechanism showing ibrutinib blocking B-cell receptor signaling at Bruton tyrosine kinase. Panel C: Venetoclax mechanism targeting BCL2 anti-apoptotic protein with characteristic tumor lysis risk requiring ramp-up dosing. Panel D: Clinical response pattern to BTK inhibitors showing lymph node shrinkage with transient lymphocytosis as cells mobilize from tissues.</image>

IX. Hairy Cell Leukemia

Hairy cell leukemia is a rare indolent B-cell neoplasm characterized by distinctive morphologic features and an excellent response to purine analog therapy, arising from mature B lymphocytes with a marked male predominance and a median age at diagnosis of 55 years. Unlike most other leukemias, hairy cell leukemia typically presents with cytopenias rather than leukocytosis, as the malignant cells infiltrate the bone marrow and spleen causing peripheral blood count suppression. Splenomegaly is a prominent feature and may be massive in some patients, while lymphadenopathy is characteristically uncommon, helping to distinguish hairy cell leukemia from CLL and lymphoma. The clinical presentation is often driven by complications of cytopenias, particularly infections related to neutropenia, with patients susceptible to unusual organisms including atypical mycobacteria and Legionella species.

The diagnosis of hairy cell leukemia relies on recognition of the distinctive morphologic, immunophenotypic, and molecular features that set this disease apart from other B-cell malignancies. On peripheral blood smear, the malignant cells demonstrate characteristic fine, hair-like cytoplasmic projections extending from the cell surface that give the disease its name. The immunophenotype shows positivity for B-cell markers CD19 and CD20, along with CD11c, CD25, CD103, and Annexin A1, a combination that is highly specific for hairy cell leukemia. The BRAF V600E mutation is present in more than 95 percent of cases and serves as a molecular confirmation of the diagnosis, also providing a therapeutic target in refractory disease. Bone marrow biopsy reveals a characteristic "fried egg" appearance from widely spaced hairy cells with abundant clear cytoplasm, and aspiration frequently yields a dry tap due to the reticulin fibrosis that accompanies marrow infiltration, while the tartrate-resistant acid phosphatase (TRAP) stain provides a classic histochemical confirmation.

The clinical features of hairy cell leukemia are dominated by pancytopenia, which is the most common presenting laboratory finding and results from a combination of marrow infiltration, splenic sequestration, and cytokine-mediated hematopoietic suppression. Monocytopenia is a particularly characteristic finding that distinguishes hairy cell leukemia from most other causes of pancytopenia and contributes to the unusual pattern of infections seen in these patients. Splenomegaly ranges from moderate to massive and reflects extensive infiltration of the splenic red pulp by hairy cells. The susceptibility to infections with atypical mycobacteria and Legionella species reflects the combined immunodeficiency from monocytopenia and neutropenia, and these opportunistic infections may be the presenting manifestation that leads to diagnosis.

Treatment of hairy cell leukemia is remarkably effective, with purine analog therapy producing complete remission rates exceeding 90 percent and durable remissions lasting many years. First-line therapy consists of cladribine (2-CdA) administered as a single course or pentostatin given over multiple cycles, both achieving excellent response rates that make hairy cell leukemia one of the most treatment-responsive hematologic malignancies. Patients who relapse after initial purine analog therapy can be retreated with the same or alternative purine analog, or may receive rituximab to achieve additional remissions. For patients with refractory disease who have failed purine analog therapy, vemurafenib, a BRAF inhibitor that targets the nearly universal BRAF V600E mutation, and moxetumomab pasudotox, an anti-CD22 immunotoxin, provide additional therapeutic options that exploit the unique molecular and surface marker profile of hairy cell leukemia.

<image>Panel A: Peripheral blood smear showing hairy cells with characteristic fine cytoplasmic projections extending from the cell surface. Panel B: Bone marrow biopsy demonstrating fried-egg appearance with widely spaced hairy cells and characteristic dry tap. Panel C: TRAP (tartrate-resistant acid phosphatase) stain showing positive cytoplasmic staining in hairy cells. Panel D: CT scan demonstrating massive splenomegaly with absent lymphadenopathy typical of hairy cell leukemia.</image>

X. Other Chronic Leukemias

T-cell prolymphocytic leukemia is a rare but aggressive chronic leukemia arising from mature T cells that presents with markedly elevated white blood cell counts, splenomegaly, and characteristic skin involvement. Despite being classified among chronic leukemias, T-PLL behaves aggressively with rapid progression and poor prognosis, distinguishing it from the typically indolent course of most chronic leukemias. Treatment with alemtuzumab, an anti-CD52 monoclonal antibody, represents the most effective initial therapy, though responses are often transient and allogeneic stem cell transplantation should be pursued in eligible patients to achieve durable disease control. The aggressive natural history of T-PLL, combined with limited treatment options and poor response to conventional chemotherapy, makes it one of the most challenging chronic leukemias to manage.

Large granular lymphocyte leukemia represents the opposite end of the clinical spectrum, characterized by an indolent clonal expansion of either CD8-positive T cells or natural killer cells that typically manifests as chronic cytopenias rather than marked leukocytosis. The most common clinical presentation is isolated neutropenia or anemia accompanied by modest splenomegaly, with the cytopenias resulting from immune-mediated suppression of normal hematopoiesis rather than marrow infiltration. A notable clinical association exists between LGL leukemia and rheumatoid arthritis, creating a clinical picture resembling Felty syndrome, the triad of rheumatoid arthritis, splenomegaly, and neutropenia. Treatment is reserved for symptomatic patients and typically employs immunosuppressive agents including methotrexate or cyclophosphamide, reflecting the immune-mediated pathophysiology rather than the need for cytotoxic chemotherapy directed at the malignant clone.

B-cell prolymphocytic leukemia is a very rare and aggressive B-cell neoplasm defined by the presence of more than 55 percent prolymphocytes in the peripheral blood, typically presenting with marked splenomegaly and high white blood cell counts. Despite sharing some features with CLL, B-PLL behaves far more aggressively and responds poorly to treatments effective in CLL, necessitating consideration of more intensive approaches. Treatment often follows protocols similar to CLL-directed therapy, though response rates are disappointingly low and the prognosis remains poor. The rarity of B-PLL has limited the ability to conduct randomized trials, and treatment decisions are largely guided by extrapolation from CLL and aggressive lymphoma experience.

A comparison of the major chronic leukemias highlights the distinctive features that guide diagnosis and treatment selection across these entities. CML arises from myeloid cells and is identified by the BCR-ABL1 molecular marker, presents with prominent splenomegaly, and achieves excellent prognosis with tyrosine kinase inhibitor therapy. CLL originates from B lymphocytes marked by CD5 and CD23 expression, demonstrates variable splenomegaly, and is treated with BTK inhibitors or venetoclax-based regimens with variable prognosis depending on genetic features. Hairy cell leukemia, also of B-lymphoid origin, is distinguished by CD103 expression and the BRAF V600E mutation, typically presents with prominent splenomegaly, and achieves excellent prognosis with purine analog therapy. These distinctions underscore the importance of accurate diagnosis in chronic leukemias, as each entity requires a fundamentally different therapeutic approach.

<image>Panel A: Large granular lymphocyte morphology showing medium-sized lymphocytes with abundant cytoplasm containing azurophilic granules. Panel B: Comparison table of CML, CLL, and HCL showing cell type, diagnostic markers, clinical features, and treatment approaches. Panel C: T-PLL with skin involvement and high WBC count contrasted with indolent LGL leukemia course. Panel D: Association of LGL leukemia with rheumatoid arthritis in Felty-like syndrome presentation.</image>

Summary

• CML: Philadelphia chromosome t(9;22); BCR-ABL1 TKI; near-normal life expectancy
• CML phases: Chronic (controlled) → accelerated → blast crisis (like AML/ALL)
• TKI monitoring: BCR-ABL1 milestones at 3, 6, 12 months; mutation analysis if failure
• CLL: Most common adult leukemia; CD5+ B cells; smudge cells
• CLL prognosis: del(17p)/TP53 adverse; IGHV mutated favorable
• CLL treatment: Watch and wait unless indicated; BTK inhibitors or venetoclax
• Richter transformation: CLL → aggressive lymphoma; poor prognosis
• Hairy cell leukemia: Hairy projections, BRAF V600E, excellent response to cladribine
• LGL leukemia: Chronic neutropenia, association with RA

Key Terms

This content is subject to the MIT License. © 2024–2026 Hibbert School of Medicine.

Read this lecture as Markdown