Use of dMLC for implementation of dynamic respiratory-gated radiation therapy

Abstract

Purpose: To simulate and evaluate the use of dynamic multileaf collimators (dMLC) in respiratory gating to compensate for baseline drift.

Methods: Tumor motion tracking data from 30 lung tumors over 322 treatment fractions was analyzed with the finite state model. A dynamic respiratory gating window was established in real-time by determining the average positions during the previous two end-of-expiration breathing phases and centering the dMLC aperture on a weighted average of these positions. A simulated dMLC with physical motion constraints was used in dynamic gating treatment simulations. Fluence maps were created to provide a statistical description of radiation delivery for each fraction. Duty cycle was also calculated for each fraction.

Results: The average duty cycle was 2.3% greater under dynamic gating conditions. Dynamic gating also showed higher fluences and less tumor obstruction. Additionally, dynamic gating required fewer beam toggles and each delivery period was longer on average than with static gating.

Conclusions: The use of dynamic gating showed better performance than static gating and the physical constraints of a dMLC were shown to not be an impediment to dynamic gating.

Figure 1

Example of dMLC gating. This figure shows the motion trace of the first three breathing cycles of a sample patient and the corresponding dMLC shapes during the treatment fraction. During the first breathing cycle the dMLC is closed. In subsequent EOE phases, the dMLC opening is informed primarily by the previous EOE phase. The graphical representations of the tumor and dMLC are placed above the EOE phases to which they correspond. The arrows indicate the motion data that are predominantly informing the location of the dMLC aperture. The change in tumor color indicates that it is recognized as being within the aperture, so radiation is being delivered.