Clinical Significance of Circulating Tumor Cells

The first discovery of circulating tumor cells (CTCs) is attributed to Ashworth in 1869, who noted the presence of cancer cells in the blood. Generally it has been difficult to measure CTCs accurately because they exist in a low frequency and separation from other cell types (red blood cells, normal epithelial cells and leukocytes) has limited sensitivity. The approaches for their detection can be classified into PCR-based and cytometric methods. PCR-based methods have, until recently, been the most widely employed methods and PCR targets must express tumor-specific DNA or mRNA sequences. For example, Schoenfeld et al. used reverse transcriptase (RT)-PCR for keratin-19 combined with immunohistochemistry to detect cancer cells in blood and micrometastases in bone marrow from patients with primary breast cancer. The cytometric methods enable morphological identification of CTCs but require cell enrichment to be feasible, using filters with pores or immunomagnetic cell enrichment [1]. The arena for CTC detection was changed with the introduction of the CellSearch System™, a US FDA-approved prognostic test that identifies CTCs in patients with metastatic breast cancer (MBC) [2]. Here, it appears to be an additional tool to the other prognostic and predictive peripheral blood assays, although advantages over simple tumor markers such as CA15-3 have not been studied. One pragmatic advantage is that blood samples can be shipped at room temperature to a facility with a machine, and CTC counts are stable for at least 72 h [3].

Metastatic breast cancer & CTCs

The detection of CTCs using this system appears to predict overall survival in patients with metastatic breast cancer, and five CTCs/7.5 ml of blood appears to have prognostic value [4–7]. Cristofanilli reported that the detection of five or more CTCs/7.5 ml of blood at baseline and at first follow up in 177 patients with MBC was associated with poor clinical outcome and the detection of elevated CTCs at any time during therapy is an accurate indication of subsequent rapid disease progression and mortality for MBC patients [6]. In addition, another study found that their changes may occur before radiologic responses: 138 patients had imaging studies carried out before and a median of 10 weeks after the initiation of therapy. CTC counts were determined approximately 4 weeks after the initiation of therapy. The median overall survival of 13 patients (9%) with radiologic nonprogression and five or more CTCs was significantly shorter than that of the 83 patients (60%) with radiologic nonprogression and less than five CTCs. The median overall survival of the 20 patients (14%) with radiologic progression and less than five CTCs was significantly longer than the 22 patients (16%) with five or more CTCs that showed progression with radiology [8].

Early breast cancer & CTCs

Using these methods, the detection of CTCs in patients with early breast cancer has been less conclusive. At Imperial College we have found that CTCs can be detected in the blood of high- and also low-risk patients during routine follow up, with healthy controls having no CTCs. The only two patients with disseminated tumor cells in the bone marrow (detected by RT-PCR) and CTCs in the blood (detected using the CellSearch system) relapsed with metastatic disease, and a challenge will be to determine the time interval from the appearance of CTCs to overt metastases (with subsequent treatment strategies based in that time window).

At the 14th European Cancer Conference in Barcelona, Julia Jückstock, from the University of Munich, presented follow up on a German breast cancer study that has shown that the detection of CTCs in peripheral blood from early breast cancer patients is possible. Here, the SUCCESS trial aims to evaluate minimal residual disease in peripheral blood from samples taken at four time points during and after the adjuvant systemic therapy (docetaxel and gemcitabine) of more than 3000 women with breast cancer. The study is important because it is one of the largest of its kind to investigate whether measuring such tumor cells can help predict the chances of cancer returning; although, the blood here is concentrated by Ficolling to save money on reagents that analyze the presence of CTCs. Based on samples taken from 1767 breast cancer patients on primary diagnosis and samples from 852 of the same patients after they had completed adjuvant chemotherapy, 10% of patients whose blood was sampled before the start of treatment had more than one CTC, and 5% of these patients had more than two CTCs in approximately 20 ml of blood. Of the patients who were initially CTC positive, only 10% remained so after chemotherapy, while the rest became negative. Of the patients who were CTC negative at diagnosis, 93% remained negative, while the remaining 7% showed CTCs in their blood after chemotherapy. Among the 852 patients who were sampled after completing chemotherapy, 7% had more than one CTC remaining in the blood after chemotherapy and the significance of one CTC requires particular evaluation. Whereas five CTCs for metastatic disease is now quoted, the relevance of one CTC during follow up of early breast cancer requires extensive clarification. The persistence of CTCs after chemotherapy treatment may be predictive of the likelihood of recurrence of cancer in these patients, but the full results from this trial should be available within 3 years.

Conclusion

The detection and number of CTCs has been proposed as a method to assess response to treatment of metastatic breast and other types of cancers, and may have clinical utility in risk stratification in early breast cancer, early detection of relapse and monitoring the response to treatment. Their significance in benign breast disease requires clarification, but overall, CTCs are likely to be a useful tool in both prognosis and prediction. However, their value over other markers requires confirmation in prospective studies [9]. CTCs are, however, likely to be useful in studying the biology of breast cancer in a single peripheral blood test. They can be routinely stained for MUC-1 and members of the EGF receptor family (EGF receptor and HER2), and thus CTC based therapeutics is a possibility. PCR of genetic material derived from CTCs and comparison with the primary tumor and metastatic lesions will hopefully answer questions concerning the identity of these cells, and whether they represent clones en route to metastases.

Financial & competing interests disclosure

The authors have no relevant affiliations or financial involvement with any organization or entity with a financial interest in or financial conflict with the subject matter or materials discussed in the manuscript. This includes employment, consultancies, honoraria, stock ownership or options, expert testimony, grants or patents received or pending, or royalties.

No writing assistance was utilized in the production of this manuscript.