Analysis of the Phase II EXTEND clinical trial suggests that adding metastasis-directed therapy (MDT)—primarily radiation therapy—to standard systemic treatment can significantly improve progression-free survival in patients with oligometastatic cancer, particularly prostate and pancreatic cancer. The treatment can also trigger measurable systemic immune responses and changes in circulating tumor DNA (ctDNA) that may help refine patient selection and treatment monitoring.
The findings come from the multicenter External Beam Radiation to Eliminate Nominal Metastatic Disease (EXTEND) trial (NCT03599765), one of the largest randomized studies to date examining MDT across multiple tumor types. Investigators reported that patients receiving MDT plus standard of care (SOC) experienced significantly longer progression-free survival compared with patients receiving standard systemic therapy alone. The study also identified histology-specific differences in benefit and uncovered translational biomarkers that could help define which patients are most likely to respond. The results were published in the Journal of Clinical Oncology.
The EXTEND trial enrolled patients with one to five metastatic lesions across six disease-specific “baskets,” including breast, pancreatic, kidney, prostate, and other tumor types. Patients were randomized to receive either standard systemic therapy alone or systemic therapy plus MDT, which consisted overwhelmingly of radiation therapy.
“This is probably the largest trial published to date in which we randomized patients to radiation versus not,” said lead investigator Chad Tang, MD, of MD Anderson Cancer Center. “We found that overall, if you add all the baskets together, there was a progression-free survival benefit with metastasis-directed therapy.”
Of 350 randomized patients, 334 were included and randomized to either standard of care SOC or SOC/MDT. Radiotherapy was used for 98% of treated metastases. After a median follow-up of 53 months, investigators found that MDT plus standard therapy reduced the risk of progression or death by 46% compared with standard therapy alone.
Not all tumor types responded equally, however.
“Some baskets, like the prostate baskets or the pancreas basket, had a big benefit for progression-free survival with metastasis-directed therapy,” Tang said. “Other baskets, like breast and kidney, there was no benefit.”
The investigators say the variability highlights the biological complexity of oligometastatic disease and underscores the need for more precise biomarkers beyond conventional imaging.
To improve disease characterization, the researchers incorporated ctDNA analyses using a methylation-based assay designed to detect molecular residual disease (MRD). The team found that detectable ctDNA at baseline was associated with significantly shorter progression-free and overall survival across tumor types. Conversely, patients whose ctDNA cleared within three months after enrollment experienced improved survival outcomes.
“If you were to clear your ctDNA—you go from MRD positive to negative—you also did much better for overall survival,” Tang said. “That may be an early endpoint showing that something positive happened for these patients.”
The findings suggest ctDNA could eventually help physicians determine which patients are appropriate candidates for MDT. “If your ctDNA is really high, maybe you shouldn’t do metastasis-directed therapy,” Tang said. “Maybe you should be changing your drug therapy.”
The study also revealed substantial differences in ctDNA shedding between tumor types. Pancreatic cancers showed the highest detection rates, while prostate cancers demonstrated relatively low ctDNA shedding, likely reflecting differences in tumor biology and treatment context.
Beyond ctDNA, the investigators conducted extensive immune profiling to better understand how radiation-based MDT might produce systemic effects. The team analyzed cytokines, flow cytometry markers, and T cell receptor (TCR) sequencing data collected before and after treatment.
One longstanding hypothesis in radiation oncology is that localized radiation can stimulate broader antitumor immune responses—sometimes referred to as the “abscopal effect.” Previous attempts to harness this phenomenon, particularly in combination with immunotherapy, have produced inconsistent clinical results. Tang believes the EXTEND findings may point to a different and potentially more clinically relevant mechanism.
“Here we’re radiating all the sites, and there’s only microscopic disease left,” he said. “The thinking is that we’re trying to turn the immune system on to control microscopic disease, which may not yet have developed a strongly immunosuppressive environment.”
Patients receiving MDT demonstrated substantially greater immune activation than those receiving systemic therapy alone. One of the strongest signals involved TCR remodeling—expansion and contraction of specific T cell clones following treatment.
“That means the T cells are getting channeled toward some antigen,” Tang explained. “Patients who had those T cell receptor modifications seemed to do better for overall survival.”
Notably, the most pronounced TCR changes occurred in pancreatic and prostate cancer baskets—the same groups that demonstrated progression-free survival benefits. By contrast, breast and kidney cancer cohorts showed minimal TCR changes and did not demonstrate clear clinical benefit from MDT.
The researchers also identified another potentially important immune biomarker: induction of proliferating CD8-positive, PD-1-positive T cells following MDT. Patients exhibiting this immune signature experienced improved progression-free survival.
“These two phenomena—TCR modification and induction of this proliferating T-cell population—were associated with good outcomes,” Tang said.
The findings may help inform future combination strategies involving radiation and immunotherapy, as well as the development of biomarkers to guide treatment intensification.
Tang said the broader message of the study is that oligometastatic disease likely represents a biologically heterogeneous state rather than a simple lesion count.
“The effect of metastasis-directed therapy seems to be very variable,” he said. “ctDNA may help us understand whether we’re actually treating the right patients, and the immune data suggest there may be ways to further enhance these responses.”
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