Table 2.

CTC classification schema

Type	Description
HD-CTC	Putative HD-CTCs are defined by having an intact nucleus, identified through DAPI; being of epithelial origin, characterized by a bright cytokeratin (CK) stain; no CD45 signal—a leukocyte marker; and a distinct morphology from the surrounding leukocytes. HD-CTCs must have a clear circumferential cytoplasm containing the entire nucleus. See Panel A in Fig. 3
CTC-small	CTC-small expresses CK at appropriate levels to be a HD-CTC, but has a similar (or smaller) nuclear size in relation to surrounding leukocytes. See Panel D in Fig. 3
CTC-low CK	CTC-low CK expresses an insufficient level of CK to consider a HD-CTC, but has a significantly larger nucleus compared to surrounding leukocytes. This group is potentially associated with cancer stem cells or cells undergoing an epithelial-to-mesenchymal transition. See Panel E in Fig. 3
CTC-cfDNA producing	Candidates with identifiable apoptotic changes, such as cytoplasmic blebbing and/or nuclear fragmentation. See Panel F in Fig. 3