Why would splitting the dose help?

Stereotactic radiosurgery, despite its name, involves no cutting. It uses 3D imaging to aim high doses of radiation precisely at a tumor while limiting exposure to surrounding healthy tissue. Radiation oncologists commonly use it to treat brain metastases after surgery.

The problem is what surgery leaves behind. Even after a surgeon removes a visible tumor, microscopic cancer cells can remain at the site.

"We see high recurrence rates in patients with brain metastases because the challenge is eliminating microscopic cancer cells left behind after surgery without exposing too much healthy brain tissue to high doses of radiation," says Paul D. Brown, M.D., a radiation oncologist at Mayo Clinic Comprehensive Cancer Center and co-principal investigator of the study.

The recurrence numbers illustrate the scale of the problem: about 30% to 40% of patients experience the cancer returning at the surgical site despite treatment with single-fraction radiosurgery.

Fractionation changes the biology of the exchange. Dividing radiation into multiple treatments lets clinicians deliver a higher total dose overall. Between sessions, healthy tissue gets time to recover. Tumor cells lack the same repair capacity, so they accumulate more damage.

Previous retrospective studies — analyses of past patient data — had suggested fractionated radiosurgery works well. But the approach had never been validated in a randomized clinical trial, the gold standard for comparing treatments. This trial supplies that evidence.

Why did patients live longer?

The survival advantage caught the researchers off guard. The trial was designed primarily to measure whether fractionated treatment reduced recurrence, not whether it extended life.

"The survival benefit was an unexpected finding, and while the exact mechanism remains unclear, we suspect that advances in systemic therapies over the past decade may be interacting with better control of disease within the brain, and overall, better outcomes for patients," Brown says.

That explanation remains a hypothesis, not a proven mechanism. Researchers will likely need further work to confirm why patients lived longer.

What does this mean for patients?

The trial results are strong enough, in the researchers' view, to change standard practice.

"The study findings give us the evidence necessary to transform clinical practice. We can now offer patients a treatment that controls their disease and is associated with improved survival," Brown says.

Robert Mutter, M.D., chair of research in the Department of Radiation Oncology and co-leader of the Novel Therapeutics and Therapeutic Modalities Research Program at Mayo Clinic Comprehensive Cancer Center, called the work "a practice-changing clinical trial in our field, demonstrating that fractionated radiosurgery not only reduces recurrence after surgery for large brain metastases but may also improve overall survival."

For patients, the practical change is modest in burden but potentially large in consequence: instead of one visit to the radiation suite, three to five visits — with no added side effects detected in the trial, better control of the tumor site, and months of additional median survival.

The study was presented as LBA 01 — the Alliance A071801 Phase III trial comparing postoperative single-fraction stereotactic radiosurgery with fractionated stereotactic radiosurgery for resected brain metastasis.