Limited brain metastases: a narrative review

Limited brain metastases refer to an oligometastatic state in the brain for which focal therapy with stereotactic radiosurgery (SRS) is appropriate. The definition of what is considered "limited" for brain metastases, however, is not well defined. Multiple recent randomized trials show that metastasis-directed therapy with surgery or stereotactic radiation can prolong survival for patients with 1-5 metastases from various primary tumors, but patients with untreated brain metastases were largely excluded from these trials. Stereotactic radiosurgery allows for the treatment of multiple brain metastases while avoiding whole brain radiation (WBRT), which has a known negative impact on neurocognitive function. Randomized trials have shown that stereotactic radiation alone is effective for the treatment of up to 4 brain metastases with decreased neurocognitive side effects and no detriment to overall survival (OS). These data have led to our current definition of "limited" brain metastases. However, more recent data suggests that patients with up to 10 brain metastases can benefit from SRS. Ongoing trials are investigating if patients with up to 15-20 brain metastases may similarly benefit from SRS alone. Advances in systemic therapies which penetrate the blood-brain barrier and have greater activity in central nervous system (CNS) may also expand the population for whom radiosurgery is an appropriate treatment. Immunotherapy may also be synergistic with radiosurgery in some cases. This narrative review will discuss the evolving definition and management of limited brain metastases.