Target dose versus extratarget dose in stereotactic radiosurgery.

In radiosurgery the dose in the periphery of the target is generally about 50-70% of the maximum dose. This type of dose distribution may be expected to be beneficial when tumours containing anoxic, more radioresistant, cells are treated. However, the impact of a high maximum dose in the target on the dose outside the target is of importance for the risk of complications. Dose distributions have been calculated for different beam geometries applicable to radiosurgery by integration of the dose from a continuum of beams. The dose distributions were calculated for beams of different diameters, and the distributions were weighted to give the same dose in the periphery of the target. With a 4 pi beam geometry it is shown that the maximum dose in the central parts of the target can be increased by a factor of 2, by using beams with smaller diameters, at the cost of a marginal increase of the dose outside the target. The average dose in the normal tissue outside increases from 29.8% with 18 mm beams to 34.1% with 14 mm beams, while the average dose within the target increases from 100 to 158%. In radiosurgery of tumours a high maximum dose in the target may give a therapeutic gain.