Performance evaluation of respiratory motion-synchronized dynamic IMRT delivery

The purpose of this study was to evaluate the capabilities of DMLC to deliver the respiratory motion-synchronized dynamic IMRT (MS-IMRT) treatments under various dose rates. In order to create MS-IMRT plans, the DMLC leaf motions in dynamic IMRT plans of eight lung patients were synchronized with the respiratory motion of breathing period 4 sec and amplitude 2 cm (peak to peak) using an in-house developed leaf position modification program. The MS-IMRT plans were generated for the dose rates of 100 MU/min, 400 MU/min, and 600 MU/min. All the MS-IMRT plans were delivered in a medical linear accelerator, and the fluences were measured using a 2D ion chamber array, placed over a moving platform. The accuracy of MS-IMRT deliveries was evaluated with respect to static deliveries (no compensation for target motion) using gamma test. In addition, the fluences of gated delivery of 30% duty cycle and non- MS-IMRT deliveries were also measured and compared with static deliveries. The MS-IMRT was better in terms of dosimetric accuracy, compared to gated and non-MS-IMRT deliveries. The dosimetric accuracy was observed to be significantly better for 100 MU/min MS-IMRT. However, the use of high-dose rate in a MS-IMRT delivery introduced dose-rate modulation/beam hold-offs that affected the synchronization between the DMLC leaf motion and target motion. This resulted in more dose deviations in MS-IMRT deliveries at the dose rate of 600 MU/min.