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Incidence and Mortality
Estimated new cases and deaths from chronic lymphocytic leukemia (CLL) in the United States in 2014:
CLL is a disorder of morphologically mature but immunologically less mature lymphocytes and is manifested by progressive accumulation of these cells in the blood, bone marrow, and lymphatic tissues. In this disorder, lymphocyte counts in the blood are usually greater than or equal to 5,000/mm3 with a characteristic immunophenotype (CD5- and CD23-positive B cells).[3,4] As assays have become more sensitive for detecting monoclonal B-CLL–like cells in peripheral blood, researchers have detected a monoclonal B-cell lymphocytosis (MBL) in 3% of adults older than 40 years and 6% in adults older than 60 years. Such early detection and diagnosis may falsely suggest improved survival for the group and may unnecessarily worry or result in therapy for some patients who would have remained undiagnosed in their lifetime, a circumstance known in the literature as overdiagnosis or pseudodisease.[6,7]
In two selected series of more than 900 patients followed prospectively for a median of 5 to 7 years, overt CLL requiring chemotherapy occurred in 7% of patients.[5,8] In a database analysis and for up to 77 months before diagnosis, almost all patients with a diagnosis of CLL had prediagnostic B-cell clones that were identified in peripheral blood when available.[4,9]
For patients with progressing CLL, treatment with conventional doses of chemotherapy is not curative; selected patients treated with allogeneic stem cell transplantation have achieved prolonged disease-free survival.[10,11,12,13,14] Antileukemic therapy is frequently unnecessary in uncomplicated early disease. The median survival for all patients ranges from 8 to 12 years in older trials with data from the 1970s through the 1990s.[15,16] There is, however, a large variation in survival among individual patients, ranging from several months to a normal life expectancy. Treatment must be individualized based on the clinical behavior of the disease.
As found in one report, CLL occurs primarily in middle-aged and elderly adults, with increasing frequency in successive decades of life. The clinical course of this disease progresses from an indolent lymphocytosis without other evident disease to one of generalized lymphatic enlargement with concomitant pancytopenia. Complications of pancytopenia, including hemorrhage and infection, represent a major cause of death in these patients. Immunological aberrations, including Coombs-positive hemolytic anemia, immune thrombocytopenia, and depressed immunoglobulin levels may all complicate the management of CLL. Prognostic factors that may help predict clinical outcome include cytogenetic subgroup, immunoglobulin mutational status, ZAP-70, and CD38.[2,21,22,23,24,25,26,27,28,29] (Refer to the Prognostic Factors section in the Stage Information for Chronic Lymphocytic Leukemia section of this summary for more information.) Patients who develop an aggressive high-grade non-Hodgkin lymphoma, usually diffuse large B-cell lymphoma and termed a Richter transformation, have a poor prognosis. Patients with CLL are also at increased risk for other malignancies, even before therapy.
Confusion with other diseases may be avoided by determination of cell surface markers. CLL lymphocytes coexpress the B-cell antigens CD19 and CD20 along with the T-cell antigen CD5. This coexpression only occurs in one other disease entity, mantle cell lymphoma. CLL B cells express relatively low levels of surface-membrane immunoglobulin (compared with normal peripheral blood B cells) and a single light chain (kappa or lambda). CLL is diagnosed by an absolute increase in lymphocytosis and/or bone marrow infiltration coupled with the characteristic features of morphology and immunophenotype, which confirm the characteristic clonal population.
The differential diagnosis must exclude hairy cell leukemia and Waldenström macroglobulinemia. (Refer to the PDQ summaries on Hairy Cell Leukemia and Adult Non-Hodgkin Lymphoma Treatment for more information.) Waldenström macroglobulinemia has a natural history and therapeutic options similar to CLL, with the exception of hyperviscosity syndrome associated with macroglobulinemia as a result of elevated immunoglobulin M. Prolymphocytic leukemia (PLL) is a rare entity characterized by excessive prolymphocytes in the blood with a typical phenotype that is positive for CD19, CD20, and surface-membrane immunoglobulin and negative for CD5. These patients demonstrate splenomegaly and poor response to low-dose or high-dose chemotherapy.[15,33]
Cladribine (2-chlorodeoxyadenosine) appears to be an active agent (60% complete remission rate) for patients with de novo B-cell prolymphocytic leukemia.[Level of evidence: 3iiiDiv] Alemtuzumab (campath-1H), an anti-CD52 humanized monoclonal antibody, has been used for 76 patients with T-cell prolymphocytic leukemia after failure of prior chemotherapy (usually pentostatin or cladribine) with a 51% response rate (95% confidence interval, 40%–63%) and median time to progression of 4.5 months (range, 0.1–45.4 months).[Level of evidence: 3iiiDiv] These response rates have been confirmed by other investigators. Patients with CLL who show prolymphocytoid transformation maintain the classic CLL phenotype and have a worse prognosis than PLL patients.
Large granular lymphocyte (LGL) leukemia is characterized by lymphocytosis with a natural killer cell immunophenotype (CD2, CD16, and CD56) or a T-cell immunophenotype (CD2, CD3, and CD8).[37,38,39] These patients often have neutropenia and a history of rheumatoid arthritis. The natural history is indolent, often marked by anemia and splenomegaly. This condition appears to fit into the clinical spectrum of Felty syndrome. A characteristic genetic finding in almost 50% of the patients with T-cell LGL involves mutations in the signal transducer and activator of the transcription 3 gene (STAT 3). Therapy includes low doses of oral cyclophosphamide or methotrexate, cyclosporine, and treatment of the bacterial infections acquired during severe neutropenia.[37,39,42,43]
Other PDQ summaries containing information about CLL include the following:
Staging is useful in chronic lymphocytic leukemia (CLL) to predict prognosis and also to stratify patients to achieve comparisons for interpreting specific treatment results. Anemia and thrombocytopenia are the major adverse prognostic variables.
CLL has no standard staging system. The Rai staging system and the Binet classification are presented below.[1,2] A National Cancer Institute (NCI)-sponsored working group has formulated standardized guidelines for criteria related to eligibility, response, and toxic effects to be used in future clinical trials in CLL.
Rai Staging System
Stage 0 CLL is characterized by absolute lymphocytosis (>15,000/mm3) without adenopathy, hepatosplenomegaly, anemia, or thrombocytopenia.
Stage I CLL is characterized by absolute lymphocytosis with lymphadenopathy without hepatosplenomegaly, anemia, or thrombocytopenia.
Stage II CLL is characterized by absolute lymphocytosis with either hepatomegaly or splenomegaly with or without lymphadenopathy.
Stage III CLL is characterized by absolute lymphocytosis and anemia (hemoglobin <11 g/dL) with or without lymphadenopathy, hepatomegaly, or splenomegaly.
Stage IV CLL is characterized by absolute lymphocytosis and thrombocytopenia (<100,000/mm3) with or without lymphadenopathy, hepatomegaly, splenomegaly, or anemia.
Clinical stage A*
Clinical stage A CLL is characterized by no anemia or thrombocytopenia and fewer than three areas of lymphoid involvement (Rai stages 0, I, and II).
Clinical stage B*
Clinical stage B CLL is characterized by no anemia or thrombocytopenia with three or more areas of lymphoid involvement (Rai stages I and II).
Clinical stage C
Clinical stage C CLL is characterized by anemia and/or thrombocytopenia regardless of the number of areas of lymphoid enlargement (Rai stages III and IV).
*Lymphoid areas include cervical, axillary, inguinal, and spleen.
The Binet classification integrates the number of nodal groups involved with the disease with bone marrow failure. Its major benefit derives from the recognition of a predominantly splenic form of the disease, which may have a better prognosis than in the Rai staging, and from recognition that the presence of anemia or thrombocytopenia has a similar prognosis and does not merit a separate stage. Neither system separates immune from nonimmune causes of cytopenia. Patients with thrombocytopenia or anemia or both, which is caused by extensive marrow infiltration and impaired production (Rai III/IV, Binet C) have a poorer prognosis than patients with immune cytopenias. The International Workshop on CLL has recommended integrating the Rai and Binet systems as follows: A(0), A(I), A(II); B(I), B(II); and C(III), C(IV). The NCI-sponsored working group has published guidelines for the diagnosis and treatment of CLL in both clinical trial and general practice settings. Use of these systems allows comparison of clinical results and establishment of therapeutic guidelines.
New prognostic markers are now available to the clinician and investigator.[6,7] The use of these markers to stratify patients in clinical trials, to help assess the need for therapy, and to help select the type of therapy continues to evolve. Prospective trials to verify and establish the role of these prognostic markers are ongoing. No large multivariable analyses exist as yet to test the relative power of these individual prognostic variables. Prognostic indices are under evaluation and will require prospective validation.
Other prognostic factors include:
Treatment of chronic lymphocytic leukemia (CLL) ranges from periodic observation with treatment of infectious, hemorrhagic, or immunologic complications to a variety of therapeutic options, including steroids, alkylating agents, purine analogs, combination chemotherapy, monoclonal antibodies, and transplant options. Because this disease is generally not curable, occurs in an elderly population, and often progresses slowly, it is most often treated in a conservative fashion. In asymptomatic patients, treatment may be deferred until the patient becomes symptomatic as the disease progresses. Since the rate of progression may vary from patient to patient, with long periods of stability and sometimes spontaneous regressions, frequent and careful observation is required to monitor the clinical course.
A meta-analysis of randomized trials showed no survival benefit for immediate versus delayed therapy for patients with early stage disease, nor for the use of combination regimens incorporating an anthracycline compared with a single-agent alkylator for advanced stage disease.[Level of evidence: 1iiA] A variety of clinical factors, including IgVH mutation, chromosomal abnormalities by fluorescent in situ hybridization analysis or cytogenetics, beta-2-microglobulin, and lymphocyte doubling time may be helpful in predicting progression of disease.
Infectious complications in advanced disease are in part a consequence of the hypogammaglobulinemia and the inability to mount a humoral defense against bacterial or viral agents. Herpes zoster represents a frequent viral infection in these patients, but infections with Pneumocystis carinii and Candida albicans may also occur. The early recognition of infections and the institution of appropriate therapy are critical to the long-term survival of these patients. A randomized study of intravenous immunoglobulin (400 mg/kg every 3 weeks for 1 year) in patients with CLL and hypogammaglobulinemia produced significantly fewer bacterial infections and a significant delay in onset of first infection during the study period. There was, however, no effect on survival. Routine chronic administration of intravenous immunoglobulin is expensive, and the long-term benefit (>1 year) is unproven.[5,6]
Second malignancies and treatment-induced acute leukemias may also occur in a small percentage of patients. Transformation of CLL to diffuse large cell lymphoma (Richter syndrome) carries a poor prognosis with a median survival of less than 1 year, though 20% of the patients may live more than 5 years after aggressive combination chemotherapy. (Refer to the PDQ summary on Adult Non-Hodgkin Lymphoma Treatment for more information.)
Autoimmune hemolytic anemia and/or thrombocytopenia can occur in patients with any stage of CLL. Initial therapy involves corticosteroids with or without alkylating agents (fludarabine can worsen the hemolytic anemia). It is frequently advisable to control the autoimmune destruction with corticosteroids, if possible, prior to administering marrow-suppressive chemotherapy because such patients may be difficult to transfuse successfully with either red blood cells or platelets. Alternate therapies include high-dose immune globulin, rituximab, cyclosporine, azathioprine, splenectomy, and low-dose radiation therapy to the spleen.[10,11] Tumor lysis syndrome is an uncommon complication (presenting in 1 out of 300 patients) of chemotherapy for patients with bulky disease.
About 1% of morphologic CLL cases express T-cell markers (CD4 and CD7) and have clonal rearrangements of their T-cell receptor genes. These patients have a higher frequency of skin lesions, more variable lymphocyte shape, and shorter median survival (13 months) with minimal responses to chemotherapy.
Computed tomographic (CT) scans have a very limited role in following patients after completion of treatment; the decision to treat for relapse was determined by CT scan or ultrasound in only 2 of 176 patients in three prospective trials for the German CLL Study Group.
Because of the indolent nature of stage 0 chronic lymphocytic leukemia (CLL), treatment is not indicated. The French Cooperative Group on CLL randomly assigned 1,535 patients with previously untreated stage A disease to receive either chlorambucil or no immediate treatment and found no survival advantage for immediate treatment with chlorambucil.[Level of evidence: 1iiA] A meta-analysis of six trials of immediate versus deferred therapy with chlorambucil (including the aforementioned trial by the French Cooperative Group) showed no difference in overall survival at 10 years.[Level of evidence: 1iiA] Whether immediate therapy with the nucleoside analogs or other newer strategies will be superior to a watchful waiting approach is uncertain.
Current Clinical Trials
Check for U.S. clinical trials from NCI's list of cancer clinical trials that are now accepting patients with stage 0 chronic lymphocytic leukemia. The list of clinical trials can be further narrowed by location, drug, intervention, and other criteria.
General information about clinical trials is also available from the NCI Web site.
Several decades of large, randomized, prospective trials of previously untreated patients have demonstrated statistically significant improvements in response rates, event-free survival (EFS), and progression-free survival (PFS) with comparison of combinations of drugs versus single-agent alkylators,[1,2] but only two trials have shown statistically significant improvement in overall survival (OS).[3,4]
The first trial, a comparison of chlorambucil versus fludarabine, after 15 years of median follow-up, showed improved median OS for patients on the fludarabine regimen at 63 months versus 59 months (P = .04), and an improved percentage of patients were alive at 8 years (31% vs. 19%, P = .04).[3,5][Level of evidence: 1iiA]
The second trial, which had 817 patients, compared FCR (fludarabine + cyclophosphamide + rituximab) versus FC (fludarabine + cyclophosphamide) with a median follow-up of 38 months and showed improved OS at 3 years for the rituximab combination (i.e., 87% vs. 83%, P = .01.[Level of evidence: 1iiA] Yet neither fludarabine nor FCR has been compared in a randomized study against watchful waiting in asymptomatic or minimally affected patients.
The improvements in response rates from more intensive regimens have maximized the clearance of minimal residual disease (MRD). In one prospective trial of 493 patients, clearance of MRD was an independent predictor of OS by multivariate analysis. The surrogate endpoint of such clearance of residual disease, while prognostic, has not been shown to improve survival in a randomized prospective trial; the necessary study would take patients who fail to completely clear the marrow with induction therapy and randomly assign them to further alternative treatment versus the same treatment later at relapse, looking at OS as the primary endpoint.[1,2]
The sequencing of the following treatment options cannot be determined from the current set of completed clinical trials. When patients become symptomatic or require treatment, FCR has become the most frequently chosen option outside of a clinical trial, mostly on the basis of the previously described prospective study.
Note: Standard options are roughly ordered by level of toxic effects, starting with the least toxic options. More recently discovered options are mentioned at the end of the list.
Several randomized trials have compared the purine analogs with chlorambucil; with cyclophosphamide, doxorubicin, and prednisone; or with cyclophosphamide, doxorubicin, vincristine, and prednisone (CHOP) in previously untreated patients.[5,23,24,25,26] All of these trials showed higher or equivalent response rates for the purine analog and most showed an improvement in PFS, with one reaching significance in OS favoring fludarabine.[3,23,24,25,26,27][Level of evidence: 1iiDiii]
A comparison of chlorambucil versus fludarabine, after 15 years' median follow-up, showed patients with improved median OS with fludarabine at 63 versus 59 months (P = .04) and an improved percentage of patients alive at 8 years (31% vs. 19%, P = .04).[Level of evidence: 1iiA] All of the trials demonstrated higher toxic effects with the purine analogs, especially granulocytopenic infections, herpes infections, autoimmune hemolytic anemia, and persistent thrombocytopenia. The increased risk of infection may persist for months or years after treatment with a purine analog.[27,29]
Although empiric evidence is lacking, some investigators recommend prophylaxis with trimethoprim-sulfa during therapy and for 6 to 12 months afterwards to prevent pneumocystis infection. In a similar way, other investigators employ prophylaxis (e.g., acyclovir) for the herpes viruses. Purine analogs cause less hair loss or nausea than combination chemotherapy, including alkylators and anthracyclines.
Bendamustine is a cytotoxic agent with bifunctional properties of an alkylator and a purine analog. In previously treated and untreated patients, bendamustine with rituximab has shown response rates around 70% to 90%.[31,32][Level of evidence: 3iiiDiii]
In a randomized comparison with chlorambucil in 319 previously treated patients, bendamustine showed a better response rate (68% vs. 31%, P < .0001) and PFS (21.6 months vs. 8 months) with a median follow-up of 35 months.[Level of evidence: 1iiDiii] The German CLL Study Group is comparing bendamustine plus rituximab versus FCR as first-line therapy in patients with CLL who require therapy.
Lenalidomide is an oral immunomodulatory agent with response rates over 50%, with or without rituximab, for patients with previously treated and untreated disease.[34,35,36,37,38,39][Level of evidence: 3iiiDiv] Prolonged, lower-dose approaches and attention to prevention of tumor lysis syndrome are recommended with this agent.[34,40] Combination therapy and long-term toxicities from using lenalidomide (such as increased myelodysplasia, as seen in myeloma patients) remain undefined for patients with CLL.
A trial of 817 patients comparing FCR versus FC with a median follow-up of 38 months showed improved OS at 3 years for the rituximab combination (87% vs. 83%, P = .01).[Level of evidence: 1iiA] FCR has never been compared with watchful waiting up front in asymptomatic or minimally affected patients. The improvements in response rates from more intensive regimens have maximized the clearance of MRD. However, the surrogate endpoint of MRD clearance has not been proven to be a valid surrogate for improved survival in a randomized prospective trial; the necessary study would take patients who fail to completely clear the marrow with induction therapy and randomly assign them to further alternative treatment versus the same treatment later at relapse looking at OS as the primary endpoint.[1,2] A cumulative incidence of 6% to 8 % for myelodysplasia is seen at 5 to 7 years in patients who received fludarabine plus cyclophosphamide, with or without rituximab.[41,42]
Other combination chemotherapy regimens include the following:
A meta-analysis of ten trials comparing combination chemotherapy (before the availability of rituximab) with chlorambucil alone showed no difference in OS at 5 years.[Level of evidence: 1iiA]
In a combination regimen, subcutaneous alemtuzumab plus fludarabine (with or without cyclophosphamide) or intravenous alemtuzumab plus alkylating agents have resulted in excess infectious toxicities and death, with no compensatory improvement in efficacy in phase II trials.[64,65,66][Level of evidence: 3iiiDiv]
In a randomized prospective study, 335 previously treated patients received intravenous alemtuzumab plus fludarabine versus fludarabine alone; with a median follow-up of 30 months, the combination of fludarabine plus intravenous alemtuzumab had better PFS, with a median of 23.7 months versus 16.5 months (hazard ratio [HR], 0.61; 95% confidence interval [CI], 0.47–0.80; P = .0003); and better OS, with a median not reached, versus 52.9 months (HR, 0.65; 95% CI, 0.45–0.94; P = .021).[Level of evidence: 1iiA] Profound and long-lasting immunosuppression has been seen, which mandates monitoring for reactivation of cytomegalovirus and prophylaxis for pneumocystis and herpes virus infections.[68,69] Antibiotic prophylaxis includes trimethoprim and sulfamethoxazole, itraconazole, and acyclovir (or ganciclovir) for asymptomatic cytomegalovirus viremia.
In a prospective randomized trial, 241 previously untreated patients younger than 66 years with advanced-stage disease received induction therapy with a CHOP-based regimen followed by fludarabine. Complete responders (105 patients) were randomly assigned to undergo autologous stem cell transplantation (ASCT) or observation, while the other 136 patients were randomly assigned to receive dexamethasone, high-dose aracytin, and cisplatin reinduction followed by either ASCT or fludarabine plus cyclophosphamide. Although the 3-year EFS favored ASCT in complete responders, there was no difference in OS in any of the randomized comparisons.[Level of evidence: 1iiDi]
Patients with adverse prognostic factors are very likely to die from CLL. These patients are candidates for clinical trials that employ high-dose chemotherapy and immunotherapy with myeloablative or nonmyeloablative allogeneic peripheral stem cell transplantation.[70,71,72,73,74,75,78,79,80,81,82,83,84,85] Although most patients who attain complete remission after ASCT eventually relapse, a survival plateau for allogeneic stem cell support suggests an additional graft-versus-leukemia effect.
Check for U.S. clinical trials from NCI's list of cancer clinical trials that are now accepting patients with stage I chronic lymphocytic leukemia, stage II chronic lymphocytic leukemia, stage III chronic lymphocytic leukemia and stage IV chronic lymphocytic leukemia. The list of clinical trials can be further narrowed by location, drug, intervention, and other criteria.
Clinical trials are appropriate and should be considered when possible. In small studies, response rates of more than 40% have been reported for lenalidomide [2,3,4] and flavopiridol.[5,6][Level of evidence: 3iiiDiv] The addition of the Bcl-2 anti-sense oligonucleotide oblimersen to fludarabine/cyclophosphamide improved complete response rates in a randomized study of 241 patients with relapsed disease.[Level of evidence: 1iiDiv] Bone marrow and peripheral stem cell transplantations are under clinical evaluation.[8,9,10,11,12,13,14]
Check for U.S. clinical trials from NCI's list of cancer clinical trials that are now accepting patients with refractory chronic lymphocytic leukemia. The list of clinical trials can be further narrowed by location, drug, intervention, and other criteria.
The PDQ cancer information summaries are reviewed regularly and updated as new information becomes available. This section describes the latest changes made to this summary as of the date above.
General Information About Chronic Lymphocytic Leukemia (CLL)
Updated statistics with estimated new cases and deaths for 2014 (cited American Cancer Society as reference 1).
Added text to state that a characteristic genetic finding in almost 50% of the patients with T-cell large granular lymphocyte leukemia involves mutations in the signal transducer and activator of the transcription 3 gene (STAT 3) (cited Koskela et al. as reference 41).
Stage I, II, III, and IV CLL
Revised text to state that lenalidomide is an oral immunomodulatory agent with response rates over 50%, with or without rituximab, for patients with previously treated and untreated disease (cited Wendtner et al. and Badoux et al. as references 38 and 39, respectively).
Added Pettitt et al. as reference 59.
Added Gritti et al. as reference 63.
Revised text to state that in a combination regimen, subcutaneous alemtuzumab plus fludarabine (with or without cyclophosphamide) or intravenous alemtuzumab plus alkylating agents have resulted in excess infectious toxicities and death, with no compensatory improvement in efficacy in phase II trials (cited Lepretre et al. as reference 66).
Added text to state that antibiotic prophylaxis includes trimethoprim and sulfamethoxazole, itraconazole, and acyclovir for asymptomatic cytomegalovirus viremia.
Added Dreger et al. as reference 76.
Added Toze et al. as reference 79.
Added text to state that ibrutinib is a selective irreversible inhibitor of Bruton tyrosine kinase, a signaling molecule located upstream in the B-cell receptor signaling cascade; other trials of relapsed and refractory CLL have shown durable responses to the oral agent in phase I and II studies (cited Advani et al. as reference 86 and level of evidence 3iiiDiii). A phase Ib–II trial (NCT01105247) of 85 patients with relapsed or refractory CLL showed a 26-month progression-free survival rate of 76%, including patients with 17p- or unmutated IgVH fluorescence in situ hybridization testing (cited Byrd et al. as reference 87 and level of evidence 3iiiDiii).
This summary is written and maintained by the PDQ Adult Treatment Editorial Board, which is editorially independent of NCI. The summary reflects an independent review of the literature and does not represent a policy statement of NCI or NIH. More information about summary policies and the role of the PDQ Editorial Boards in maintaining the PDQ summaries can be found on the About This PDQ Summary and PDQ NCI's Comprehensive Cancer Database pages.
Purpose of This Summary
This PDQ cancer information summary for health professionals provides comprehensive, peer-reviewed, evidence-based information about the treatment of chronic lymphocytic leukemia. It is intended as a resource to inform and assist clinicians who care for cancer patients. It does not provide formal guidelines or recommendations for making health care decisions.
Reviewers and Updates
This summary is reviewed regularly and updated as necessary by the PDQ Adult Treatment Editorial Board, which is editorially independent of the National Cancer Institute (NCI). The summary reflects an independent review of the literature and does not represent a policy statement of NCI or the National Institutes of Health (NIH).
Board members review recently published articles each month to determine whether an article should:
Changes to the summaries are made through a consensus process in which Board members evaluate the strength of the evidence in the published articles and determine how the article should be included in the summary.
The lead reviewers for Chronic Lymphocytic Leukemia Treatment are:
Any comments or questions about the summary content should be submitted to Cancer.gov through the Web site's Contact Form. Do not contact the individual Board Members with questions or comments about the summaries. Board members will not respond to individual inquiries.
Levels of Evidence
Some of the reference citations in this summary are accompanied by a level-of-evidence designation. These designations are intended to help readers assess the strength of the evidence supporting the use of specific interventions or approaches. The PDQ Adult Treatment Editorial Board uses a formal evidence ranking system in developing its level-of-evidence designations.
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The preferred citation for this PDQ summary is:
National Cancer Institute: PDQ® Chronic Lymphocytic Leukemia Treatment. Bethesda, MD: National Cancer Institute. Date last modified <MM/DD/YYYY>. Available at: http://www.cancer.gov/cancertopics/pdq/treatment/CLL/healthprofessional. Accessed <MM/DD/YYYY>.
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Last Revised: 2014-04-16
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