Hairy cell leukaemia: ESMO Clinical Practice Guidelines for diagnosis, treatment and follow-up
Authors: T. Robak, E. Matutes, D. Catovsky, P.L. Zinzani, C. Buske
Journal: Annals of Oncology (2015)

Classical hairy cell leukaemia (HCL) is a B-cell chronic lymphoproliferative disorder characterised by splenomegaly, pancytopaenia and bone marrow involvement with fibrosis. HCL represents 2% of adult leukaemia. Approximately 1600 new cases per year are diagnosed in Europe [1.Ferlay J. Steliarova-Foucher E. Lortet-Tieulent J. et al.Cancer incidence and mortality patterns in Europe: estimates for 40 countries in 2012.Eur J Cancer. 2013; 49: 1374-1403Abstract Full Text Full Text PDF PubMed Scopus (4108) Google Scholar], with a median age of 52 years at the time of diagnosis. The disease occurs more often in men than in women, with a ratio of approximately 4:1 [2.Morton L.M. Wang S.S. Devesa S.S. et al.Lymphoma incidence patterns by WHO subtype in the United States, 1992–2001.Blood. 2006; 107: 265-276Crossref PubMed Scopus (1217) Google Scholar]. In the USA, a higher frequency of HCL is observed among white Americans than among African-Americans or Asians, as well as in patients following exposure to the herbicide ‘Agent Orange’, used during the Vietnam War [3.Grever M.R. Blachly J.S. Andritsos L.A. Hairy cell leukemia: update on molecular profiling and therapeutic advances.Blood Rev. 2014; 28: 197-203Crossref Scopus (28) Google Scholar]. HCL variant (HCL-V) is classified among the unclassifiable splenic B-cell leukaemia/lymphoma that is no longer biologically related to classical HCL. It is included in the World Health Organization (WHO) classification as a provisional entity [4.Swerdlow S.H. Campo E. Harris N.L. et al.World Health Organization (WHO) Classification of Tumours: Pathology and Genetics of Haematopoietic and Lymphatic Tissues.4th edition. IARC Press, Lyon, France2008Google Scholar]. HCL-V is an uncommon disorder, accounting for approximately 0.4% of chronic lymphoid malignancies and 10% of all HCL cases, without sexual predominance. The median age of the patients is 71 years. The examination of peripheral blood films and immunophenotyping allows for a diagnosis to be established in most cases (Table 1) [I, C] [5.Del Giudice I. Matutes E. Morilla R. et al.The diagnostic value of CD123 in B-cell disorders with hairy or villous lymphocytes.Haematologica. 2004; 89: 303-308PubMed Google Scholar, 6.Matutes E. Immunophenotyping and differential diagnosis of hairy cell leukemia.Hematol Oncol Clin North Am. 2006; 20: 1051-1063Abstract Full Text Full Text PDF PubMed Scopus (69) Google Scholar]. The neoplastic cells are twice the size of a lymphocyte and have a round or kidney-shaped nucleus with loose chromatin and abundant pale cytoplasm with projections. Monocytopaenia and macrocytosis are very common; other cytopaenias may be present. A diagnosis of HCL based on cytology can be effectively confirmed by flow cytometry studies using anti-B-cell monoclonal antibodies such as CD19, CD20 or CD22, together with a panel of antibodies such as CD11c, CD25, CD103 and CD123, which are more specific to HCL; this combination will allow for the differentiation of HCL from other B-cell leukaemias and lymphomas with circulating villous cells [5.Del Giudice I. Matutes E. Morilla R. et al.The diagnostic value of CD123 in B-cell disorders with hairy or villous lymphocytes.Haematologica. 2004; 89: 303-308PubMed Google Scholar, 6.Matutes E. Immunophenotyping and differential diagnosis of hairy cell leukemia.Hematol Oncol Clin North Am. 2006; 20: 1051-1063Abstract Full Text Full Text PDF PubMed Scopus (69) Google Scholar]. In addition, strong expression of CD200 is characteristic of HCL and may be useful for the diagnosis in difficult cases [7.Pillai V. Pozdnyakova O. Charest K. et al.CD200 flow cytometric assessment and semiquantitative immunohistochemical staining distinguishes hairy cell leukemia from hairy cell leukemia-variant and other B-cell lymphoproliferative disorders.Am J Clin Pathol. 2013; 140: 536-543Crossref PubMed Scopus (35) Google Scholar].Table 1Diagnostic work-up of classical HCLPeripheral blood film morphology [I, C]Flow cytometry in peripheral blood and bone marrow aspirate [I, C]Bone marrow trephine biopsy with immunohistochemistry [I, C]BRAF mutation of exon 15 in difficult cases [II, C]HCL, hairy cell leukaemia. Open table in a new tab HCL, hairy cell leukaemia. HCL-V typically presents with high lymphocyte counts, with the cells being nucleolated and lacking monocytopaenia. Flow cytometry will indicate the cells to be CD11c+ and often CD103+ but very rarely CD25+ and CD123+. There is an overlap with splenic diffuse red pulp lymphoma, and distinguishing between these two diseases may be difficult. A bone marrow examination is required for the diagnosis, particularly after treatment, to assess response [I, C]. As the bone marrow can rarely be aspirated (‘dry-tap’) in classical HCL, diagnosis is typically performed by a bone marrow trephine biopsy. The degree and pattern of infiltration varies from mild interstitial to diffuse, and the lymphoid cells are surrounded by a clear halo due to the abundant cytoplasm, giving the characteristic ‘fried egg’ pattern. In HCL-V, the infiltration is either intrasinusoidal or interstitial. Immunohistochemistry with the monoclonal antibodies CD20, CD72 (DBA44), CD11c, CD25, CD103, annexin A1 and tartrate-resistant acid phosphatase stain will highlight the lymphoid infiltrates and support the diagnosis of classical HCL [6.Matutes E. Immunophenotyping and differential diagnosis of hairy cell leukemia.Hematol Oncol Clin North Am. 2006; 20: 1051-1063Abstract Full Text Full Text PDF PubMed Scopus (69) Google Scholar]. However, annexin A1 is not suitable for detecting residual disease after treatment as it stains myeloid cells also. Cyclin D1 may be weakly positive but differential diagnosis with mantle cell lymphoma rarely arises. Recently, monoclonal antibodies that detect the mutated BRAF protein have been developed and shown to be useful for the diagnosis and detection of minimal residual disease (MRD) [8.Andrulis M. Penzel R. Weichert W. et al.Application of a BRAF V600E mutation-specific antibody for the diagnosis of hairy cell leukemia.Am J Surg Pathol. 2012; 36: 1796-1800Crossref PubMed Scopus (112) Google Scholar]. Although this requires validation, preliminary data suggest that this marker has high specificity and sensitivity for HCL [II, C]. The main distinguishing features between HCL and HCL-V are outlined in Table 2. HCL does not have a distinct chromosomal abnormality. The majority of the cases have mutations of the immunoglobulin heavy chain (IGHV) gene, suggesting that the disease arises on a memory B cell. Unlike other splenomegalic disorders such as splenic marginal zone lymphoma or HCL-V, there is no evidence of specific IGHV, IGHD, IGHJ repertoires or stereotypes in HCL [9.Hockley S.L. Giannouli S. Morilla A. et al.Insight into the molecular pathogenesis of hairy cell leukaemia, hairy cell leukaemia variant and splenic marginal zone lymphoma provided by the analysis of their IGH rearrangements and somatic hypermutation patterns.Br J Haematol. 2010; 148: 666-669Crossref PubMed Scopus (44) Google Scholar, 10.Forconi F. Sozzi E. Cencini E. et al.Hairy cell leukemias with unmutated IGHV genes define the minor subset refractory to single-agent cladribine and with more aggressive behaviour.Blood. 2009; 114: 4696-4702Crossref PubMed Scopus (97) Google Scholar, 11.Arons E. Suntum T. Stetler-Stevenson M. Kreitman R.J. VH4–34+ hairy cell leukemia, a new variant with poor prognosis despite standard therapy.Blood. 2009; 114: 4687-4695Crossref PubMed Scopus (121) Google Scholar]. In 2011, Tiacci et al. [12.Tiacci E. Trifonov V. Schiavoni G. et al.BRAF mutations in hairy cell leukemia.N Engl J Med. 2011; 364: 2305-2315Crossref PubMed Scopus (814) Google Scholar] reported the presence of the V600E mutation of the BRAF gene in exon 15 in all 47 investigated HCL cases. The mutation rarely occurs in exon 11. The BRAF mutation leads to the activation of the RAF/MEK-ERK pathway, resulting in enhanced cell proliferation and survival. These results have been validated by other groups and new simpler and more sensitive methods to detect the mutation, such as quantitative real-time polymerase chain reaction (q-PCR) have been developed. This finding is relevant to the diagnosis and pathogenesis of HCL with potential therapeutic implications. In the near future, testing for this mutation may be incorporated on a routine basis into the diagnostic work-up of HCL [II, C]. Although HCL-V patients lack the BRAF mutation, TP53 mutations are present in one-third of cases [13.Hockley S.L. Else M. Morilla A. et al.The prognostic impact of clinical and molecular features in hairy cell leukaemia variant and splenic marginal zone lymphoma.Br J Haematol. 2012; 158: 347-354Crossref PubMed Scopus (46) Google Scholar].Table 2Diagnostic criteria for HCL and HCL-VCharacteristicsClassical HCLHCL-VBone marrow aspirationDifficult (often dry tap)EasyLymphocytosis-+Monocytopaenia+-Prominent nucleoli-+Cytoplasmic projections++CD25+-FMC7, CD20, CD22, CD11c++CD103, CD123+VariableAnnexin+-BRAF V600E mutation+-Splenomegaly++Response to purine analoguesGoodPoorHCL, hairy cell leukaemia; HCL-V, HCL variant. Open table in a new tab HCL, hairy cell leukaemia; HCL-V, HCL variant. There is no worldwide accepted staging system for HCL. In addition to the diagnostic tests on the blood and bone marrow trephine biopsy, a staging work-up should include full blood cell counts with differential and reticulocytes, renal and liver biochemistry, serum immunoglobulins, β2 microglobulin, direct antiglobulin test (DAT), Coombs test and hepatitis virus B and C and human immunodeficiency virus screening. Computed tomography (CT) imaging is desirable at the time of diagnosis (as around 10% of HCL patients have abdominal lymphadenopathy) and should be performed at relapse [14.Mercieca J. Puga M. Matutes E. et al.Incidence and significance of abdominal lymphadenopathy in hairy cell leukaemia.Leuk Lymphoma. 1994; 14: 79-83PubMed Google Scholar]. There is no international prognostic system for risk stratification of HCL. Clinical variables that have been considered to have an adverse prognosis include the degree of cytopaenias (Hb < 10 g/dl, platelets < 100 × 109/l) and neutrophils (<1 × 109/l), as well as the presence of lymphadenopathy, which predicts a poor response to purine analogues [15.Mercieca J. Matutes E. Emmett E. et al.2-chlordeoxyadenosine in the treatment of hairy cell leukaemia: differences in response in patients with and without abdominal lymphadenopathy.Br J Haematol. 1996; 93: 409-411Crossref PubMed Scopus (32) Google Scholar]. Patients who achieve a complete response (CR) have a significantly longer disease-free survival than those who achieve a partial response (PR) [II, B] [16.Else M. Dearden C.E. Matutes E. et al.Long-term follow-up of 233 patients with hairy cell leukaemia, treated initially with pentostatin or cladribine, at a median of 16 years from diagnosis.Br J Haematol. 2009; 145: 733-740Crossref PubMed Scopus (205) Google Scholar]. Biological factors that have been associated with a poor outcome are the presence of TP53 mutations and the lack of somatic mutations in the IGVH genes, which occur in a minor proportion of cases [IV, C] [10.Forconi F. Sozzi E. Cencini E. et al.Hairy cell leukemias with unmutated IGHV genes define the minor subset refractory to single-agent cladribine and with more aggressive behaviour.Blood. 2009; 114: 4696-4702Crossref PubMed Scopus (97) Google Scholar] and the VH4-34 family usage, a feature more frequent in HCL-V [11.Arons E. Suntum T. Stetler-Stevenson M. Kreitman R.J. VH4–34+ hairy cell leukemia, a new variant with poor prognosis despite standard therapy.Blood. 2009; 114: 4687-4695Crossref PubMed Scopus (121) Google Scholar]. The presence of TP53 mutations (but not VH4-34) usage appears to be an adverse prognostic factor in HCL-V [IV, C] [13.Hockley S.L. Else M. Morilla A. et al.The prognostic impact of clinical and molecular features in hairy cell leukaemia variant and splenic marginal zone lymphoma.Br J Haematol. 2012; 158: 347-354Crossref PubMed Scopus (46) Google Scholar]. Treatment is not indicated in asymptomatic patients [V, B]. However, untreated patients should be closely monitored with a complete history, physical examination, and complete blood cell count with a differential test every 3–6 months. In contrast to chronic lymphocytic leukaemia, asymptomatic patients, who may be diagnosed by chance, are rare and in practice most patients need treatment shortly after diagnosis, either because of symptoms or to correct cytopaenias including monocytopaenia. Treatment should be initiated in patients with symptomatic disease manifested by bulky or progressive, symptomatic splenomegaly cytopaenias (haemoglobin <10 g/dl and/or platelets <100 × 109/l and/or neutrophils <1 × 109/l), recurrent or severe infections and/or systemic symptoms [II, A] [17.Grever M.R. How I treat hairy cell leukemia.Blood. 2010; 115: 21-28Crossref PubMed Scopus (123) Google Scholar, 18.Cornet E. Delmer A. Feugier P. et al.Recommendations of the SFH (French Society of Haematology) for the diagnosis, treatment and follow-up of hairy cell leukaemia.Ann Hematol. 2014; 93: 1977-1983Crossref PubMed Scopus (36) Google Scholar]. Purine analogues, cladribine (2-CldA) or pentostatin (DCF), are recommended as initial treatment of symptomatic HCL patients who are young and fit (Figure 1) [II, A]. 2-CldA induces durable and unmaintained response in 87%–100% patients, including CR in 85%–91%, after a single course of therapy [19.Saven A. Burian C. Koziol J.A. Piro L.D. Long-term follow-up of patients with hairy cell leukemia after cladribine treatment.Blood. 1998; 92: 1918-1926Crossref PubMed Google Scholar]. 2-CldA is administered either as a continuous intravenous (i.v.) infusion at a dose of 0.09 mg/kg over a 5–7 day period, or as a 2-h i.v. infusion at a dose of 0.12–0.14 mg/kg for 5–7 days [20.Cheson B.D. Sorensen J.M. Vena D.A. et al.Treatment of hairy cell leukemia with 2-chlorodeoxyadenosine via the Group C protocol mechanism of the National Cancer Institute: a report of 979 patients.J Clin Oncol. 1998; 16: 3007-3015Crossref PubMed Scopus (146) Google Scholar, 21.Robak T. Blasińska-Morawiec M. Krykowski E. et al.2-chlorodeoxyadenosine (2-CdA) in 2-hour versus 24-hour intravenous infusion in the treatment of patients with hairy cell leukemia.Leuk Lymphoma. 1996; 22: 107-111Crossref PubMed Scopus (50) Google Scholar]. 2-CldA is also an effective drug when administered at a dose of 0.12–0.15 mg/kg in 2-h infusion once a week over 6 courses [I, B] [22.Robak T. Jamroziak K. Gora-Tybor J. et al.Cladribine in a weekly versus daily schedule for untreated active hairy cell leukemia: final report from the Polish Adult Leukemia Group (PALG) of a prospective, randomized, multicenter trial.Blood. 2007; 109: 3672-3675Crossref PubMed Scopus (71) Google Scholar, 23.Zenhäusern R. Schmitz S.F. Solenthaler M. et al.Randomized trial of daily versus weekly administration of 2-chlorodeoxyadenosine in patients with hairy cell leukemia: a multicenter phase III trial (SAKK 32/98).Leuk Lymphoma. 2009; 50: 1501-1511Crossref PubMed Scopus (30) Google Scholar]. Both CR and overall response (OR) rates are similar in weekly and daily administration. Neither less frequent infections nor haematological toxicity have been confirmed in the weekly schedule by randomised trials [I, B] [23.Zenhäusern R. Schmitz S.F. Solenthaler M. et al.Randomized trial of daily versus weekly administration of 2-chlorodeoxyadenosine in patients with hairy cell leukemia: a multicenter phase III trial (SAKK 32/98).Leuk Lymphoma. 2009; 50: 1501-1511Crossref PubMed Scopus (30) Google Scholar, 24.von Rohr A. Schmitz S.F. Tichelli A. et al.Treatment of hairy cell leukemia with cladribine (2-chlorodeoxyadenosine) by subcutaneous bolus injection: a phase II study.Ann Oncol. 2002; 13: 1641-1649Abstract Full Text Full Text PDF PubMed Scopus (53) Google Scholar]. Similar results were achieved when the drug was given as a subcutaneous injection [II, B] [24.von Rohr A. Schmitz S.F. Tichelli A. et al.Treatment of hairy cell leukemia with cladribine (2-chlorodeoxyadenosine) by subcutaneous bolus injection: a phase II study.Ann Oncol. 2002; 13: 1641-1649Abstract Full Text Full Text PDF PubMed Scopus (53) Google Scholar, 25.Lauria F. Cencini E. Forconi F. Alternative methods of cladribine administration.Leuk Lymphoma. 2011; 52: 34-37Crossref PubMed Scopus (15) Google Scholar]. A subcutaneous 2-CldA is given at a dose of 0.1 mg/kg/day for 5–7 days or 0.14 mg/kg/day for 5 days as a single course [24.von Rohr A. Schmitz S.F. Tichelli A. et al.Treatment of hairy cell leukemia with cladribine (2-chlorodeoxyadenosine) by subcutaneous bolus injection: a phase II study.Ann Oncol. 2002; 13: 1641-1649Abstract Full Text Full Text PDF PubMed Scopus (53) Google Scholar, 25.Lauria F. Cencini E. Forconi F. Alternative methods of cladribine administration.Leuk Lymphoma. 2011; 52: 34-37Crossref PubMed Scopus (15) Google Scholar]. Grade 3–4 toxicity, largely represented by neutropaenic fevers and infections, were less frequent when lower total doses were used (0.5 mg/kg) than higher doses (0.7 mg/kg) with similar OR [25.Lauria F. Cencini E. Forconi F. Alternative methods of cladribine administration.Leuk Lymphoma. 2011; 52: 34-37Crossref PubMed Scopus (15) Google Scholar]. Subcutaneous administration does not usually require hospitalisation, and seems to be an easier way of delivering the drug than i.v. administration. Although no randomised trial comparing i.v. versus subcutaneous 2-CldA administration has been performed, the efficacy of both methods is similar [II, B]. A CR following 2-CldA administration is durable even without maintenance therapy [16.Else M. Dearden C.E. Matutes E. et al.Long-term follow-up of 233 patients with hairy cell leukaemia, treated initially with pentostatin or cladribine, at a median of 16 years from diagnosis.Br J Haematol. 2009; 145: 733-740Crossref PubMed Scopus (205) Google Scholar]. In patients demonstrating a PR after the first course of 2-CldA, a second course should be repeated to achieve a CR at least 6 months after the end of the first course, with or without rituximab [IV, B] [26.Dearden C.E. Else M. Catovsky D. Long-term results for pentostatin and cladribine treatment of hairy cell leukemia.Leuk Lymphoma. 2011; 52: 21-24Crossref PubMed Scopus (52) Google Scholar]. Similarly to 2-CldA, DCF induces a high rate of long-lasting CR. In patients with a normal creatinine clearance (>60 ml/min), DCF is usually given at a dose of 4 mg/m2 i.v. every second week until CR, plus one or two consolidating injections [27.Flinn I.W. Kopecky K.J. Foucar M.K. et al.Long-term follow-up of remission duration, mortality and second malignancies in hairy cell leukemia patients treated with pentostatin.Blood. 2000; 96: 2981-2986PubMed Google Scholar]. After 8–9 courses, the full blood count usually normalises, and the bone marrow biopsy should be performed to confirm a CR [III, B]. If a CR is documented, one or two further DCF injections are indicated [16.Else M. Dearden C.E. Matutes E. et al.Long-term follow-up of 233 patients with hairy cell leukaemia, treated initially with pentostatin or cladribine, at a median of 16 years from diagnosis.Br J Haematol. 2009; 145: 733-740Crossref PubMed Scopus (205) Google Scholar]. DCF and 2-CldA appear to induce similar high response rates, duration of response, recurrence rates and adverse events [III, B] [16.Else M. Dearden C.E. Matutes E. et al.Long-term follow-up of 233 patients with hairy cell leukaemia, treated initially with pentostatin or cladribine, at a median of 16 years from diagnosis.Br J Haematol. 2009; 145: 733-740Crossref PubMed Scopus (205) Google Scholar]. However, no randomised, direct comparison between the two drugs has been performed. The advantage of DCF over interferon-α (IFN-α) in HCL patients has been confirmed in a multicentre, randomised trial [I, A] [28.Grever M. Kopecky K. Foucar M.K. et al.Randomized comparison of pentostatin versus interferon alfa-2a in previously untreated patients with hairy cell leukemia: an intergroup study.J Clin Oncol. 1995; 13: 974-982Crossref PubMed Scopus (249) Google Scholar]. 2-CldA administration is more convenient than DCF and is used more frequently. As purine analogues produce higher and more durable remissions, and are more convenient to patients, the use of IFN-α in the treatment of HCL is limited. However, IFN-α may still have a place in the treatment of HCL in pregnancy [V, B]. It can also be used in patients presenting with very severe neutropaenia (neutrophil count <0.2 × 109/l) to increase the neutrophil count prior to nucleoside analogue therapy [V, C] [29.Habermann T.M. Rai K. Historical treatments of in hairy cell leukemia, splenectomy and interferon: past and current uses.Leuk Lymphoma. 2011; 52: 18-20Crossref Scopus (22) Google Scholar]. Responses are defined according to the ‘Consensus Resolution’ criteria (Table 3) [30.Anonymous Consensus resolution: proposed criteria for evaluation of response to treatment in hairy cell leukemia.Leukemia. 1987; 1: 405Google Scholar]. Response evaluation includes careful physical examination and a blood cell count. A marrow biopsy is recommended to establish a CR. A chest X-ray and an abdominal ultrasound or CT for response evaluation should be performed. A CR requires the morphological absence of hairy cells in peripheral blood and bone marrow aspiration or biopsy specimens, and normalisation of any organomegaly and cytopaenia. Immunophenotypic analysis of peripheral blood or bone marrow is not required but is useful to detect MRD. A PR is defined as normalisation of peripheral counts, associated with at least a 50% reduction in organomegaly and bone marrow hairy cells, and <5% circulating hairy cells. All other outcomes are considered as nonresponse. The eradication of MRD is generally not recommended in routine clinical practice. Assessment of response should be performed 4–6 months after treatment with 2-CldA and after 8–9 courses of DCF [31.Jones G. Parry-Jones N. Wilkins B. et al.Revised guidelines for the diagnosis and management of hairy cell leukaemia and hairy cell leukaemia variant.Br J Haematol. 2012; 156: 186-195Crossref PubMed Scopus (69) Google Scholar]. Relapse is defined as any deterioration in blood counts related to the detection of hairy cells in peripheral blood and/or bone marrow.Table 3Response criteria for HCLDefinition of response categoriesComplete responseNo hairy cells on peripheral blood and bone marrow aspiration or biopsy specimens, normalisation of organomegaly and peripheral blood countsPartial responseNormalisation of peripheral blood counts, at least 50% reduction in organomegaly and bone marrow hairy cells, and <5% circulating hairy cellsRelapseAny deterioration in blood counts related to the detection of hairy cells in peripheral blood and/or bone marrow and/or increasing splenomegalyHCL, hairy cell leukaemia. Data from [30.Anonymous Consensus resolution: proposed criteria for evaluation of response to treatment in hairy cell leukemia.Leukemia. 1987; 1: 405Google Scholar]. Open table in a new tab HCL, hairy cell leukaemia. Data from [30.Anonymous Consensus resolution: proposed criteria for evaluation of response to treatment in hairy cell leukemia.Leukemia. 1987; 1: 405Google Scholar]. Relapsed patients can be successfully retreated with 2-CldA or DCF if relapse occurs after 12–18 months [IV, B] [32.Naik R.R. Saven A. My treatment approach to hairy cell leukemia.Mayo Clin Proc. 2012; 87: 67-76Abstract Full Text Full Text PDF PubMed Scopus (40) Google Scholar] (Figure 2). The alternative nucleoside analogue can be used in early relapse within 2 years after the first-line treatment [31.Jones G. Parry-Jones N. Wilkins B. et al.Revised guidelines for the diagnosis and management of hairy cell leukaemia and hairy cell leukaemia variant.Br J Haematol. 2012; 156: 186-195Crossref PubMed Scopus (69) Google Scholar]. The ability to attain CR decreases with each course of therapy, but CR duration appears to be similar after first-, second- or third-line therapy [16.Else M. Dearden C.E. Matutes E. et al.Long-term follow-up of 233 patients with hairy cell leukaemia, treated initially with pentostatin or cladribine, at a median of 16 years from diagnosis.Br J Haematol. 2009; 145: 733-740Crossref PubMed Scopus (205) Google Scholar, 33.Zinzani P.L. Pellegrini C. Stefoni V. et al.Hairy cell leukemia: evaluation of the long-term outcome in 121 patients.Cancer. 2010; 116: 4788-4792Crossref PubMed Scopus (51) Google Scholar]. Rituximab at a dose of 375 mg/m2 for 4–8 doses given weekly as i.v. infusions can be used in early relapsed patients [III, B] [34.Hagberg H. Lundholm L. Rituximab, a chimaeric anti-CD20 monoclonal antibody in the treatment of hairy cell leukaemia.Br J Haematol. 2001; 115: 609-611Crossref PubMed Scopus (101) Google Scholar, 35.Nieva J. Bethel K. Saven A. Phase 2 study of rituximab in the treatment of cladribine-failed patients with hairy cell leukemia.Blood. 2003; 102: 810-813Crossref PubMed Scopus (123) Google Scholar, 36.Forconi F. Toraldo F. Sozzi E. et al.Complete molecular remission induced by concomitant cladribine--rituximab treatment in a case of multi-resistant hairy cell leukemia.Leuk Lymphoma. 2007; 48: 2441-2443Crossref PubMed Scopus (14) Google Scholar]. However, rituximab alone is inferior to purine analogues and is not the treatment of choice as a single agent in relapsed patients. Outcomes for patients with recurrent HCL appear to be better when a combination of rituximab and 2-CldA or DCF is used rather than the purine analogue alone [III, B] [37.Else M. Dearden C.E. Matutes E. et al.Rituximab with pentostatin or cladribine: an effective combination treatment for hairy cell leukemia after disease recurrence.Leuk Lymphoma. 2011; 52: 75-78Crossref PubMed Scopus (42) Google Scholar, 38.Ravandi F. O'Brien S. Jorgensen J. et al.Phase 2 study of cladribine followed by rituximab in patients with hairy cell leukemia.Blood. 2011; 118: 3818-3823Crossref PubMed Scopus (84) Google Scholar]. Concurrent therapy of rituximab and a purine analogue induces higher response rates, and higher rates of toxic events than in the sequential administration [III, B] [39.Else M. Osuji N. Forconi F. et al.The role of rituximab in combination with pentostatin or cladribine for the treatment of recurrent/refractory hairy cell leukemia.Cancer. 2007; 110: 2240-2247Crossref PubMed Scopus (46) Google Scholar]. IFN-α is also a possible option for selected patients relapsing after purine analogue therapy [IV, B] [40.Seymour J.F. Estey E.H. Keating M.J. Kurzrock R. Response to interferon-alpha in patients with hairy cell leukemia relapsing after treatment with 2-chlorodeoxyadenosine.Leukemia. 1995; 9: 929-932PubMed Google Scholar, 41.Hoffman M.A. Interferon-alpha is a very effective salvage therapy for patients with hairy cell leukemia relapsing after cladribine: a report of three cases.Med Oncol. 2011; 28: 1537-1541Crossref Scopus (9) Google Scholar]. Patients refractory to purine analogue therapy should be enrolled on clinical trials that use new agents, whenever possible. Fludarabine at a dose of 40 mg/m2 oral (p.o.) for five consecutive days in combination with an i.v. injection of 375 mg/m2 rituximab on day 1, every 28 days for four cycles, can be a therapeutic option in relapsed or refractory patients previously treated with 2-CldA [IV, B]. After a median follow-up of 35 months with 14 progression-free patients, a 5-year progression-free survival (PFS) of 89%, an overall survival (OS) of 83% and a recurrence rate of 7% were observed in 15 treated patients [42.Gerrie A.S. Zypchen L.N. Connors J.M. Fludarabine and rituximab for relapsed or refractory hairy cell leukemia.Blood. 2012; 119: 1988-1991Crossref Scopus (31) Google Scholar]. Bendamustine at 70–90 mg/m2 combined with rituximab is another therapeutic option in multiply relapsed/refractory HCL, and could be considered in HCL patients after the failure of standard therapies [IV, B] [43.Burotto M. Stetler-Stevenson M. Arons E. et al.Bendamustine and rituximab in relapsed and refractory hairy cell leukemia.Clin Cancer Res. 2013; 19: 6313-6321Crossref PubMed Scopus (58) Google Scholar]. Other promising drugs active in purine analogue refractory HCL patients include moxetumomab pasudotox, an anti-CD22 recombinant immunotoxin, and vemurafenib, a BRAF V600E inhibitor [44.Kreitman R.J. Tallman M.S. Robak T. et al.Phase I trial of anti-CD22 recombinant immunotoxin moxetumomab pasudotox (CAT-8015 or HA22) in patients with hairy cell leukemia.J Clin Oncol. 2012; 30: 1822-1828Crossref PubMed Scopus (251) Google Scholar, 45.Tiacci E. De Carolis L. Zinzani P.L. et al.Vemurafenib is safe and highly active in hairy cell leukemia patients refractory to or relapsed after purine analogs: a phase-2 Italian clinical trial.Haematologica. 2014; 99 (Abstract S696)Google Scholar, 46.Samuel J. Macip S. Dyer M.J. Efficacy of vemurafenib in hairy-cell leukemia.N Engl J Med. 2014; 370: 286-288Crossref PubMed Scopus (45) Google Scholar]. A phase I trial of moxetumomab pasudotox in relapsed/refractory HCL induced an 86% OR rate, and a 46% CR [44.Kreitman R.J. Tallman M.S. Robak T. et al.Phase I trial of anti-CD22 recombinant immunotoxin moxetumomab pasudotox (CAT-8015 or HA22) in patients with hairy cell leukemia.J Clin Oncol. 2012; 30: 1822-1828Crossref PubMed Scopus (251) Google Scholar]. However, moxetumomab pasudotox is not licensed in Europe yet. Vemurafenib also showed remarkable activity in multiply relapsed and refractory HCL patients with rapidly decreased splenomegaly, increased platelet counts and normalisat