In this paper, we examine the role of circulating tumor cells (CTCs) in breast cancer. CTCs are tumor cells present in the peripheral blood. They are found in many different carcinomas but are not present in patients with benign disease. Recent advances in theories regarding metastasis support the role of early release of tumor cells in the neoplastic process. Furthermore, it has been found that phenotypic variation exists between the primary tumor and CTCs. Of particular interest is the incongruency found between primary tumor and CTC HER2 status in both metastatic and early breast cancer. Overall, CTCs have been shown to be a poor prognostic marker in metastatic breast cancer. CTCs in early breast cancer are not as well studied, however, several studies suggest that the presence of CTCs in early breast cancer may also suggest a poorer prognosis. Studies are currently underway looking at the use of CTC level monitoring in order to guide changes in therapy. 1. Introduction Breast cancer is one of the most common cancers affecting women. It is estimated that one in eight women will develop an invasive breast cancer at some point during her lifetime. In 2010, according to the American Cancer Society, approximately 207, 090 new cases of invasive breast cancer will be diagnosed and 39, 840 women will die from metastatic disease. In this era of molecular medicine, novel approaches are needed in the management of breast cancer. In the last several decades, circulating tumor cells (CTCs) have emerged as a unique target for understanding disease progression, prognosis, and treatment in breast cancer pathogenesis. CTCs are tumor cells present in the peripheral blood. They are found in many different carcinomas but are not present in patients with benign disease [1]. Much of the research involving CTCs stems from studies involving disseminated tumor cells (DTCs). DTCs are tumor cells present in the bone marrow. Briefly, several studies have shown that patients with DTCs at the time of diagnosis have larger tumors, higher histologic grade, and a higher incidence of lymph-node metastasis, distance metastasis, and cancer-related death versus those patients without DTCs [2, 3]. Furthermore, detection of DTCs after systemic treatment is associated with increased risk of recurrence and decreased disease-free survival as well as decreased breast cancer-specific survival [4, 5]. Though DTCs have been more thoroughly studied, there are several studies that have documented a correlation between the occurrence of DTCs and CTCs in both primary and metastatic breast cancer
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