Hairy Cell Leukemia (HCL)
Hairy cell leukemia is a rare, slow-growing cancer of B-cells, characterized by hair-like cytoplasmic projections seen on peripheral blood smear. Hairy cell leukemia most frequently occurs in older men. It is commonly associated with an activating BRAF mutation, and the enzyme TRAP is upregulated in the malignant or hairy B-cells. Clinically, hairy cell leukemia commonly presents with typical leukemic symptoms, including pancytopenia due to bone-marrow invasion of malignant B-cells. Repeated bone marrow damage can cause bone marrow fibrosis, which manifests clinically as dry bone marrow taps. Splenic infiltration of malignant B-cells can also result in splenomegaly. The purine analog cladribine is the treatment of choice.