A blood test performed only weeks after cancer surgery could help doctors decide who should receive chemotherapy and who may be able to avoid it, according to new research presented at the ESMO Gastrointestinal Cancers Congress 2026.
Colorectal cancer affects the large intestine or rectum and is the third most common cancer worldwide. It often spreads to the liver, making treatment more difficult. Surgeons can sometimes remove these liver tumors completely, giving patients a chance for long-term survival.
Even so, hidden cancer cells may remain behind. For this reason, many patients receive chemotherapy after surgery, although only a small number appear to gain a lasting benefit while almost everyone experiences treatment side effects.
Researchers from Hyogo Medical University in Japan, working with colleagues including the University of Oxford, investigated whether circulating tumor DNA could help solve this problem.
Circulating tumor DNA, known as ctDNA, consists of tiny pieces of genetic material released into the bloodstream by cancer cells. Detecting these fragments after surgery may indicate that cancer cells are still present even when scans appear clear.
The Phase II GALAXY study included 298 patients who underwent surgery for colorectal cancer that had spread to the liver. Blood samples were collected between two and ten weeks after surgery using a personalized ctDNA test based on each patient’s own tumor. Some participants had surgery before any chemotherapy, while others received chemotherapy before surgery.
For patients who had surgery first, the results were striking. Detectable ctDNA strongly predicted cancer recurrence and a higher risk of death.
However, those with positive ctDNA who received chemotherapy after surgery experienced far better long-term outcomes than similar patients who did not receive additional treatment. Their chances of remaining alive and cancer-free were substantially improved.
Patients whose ctDNA tests were negative generally did well regardless of whether chemotherapy was given. This finding suggests that some people may not need extra treatment after surgery, reducing unnecessary exposure to side effects.
Among patients who had chemotherapy before surgery, ctDNA continued to predict prognosis, but giving more chemotherapy afterward did not improve outcomes. This difference highlights the importance of tailoring treatment according to each patient’s clinical situation.
Cancer specialists welcomed the findings but emphasized that ctDNA testing is not yet ready for routine clinical practice. Additional randomized studies will be needed to confirm that treatment decisions based on ctDNA testing truly improve patient outcomes.
The research was presented at the ESMO Gastrointestinal Cancers Congress 2026.
This study is important because it moves cancer treatment closer to precision medicine, where therapy is matched to each patient’s individual risk. Although the evidence is still developing, the results suggest ctDNA testing could eventually help doctors deliver chemotherapy only to those most likely to benefit while sparing others unnecessary treatment and side effects.
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Source: Hyogo Medical University.
