Understanding Oligometastatic Disease and How Stereotactic Radiotherapy Can Help

What Is Oligometastatic Disease?

Oligometastatic disease describes cancer that has spread beyond its original site but remains limited to a relatively small number of metastases. International consensus commonly describes this as one to five metastatic lesions, although the number alone does not determine whether treatment is appropriate. The location of the disease, the type of cancer, its behaviour over time and whether all visible sites can be safely treated are equally important.

This represents a potentially important stage between localised cancer and widespread metastatic disease. In some patients, the cancer may have developed the ability to spread but has not yet produced extensive disease throughout the body. Modern PET, CT and MRI imaging has made this state increasingly visible by detecting small metastatic deposits that previously would not have been identified.

For selected patients, this creates an opportunity to treat the individual sites of disease directly rather than relying on systemic therapy alone.

Why Treat Individual Metastases?

One reason oligometastatic disease is important is that metastases are not always simply the end result of cancer spread. Genetic studies comparing multiple metastases from the same patient have shown that one metastatic deposit can sometimes seed another. By analysing the pattern of mutations within different tumours, researchers have been able to reconstruct these evolutionary pathways and show how cancer can spread progressively from one site to the next.

This provides part of the biological rationale for metastasis-directed treatment. By ablating an active metastatic site with SABR, we may not only control that particular tumour but also remove a potential source of further spread. This does not guarantee that new metastases will not appear, as microscopic disease may already exist elsewhere, but it helps explain why treating a limited number of metastatic sites can potentially influence the subsequent course of the disease.

SABR is particularly suited to this approach because it can deliver a highly focused dose to individual metastases over a small number of treatments. In oligoprogressive disease, where most of the cancer remains controlled by systemic therapy but one or a few sites begin to grow, treating those resistant areas can also sometimes allow an otherwise effective treatment to continue for longer before the next line of therapy is required.

What Does the Evidence Show?

The concept of treating oligometastatic disease has now been investigated in randomised clinical trials.

One of the landmark studies was SABR-COMET, which included patients with a limited number of metastases from several different primary cancers. Long-term follow-up demonstrated better progression-free and overall survival among patients who received SABR to all known metastatic sites compared with standard care alone. At eight years, overall survival was approximately 27% with SABR compared with 14% with standard care.

The study was important because it supported the idea that treating individual metastatic deposits can influence the broader course of disease rather than simply providing local tumour control. It was nevertheless a relatively small phase II study involving several different cancer types, so its results should not automatically be applied to every patient with metastatic cancer.

Since then, disease-specific trials have strengthened the evidence in areas including prostate and lung cancer. Increasingly, the benefits of metastasis-directed treatment are also being measured by outcomes such as local control, progression-free survival and the time before additional systemic treatment is required.

These endpoints are particularly relevant because the purpose of SABR in oligometastatic disease is not always to eradicate every cancer cell permanently. For many patients, the clinically meaningful goal is to control the known sites of disease and delay the next stage of treatment for as long as reasonably possible.

Our Approach to Oligometastatic Disease

At Pinpoint Radiotherapy, the decision to use SABR for oligometastatic disease involves much more than simply counting the number of lesions on a scan.

We consider the type and biology of the cancer, where the metastases are located, how quickly the disease has changed, the quality of the imaging, previous and current systemic treatments and whether all clinically important sites can be treated safely.

The pattern of disease over time can be particularly informative. A patient with one slowly developing metastatic site after several years of otherwise controlled disease represents a very different biological situation from someone developing multiple new lesions over a few weeks, even if both initially appear to have a similar number of metastases.

For selected patients, the aim may be durable control of all currently visible disease. For others, particularly those with oligoprogressive disease, the goal may be to eliminate resistant sites and delay the need to change an otherwise effective systemic treatment.

Modern SABR can target metastases in areas including lymph nodes, bone, lung, liver, kidney, brain and spine with high precision. The important question is therefore not simply whether a patient has a small number of metastases, but whether treating those sites is likely to make a meaningful difference to the subsequent course of their cancer.