A 65-year-old woman with a history of stage IIIA triple-negative breast carcinoma, status post mastectomy, treated with checkpoint inhibitor and chemotherapy, presented one year later with altered mental status, seizure, fever, and poor oral intake. Laboratory evaluation revealed hypercalcemia, anemia, thrombocytopenia, and leukocytosis (WBC 13.07 × 10³/µL). Peripheral blood smear demonstrated rare atypical cells with prominent nucleoli and vacuolated cytoplasm concerning for malignancy.

Bone marrow biopsy showed near-complete absence of hematopoietic tissue with ~20% of the marrow space replaced by reticulin and collagen. The infiltrating tumor was widely disseminated, undifferentiated, and immunohistochemically positive for AE1/AE3, strongly Ki-67 immunoreactive, and focally positive for GATA-3; PAS and mammaglobin were negative. Findings were diagnostic of metastatic carcinoma consistent with breast primary involving bone marrow. The patient elected palliative care with home health services.

Rare circulating tumor cells were also identified in the peripheral smear. While this confirms hematogenous dissemination, it is distinct from carcinocythemia, a rare entity defined by leukemic-phase spread of solid tumors, classically reported in small cell carcinoma of the lung and Merkel cell carcinoma.