The review, led by Chimbabantu Kaoma and colleagues at the Nuclear Medicine Research Infrastructure and the University of Pretoria in South Africa, synthesises evidence on circulating tumour DNA, or ctDNA, across the four major treatment classes in mCRPC: androgen receptor pathway inhibitors, taxane chemotherapy, PARP inhibitors, and PSMA-targeted radioligand therapy. The authors’ central message is nuanced. ctDNA is undeniably informative, but most of its proven value today is prognostic rather than predictive, meaning it tells clinicians how aggressive a patient’s disease is rather than which specific drug will work best.

The biological logic behind ctDNA is elegant. When tumour cells die, through apoptosis, necrosis, or therapy-induced destruction, they release fragments of their DNA into the bloodstream. This tumour-derived DNA mixes with cell-free DNA shed by normal cells, predominantly from white blood cells and their precursors. In healthy individuals, cell-free DNA consists mainly of short fragments around 40 to 200 base pairs. In patients with metastatic cancer, a variable proportion of this circulating DNA originates from the tumour itself, and that proportion, known as the tumour fraction, can range from less than 1 percent to more than 50 percent in advanced mCRPC. Because ctDNA has a half-life of roughly one to two hours, cleared rapidly by the liver, kidneys, nucleases, and macrophages, its levels reflect tumour dynamics almost in real time.

What makes ctDNA particularly powerful in mCRPC is that it captures the tumour’s evolutionary story. Prostate cancer under therapeutic pressure is a textbook example of Darwinian selection. Androgen receptor signalling remains the dominant oncogenic driver even after castration, and tumours accumulate gain-of-function mutations in the androgen receptor’s ligand-binding domain, such as T878A, H875Y, L702H, and F877L, that broaden ligand specificity or even convert drugs like enzalutamide from antagonists into agonists. The F877L mutation, for instance, can flip enzalutamide and apalutamide into activators of the very receptor they were designed to block, while L702H allows glucocorticoids given during treatment to activate the receptor. Serial ctDNA sampling can track these resistant subclones as they emerge, something a single archival tissue biopsy taken years earlier simply cannot do.

On the technical side, next-generation sequencing has transformed what is possible. Targeted hybrid-capture panels, amplicon-based deep sequencing, droplet digital PCR for hotspot mutations, and low-pass whole-genome sequencing for tumour fraction estimation can now detect variant allele frequencies below 0.1 percent under optimised conditions, thanks to molecular barcoding and error-suppression strategies. But the review is careful to distinguish analytical sensitivity from biological sensitivity. A technically exquisite assay still fails when the tumour sheds little DNA, as is common in bone-predominant disease, or when the relevant resistant clone sits below the assay’s limit of detection. A negative ctDNA result, the authors stress, never excludes active disease.

One of the most important confounders the review highlights is clonal haematopoiesis of indeterminate potential, or CHIP. As people age, some haematopoietic stem cells acquire somatic mutations in genes like TP53, ASXL1, DNMT3A, and TET2, and these non-malignant clones release mutated DNA into the plasma that can be mistaken for tumour-derived signal. Matched white blood cell sequencing is therefore essential to filter out this noise. Pre-analytical variability, differences in tube type, processing time, and storage conditions, plus a lack of harmonisation across platforms, further complicate interpretation and currently prevent universal thresholds from being applied in the clinic.

Despite these caveats, the prognostic evidence for ctDNA tumour fraction is remarkably consistent across every treatment class examined. In landmark trials of abiraterone and enzalutamide, ctDNA-positive patients had median overall survival reduced from 47.4 to 27.3 months and median radiographic progression-free survival reduced from 33.0 to 17.8 months, independent of PSA and other clinical variables. Each 10 percent increase in ctDNA aneuploidy fraction was associated with a proportional increase in the hazard of progression and death. Similar patterns emerged in the TALAPRO-2 trial of talazoparib plus enzalutamide, in the FIRSTANA and PROSELICA taxane trials, and in the PSMAfore and TheraP radioligand therapy studies, where baseline ctDNA fraction above 0.5 percent independently predicted inferior outcomes regardless of treatment assignment.

The one domain where ctDNA has achieved genuine predictive, treatment-defining status is in homologous recombination repair testing for PARP inhibitors. In the PROfound trial, ctDNA testing showed substantial concordance with tissue sequencing for BRCA1, BRCA2, and ATM alterations, and this evidence contributed directly to regulatory approval of olaparib and to the FDA clearance of FoundationOne Liquid CDx as a companion diagnostic. Beyond selection, ctDNA also monitors response in this setting: in TOPARP-A, a fall of at least 50 percent in cell-free DNA concentration at four to eight weeks was associated with improved survival, while rising ctDNA identified non-responders and revealed resistance mechanisms such as BRCA2 and PALB2 reversion mutations that restore the broken repair pathway.

For PSMA-targeted radioligand therapy with lutetium-177 PSMA-617, the review frames ctDNA as complementary to, not a replacement for, PSMA PET imaging. PSMA PET determines eligibility by confirming sufficient target expression across metastatic sites, while ctDNA refines molecular risk assessment and tracks clonal evolution. Exploratory analyses from TheraP suggest that ATM alterations may predict greater benefit from radioligand therapy over cabazitaxel, plausibly because impaired DNA repair enhances radiosensitivity, while AR amplification and alterations in TP53, RB1, and PTEN are enriched in poor responders, though these signals remain investigational. Early on-treatment ctDNA kinetics, measured at cycle 2 day 1 in PSMAfore, correlated more strongly with progression-free and overall survival than baseline levels and were largely independent of PSA, suggesting ctDNA can detect molecular response to radiation-induced DNA damage before anatomical changes become visible.

The review’s conclusion is measured but forward-looking. ctDNA is best positioned today as a complementary tool for risk stratification and disease monitoring, with tumour fraction serving as a pan-therapy prognostic biomarker and genomic alterations providing therapy-specific predictive information only in validated contexts such as HRR testing. Emerging technologies, including fragmentomics, DNA methylation profiling capable of detecting neuroendocrine transformation, multi-omic liquid biopsy integrating circulating tumour cells and extracellular vesicles, and machine learning approaches to integrate complex datasets, promise to expand ctDNA’s clinical reach. But the authors are clear that routine implementation will require prospective biomarker-driven trials, assay standardisation, population-diverse validation, and a demonstration that acting on ctDNA results genuinely improves patient outcomes. Until then, the blood test that reads a tumour’s evolutionary diary remains a powerful adjunct rather than a standalone decision-maker, one whose full potential in precision oncology is still being written.

Subject of Research: Circulating tumour DNA as a liquid biopsy biomarker for monitoring and prognostication in metastatic castration-resistant prostate cancer.

Article Title: Circulating tumour DNA in mCRPC: bridging tumour biology and real-time treatment monitoring

Article References: Circulating tumour DNA in mCRPC: bridging tumour biology and real-time treatment monitoring. (n.d.). https://doi.org/10.3389/fmmed.2026.1852641

Image Credits: AI Generated

DOI: 10.3389/fmmed.2026.1852641

Keywords: circulating tumour DNA, liquid biopsy, metastatic castration-resistant prostate cancer, androgen receptor pathway inhibitors, PARP inhibitors, PSMA radioligand therapy, tumour fraction, clonal haematopoiesis, clonal evolution, precision oncology, biomarker validation, next-generation sequencing

Circulating tumour DNA is exactly what its name suggests: genetic material shed by tumour cells into the bloodstream as cancer cells die and release their contents. Unlike a traditional tissue biopsy, which captures only a single snapshot of a single tumour site, ctDNA offers a minimally invasive, real-time window into the entire tumour burden across the body. Because it can be sampled repeatedly from a simple vial of blood, it has the theoretical potential to reveal whether a treatment is working long before changes become visible on a CT scan or PET image. That matters enormously in advanced breast cancer, where treatment strategies are evolving rapidly but where decisions about when to switch therapies still rest largely on imaging-based assessments such as the RECIST criteria, which measure tumour size on radiographic scans.

To establish whether ctDNA truly delivers on this promise, the Aarhus team conducted a rigorous search of the Medline, Embase and Cochrane databases covering studies published up to February 2025. They registered the review prospectively in PROSPERO and evaluated the quality of each included study using the Quality in Prognosis Studies tool, a standard instrument designed to guard against bias in prognostic research. From the eligible literature, they extracted hazard ratios — statistical measures that describe how much a biomarker changes the risk of an event such as disease progression or death — along with their 95 percent confidence intervals, and then pooled the results using random-effects models, which account for variability between studies.

The headline numbers are compelling. Across studies encompassing 2,586 patients, elevated ctDNA levels measured at the start of treatment were associated with roughly a doubling of the risk of disease progression, with a pooled hazard ratio of 2.0 for progression-free survival. For overall survival, the association was even stronger: among 2,454 patients, high baseline ctDNA carried a pooled hazard ratio of 2.6, meaning those patients faced more than two and a half times the risk of death compared with patients whose baseline ctDNA was low. In practical terms, a patient walking into the clinic with abundant tumour DNA in her plasma is statistically likely to fare worse than a patient with barely detectable levels, regardless of the treatment she ultimately receives.

Perhaps even more clinically meaningful is what happens to ctDNA during therapy. The meta-analysis found that unfavourable changes in ctDNA levels over the course of treatment — rising concentrations, or a failure to clear the biomarker once therapy begins — were associated with reduced progression-free survival, with a pooled hazard ratio of 2.5 across 710 patients, and reduced overall survival, with a pooled hazard ratio of 2.4 across 154 patients. This dynamic dimension is where ctDNA distinguishes itself from static prognostic markers. A single measurement tells you something about tumour burden; a series of measurements tells you whether the tumour is responding to the drug in real time. In the era of targeted therapies such as CDK4/6 inhibitors and PI3K inhibitors, where treatment sequences are complex and expensive, an early molecular signal of failure could spare patients months of ineffective therapy and its side effects.

The review also examined whether specific genetic alterations detected in ctDNA carry prognostic information of their own. The answer, broadly, is yes — but with caveats. Mutations in genes such as ESR1, the oestrogen receptor gene that frequently evolves resistance to endocrine therapy, and PIK3CA and TP53, both central players in breast cancer biology, were generally associated with less favourable outcomes when detected at baseline. Studies tracking ESR1 mutations in patients treated with fulvestrant and CDK4/6 inhibitors, for example, have repeatedly shown that the emergence of these mutations in plasma heralds clinical progression. However, the authors note that results across the literature were not entirely consistent, reflecting differences in assay methods, patient populations, treatment contexts and the specific variants analysed. The heterogeneity is a reminder that ctDNA is not a single test but a family of technologies, ranging from digital droplet PCR, which hunts for known mutations with exquisite sensitivity, to whole-exome and whole-genome sequencing, which survey the tumour genome more broadly.

Methodologically, the Danish analysis stands out for its scale and discipline. Sixty-four studies is a large evidence base for a prognostic biomarker review, and the use of the QUIPS tool for quality assessment, together with random-effects meta-analysis and formal measures of statistical heterogeneity, follows the highest standards in the field, including the PRISMA 2020 reporting guidelines. The authors also situate their work within a rapidly moving landscape: clinical guidelines from the American Society of Clinical Oncology now recommend testing for ESR1 mutations to guide therapy in hormone receptor-positive metastatic disease, and proposals for ctDNA-based response criteria — sometimes dubbed ctDNA-RECIST — are actively being developed to complement, and perhaps eventually rival, traditional imaging endpoints.

Still, the researchers are careful about what their findings do and do not establish. A strong prognostic association — showing that ctDNA levels predict outcomes — is not the same as demonstrated clinical utility, meaning proof that acting on the biomarker actually improves patients’ lives. That distinction, long emphasised in the biomarker literature, requires prospective interventional trials in which treatment decisions are randomised based on ctDNA results. The review also highlights gaps in the evidence: overall survival data for dynamic ctDNA changes rest on relatively few patients, subgroup analyses by breast cancer subtype were limited by inconsistent reporting, and the field lacks standardisation in how ctDNA is quantified and what thresholds define ‘elevated’ or ‘unfavourable’ change. Until those issues are resolved, ctDNA remains a powerful prognostic signal rather than a validated decision-making tool for every patient.

Even so, the trajectory is unmistakable. The pooled effect sizes reported here — hazard ratios of 2.0 to 2.6 across thousands of patients — place ctDNA among the most robust prognostic biomarkers described in advanced breast cancer. Combined with the growing arsenal of therapies that can be matched to specific mutations found in plasma, the technology points toward a future in which a blood test drawn at diagnosis and repeated throughout treatment provides oncologists with a continuous, molecular readout of the disease. For patients with advanced breast cancer, whose options depend on catching treatment failure early, that future cannot come soon enough. The Danish team’s synthesis does not close the chapter on ctDNA, but it firmly establishes the scientific foundation on which the next generation of trials — and, ultimately, clinical practice — will be built.

Subject of Research: Prognostic value of circulating tumour DNA in advanced breast cancer

Article Title: Circulating tumour DNA and its clinical relevance in predicting progression-free survival and overall survival in patients with advanced breast cancer: a systematic review and meta-analysis

Article References: Circulating tumour DNA and its clinical relevance in predicting progression-free survival and overall survival in patients with advanced breast cancer: a systematic review and meta-analysis. (n.d.). https://doi.org/10.1007/s10549-026-08087-0

Image Credits: AI Generated

DOI: 10.1007/s10549-026-08087-0

Keywords: ctDNA, advanced breast cancer, liquid biopsy, progression-free survival, overall survival, meta-analysis, biomarkers, ESR1 mutations, systematic review, prognosis, precision oncology, treatment monitoring