Cancer of Unknown Primary is one of oncology’s most frustrating diagnoses: a patient has metastatic cancer, yet conventional tests cannot identify the organ where the disease began. A new prospective national study, CUP-COMP, is now evaluating whether a blood test can help overcome that uncertainty by identifying molecular signals released by tumours into the bloodstream. The trial, reported by Conway, Robinson, Concannon and colleagues in the British Journal of Cancer, examines the practical feasibility of using blood-based molecular profiling to guide precision medicine for people with CUP.
CUP is not a single disease but a clinical condition in which cancer has spread while its original site remains hidden. In many patients, doctors use imaging, pathology, immunohistochemistry and molecular tests to search for the primary tumour. Even after extensive investigation, however, the source may remain unknown. This creates a major treatment challenge because cancer therapies are often selected according to the tissue in which a tumour originated. A cancer that began in the lung, breast, bowel or pancreas may respond to very different drugs, yet CUP can prevent clinicians from making that distinction with confidence.
The CUP-COMP trial is focused on a technology known as liquid biopsy. Instead of requiring a tumour sample obtained through surgery or an invasive biopsy, liquid biopsy analyses biological material circulating in the blood. Tumour cells can release fragments of DNA into the bloodstream, including circulating tumour DNA, or ctDNA. These fragments may contain mutations, copy-number changes and other molecular abnormalities that reflect the biology of the cancer. By sequencing this material, researchers can search for patterns that may help classify a tumour, reveal potentially targetable alterations or indicate how the disease is changing over time.
The central question is not simply whether such a test can produce a technically impressive molecular profile. The trial is designed to investigate whether blood-based profiling can be delivered reliably and usefully in routine clinical pathways for patients with CUP. That distinction is crucial. A test may work in a laboratory but prove difficult to implement at a national scale if blood samples arrive too late, contain too little tumour-derived DNA, fail quality-control checks or produce results that clinicians cannot interpret within the time available for treatment decisions.
A prospective design allows the researchers to evaluate these issues as they occur, rather than relying only on stored samples or retrospective records. Patients can be followed through the process of consent, blood collection, sample transport, laboratory analysis and clinical reporting. This approach can reveal where delays and failures arise and whether the information generated is available at a moment when it might influence patient care. It also provides a framework for measuring how often blood samples yield an interpretable molecular profile and how consistently testing can be integrated across participating centres.
Technically, the approach may combine several layers of genomic information. DNA sequencing can identify mutations in genes that drive tumour growth or create vulnerabilities to targeted drugs. Copy-number analysis can detect gains and losses of DNA segments, while broader molecular signatures may offer clues about tumour lineage. Some platforms can also estimate the fraction of DNA in a blood sample that originates from the tumour, a measurement known as tumour fraction. When this fraction is low, a negative result may not mean that a mutation is absent; it may simply indicate that the test did not receive enough tumour-derived material to detect it.
That limitation is particularly important in CUP, where disease biology can vary widely and metastatic deposits may release unequal amounts of DNA into the circulation. Tumour burden, the location of metastases, treatment exposure and the biology of individual cancers can all affect ctDNA levels. Blood-based profiling therefore does not eliminate the need for clinical assessment, imaging or tissue pathology. Instead, it is being evaluated as an additional source of evidence that could complement established diagnostic methods and potentially reduce the time required to obtain molecular information.
The precision-medicine element of CUP-COMP reflects a broader shift in cancer treatment. Rather than assigning therapy solely according to the organ where a tumour started, oncologists increasingly seek molecular features that can be targeted directly. Alterations in genes involved in DNA repair, cell signalling or immune regulation may appear across cancers from different organs. If these abnormalities are detected in a patient with CUP, they could provide a rationale for considering a targeted therapy or an immunotherapy, although the clinical value of any proposed treatment must still be assessed through evidence, eligibility criteria and multidisciplinary review.
The study also addresses an important question of equity and scalability. Advanced molecular testing is not useful if it is available only at specialist institutions or to patients who can access highly centralised services. A national trial can test whether samples collected in different hospitals can be processed through a coordinated system and whether results can be returned in a consistent format. The findings may help establish the logistical requirements for wider adoption, including laboratory capacity, data interpretation, reporting standards and communication between molecular scientists and treating teams.
For patients facing a diagnosis in which the primary tumour cannot be found, the promise of a blood-based test is therefore measured not only in scientific novelty but in speed, accessibility and clinical clarity. CUP-COMP is evaluating whether molecular information can be obtained from a relatively simple blood draw and incorporated into real-world decision-making. Its significance will ultimately depend on whether the approach produces dependable results, identifies actionable biology and fits the demanding timelines of cancer care. By testing those questions prospectively, the study may help determine whether liquid biopsy can become a practical component of precision medicine for one of oncology’s most uncertain and difficult diagnoses.
Subject of Research: Blood-based molecular profiling and precision medicine for patients with Cancer of Unknown Primary (CUP).
Article Title: A prospective national precision medicine trial evaluating the feasibility of blood-based molecular profiling in patients with Cancer of Unknown Primary (CUP-COMP).
Article References: Conway, AM., Robinson, M., Concannon, M. et al. A prospective national precision medicine trial evaluating the feasibility of blood-based molecular profiling in patients with Cancer of Unknown Primary (CUP-COMP). Br J Cancer (2026). https://doi.org/10.1038/s41416-026-03519-6
Image Credits: AI Generated
DOI: 10.1038/s41416-026-03519-6
Keywords: Cancer of Unknown Primary, CUP, liquid biopsy, circulating tumour DNA, molecular profiling, precision medicine, cancer genomics, oncology, blood-based testing.
