Oligometastatic breast cancer: where are we now and where are we headed?-a narrative review

Metastatic breast cancer has traditionally been considered incurable, with treatments focused on systemic therapies and palliative local treatment. However, evidence is emerging that in some patients with limited metastatic disease, or "oligometastatic disease," often defined as five or fewer metastases diagnosed on imaging, aggressive metastasis-directed therapy (MDT) with surgery and/or hypofractionated image-guided radiation therapy (HIGRT) improves outcomes and may even be curative. This practice is becoming more common as evidence has grown to support the approach and as technology has made it more feasible. Treatment of certain oligometastatic breast cancers in particular (i.e., hormone receptor positive and bone-only metastases) may be especially useful given the long natural history of the disease in some of these patients. Recently, high quality data supporting ablative MDT in patients with oligometastatic disease has emerged from randomized trials for specific sites such as non-small cell lung cancer and prostate cancer, as well as from histology agnostic studies (i.e., SABR-COMET). However, randomized data in breast cancer specifically is currently lacking. Retrospective series and subgroup analysis from prospective trials have demonstrated improved outcomes with MDT for oligometastatic breast cancer. The ongoing phase II/III NRG BR002 trial seeks to provide the first randomized data to determine whether MDT in oligometastatic breast cancer improves outcomes. This may be especially important as improved systemic therapies such as targeted agents and immunotherapy prolong the disease course. Alternatively, if improved systemic therapies render patients disease free, MDT may not be necessary and only adds toxicity. However, MDT may also provide non-curative benefits for patients such as palliation of symptoms and extended time off systemic therapy. For now, aggressive MDT for certain favorable subgroups of oligometastatic breast cancer such as those with few metastases, hormone positive disease, and/or bone-only metastases is reasonable and may improve outcomes. We eagerly anticipate the results of NRG BR002 to further clarify the role of ablative therapy to all sites of disease in these patients.