Ibrutinib in Treating Patients With Relapsed Hairy Cell Leukemia

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 * Enlarging lymphadenopathy \>= 2 cm * Absolute lymphocyte count \> 5,000/mL * 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 with classic hairy cell leukemia 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 * Because there is no recognized standard of care for patients with variant hairy cell leukemia, both previously treated and previously untreated patients with this diagnosis will be eligible * Age ≥ 18 years. Because no dosing or adverse event data are currently available on the use of ibrutinib in patients \< 18 years of age, children are exclud…