Techniques and Thresholds for Quantifying Circulating Tumor Cells in Breast Cancer

To the Editor We read with great interest the article by Sparano et al, who evaluated the association of the presence of circulating tumor cells (CTCs) in a peripheral blood sample obtained approximately 5 years after diagnosis with late clinical recurrence of operable human epidermal growth factor receptor 2–negative breast cancer (BC). The authors concluded that a positive CTC assay result 5 years after diagnosis of estrogen receptor–positive BC provided independent prognostic information for late clinical recurrence. This impressive finding raises 2 important issues.

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