ctDNA Flags Peritoneal Prognosis
- Conference highlights reported that plasma ctDNA levels predicted survival in patients with peritoneal metastases. (x.com) - Specifically, negative ctDNA after treatment correlated with substantially better survival in colorectal and appendiceal peritoneal disease. (x.com) - The result implies liquid biopsy could complement or refine peritoneal cytology for staging and prognosis, per AACR26 notes. (x.com)
A blood test that looks for fragments of tumor DNA tracked survival in patients with cancer spread across the abdominal lining, according to data presented at the American Association for Cancer Research meeting in San Diego in April 2026. (aacrjournals.org) The test measures circulating tumor DNA, or ctDNA: bits of cancer DNA shed into the bloodstream, often called a “liquid biopsy” because it samples a tube of blood instead of a tumor. In Abstract 6444, investigators studied 61 patients treated between 2023 and 2025 for peritoneal metastases from colorectal cancer or high-grade appendiceal cancer. (aacrjournals.org) Of those 61 patients, 41 had colorectal cancer and 20 had high-grade appendiceal cancer. Median follow-up after surgery was 18 months, and 82% of patients were alive at last follow-up. (aacrjournals.org) Before surgery, 17 patients, or 33%, had detectable ctDNA in plasma. Patients who were ctDNA-negative before surgery had an estimated 2-year overall survival of 91.2%. (aacrjournals.org) The signal held even in patients with disease beyond the peritoneum. Among ctDNA-negative patients, estimated 2-year overall survival was 83.3% for those with multisite disease and 95.6% for those with peritoneal-only disease. (aacrjournals.org) The peritoneum is the thin lining of the abdominal cavity, and tumors there can be hard to measure with scans or standard blood tests. That has left clinicians relying on surgery, pathology, and peritoneal cytology — looking for cancer cells in abdominal fluid under a microscope — to estimate risk. (link.springer.com, pmc.ncbi.nlm.nih.gov) That gap is one reason ctDNA has drawn attention in this setting, but the evidence has been thin. A 2025 consensus guideline for colorectal cancer with peritoneal metastases gave only a weak recommendation for ctDNA testing because published data were still limited. (link.springer.com) Earlier small studies pointed in the same direction. In a 2020 prospective series of 11 patients undergoing cytoreductive surgery and heated intraperitoneal chemotherapy, every patient with positive plasma ctDNA after treatment relapsed systemically, while patients with negative plasma ctDNA after treatment did not relapse. (pmc.ncbi.nlm.nih.gov) Appendiceal cancer adds another complication because it often sheds less tumor DNA into blood than colorectal cancer. A 2025 study reported detectable ctDNA in 38% of patients with metastatic appendiceal adenocarcinoma and found worse overall survival when ctDNA was detectable. (aacrjournals.org) The new AACR result does not settle how doctors should use the test in practice, because it is a conference abstract with 61 patients and short follow-up. But it gives surgeons and oncologists a clearer number to watch: whether ctDNA stays negative after treatment in a disease where prognosis is often hardest to read from the bloodstream. (aacrjournals.org, link.springer.com)
