Localization accuracy of 6-second CBCT for lung IGRT with various breathing patterns.

Purpose: The 6-second cone beam computed tomography (CBCT) acquisition of the Ethos HyperSight (Varian Medical Systems, Inc. Palo Alto, CA, USA) on-board imaging system offers benefits, but could be too fast to accurately capture an average target position in a free-breathing lung cancer patient. This study aimed to ascertain whether a 6-second acquisition is appropriate for regularly breathing patients with varying respiration periods. Additionally, breathing patterns that could lead to clinically impactful inaccuracies for image-guided radiation therapy (IGRT) shifts were investigated.

Methods: Nine regular (sinusoidal) breathing patterns with different respiration rates (8-20 breaths-per-minute) and amplitudes, along with five irregular breathing patterns including a gasp/cough scenario, were tested using a respiratory motion phantom with a 3 cm diameter spherical target. Once the phantom was aligned using the external chassis to remove any default shifts, the 6-second CBCTs with Acuros reconstruction were acquired. Rigid registrations were performed using the 4DCT simulation average/untagged reconstructed image as reference to align to target (simulating clinical lung IGRT setup). Sixty-second CBCTs were also tested to verify the average offsets with conventional practice. The IGRT shifts were compared to 5 mm, a PTV margin typically used for lung tumors.

Results: Minimal (<1.0 mm) shifts were observed for all regular breathing patterns with both 6-second and 60-second CBCT acquisitions. For moderately irregular breathing patterns and the gasp/cough scenario, all shifts were less than 5 mm. Shifts larger than 5 mm were observed in highly irregular breathing patterns, with both 6-second (14%) and 60-second (24%) CBCT acquisitions. Statistical tests showed no significant differences (p > 0.05) between the sizes of shifts made with the two CBCT durations.

Conclusion: The 6-second CBCT can effectively and reliably localize a mobile target for regular and moderately-irregular breathers. Cautions should be used for highly irregular breathers, regardless of the duration of CBCT acquisition.