A 57-year-old woman presented with persistent fever, marked hypoproteinemia, and right breast nodules. Positron emission tomography-computed tomography scan revealed high-density nodules within the mammary gland. The breast biopsy revealed large highly atypical cells, which were confined to the blood vessels’ lumina (panels A-C; hematoxylin and eosin, 10× objective, total magnification ×100 [A]; 20× objective, total magnification ×200 [B]; 40× objective, total magnification ×400 [C]). The histological morphology was uncannily like breast carcinoma. Using CD34 immunostaining, we observed apparent delineation of the vascular lumina (panel D; 20× objective, total magnification ×200). Strong reactivity was evident with CD3 (panel E; 20× objective, total magnification ×200), CD2, CD7, CD56, and granzyme B, which suggested a T and cytotoxic phenotype. In contrast, the CD20, CD79a, PAX5, CD5, ALK, and cytokeratins were negative. The Epstein-Barr virus–encoded RNA in situ hybridization was diffusely positive (panel F; 40× objective, total magnification ×400). Immunohistochemistry was crucial for making the correct diagnosis. Although the negative cytokeratin signal aided in the diagnosis, the patient died within 2 weeks of her disease.

Intravascular natural killer/T-cell lymphoma (INK/TL) is an uncommon tumor. So far, most reports are of skin and/or central nervous system involvement, and one report of renal involvement. This report of breast INK/TL is the first known published report of its kind.