Adaptation of High-Precision Radiotherapy to Moving Target Volumes in Real-Time Using Dynamic Multileaf Collimators

Abstract

High quality dose distributions achievable with advanced radiation therapy techniques such as IMRT require a precise and accurate delivery of modulated fluence patterns to the target volume. Intrafractional target motion, however, considerably deteriorates the geometric accuracy of the delivery process. Therefore, a dynamic control system was developed which enabled the real-time adaptation of the dose delivery for moving target volumes. For this purpose, algorithms were developed which calculated an optimized multileaf collimator (MLC) aperture compensating for target motion and deformation. In addition, it is possible to completely spare adjacent organs-at-risk. Delay times of the MLC control system are accounted for by a linear prediction filter. Furthermore, different field definition modes were designed and included. The algorithms were used to develop a dynamic target tracking control system for the Siemens 160 MLC. To assess the quality of the tracking technique, experiments were performed with different phantoms. The experiments proved that 2-dimensional target motion can be compensated for with the new control system. The congruence within 2 % / 2 mm with a static reference delivery was increased from 18.75 % to 76.79 % for an IMRT dose distribution. For the delivery of a complete treatment fraction to a lung phantom, similar improvements were observed. However, the system's latency times reduced the accuracy depending on the specific target motion. It was demonstrated that the developed tracking concepts algorithms and the new control system can effectively compensate for target motion in real-time and therefore significantly increase the accuracy of the treatment delivery.