Addressing relative motion of tumors and normal tissue during dynamic MLC tracking delivery

A general methodology for addressing relative motion of targets and normal tissue for dynamic-MLC (DMLC) tracking is discussed. The basic idea is to exploit the extra degrees of freedom that tissue motion introduces into DMLC delivery. This principle is illustrated through the use of a simple example which uses a moving rigid target whose projection in the beam's eye view intersects a stationary critical organ. DMLC delivery which tracks the target motion is simulated, and it is shown that the choice of leaf motion and the time of the onset of delivery can have a large impact on the exposure of the critical organ. Depending on the choice of these parameters, the integral MU delivered to the critical organ ranged from -75% to +250% relative to the delivery planned for static geometry. These results indicate that deliveries using DMLC tracking should accommodate any normal tissue that is moving with respect to the tumor. This should be done to (1) avoid deliveries which result in large over-exposures of critical organs, and (2) to seek out deliveries that result in critical organ exposures that are lower than what is possible with any static geometry (4D therapy).