Geometric accuracy of dynamic MLC tracking with an implantable wired electromagnetic transponder

Background. Tumor motion during radiotherapy delivery can substantially deteriorate the target dose distribution. A promising method to overcome this problem is dynamic multi-leaf collimator (DMLC) tracking. The purpose of this phantom study was to integrate a wired electromagnetic (EM) transponder localization system with DMLC tracking and to investigate the geometric accuracy of the integrated system. Material and methods. DMLC tracking experiments were performed on a Trilogy accelerator with a prototype DMLC tracking system. A wired implantable EM transponder was mounted on a motion stage with a 3 mm tungsten sphere used for target visualization in continuous portal images. The three dimensional (3D) transponder position signal was used for DMLC aperture adaption. The motion stage was programmed to reproduce eight representative patient-measured trajectories for prostate and for lung tumors. The tracking system latency was determined and prediction was used for the lung tumor trajectories to account for the latency. For each trajectory, three conformal fields with a 10 cm circular MLC aperture and 72 s treatment duration were delivered: (1) a 358 arc field; (2) an anterior static field; and (3) a lateral static field. The tracking error was measured as the difference between the marker position and the MLC aperture in the portal images. Results. The tracking system latency was 140 ms. The mean root-mean-square (rms) of the 3D transponder localization error was 0.53/0.54 mm for prostate/lung tumor trajectories. The mean rms of the two dimensional (2D) tracking error was 0.69 mm (prostate) and 0.98 mm (lung tumors) with tracking and 3.4 mm (prostate) and 5.3 mm (lung tumors) without tracking. Conclusions. DMLC tracking was integrated with a wired EM transponder localization system and investigated for arc and static field delivery. The system provides sub-mm geometrical errors for most trajectories.