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comparison of survival rates between two histological subgroups in

these studies.

These study results indicate that CTCs possess distinct bio-

logical characteristics in esophageal adenocarcinomas when

compared to esophageal squamous cell carcinoma. In addition,

the CTCs potentially act as a better biomarker for this carcinoma

for predicting cancer stages, metastasis, and thereby prognosis

and treatment.

The expanding investigation into circulating tumor cells has not

been restricted to detection and quantification. In vivo models

have also been utilized to investigate the biological functions of

isolated proliferative CTCs [ 11 ]. Following the isolation of

CTCs from murine blood, the cells were cultured and then

implanted into other mice. After the tumor growth in the

injected mouse mode, additional CTCs were extracted from the

resultant mouse model. Subsequent implanting and culturing in

another mouse highlighted the stem cell-like properties of CTCs.

Results illustrated heightened proliferative characteristics in the

isolated and cultured CTCs, represented by increased colony

formation when compared to the parental cultured cells.

Furthermore, the xenografts generated using the sequestered

CTCs metastasized faster than the original cell line, resulting in

larger and heavier metastases.

The most characterized method for CTC enrichment based on

physical classification is through density gradient centrifugation

using Ficoll solution (a neutral, highly branched, high-mass,

hydrophilic polysaccharide which dissolves readily in aqueous solu-

tions). The theory behind this cell separation method relies on the

differential centrifugal migration according to cellular size and

density resulting in the cellular fractionation. Exposing whole

blood samples to centrifugation results in the fractionation of three

distinct layers: top layer consisting of plasma, followed by nucle-

ated cells comprising white blood cells, platelets, and CTCs fol-

lowed by red blood cells on the bottom layer. This method of CTC

enrichment has been utilized in conjunction with hemolysis since

as early as 1917 to isolate circulating tumor cells from blood sam-

ples [ 21 ]. OncoQuick utilizes a density gradient-based method of

enrichment whereby a porous membrane prevents cross-

contamination of cellular fractions without reducing the rate of

CTC recovery [ 22 ].

Additionally, by means of physical separation, the ISET

(Isolation by size of Epithelial Tumor Cells) utilizes an automated

filter-based method [ 14 ]. The mechanism of the method is based

on the assumption that CTCs are identified as having a larger

diameter (30 μm in breast cancer cells) when compared with the

majority of hematopoietic cells that are characteristically smaller

(8–11 μm) [ 23 ]. Based on these physical differences, when the

1.4 CTC Enrichment
and Isolation

Circulating Tumour Cells