AbstractCirculating tumour cell (CTC) clusters have been proposed to be major players in the metastatic spread of breast cancer, particularly during advanced disease stages. Yet, it is unclear whether or not they manifest in early breast cancer, as their occurrence in patients with metastasis-free primary disease has not been thoroughly evaluated. In this study, exploiting nanostructured titanium oxide-coated slides for shear-free CTC identification, we detect clustered CTCs in the curative setting of multiple patients with early breast cancer prior to surgical treatment, highlighting their presence already at early disease stages. These results spotlight an important aspect of metastasis biology and the possibility to intervene with anti-cluster therapeutics already during the early manifestation of breast cancer.