## 1 Introduction  
Breast cancer is the leading cause of cancer deaths among women worldwide [1]. Breast cancer metastasis accounts for the majority of deaths from breast cancer. The detection of metastases at the earliest stage is important for the management and estimation of breast cancer progression [2]. Breast cancer treatments today are based on the absence or presence of the hormone estrogen receptor (ER), the progesterone receptor (PR) and the expression of human epidermal growth factor receptor 2 (HER2) [3, 4]. A tumor with the absence of all of these three receptors, also called triple negative breast cancer, is an aggressive form of breast cancer with a high risk of relapse and metastasis, and is associated with a poor clinical outcome [5, 6, 7].  
Epithelial cells may undergo epithelial-to-mesenchymal transition (EMT), allowing the cells to gain new abilities to cross the extracellular matrix, migrate and become circulating cells [8,9]. The presence of circulating tumor cells (CTC) in patients with metastatic breast cancer is indicative of poor prognosis [10]. The survival rate of women with breast cancer is highly dependent on the absence of metastatic cells and the tumor grade [2].  
The most common marker for CTCs that is used today is the epithelial cell adhesion molecule (EpCAM), but for some malignancies such as aggressive breast cancer metastases that are often of non-epithelial origin or have transitioned through EMT, the EpCAM detection method is not useful [11]. This highlights the need for more reliable CTC markers. The adhesion receptor CD44 and EpCAM function together in preparing the pre-metastatic niche [12], while CD45 is a leukocyte marker that is exclusively expressed on all nucleated cells of the hematopoietic system. The absence of CD45 on epithelial cells emphasizes the fact that it is an appropriate cell surface marker for distinguishing between CTCs and hematopoietic cells [13].  
CTCs are known to have morphological properties that distinguish them from other circulating cells, such as a larger size than leukocytes and different nuclear morphology [14]. CTCs have been isolated by size, using different methods such as density gradient centrifugation and micropore filtration [15, 16, 17, 18, 19]. A more cell-friendly and speedy method of investigating morphological differences would facilitate CTC research. Digital holographic cytometry (DHC) is a powerful tool for label-free cell observations and the evaluation of cell morphological and dynamical parameters in vitro [20, 21, 22, 23, 24, 25, 26]. Studies using DHC include different cell types, from protozoa, bacteria and plant cells to mammalian cells such as nerve cells, stem cells and tumor cells [23]. DHC has also become popular in the diagnostic field such as for the screening of malaria-infected red blood cells and cervical cancer [21, 27, 28, 29].  
The aim of the present study was to use DHC to investigate the morphological differences between a collection of breast cancer cell lines and white blood cell (WBC) cancer cell lines. The flow cytometry analysis of the cell markers EpCAM and CD45 was used to characterize the cell lines. We show that cell types lacking CD45 expression have significantly larger cell area, volume and thickness than CD45\({}^{+}\) cells without EpCAM expression. In conclusion, DHC is a powerful technique for discriminating cancer cells from WBCs by performing non-invasive measurements of cell area, volume and thickness.