After primary cancer cells break away from the original site of the cancer, circulate the body through the bloodstream and reach the brain, it is thought that the entire brain is “seeded” with metastases. This may be true even if only a single tumor is found. As a result, the preferred conventional radiation therapy treatment for brain metastases has been whole brain radiotherapy (WBRT).
WBRT is just what it sounds like—giving radiation to the entire brain, even to healthy tissue. In WBRT, both the healthy brain cells and cancer cells receive X-Ray radiation, but the amount of radiation dose that can be given is limited by the potential side effects experienced by the patient.
In addition, WBRT must be spread across many sessions or fractions. Anywhere from five to 30 fractions are given on separate days, allowing healthy tissue to recover between each dose. While the intent of whole brain radiation therapy is to limit the growth of metastatic cancer cells, it is well known that this treatment does not provide long term control. [1-2]
 Kondziolka et al. Stereotactic radiosurgery plus whole brain radiotherapy versus radiotherapy alone for patients with multiple brain metastases. Int J Radiat Oncol Biol Phys. 1999 Sep 1;45(2):427-34.
 Choosing Wisely recommendations from the American Society for Radiation Oncology