> ## Content Index > Fetch the complete content index at: https://compounded.ghost.io/llms.txt > Use this file to discover other available public pages before exploring further. # A Blood Test Can Find Cancer Cells Left After Surgery. Acting on It Is the Unsolved Part. - URL: https://compounded.ghost.io/a-blood-test-can-find-cancer-cells-left-after-surgery-acting-on-it-is-the-unsolved-part/ - Published: 2026-10-24T11:22:00.000Z - Updated: 2026-10-24T11:22:08.000Z - Author: Connor Hayes After a colon tumour is removed, the central question is whether anything was left behind. For decades the only answers were a pathology report describing how far the tumour had spread and a schedule of scans that would eventually show a recurrence, usually once it was large enough to matter. Circulating tumour DNA testing changed the first half of that. A blood sample taken weeks after surgery can now detect fragments of tumour DNA at concentrations far below what any scan can see. The prognostic power is not in dispute. In a Japanese registry of more than two thousand patients, those with detectable tumour DNA after surgery had roughly a twelvefold higher risk of recurrence. What remains unsettled, after several randomized trials, is what to do about a positive result. ### From the Lab to the Ledger Tumours shed DNA into the bloodstream as cells die. That DNA carries the tumour's own mutations, which distinguishes it from the far larger quantity of normal DNA circulating at the same time. Modern assays either sequence the resected tumour first and then hunt for that patient's specific mutations, or screen for a fixed panel of common alterations. The first approach is more sensitive and slower; the second is faster and less personalized. Finding any of it after surgery means cancer cells remain somewhere in the body, below the resolution of imaging. The clinical logic seems obvious: treat the people who have it, spare chemotherapy to those who do not. Both halves of that logic have now been tested, and only one has held up. The de-escalation half worked in early-stage disease. In the DYNAMIC trial in stage II colon cancer, using the test to decide who needed chemotherapy cut chemotherapy use from about 28 percent of patients to about 15 percent, with five-year recurrence-free survival essentially unchanged at 88 percent against 87 percent. That is a meaningful reduction in toxicity for the same outcome. The escalation half has not worked. In stage III disease, intensifying chemotherapy for patients with detectable tumour DNA produced no benefit, and a separate randomized trial of an additional drug in test-positive patients after standard chemotherapy missed its primary endpoint. Knowing the cancer is still there has not yet translated into a treatment that removes it. ### Bio-Pipeline Ledger ctDNA as a prognostic marker after colorectal surgery: strongly validated. Detectable tumour DNA after surgery is among the most powerful predictors of recurrence yet measured in this disease. ctDNA-guided de-escalation in stage II colon cancer: supported by one randomized trial showing roughly half as much chemotherapy with comparable recurrence-free survival. Promising and not yet standard of care. ctDNA-guided de-escalation in stage III colon cancer: tested and non-inferiority was not met, so withholding chemotherapy from test-negative patients at this stage remains investigational. ctDNA-guided treatment intensification: tested in randomized trials and so far unsuccessful. No therapy triggered by a positive result has been shown to reduce recurrence or extend survival. Commercial MRD testing panels: clinically available and ordered in practice, with assay standardization, cost and access still unresolved across platforms. ### The Clinical Reality Check The guidelines are unusually direct here, and they are worth quoting in substance. The major oncology bodies treat this as a prognostic marker and advise that it should not drive adjuvant treatment decisions outside a clinical trial. The FDA has stated that clearing tumour DNA from the blood has not been validated as a surrogate for survival in any solid tumour. That is the current consensus, and it sits uncomfortably alongside how often the test is being ordered. The uncomfortable part for patients is the asymmetry. A negative result is reassuring but imperfect, because recurrences in the lung and the lining of the abdomen shed less DNA and are more likely to be missed. A positive result is reliable and, right now, frequently actionable only in the sense that it tells you to expect something, not what to do about it. Several specialists have described the psychological weight of that gap, and it is a legitimate thing to weigh before testing. What makes this worth following rather than dismissing is that the trials now running are the right ones. Large studies are testing escalation with different drug combinations and de-escalation in higher-risk groups, and the open question is specific enough to be answerable. If treating a positive result ever does improve survival, this becomes one of the most consequential tools in cancer care. For anyone facing this decision after colorectal surgery, the useful question for the oncologist is not whether the test is accurate, because it is. It is what, concretely, would change if the result came back positive, and whether a trial is available. If the honest answer is that nothing would change, that is worth knowing before the blood is drawn rather than after. ![](https://storage.ghost.io/c/93/20/932004ad-b501-4cef-8a02-28e1473c42cb/content/images/2026/10/ctdna-residual-disease-testing-prognosis-vs-treatment-2-cinematic.jpg) ###