A 42-year-old woman with hypothyroidism presented with new onset leukocytosis with a white blood cell count of 11.5 × 10⁹/L with 37% atypical lymphoid cells. Wright-Giemsa staining of the peripheral blood (PB) showed frequent small- to medium-sized atypical lymphoid cells with relative ample cytoplasm exhibiting circumferential cytoplasmic projections (panel A; objective 100× with total magnification [TM] ×1000). Flow cytometry of the PB showed the atypical cells (∼30% to 33%) were positive for CD10 (panel B), CD11c (panel C), CD19 (panel B), CD20, CD25 (panel C), CD103 (partial and dim) (panel D), and λ-restricted (panel E). A bone marrow biopsy showed extensive interstitial infiltrate with atypical lymphoid cells exhibiting “fried egg” appearance (panel F; objective 40× with TM ×400). By immunohistochemistry, these B cells were positive for BCL1, CD10, CD79a, DBA44, and TRAP (panel G; objective 40× with TM ×400) (rest not shown) but negative for annexin A1 (panel H; objective 40× with TM ×400). Next-generation sequencing of the PB showed BRAF V600E with a variant allele frequency of 24%. Ultrasound of the abdomen showed 20 cm heterogeneously enlarged spleen with multiple hypoechoic areas (not shown). The overall findings are consistent with hairy cell leukemia (HCL). The patient was treated with cladribine and achieved complete remission.

This HCL has several atypical features, namely partially and dimly positive for CD103 and positive for CD10 but negative for annexin A1. While ∼10% to 20% of HCLs are positive for CD10, lack of annexin A1 is extremely rare, and this is a rarely documented HCL with no annexin A1.