Inter-breath-hold motion in gated cone-beam CT introducing uncertainties in image-guided radiotherapy for breath-hold treatment

Active breathing coordination (ABC) devices have been used to effectively manage respiratory motion by temporarily holding a patient’s breath. To acquire a gated cone beam computed tomography (CBCT) with ABC, multiple breath holds (BH) (∼3) were required because of slow gantry rotation and short breath-hold periods. The variability of inter-BH tumor position introduced artifacts in the reconstructed CBCT images and affected tumor localization. This study aimed to quantify the effect of inter-BH tumor displacement on target localization in ABC-gated CBCT image guidance. Numerical phantoms with different sized “tumors” and a physical phantom with programmable moving inserts were used for this study. The cone-beam projection images of the numerical/physical phantom were obtained from a full 360° CBCT using the half-fan mode. The off-set detector geometry in the half-fan mode resulted in “missing” projections through the tumor for some gantry angles. We first investigated the gated CBCT data acquisition consisted of three angular sectors corresponding to the three breath holds. Three inter-BH displacements in the longitudinal direction, denoted as s i (i = 1, 2, 3), were introduced in each breath hold. Two inter-BH motion patterns (type 1: s 1 = s 3 ≠ s 2 and type 2: s 1 ≠ s 2 ≠ s 3 ) were tested. The effect of inter-BH motions on target localization was quantified by calculating the centroid shifts of the target in CBCT images. From numerical simulations, we observed that the inter-BH motion magnitudes in the gated CBCT scan affect the relative tumor centroid shifts for both types of motion patterns. The relative tumor centroid shifts were linearly correlated with the weighted average motion magnitude during the CBCT scan. The slopes were close to one and depended on the tumor location and size. The severity of the image artifacts was correlated to the absolute difference of the inter-BH motion magnitude of two “relevant” breath holds, which have minimal overlap with the gantry angles with “missing” information of tumor, and depended on the tumor location and size. The relative tumor centroid shifts can be up to the maximum magnitude of the inter-BH motion. The image artifacts were visible if the absolute difference of the motion magnitude in the two “relevant” breath holds exceeding 2 mm. Numerical simulations were also performed to study the impact of total number of breath hold (three, four, five, and six breath holds) in one gated CBCT scan. The linear relationship between the relative centroid shifts and the weighted average inter-BH motion did not depend on the total number of breath hold used in one CBCT scan. Numerical simulations for full-fan scan also demonstrated that the relative centroid shifts were proportional to the weighted average motion magnitudes with equal weights. For a gated CBCT with three breath holds, physical phantom studies confirmed the observations in the numerical simulations for half-fan gated CBCT. Inter-BH movement caused tumor location variation and could be detected in gated CBCT in both half-fan and full-fan modes. The displacement obtained from the half-fan acquisition deviated from the average inter-BH motion obtained from the full-fan acquisition. Tumor displacement was sensitive to the complexity of the inter-BH motion pattern with half-fan CBCT due to “missing data” in the data acquisition. Patient specific planning margins were recommended to take into account the inter-BH motion.