In principle, infiltrative tumors should not be considered to be suitable for a focal treatment. This statement is particularly true for highgrade gliomas that infiltrate far from the main mass. We rarely use this technique in such cases. However, in the case of metastases, local infiltration into the normal tissue is fairly limited and thus both surgery and radiosurgery confer a benefit by local cytoreduction. Furthermore, radiosurgery treatment has a penumbral effect, as opposed to the truly sharp boundary with surgical excision, and this may even result in superior local control compared with surgery alone. A multicenter randomized trial is in progress in Europe, which compares gamma knife radiosurgery alone to surgery with whole-brain radiotherapy.

This pathology is especially important for healthcare planners. During the course of their disease, 20-40% of cancer patients will develop brain metastases, and this predicts almost half a million patients with brain metastases per year in the USA alone. Given the number of cases, the following question arises: "How many of these cases are appropriate for radiosurgery?" Appropriate is defined as "the patient stands a reasonable chance of obtaining a benefit in terms of control of brain disease and retention/improvement in quality of life with treatment". In order to identify a group of patients who might benefit from more aggressive treatment, such as radiosurgery, one can use the experience of the RTOG (Radiation Therapy Oncology Group) protocol 79-16, a radiation therapy protocol on brain metastases. The four factors predictive of improved survival were:

Karnofsky performance status >70%

Absent or controlled primary disease

Age <60 years

Evidence of metastasis to brain only

In our unit we use these criteria to include patients in the radiosurgery program, but also consider the number of metastases visible on MRI. We are reluctant to treat in cases where there are more than three lesions as the benefits of focal treatment are lost when there is an obvious generalized disease. Technically speaking, most metastases are close to ideal for radio-surgery. They are easily visible on imaging, they are rarely beyond the suitable size-range, and they are usually spherical with sharp outlines, therefore the matching of the shape of the lesion is easy. There is published evidence from a multicenter study [23] showing that there was a 2.84 times higher risk of local recurrence after LINAC treatment than after gamma knife treatment. The reason for this finding is controversial but it may question in the future the acceptability of LINAC use, as opposed to gamma knife use, for this indication.

It is particularly important in this patient group to state that the aim of radiosurgery is only to achieve local tumor control. This may be achievable in up to 90% of cases. Prospective randomized trials are needed to properly define the role of radiosurgery for cerebral metastases.

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