Published online Dec 28, 2019. doi: 10.3748/wjg.v25.i48.6939
Peer-review started: October 29, 2019
First decision: November 22, 2019
Revised: December 5, 2019
Accepted: December 22, 2019
Article in press: December 22, 2019
Published online: December 28, 2019
Around 50% of patients with colorectal cancer (CRC) are found to have synchronous liver or extrahepatic metastases at the time of diagnosis, or will develop metachronous metastases within several years after surgery. It is known that radical surgical resection, the R0 resection which involves a complete removal of the diseased tissue, is the only effective treatment option, ideally in combination with perioperative chemotherapy. Besides removal of colorectal cancer metastases, postoperative patient follow-up is no less important as it allows for the timely identification of any progression or recurrence of the disease and prompt therapeutical response. The monitoring of patients is predominantly based on imaging techniques, usually with assessments of serum tumor markers. One of the promising tools for long-term postoperative follow-up is the detection of circulating tumor DNA (ctDNA) in the peripheral blood. Sometimes referred to as a "liquid (re)biopsy" it is a minimally invasive procedure and can be performed repeatedly at relatively short intervals (months or even weeks). The presence of the disease and the actual extent of the tumor burden (tumor mass) within the patient’s body can be monitored. This is of particular importance, especially when evaluating radicality of surgical treatment as well as for early detection of disease progression or recurrence.
Radicality of surgery is normally assessed using histological examination of a resected sample and based on the results of imaging methods. However, these examinations cannot indicate any potential presence of circulating tumor cells or microscopic metastases. Also, the main tools used for postoperative patient follow-up, imaging methods and tumor markers, are often not sufficient in early detection of disease progression or recurrence. Tumor markers have low specificity and sensitivity so that they cannot be used alone to diagnose recurrence. Imaging methods are known to fail to detect small foci (micrometastases) or, on the contrary, find abnormalities that are not cancer. Moreover, they cannot be performed frequently due to the radiation exposure. The recently introduced ctDNA testing could present a useful alternative tool. It has proven to be very promising for long-term, postoperative follow-ups of patients with CRC, particularly in advanced stages. Being minimally invasive, it can be repeated frequently for a long time with no significant burden to the patient, and moreover, this test shows high sensitivity and specificity to patients with preoperative ctDNA positivity.
The main objectives of the study were to confirm the radicality of surgery using ctDNA and to compare available methods for detection of recurrence in metastatic CRC (mCRC). It is important to verify whether ctDNA can be used in clinical practice, particularly for the evaluation of the radicality of surgery and for the timely detection of any progression or recurrence of the disease.
A total of 47 patients with detected ctDNA and indications for resection of mCRC were enrolled in the multicenter study involving three surgical centers. Standard postoperative follow-ups using imaging techniques and the determination of tumor markers were supplemented by ctDNA sampling. In addition to the baseline ctDNA testing prior to surgery, a postoperative observation was conducted by evaluating ctDNA presence up to a week after surgery and subsequently at approximately three-month intervals. The presence of ctDNA was correlated with radicality of surgical treatment and the actual clinical status of the patient. To test ctDNA, we performed a sensitive, efficient, rapid and affordable method based on the formation of heteroduplexes with subsequent detection using denaturing capillary electrophoresis (DCE). This method can be used to detect a mutated locus in plasma with the sensitivity of 0.03% to 1% depending on the mutation to be determined with input DNA amount of tens of pg. The specificity of this ctDNA test is 100%, which means that the presence of ctDNA always provides evidence that a tumor residue or tumor cells are present. As previously shown by us, ctDNA testing using DCE allows a patient follow-up in real time, while ctDNA levels correlate very well with the current condition of the patient.
Among the monitored patients, the R0 (curative) resection correlated with postoperative ctDNA negativity in 26 out of 28 cases of surgical procedures (93%). In the remaining cases of R0 surgeries that displayed ctDNA, both patients were diagnosed with a recurrence of the disease after 6 mo. In 7 patients who underwent an R1 resection, 4 ctDNA positivities (57%) were detected after surgery and associated with the confirmation of early disease recurrence (after 3-7 mo). All 15 patients undergoing R2 resection remained constantly ctDNA positive during the entire follow-up period. In 22 cases of recurrence, ctDNA positivity was detected 22 times (100%) compared to 16 positives (73%) by imaging methods and 15 cases (68%) of elevated tumor markers.
Although ctDNA detection cannot replace traditional methods used in the follow-up scheme, it might represent a useful supplementary instrument for both the prediction and earlier detection of recurrences, particularly in patients with a higher risk of liver metastatic recurrence, and thus it may actually improve the overall survival odds of such patients. And it has also been shown that the ctDNA test is a highly specific and sensitive tool for confirming the radicality of surgical treatment and for the potential prediction of early disease recurrence after R0/R1 surgeries.
In this study, the high sensitivity of the methodology used to test ctDNA after curative mCRC surgical treatment and also to detect recurrence of the disease was shown. However, to confirm this hypothesis, testing on a larger sample set is required. In particular, it is desirable to obtain a greater number of ctDNA positive and negative results after R1 resections that could be correlated with time to disease recurrence. Similarly, expanding the set of long-term follow-up patients using standard approaches supplemented with ctDNA sampling is warranted.