**4.1. Breast cancer**

One meta‐analysis reported by Liao et al. [107], 14 studies with 2336 patients were enrolled and found that presence of CTCs in peripheral blood was significantly associated with the size of tumor [OR 0.68, 95% confidence interval (CI) (0.54, 0.87), *P* = 0.002], tumor grade [OR 0.71, 95% CI (0.55, 0.91), *P* = 0.006], estrogen receptor (ER) status [OR 0.72, 95% CI (0.57, 0.91), *P* = 0.007], and progesterone receptor (PR) of tumor status [OR 0.78, 95% CI (0.61, 0.98), *P* = 0.04]. In addition, the presence of CTCs is highly correlated with tumor size, tumor grade, ER, and PR status in patients with breast cancer. Although the analysis did not consider the method of isolation which might be one of the downsides and biases of the analysis, the results suggested a trend of physical (tumor size), functional (tumor grade) and status of drugable targets (ER, PR status), which are very useful clinically.

In Zhao et al. [234] performed a meta‐analysis collecting 24 trials with 4013 breast cancer patients and 1333 controls. Poor overall survival was found to be associated with the positive CTC detection (HR = 3.00 [95% CI 2.29–3.94], *P* < 0.0001) and recurrence‐free survival as well (HR = 2.67 [95% CI 2.09–3.42], *P* < 0.0001). CTC‐positive breast cancers were significantly associated with high histological grade (HR = 1.21 [95% CI 1.09–1.35], *P* < 0.0001), tumor size (>2 cm) (HR = 1.12 [95% CI 1.02–1.22], *P* = 0.01), and nodal status (≥1) (HR = 1.10 [95% CI 1.00– 1.21], *P* = 0.037). The studies, different to that of Liao et al. [107], mentioned about prognostic values of CTC testing. However, the two reports did not mention about the isolation methods and might neglect the biases from CTC number is highly correlated to the method of isolation.

For the purpose of technical standardization, Janni et al. [235] conducted a pooled analysis of individual data from 3173 patients with nonmetastatic (stages I–III) breast cancer from five breast cancer institutions. The prevalence and numbers of CTCs were assessed at the time of primary diagnosis with the FDA‐cleared CellSearch System. Results confirmed that ≥1 CTC(s) were detected in 20.2% of the patients and CTC‐positive patients had larger tumors, increased lymph node involvement, and a higher histologic tumor grade than did CTC‐negative patients (all *P* < 0.002). Multivariate Cox regressions confirmed that the presence of CTCs was an independent prognostic factor for disease‐free survival [HR, 1.82; 95% confidence interval (CI), 1.47–2.26], distant disease‐free survival (HR, 1.89; 95% CI, 1.49–2.40), breast cancer‐specific survival (HR, 2.04; 95% CI, 1.52–2.75), and overall survival (HR, 1.97; 95% CI, 1.51–2.59). The study addressed the clinical impacts of CellSearch™ system in breast cancer patients and it has confirmed the positive results from a large pooled database.

For a subset in breast cancers, Rack et al. [91] addressed the role of CTCs isolated by Cell‐ Search™ in a prospective trial enrolling 2026 early average‐to‐high risk breast cancer patients and found an independent prognostic relevance of CTCs both before and after adjuvant chemotherapy. The study successfully proved the prognostic role of CTCs in adjuvant settings. The next direction of future studies should be designed to answer the question that whether if extended adjuvant therapy is needed and whether if the extended therapy did reduce the risk of recurrence or not.

For the role of CTCs in a novel and specific therapy in breast cancer, Paoletti et al. [280] reported that heterogeneous mechanisms of resistance to fulvestrant, including estrogen receptor alpha gene (ESR1) mutation. CTC enumeration, phenotyping, and genotyping might identify patients who would benefit from fulvestrant dose escalation versus switching to alternative therapies. The CTCs could possibly help find the resistance genes during the therapy and warn the clinicians to change therapy in time before the tumor already gets progression.

In triple negative breast cancer (TNBC) who lacks of drugable targets (hormone therapy) in breast cancer, Hall et al. (2015) enrolled 44 TNBC patients using CellSearch™ for CTC testing and found that ≥1 CTC in each sample was identified in 30% of patients completing neoadju‐ vant chemotherapy (NACT). Multivariate analysis demonstrated that detection of ≥1 CTC predicted decreased RFS (log‐rank *P* = 0.03, HR 5.25, 95% CI 1.34–20.56) and OS (log‐rank *P* = 0.03, HR 7.04, 95% CI 1.26–39.35). The results suggested a modification of clinical management for TNBC patients with positive CTC detection after NACT, including extension of NACT or adding another anti‐cancer therapy before tumor recurs.
