Rituximab+ Zanubrutinib for Patients With Hairy Cell Leukemia and Hairy Cell Variant

Inclusion Criteria:• Histologically confirmed diagnosis of hairy cell leukemia or variant according to World Health Organization (WHO) criteria with any of the following indications for therapy: * Hemoglobin \< 11 g/dL, Platelet count \< 100,000/mL, Absolute neutrophil count \< 1,000/mL * Progressive or symptomatic splenomegaly or hepatomegaly or Enlarging lymphadenopathy \>= 2 cm * Disease related constitutional symptoms consisting of unexplained weight loss exceeding 10% of body weight over the preceding 6 months, Cancer Therapy Evaluation Program (CTEP) active version of the Common Terminology Criteria for Adverse Events (CTCAE) grade 2 or 3 fatigue, fevers \> 100.5 degrees Fahrenheit (F) or night sweats for greater than 2 weeks without evidence of infection • Patients may receive therapy under the following conditions: * After at least 1 prior purine nucleoside analog-containing regimen (fludarabine, pentostatin, or cladribine), or * Relapsed or de novo disease if deemed medically unfit for therapy with a purine nucleoside analog * Eastern Cooperative Oncology Group (ECOG) performance status =\< 2 (Karnofsky \>= 60%) * Creatinine =\< 2.0 mg/dL, and/or creatinine clearance (estimated glomerular filtration rate \[GFR\] \[Cockcroft-Gault\]) \>= 30 mL/min (Annex 1) * Total bilirubin =\< 1.5 x upper limit of normal (ULN) (unless disease related or due to Gilbert's disease) or Aspartate aminotransferase (AST) =\< 3.0 x ULN (unless disease related) * The effects of …