Circulating tumor cells (CTC) are an independent prognostic factor in metastatic breast cancer patients (MBC). However, CTC are undetectable in one third of patients. The aim of this study was to assess the prognostic factors in MBC patients without detectable CTC. This retrospective study included 292 MBC patients evaluated between January 2004 and December 2007. CTC were enumerated before patients started a new line of treatment using the CellSearch™. Overall survival (OS) was calculated from the date of CTC measurement and estimated by the Kaplan-Meier product limit method. CTC were not detected in 35.96% patients, whereas 40.75% patients had CTC ≥ 5. Undetectable CTC status was positively correlated with presence of brain metastasis (OR: 6.17, 95%CI = 2.14-17.79; p = 0.001), and inversely correlated with bone metastasis (OR: 0.47; 95%CI = 0.27-0.80; p = 0.01). In multivariate analysis, hormone receptors, number of metastatic sites and lines of therapy were independent prognostic factors for OS in patients without detectable CTC. Patients without detectable CTC before starting of a new line of therapy comprise a heterogeneous group with substantially different prognosis. We showed that some important metastatic disease characteristics are predictive of undetectable CTC status in MBC.
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