The physical properties of lesions best suited for SRS are:
small size sharply defined margins, and favorable shape.
On size, it is customary to quote 3-3.5 cm maximum diameter as the upper limit. In spite of the high precision of targeting ("hitting" the target point: 0.1 mm for gamma knife and 1-2 mm for Linac) and conformal planning, the outer margin of the radiation field is not absolutely sharp. The radiation dose drops very rapidly at the margins but there is a definable penumbra, which receives potentially harmful radiation. The steepness of the margin becomes less with larger lesions. Also, the surface of the lesion increases in proportion to the square of the radius, and thus the volume of adjacent normal brain receiving potentially harmful radiation would become unacceptable for a large target. In these cases single-dose radiation has to be given in lower doses and thus efficacy suffers. This is true whichever equipment one uses.
The margins of the lesion have to be well defined. An infiltrative glioma or a diffuse AVM does not lend itself to precise dose planning. If these lesions are nevertheless treated, the treatment must be considered to be palliative, whereby the aim is to achieve an effect only in the limited volume covered by the treatment plan.
A very flat shape may also pose a dose-planning problem. Examples of these lesions would include "en-plaque" extensions of meningiomas or dural AVMs.
There are many lesions that are adjacent to highly eloquent structures, e.g. the optic chiasm. Good dose planning can minimize the risk to those structures. The precise details of the techniques achieving this are beyond the scope of
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