Integrated radiotherapy imaging system (IRIS): design considerations of tumour tracking with linac gantry-mounted...

The design of an integrated radiotherapy imaging system (IRIS), consisting of gantry mounted diagnostic (kV) x-ray tubes and fast read-out flat-panel amorphous-silicon detectors, has been studied. The system is meant to be capable of three main functions: radiographs for three-dimensional (3D) patient set-up, cone-beam CT and real-time tumour/marker tracking. The goal of the current study is to determine whether one source/panel pair is sufficient for real-time tumour/marker tracking and, if two are needed, the optimal position of each relative to other components and the isocentre. A single gantry-mounted source/imager pair is certainly capable of the first two of the three functions listed above and may also be useful for the third, if combined with prior knowledge of the target's trajectory. This would be necessary because only motion in two dimensions is visible with a single imager/source system. However, with previously collected information about the trajectory, the third coordinate may be derived from the other two with sufficient accuracy to facilitate tracking. This deduction of the third coordinate can only be made if the 3D tumour/marker trajectory is consistent from fraction to fraction. The feasibility of tumour tracking with one source/imager pair has been theoretically examined here using measured lung marker trajectory data for seven patients from multiple treatment fractions. The patients' selection criteria include minimum mean amplitudes of the tumour motions greater than 1 cm peak-to-peak. The marker trajectory for each patient was modelled using the first fraction data. Then for the rest of the data, marker positions were derived from the imager projections at various gantry angles and compared with the measured tumour positions. Our results show that, due to the three dimensionality and irregular trajectory characteristics of tumour motion, on a fraction-to-fraction basis, a 'monoscopic' system (single source/imager) is inadequate for consistent real-time tumour tracking, even with prior knowledge. We found that, among the seven patients studied with peak-to-peak marker motion greater than 1 cm, five cases have mean localization errors greater than 2 mm and two have mean errors greater than 3 mm. Because of this uncertainty associated with a monoscopic system, two source/imager pairs are necessary for robust 3D target localization. Dual orthogonal x-ray source/imager pairs mounted on the linac gantry are chosen for the IRIS. We further studied the placement of the x-ray sources/panel based on the geometric specifications of the Varian 21EX Clinac. The best configuration minimizes the localization error while maintaining a large field of view and avoiding collisions with the floor/ceiling or couch.