Determining parameters for respiration-gated radiotherapy

Respiration-gated radiotherapy for tumor sites affected by respiratory moti on will potentially improve radiotherapy outcomes by allowing reduced treat ment margins leading to decreased complication rates and/or increased tumor control. Furthermore, for intensity-modulated radiotherapy (IMRT), respira tory gating will minimize the hot and cold spot artifacts in dose distribut ions that may occur as a result of interplay between respiratory motion and leaf motion. Most implementations of respiration gating rely on the real t ime knowledge of the relative position of the internal anatomy being treate d with respect to that of an external marker. A method to determine the amp litude of motion and account for any difference in phase between the intern al tumor motion and external marker motion has been developed. Treating pat ients using gating requires several clinical decisions, such as whether to gate during inhale or exhale, whether to use phase or amplitude tracking of the respiratory signal, and by how much the intrafraction tumor motion can be decreased at the cost of increased delivery time. These parameters may change from patient to patient. A method has been developed to provide the data necessary to make decisions as to the CTV to PTV margins to apply to a gated treatment plan. (C) 2001 American Association of Physicists in Medic ine.