Despite advances in multimodal treatment, CRC is the second most common cause of cancer death worldwide (1). CTCs play a pivotal role in disease progression, metastasis, and recurrence (20). Aside from TNM classification and residual tumor status, novel tools and staging systems are needed for adequate prognostic staging and for guiding multimodal therapy (21). CTC identification has proven to be an important prognostic factor in breast (22). For this reason, a decade ago, 7th AJCC Cancer Staging Manual introduced the new category M0(i) for breast cancer, which is defined by the presence of circulating or disseminated tumor cells not exceeding 0.2 mm detectable in bone marrow, circulating blood or other non-regional tissues of non-metastatic patients. In contrast, such a category is still lacking in the current CRC Staging system (23) due to the fact that the assessment of CTCs in CRC is still controversial (16) and different technical platforms have provided conflicting results (24–26). Here, we applied the FDA-cleared Cellsearch® system, which uses immunomagnetic enrichment (EpCAM) and immunocytochemical (cytokeratin, CD45, DAPI) analysis. Compared to immunocytochemistry, CS is standardized, more time efficient and provides better CTC enrichment(16, 18, 27). In our cohort, we found CTCs in 45.6% % of the patients before surgical resection. CTCs were present in all stages of CRC. Even in early stage CRC, UICC I (T1-2, N0, M0), one of four patients had detectable CTCs.
Up to now, there is no consensus regarding the threshold used to define CTC positivity in CRC(16). Cohen et al have used the cut-off of ≥ 3 CTC/7.5 ml for defining CTC positivity in metastatic CRC(12), while others have shown that cut-offs ≥ 1 CTC/7.5 ml and ≥ 2 CTC/7.5 ml were also associated with poor prognosis in CRC(28, 29).
We used a strict cut-off of ≥ 1 CTC/7.5 ml. CTC detection before surgery was associated significantly with a shorter progression-free (P = 0.008) and overall survival (P = 0.008). Multivariate analyses identified CTCs as a strong, independent, prognostic indicator for overall survival.
Many studies have shown inferior survival for right-sided tumors (32, 33), so we assessed the effect of tumor location on CTC enumeration. Right-sided colon cancer (RCC), left-sided colon cancer (LCC) and rectal cancer (RC) were considered independently, in light of the “three entities” hypothesis in colorectal cancer(34–36). Although more CTCs were present in LCC compared to RCC and RC, this was not statistically significant. A previous publication from Nicolazzo et al. showed similar results (34). However, larger cohorts are needed to deliver a robust conclusion about tumor sidedness and CTC detection.
From a clinical perspective, using CTCs in peripheral blood allows assessing cancer prognosis in a non-invasive and easily applicable method (16) that allows real-time monitoring of tumor dynamics (37). Our present study supports that preoperative CTC detection in CRC identifies patients with shorter OS and PFS, which might contribute to an improved stratification of high-risk CRC patients. Nevertheless, larger validation studies are required before implementation of CTC detection into tumor staging classification of CRC.