Rectal Cancer Tumor DNA Fell From 45.8% to 8.7% After Chemotherapy

All four patients with early-stage rectal cancer who still had circulating tumor DNA (ctDNA) in their blood after chemotherapy were recommended a major surgery called total mesorectal excision (TME), compared with 38.1% of patients without detectable ctDNA, according to a study of the phase 2 CO.28 trial published in JCO Precision Oncology.

The Canadian Cancer Trials Group (CCTG) CO.28 trial enrolled patients with node-negative rectal cancer, meaning the cancer had not spread to nearby lymph nodes. Patients received three months of chemotherapy without radiation, followed by transanal endoscopic surgery (TES), a less extensive procedure that removes the tumor through the anus and can preserve the rectum. The researchers wanted to know whether ctDNA, small pieces of tumor DNA that circulate in the blood, could help guide decisions about which patients need TME.

The question matters because TME can affect quality of life, according to the researchers, who noted that the surgery can lead to a temporary or permanent stoma (an opening in the abdomen for waste to leave the body) and reduced bowel and sexual function.

What did ctDNA testing show in the CO.28 rectal cancer trial?

Before chemotherapy, the blood test detected ctDNA in 45.8% of patients (22 of 48). After chemotherapy, detection dropped to 8.7% (four of 46), a decline large enough that it was unlikely to have come about by chance. Among 41 patients tested at both points, 17 went from detectable ctDNA to none at all after chemotherapy.

The four patients who still had ctDNA after chemotherapy all had a poor response or persistent cancer and were recommended TME. Two had the surgery and two declined it. Three of the four had no ctDNA on later blood tests and were alive more than three years later. The fourth patient, who had TME, later had the cancer return in the lymph nodes and died about 3.5 years after TES.

"ctDNA testing identified cancers with inadequate response to chemotherapy in patients for whom TME was recommended," the researchers wrote.

Five patients had their cancer return during follow-up, two in the rectum and three in distant sites (the lungs or lymph nodes). The study did not find a clear difference in recurrence between patients with and without ctDNA, although the researchers noted the small number of patients affected that comparison. Three years after enrollment, 75.0% of patients with ctDNA after chemotherapy were free of recurrence, compared with 90.5% of those without it, but the gap was small enough that it could have come about by chance.

No ctDNA was found in any of the 90 follow-up blood samples from patients whose cancer did not return. Both patients whose cancer came back in the rectum had blood drawn at recurrence, and ctDNA was detected in one of them.

How did researchers test for ctDNA in the CO.28 rectal cancer trial?

Of the 58 patients enrolled in CO.28, 53 (91.4%) had at least one blood sample available for this analysis. All had node-negative rectal cancer that was stage T1 to T3, and the median age was 66 years. Patients received three months of chemotherapy, either capecitabine and oxaliplatin (CAPOX) or fluorouracil, leucovorin and oxaliplatin (FOLFOX), before TES.

Blood was drawn before chemotherapy, after chemotherapy but before TES, at recurrence, and once a year for three years. Patients were recommended TME if their tumors responded poorly to chemotherapy or showed high-risk features at the time of TES, and 23 patients received that recommendation.

The researchers used the Guardant Reveal assay, a tissue-free test that looks for chemical tags on DNA across more than 20,000 regions and does not require a sample of the tumor. The researchers noted that a tissue-free test may return results faster than tests built from a patient's tumor tissue. In earlier work in the same patients, about 10% did not have enough tumor tissue to build a tumor-informed test, they added. The researchers also noted that the tissue-free test detected ctDNA before chemotherapy in more patients than the tumor-informed test had (45.8% versus 24.5%).

What are the limitations of the CO.28 ctDNA study?

The researchers noted that the small number of patients, the low number of recurrences and missing follow-up blood samples for some patients may have led them to underestimate how well the test picks up cancer and how early it can signal a recurrence. Three of the five patients whose cancer returned had no ctDNA detected before or after chemotherapy. The analysis was also done after the fact, using blood samples collected during the trial.

What do the CO.28 ctDNA results mean for patients with early rectal cancer?

The researchers concluded that ctDNA blood tests could help doctors decide which patients with early-stage rectal cancer can safely avoid TME, used alongside the MRI scans and endoscopy exams that are currently the standard way to check response. They noted that patients who decline TME face the highest risk of recurrence and stand to gain the most from added decision tools.

The researchers called for larger studies with longer follow-up before ctDNA testing is used this way. Chemotherapy followed by TES, the approach tested in CO.28, is now being compared with chemoradiation followed by local excision in a randomized phase 3 trial.

Reference

1. Loree JM, et al. "Tumor-Free Circulating Tumor DNA in Node-Negative Rectal Cancers Undergoing Neoadjuvant Chemotherapy, Excision, and Observation in the Phase II NEO Trial (CO.28)." JCO Precision Oncology. Published October 2, 2026. doi.org/10.1200/PO-26-00459

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