Evaluation of tumor localization accuracy on fast ring-gantry cone-beam computed tomography using patient-specific breathing curves and a dynamic anthropomorphic thorax phantom.

Background and purpose: Fast cone beam computed tomography (CBCT) on ring-gantry systems allows for improved image quality and fast 6-second acquisition. However, 6-second acquisition might pose challenges regarding target delineation and capturing the full range of motion of moving lung tumors. Especially, in patients with slow (period > 6 s) or irregular breathing, capturing the entire tumor motion might not be guaranteed. Materials and methods: This study evaluated localization and volumetric accuracy of 6- and 60-second CBCT scans in an in-house dynamic anthropomorphic thorax phantom, with synchronized imaging capabilities. The phantom was scanned for a sinusoidal and four patient-derived breathing patterns, including regular, slow or irregular breathing. Target position and volume for 6- and 60-second acquisitions were compared to ground truth delineation on time-averaged 4D computed tomography (4DCT) reconstruction, assessing if 6-second acquisition is sufficient to accurately capture the tumor motion. Results: For sinusoidal and regular patient-derived motion, both 6- and 60-second CBCT acquisition captured target motion, compared to 4DCT (Dice Similarity Coefficient, DSC > 0.9). For large amplitudes, only one out of three 6-second scans fully captured target motion (DSC > 0.85). For slow and irregular patient-derived patterns, localization errors and volume differences up to 10.3 mm and 119% were observed using 6-second acquisition, compared to 4DCT, with superior localization and volumetric accuracy of the 60-second acquisition. Conclusion: The 6-second protocol showed accurate results, capturing full target motion for regular breathing patterns. Adaptive protocols, taking into account patient-specific breathing periods and irregularities may be preferred in patients exhibiting slow or irregular breathing.