A 58-year-old man underwent surgical resection of a bleeding pilonidal cyst ulcerating the anal skin (panel A). He had no other symptoms and no other skin lesions. Histology confirmed an inflamed pilonidal cyst. However, the skin was also diffusely infiltrated by small- to medium-sized cells with clear cytoplasm (panel B; original magnification ×40, hematoxylin and eosin [H&E] stain; panel C; original magnification ×400, H&E stain). The cells were positive for CD20 (panel D; original magnification ×200, immunohistochemical stains), BRAF VE1 (panel E; original magnification ×200, immunohistochemical stains), TRAP (panel F; original magnification ×200, immunohistochemical stains), and CD25 and were negative for CD3, CD138, and cytokeratins. The BRAF gene mutation analysis revealed a V600E mutation, so a diagnosis of hairy cell leukemia (HCL) localized to the skin was made. A bone marrow biopsy was performed showing diffuse involvement, confirmed by flow cytometry. The peripheral blood was also involved.

HCL is a rare chronic B-cell neoplasm, accounting for 2% to 3% of all leukemias. The direct infiltration of the skin is extremely rare in patients with HCL with only 7 other cases of HCL localized to the skin having been reported, all in patients with systemic disease. In 6 of these cases, skin involvement was present at diagnosis. Our case is also unique in that it was a single lesion in a pilonidal cyst.