Positional accuracy of novel x-ray-image-based dynamic tumor-tracking irradiation using a gimbaled MV x-ray head of a Vero4DRT (MHI-TM2000).

PURPOSE To verify the positional accuracy of a novel x-ray-image-based dynamic tumor-tracking (DTT) irradiation technique using the gimbaled MV x-ray head of a Vero4DRT (MHI-TM2000). METHODS Verification of the x-ray-image-based DTT was performed using three components: a three-dimensional moving phantom with a steel ball target, a laser displacement gauge, and an orthogonal kV x-ray imaging subsystem with a gimbaled MV x-ray head and the system controller of the Vero4DRT. The moving phantom was driven based on seven periodic patterns [peak-to-peak amplitude (A): 20-40 mm, breathing period (T): 2-5 s] and 15 patients' aperiodic respiratory patterns (A: 6.5-22.9 mm, T: 1.9-5.8 s). The target position was detected in real time with the orthogonal kV x-ray imaging subsystem using the stereo vision technique. Subsequently, the Vero4DRT predicted the next position of the target, and then the gimbaled MV x-ray head tracked the corresponding orientation of the target. The displacements of the target were measured synchronously using the laser displacement gauge. The difference between the target positions predicted by the Vero4DRT and those measured by the laser displacement gauge was computed as the prediction error (E(P)), and the difference between the target positions tracked by the gimbaled MV x-ray head and predicted target positions was computed as the mechanical error (E(M)). Total tracking system error (E(T)) was defined as the difference between the tracked and measured target positions. RESULTS The root mean squares (RMSs) of E(P), E(M), and E(T) were up to 0.8, 0.3, and 0.7 mm, respectively, for the periodic patterns. Regarding the aperiodic patterns, the median RMSs of E(P), E(M), and E(T) were 1.2 (range, 0.9-1.8) mm, 0.1 (range, 0.1-0.5) mm, and 1.2 (range, 0.9-1.8) mm, respectively. From the results of principal component analysis, tracking efficiency, defined as the ratio of twice the RMS of E(T) to A, was improved for patients with high respiratory function (R = 0.91; p < 0.01). CONCLUSIONS The present study demonstrated that the Vero4DRT is capable of high-accuracy x-ray-image-based DTT. E(T) was caused primarily by E(P), and E(M) was negligible. Furthermore, principal component analysis showed that tracking efficiency could be improved with this system, especially for patients with high respiratory function.