Organ and tumor motion: an overview.

An overview is presented of some of the issues that arise when considering how to manage motion of the patient (including setup errors), the tumor within the patient, and normal tissues that are sufficiently close to the tumor as to be likely to be at least partially irradiated. Problems arise in 3 areas: (1) at the tumor periphery where the question of just where the tumor surface is must be decided, (2) within the tumor proper where interplay effects between tumor motion and the radiation application (eg, in intensity-modulated radiation therapy) may result in dose inhomogeneity within the tumor, and (3) just outside the target volume where the extent to which organs at risk are to be irradiated (which needs to be based on an estimate of the consequences to them) must be decided. In the context of these problems, one must decide how to deal with potential motion. This involves 3 issues: (1) how to limit motion-the available techniques ranging from simple and unobtrusive to complicated, quasi-invasive, and quite costly; (2) how to handle the residual motion; and (3) how to calculate and present the consequences of the residual motion. These issues are discussed, and the importance of an explicit estimation of uncertainty is emphasized.