Background
Despite increasing efforts being made in the past with prevention programs and strategies to improve therapeutic efficiency with neoadjuvant chemoradiation, adenocarcinoma of the rectum is still one of the most common malignancies in the western world. A five-year relative survival rate of nearly 60 % in Europe in the last years [
1] implies that several patients develop disease recurrence, primarily with liver and lung metastases.
Recently, a variety of molecular biomarkers and high-risk gene signatures have been introduced that may provide further information regarding prognosis and risk stratification to neoadjuvant treatment or adjuvant therapy of patients with colorectal cancer (CRC) [
2,
3]. None of these parameters have been implemented in routine clinical practice with the exception of mutational
KRAS status in patients with advanced CRC [
4]. The existence of circulating tumor cells (CTC) has been known for years. With the improvement of molecular detection technologies of CTC during the last years, making tests easier and more reliable, the clinical perception has been underlined. Recently the identification of single tumor cells in the blood or bone marrow has been proposed as a prognostic biomarker for colorectal cancer and other malignancies [
5]. It has been shown for colorectal cancer that patients with circulating tumor cells in the blood have a shorter overall survival (OS) [
6]. This was supported by a recent meta-analysis that could reveal CTC as a significant negative prognostic factor in a pooled analysis [
7]. We were also able to show that disseminated tumor cells (DTC) in the bone marrow negatively influence OS in patients after complete resection of colorectal liver metastasis [
8]. Furthermore, liver resection and radiofrequency ablation for liver metastases can considerably change the level of CTC in the blood [
9]. On the other hand, test methods and markers for CTC and DTC in CRC patients are still not standardized and prospective, multicenter trials with large patient numbers are needed.
Response to preoperative chemoradiation can sometimes even achieve a complete pathological remission, which is an important prognostic factor for locally advanced rectal cancer. During the last years, several studies have described markers and gene expression profiles to predict response to neoadjuvant chemoradiation [
10,
11]. Despite some encouraging results in defining markers/ gene profiles, there is still controversy between different studies.
A small series of 26 patients with rectal cancer undergoing neoadjuvant chemoradiation was able to show that responders to RCTX have a higher rate of CTC before initiation of RCTX compared to non-responders, and that RCTX induces a significant decrease in the detection rate of CTC in responders [
12]. Thus, we designed this study to further evaluate the prognostic value of CTC and to elucidate the impact of CTC in predicting response after RCTX. To our knowledge, this is the largest series of patients evaluating the impact of CTC in rectal cancer patients.
Discussion
According to current guidelines most patients with locally advanced rectal cancer are scheduled for neoadjuvant chemoradiation. The problem of local recurrence has been effectively reduced by this multimodal treatment. Even so, patients with advanced rectal cancer are still at high risk to develop distant metastases [
16]. It is a widely accepted hypothesis, that dissemination of tumor cells from the primary tumor is a precondition for distant metastases and tumor recurrence. According to the “revisited” hypothesis of “seed and soil”, it does not only depend on the cell itself, but also on local environmental factors, whether circulating tumor cells can develop and grow out into liver and lung metastases [
17]. Despite the inconsistency in the detection methods of CTC, the majority of studies published in the last years reported poor prognosis when CTCs are detected in colorectal cancer patients [
7].
In addition to the prognostic value of CTC, the detection of CTC may be useful to serve as a predictive marker for therapy response. There is a wide spectrum of response to neoadjuvant chemoradiation, which creates an urgent necessity to predict the responders and non-responders in order to keep non-responders form unnecessary, potentially harmful treatment.
Only a few studies have evaluated the role of CTC in patients with locally advanced rectal cancer being candidates for neoadjuvant chemoradiation. Kienle et al. were the first to demonstrate in a small cohort of patients that RCTX leads to a clearance of CTC, which is associated with a decreased detection rate of CTC [
18]. These results have been confirmed by other small monocentric trials [
12,
19,
20], whereas only two studies have shown that CTC can be serve as marker for response after RCTX [
12,
19]. We have shown in this so far largest series of patients that there is a correlation of response after neaodjuvant RCTX and detection of CTC.
In our study we used Ficoll extraction of MNC’s followed by CK20 RT-PCR to detect CTC. With an overall detection rate of 30 % this system is more sensitive than anti-EpCAM based binding capture technique (i.e., CellSearch™) which has recently been introduced. For rectal cancer patients detection rates of 19 % with the CellSearch™ system have been reported [
19]. Recently, it has been shown that including CK20 as a biomarker for detection of CTC, the sensitivity of the CellSearch
TM system is efficiently enhanced [
21], underlying the important role of CK20 in detecting CTC. Probably many CTC stay undetected due to an epithelial mesenchymal transition process (EMT) [
22] of tumor cells or due to a CTC population with atypical characteristics which has been described earlier [
23]. This group of CTC may prove to be very important for the treatment of the metastatic disease, as they represent stem cell-like cancer cells that most likely do not respond well to current therapeutic regimens.
In our series of patients with rectal cancer we were not able to show a correlation between CTC and tumor stage or overall survival. These results are contradictory to our own analysis of more than 500 colon cancer patients, demonstrating a negative correlation between detection of CTC and overall survival (data not shown). Most of the published series that have shown a negative prognostic impact of CTC (for overview see meta analysis [
7]) do not differentiate between colon and rectal cancer patients. Our results are consistent with recent results from studies focusing only on rectal cancer patients, which were also not able to show a correlation between CTC and overall survival or tumor stage [
12,
19,
20]. Even analyzing subgroups of our data (tumor localisation in the upper 1/3 of the rectum; only patients without neoadjuvant RCTX) we found no negative prognostic impact of CTC. Therefore, we have to conclude that tumor biology with regard to the impact of CTC is diverse between colon and rectal cancer. The blood drainage from the tumor might be an explanation for this different behavior. It has already been shown, that in patients with low rectal cancer the detection rate of CTC is higher in central venous blood than in the blood from the mesenteric vein compared to tumors in the middle and upper rectum [
24]. In our series of patients the detection rate of CTC in the peripheral blood was not significantly different between tumors of the lower part compared to tumors in the middle and upper part of the rectum (data not shown).
The limitation of our study is, that we only measured CTC prior to surgery and not at different time points before and during neoadjuvant RCTX. Historically, that was not planned, because the focus was initially on patients with colon cancer, scheduled directly for the operation without any neoadjuvant treatment. We know from previous reports, that neoadjuvant RCTX is associated with a decreased detection rate of CTC [
19]. We have shown a significantly decreased rate of CTC in responders after neoadjuvant RCTX. It would be very interesting, if this difference is already apparent before the treatment or at which time point after initiation of RCTX a significant difference in the detection rate can be detected. Goal for the future is to implement serial measurements of CTC before, during and after RCTX as a “liquid biopsy” to evaluate in which patients the CTC are effectively cleared from the blood during RCTX. Corresponding to PET-CT evaluation of patients with esophageal cancer at the beginning of neoadjuvant chemotherapy to guide the treatment [
25], measurement of CTC during the beginning of neoadjuvant RCTX might be helpful to distinguish responders from non-responders and possibly preventing non-responders from potentially harming, ineffective RCTX.
Competing interests
The authors declare that they have no competing interest.
Authors’ contributions
SH, HK and TB designed the study. SH analyzed the data and drafted the manuscript. CR carried out the RT-PCR analysis. JT designed the study and helped to draft the manuscript. AH and CS helped analyzing the data. All authors read and approved the final manuscript.