📋 Key Information Summary
- Rare, indolent B-cell neoplasm (≈2% of lymphoid leukaemias) defined by the BRAF V600E mutation in >90% of classical cases.
- Classic presentation: pancytopenia, splenomegaly (often massive), and circulating 'hairy' lymphoid cells with cytoplasmic projections.
- Diagnosis requires immunophenotyping: CD19+, CD20+, CD22+, CD25+, CD103+, CD123+, Annexin A1+; and may show TRAP positivity.
- First-line treatment for symptomatic or cytopenic disease: purine analogue cladribine (2-CdA). Single-course achieves durable complete remission in >80%.
- Pentostatin is an alternative purine analogue; choice depends on comorbidities and local protocols.
- Anti-CD20 monoclonal antibody rituximab is used as consolidation for residual disease, or in relapsed/refractory settings.
- BRAF inhibitors (vemurafenib, dabrafenib ± trametinib) are effective in multiply relapsed/refractory BRAF V600E-mutated HCL.
- Asymptomatic patients with minimal cytopenias may be observed with regular monitoring.
- Major risk is profound, prolonged immunosuppression post-purine analogue therapy; prophylactic antimicrobials and live vaccine avoidance are essential.
- Median age at diagnosis ~55 years; male:female ratio ≈4:1. No established screening; diagnosed via blood film and bone marrow biopsy.
- ATSI considerations: address geographic barriers to specialist care, ensure culturally safe communication, and consider higher risk of infections in remote communities.
Introduction & Australian Epidemiology
Hairy cell leukaemia (HCL) is a rare, indolent B-cell lymphoproliferative neoplasm accounting for approximately 2% of all leukaemias. It is characterised by the accumulation of small B-lymphocytes with circumferential cytoplasmic projections ('hairy' cells) in the peripheral blood, bone marrow, and spleen. The disease has a distinctive molecular signature, with the BRAF V600E mutation present in the vast majority of classical cases.
In Australia, the age-standardised incidence is estimated at 0.3–0.4 per 100,000 population per year. The disease predominantly affects middle-aged to older adults, with a median age at diagnosis of approximately 55 years. There is a marked male predominance, with a male-to-female ratio of about 4:1. While rare, its high responsiveness to specific therapies makes accurate diagnosis and management crucial. The majority of patients present with symptoms related to cytopenias (fatigue, infection, bleeding) or splenomegaly.
Pathogenesis (BRAF V600E Mutation)
The discovery of the BRAF V600E mutation has revolutionised the understanding of HCL pathogenesis and provided a targeted therapeutic avenue.
Central driver: The somatic BRAF V600E mutation, leading to constitutive activation of the MAPK/ERK signalling pathway, is present in >90% of classical HCL cases. This mutation is considered the initiating event.
The mutation promotes cell survival, proliferation, and the characteristic morphology and immunophenotype of the hairy cell. It is highly specific for classical HCL and is not found in HCL-variant (HCL-v) or most other B-cell lymphomas, making it a key diagnostic marker.
The downstream signalling results in expression of specific markers (CD25, CD123) and cytokine production contributing to the fibrotic marrow environment and pancytopenia. Understanding this pathway underpins the use of BRAF inhibitors in relapsed disease.
Clinical Features & Blood Film
Presentation is often insidious. Symptoms are primarily due to bone marrow failure and splenomegaly.
Typical Clinical Features
- Constitutional: Fatigue, weakness, weight loss, night sweats (less common than in aggressive lymphomas).
- Due to pancytopenia: Recurrent infections (neutropenia), bleeding/bruising (thrombocytopenia), pallor (anaemia).
- Splenomegaly: Present in >80% of cases, often massive and causing left upper quadrant discomfort or early satiety. Hepatomegaly is less common.
- Autoimmune complications: Vasculitis, polyarthritis, and immune-mediated haemolytic anaemia may occur.
Peripheral Blood Film Findings
The blood film is a critical first clue to diagnosis.
- Pancytopenia: Typical, though some patients may have only one or two lineages affected.
- 'Hairy' cells: Mononuclear cells with abundant cytoplasm and irregular, 'hairy' projections. They are often present in low numbers (may be <10% of circulating leucocytes).
- Monocytopenia: An almost universal and characteristic feature, helping to distinguish HCL from other low-grade lymphomas.
Diagnosis (TRAP Stain, Immunophenotyping)
Diagnosis is made by a combination of morphology, cytochemistry, immunophenotyping, and molecular testing. Bone marrow biopsy is usually required.
Key Diagnostic Investigations
Essential
Bone marrow aspirate & trephine biopsy
Aspirate often 'dry tap' due to marrow fibrosis. Trephine shows diffuse infiltration by cells with abundant cytoplasm and oval nuclei, creating a 'fried egg' or 'honeycomb' pattern. Reticulin fibrosis is characteristic.
Essential
Immunophenotyping by Flow Cytometry
The cornerstone of diagnosis. Classic HCL phenotype: CD19+, CD20+ (bright), CD22+, CD79a+, CD11c+ (bright), CD25+, CD103+, CD123+. Annexin A1 positivity by immunohistochemistry is highly specific.
Available
TRAP (Tartrate-Resistant Acid Phosphatase) Cytochemistry
Historically used; positivity is seen in HCL cells but is now largely superseded by immunophenotyping. It is not required if flow cytometry is diagnostic.
Essential
BRAF V600E Mutation Analysis
Can be performed on peripheral blood or bone marrow. Detects the pathognomonic mutation in >90% of classical HCL. Useful for diagnosis and as a therapeutic target. Available via PCR or next-generation sequencing in major centres.
Differential diagnosis: HCL-variant (HCL-v, lacking CD25, BRAF V600E negative), splenic marginal zone lymphoma, and other small B-cell leukaemias must be excluded via comprehensive immunophenotyping and molecular studies.
Risk Stratification / Severity Scoring
HCL is generally indolent, but stratification guides treatment timing and choice.
Low Tumour Burden
Asymptomatic / Minimal Cytopenias
No significant splenomegaly, Hb >100 g/L, platelets >100 x 10⁹/L, neutrophils >1.0 x 10⁹/L.
Management: Active observation with 3-6 monthly clinical and blood count review.
Moderate Tumour Burden
Symptomatic / Cytopenias
Symptomatic splenomegaly, recurrent infections, Hb <100 g/L, platelets <100 x 10⁹/L, neutrophils <1.0 x 10⁹/L.
Management: Initiation of first-line therapy (cladribine).
High Risk
Relapsed/Refractory Disease
Early relapse (<24 months), failure to achieve CR with first-line purine analogue, or multiply relapsed disease.
Management: Re-treatment with purine analogue ± rituximab, or switch to BRAF inhibitor-based therapy.
Management (Cladribine, Rituximab)
First-Line Therapy: Purine Analogues
Treatment is indicated for symptomatic disease or significant cytopenias. Purine analogues are the standard of care.
Cladribine (2-Chlorodeoxyadenosine)
Leustatin® · Purine analogue
Adult dose
0.1 mg/kg/day continuous IV infusion for 7 days, OR 0.14 mg/kg/day SC/IV bolus for 5 days.
Key Monitoring
FBC weekly for 4-6 weeks. Monitor for profound, prolonged neutropenia and risk of opportunistic infections.
PBS status
✔ PBS Restricted Benefit
Pentostatin (2'-Deoxycoformycin)
Nipent® · Purine analogue
Adult dose
4 mg/m² IV every 2 weeks for 3-6 months until maximum response + 2 further cycles.
Key Monitoring
Renal function (dose adjust for CrCl <60 mL/min). Neurotoxicity and renal toxicity possible.
PBS status
✔ PBS Restricted Benefit
Critical Safety: Purine analogues cause profound and prolonged (6-12 months) T-cell depletion. All patients require:
- Pneumocystis jirovecii prophylaxis (e.g., trimethoprim/sulfamethoxazole 3 times/week) for at least 3-6 months post-therapy.
- Antiviral prophylaxis (e.g., valaciclovir) for at least 3 months.
- Avoidance of live vaccines indefinitely.
Consolidation & Relapsed/Refractory Therapy
Rituximab plays a key role in managing residual disease and relapse.
Rituximab
MabThera® · Anti-CD20 monoclonal antibody
Adult dose
375 mg/m² IV weekly for 4-8 doses (consolidation), or as part of combination regimens for relapse.
PBS status
✔ PBS Authority Required
Vemurafenib ± Rituximab
Zelboraf® · BRAF inhibitor
Adult dose
960 mg PO BD. Duration variable (typically 3-6 months). Often combined with rituximab.
PBS status
✔ PBS Authority Required (Specialist)
Monitoring
Response assessment and long-term follow-up are critical.
- Response Assessment: Blood counts at 3-6 months post-therapy. Bone marrow biopsy and imaging (CT for splenomegaly) are performed if blood counts do not normalise, to assess for complete remission (CR).
- Definition of CR: Absence of hairy cells in blood/bone marrow, resolution of splenomegaly, and recovery of counts (Hb >110 g/L, platelets >100 x 10⁹/L, neutrophils >1.5 x 10⁹/L) for at least 4 weeks.
- Long-term Follow-up: Clinical review and FBC every 3-6 months for the first 2 years, then annually. Monitor for late relapse (can occur >10 years post-treatment) and long-term complications (secondary malignancies, infections).
Special Populations
Pregnancy
HCL is rare in women of childbearing age. Pregnancy is not recommended during active treatment.
If asymptomatic, observation through pregnancy is preferred. For urgent treatment, discuss with haematology and obstetric teams; interferon-alpha has been used in this setting.
Renal Impairment
Cladribine: No dose adjustment required, but use with caution in severe renal impairment.
Pentostatin: Contraindicated if CrCl <60 mL/min due to increased nephrotoxicity.
Elderly / Comorbid
HCL often presents in older adults. Purine analogues remain effective but toxicity risks are higher.
Consider subcutaneous cladribine (shorter infusion) and meticulous supportive care. Rituximab monotherapy may be considered in frail patients.
Immunocompromised
The disease itself and its treatment cause severe immunosuppression.
Strict adherence to antimicrobial prophylaxis is mandatory. Monitor for opportunistic infections (PCP, HSV, VZV, fungal).
Aboriginal and Torres Strait Islander Health Considerations
Aboriginal and Torres Strait Islander Health
📚 References
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