To apply radiosurgical methods to tumors in the chest and abdomen, it is necessary to take into account respiratory motion. Respiratory motion can move the tumor by more than 1 cm. Without compensation for respiratory motion, it is necessary to enlarge the target volume with a safety margin. For small targets, an appropriate safety margin produces a very large increase in treated volume. For a spherical tumor of radius 1 cm, a safety margin of 0.5–1 cm would have to be added to ensure that the tumor remains within the treated volume at all times. The ratio between the radius and volume of a sphere is cubic; thus, a margin of 1 cm will cause an eightfold increase in treated volume. Furthermore, observed motion ranges (>3 cm) suggest that a 1-cm margin may not be suffi cient in all cases. An enlarged margin of 2 cm would result in a 27-fold increase of dose in this example; thus, an accurate method capable of compensating for respiratory motion would be of utmost clinical relevance.