A Time-Limited Approach to Treatment With Ibrutinib for Chronic Lymphocytic Leukemia and Small Lymphocytic Lymphoma

Inclusion Criteria: * ≥ 18-years-old * Diagnosis of CLL or SLL confirmed by the enrolling institution * Negative del(17p) test confirmed by FISH testing prior to initiating treatment with ibrutinib * Receiving commercial treatment with an ibrutinib-based regimen in the front-line setting as defined below with the intent to discontinue therapy on C1D1: 1. Ibrutinib monotherapy 2. Ibrutinib in combination with anti-CD20 monoclonal antibody (Patients must have completed the anti-CD20 monoclonal antibody portion of the regimen prior to signing consent) * Patients must have received ibrutinib-based therapy for at least 15 months. Dose interruptions and reductions during this treatment period may have been carried out per treating provider discretion. * Response evaluation performed by radiology assessment (CT or MRI imaging of neck / chest / abdomen / pelvis) confirming complete remission or partial remission by iwCLL criteria. * Peripheral blood (by flow cytometry per institutional standards) testing for MRD confirming U-MRD to a sensitivity of 10\^-4. MRD testing must be confirmed to follow ERIC consensus criteria. 1. CR with or without U-MRD in peripheral blood 2. PR with U-MRD in peripheral blood Exclusion Criteria: * Any life-threatening illness, medical condition, or organ system dysfunction that, in the investigator's opinion, could compromise the subject's safety or put the study outcomes at undue risk.. * Unwilling or unable to participate in all required study evaluations and procedures. Unable to understand the purpose and risks of the study and to provide a signed and dated informed consent form (ICF) and authorization to use protected health information (in accordance with national and local patient privacy regulations).