Evaluation of two synchronized external surrogates for 4D CT sorting

The purpose of this study was to quantify the performance and agreement between two different external surrogate acquisition systems: Varian's Real-Time Position Management (RPM) and Philips Medical Systems' pneumatic bellows, in the context of waveform and 4D CT image analysis. Eight patient displacement curves derived from RPM data were inputted into a motion platform with varying amplitudes (0.5 to 3 cm) and patterns. Simultaneous 4D CT acquisition, with synchronized X-ray on detection, was performed with the bellows and RPM block placed on the platform. Bellows data were used for online retrospective phase-based sorting, while RPM data were used for off-line reconstruction of raw 4D CT data. RPM and bellows breathing curves were resampled, normalized, and analyzed to determine associations between different external surrogates, relative amplitude differences, and system latency. Maximum intensity projection (MIP) images were generated, phantom targets were delineated, and volume differences, overlap index, and Dice similarity coefficient differences were evaluated. A prospective patient study of ten patients was performed and waveforms were evaluated for latency (i.e., absolute time differences) and overall agreement. 4D CT sorting quality and subtraction images were assessed. Near perfect associations between the RPM and bellows-acquired breathing traces were found (Pearson's r = 0.987-0.999). Target volumes were 200.4 ± 12 cc and 199.8 ± 12.6 cc for RPM and bellows targets, respectively, which was not significantly different (U = 33, p > 0.05). Negligible centroid variations were observed between bellows and RPM-contoured MIP targets (largest discrepancy = -0.24 ± 0.31 mm in superior-inferior direction). The maximum volume difference was observed for an RPM target 2.5 cc (1%) less than bellows, yielding the largest difference in centroid displacement (0.9 mm). Strong correlations in bellows and RPM waveforms were observed for all patients (0.947 ± 0.037). Latency between external surrogates was < 100 ms for phantom and patient data. Negligible differences were observed between MIP, end-exhale, and end-inhale phase images for all cases, with delineated RPM and bellows lung volumes demonstrating a mean difference of -0.3 ± 0.51%. Dice similarity coefficients and overlap indices were near unity for phantom target volumes and patient lung volumes. Slight differences were observed in waveform and latency analysis between Philips bellows and Varian's RPM, although these did not translate to differences in image quality or impact delineations. Therefore, the two systems were found to be equivalent external surrogates in the context of 4D CT for treatment planning purposes.