Flow Cytometric Detection of CK19-Positive Circulating Tumor Cells in Bone Marrow and Peripheral Blood in Metastatic Breast Cancer: A Case Report

Metastatic breast cancer represents the most advanced stage, where the accurate and rapid identification of neoplastic cells is essential for clinical management and prognostic assessment. Although immunohistochemistry is the gold standard, flow cytometry emerges as a robust and sensitive complementary diagnostic method for detecting circulating tumor cells. We report a breast cancer patient with bone marrow metastasis and demonstrate the utility of a novel flow cytometry protocol for tumor cell detection. A 39-year-old woman, with no family history of cancer, identified a right breast nodule in January 2025. Imaging studies and biopsy with immunohistochemistry confirmed invasive lobular carcinoma (ER+, PR+, HER2−). Bone scintigraphy revealed multiple metastatic lesions. Bone marrow aspiration showed infiltration by tumor cells, with flow cytometry identifying 4.0% of non-hematologic (CD45−) large cells expressing epithelial markers (Ber-EP4+, Pan-CK+++) and breast tumor markers (GCDFP-15+, CK-19+++). Peripheral blood analysis using the Bulk Lysis protocol detected 2.5% circulating tumor cells. This case highlights that standardized flow cytometry protocols provide a rapid and, reliable and non-invasive approach for detecting tumor cells in different samples. The integration of this technique into clinical practice can significantly enhance disease monitoring and early detection of metastasis in breast cancer patients.