Among 44 patients included in this study, 118 pairs of images from 23 patients (without stent/alignment) with both CBCT and kV-orthogonal images positioned with vertebral bones and 53 pairs of images from 9 patients (with stent/alignment) with CBCT and kV-orthogonal images aligned separately with vertebral bodies and with stent/lipiodol were analyzed. Pearson’s correlation coefficient was calculated to compare the accuracy of the two alignment methods on kV-orthogonal images. As shown in Fig. 4, correlation was significantly better with stent/lipiodol than with vertebral bone in the AP, (r2 = 0.991, p < 0.001 vs. r2 = 0.280, p < 0.001), CC (r2 = 0.991, p < 0.001 vs. r2 = 0.295, p < 0.001), and RL axis (r2 = 0.964, p < 0.001 vs. r2 = 0.257, p = 0.933).
The 28 patients using the SDX system maintained a median breath-hold interval of 30 seconds (range: 25–40 seconds). The average BHL was 1.41 L (range: 0.76–2.08 L). ΔBHL ranged from 0.015 L to 0.045 L, with a median ΔBHL of 0.03 L. The positional errors in the AP, CC, and RL directions were 0.28 ± 0.15 cm, 0.40 ± 0.22 cm and 0.23 ± 0.14 cm, respectively.
Patients with mean BHL > 1.4 L were significantly taller (167.6 cm vs. 161.6 cm, p = 0.03) and heavier (67.1 kg vs. 57.4 kg, p = 0.02) than those with BHL < 1.4 L (Fig. 5A). In addition, significantly larger positional errors in the CC direction (-0.26 cm [caudally] vs. +0.09 cm [cranially], p = 0.01), but similar shifts in the AP (-0.13 cm vs. +0.04 cm, p = 0.08) and RL direction (+ 0.00 cm vs. +0.08 cm, p = 0.35), were found in patients with mean BHL > 1.4 L compared to those with BHL < 1.4 L, respectively (Fig. 5B). Other patient-related factors, including age, liver volume, and gross tumor volume, were not significantly associated with positional error (Table 2).
Table 2
Comparison in positional errors between subgroups
| | Average positional shift ± SD (mm) |
Subgroup | n | AP | CC | RL |
Overall | 28 | -0.48 ± 2.54 | -0.85 ± 3.72 | + 0.41 ± 2.35 |
Age | | | | |
≤ 60 y/o | 13 | -0.17 ± 2.71 | -1.89 ± 2.95 | + 0.71 ± 2.45 |
> 60 y/o | 15 | -0.74 ± 2.35 | + 0.06 ± 4.07 | + 0.14 ± 2.23 |
p value | | 0.58 | 0.17 | 0.54 |
BMI | | | | |
≤ 22.9 | 14 | -0.73 ± 2.73 | -0.19 ± 2.32 | + 0.66 ± 2.70 |
> 22.9 | 14 | -0.22 ± 2.30 | -1.50 ± 4.63 | + 0.27 ± 1.91 |
p value | | 0.61 | 0.38 | 0.59 |
Mean BHL | | | | |
≤ 1.4L | 14 | -1.34 ± 2.00 | + 0.90 ± 2.64 | -0.03 ± 2.30 |
> 1.4L | 14 | + 0.39 ± 2.72 | -2.59 ± 3.82 | + 0.84 ± 2.32 |
p value | | 0.08 | 0.01 | 0.35 |
ΔBHL | | | | |
≤ 0.03L | 18 | -0.15 ± 2.87 | -0.81 ± 4.25 | + 0.66 ± 2.43 |
> 0.03L | 10 | -1.06 ± 1.65 | -0.90 ± 2.50 | -0.06 ± 2.14 |
p value | | 0.31 | 0.95 | 0.45 |
Liver volume | | | | |
≤ 1.3L | 13 | -0.43 ± 1.59 | -1.38 ± 3.80 | + 0.65 ± 1.92 |
> 1.3L | 15 | -0.51 ± 3.14 | -0.39 ± 3.59 | + 0.20 ± 2.65 |
p value | | 0.93 | 0.50 | 0.62 |
GTV | | | | |
≤ 60 cm3 | 14 | -0.60 ± 2.06 | -1.49 ± 3.30 | + 1.09 ± 1.72 |
> 60 cm3 | 14 | -0.35 ± 2.94 | -0.20 ± 4.00 | -0.28 ± 2.68 |
p value | | 0.80 | 0.38 | 0.13 |
Under the rigorous protocol for breath-hold precision, the positional errors did not significantly differ between patients with ΔBHL < 0.03 L and ΔBHL > 0.03 L in the AP (-0.026 cm vs. -0.07 cm, p = 0.66), CC (-0.067 cm vs. -0.102 cm, p = 0.81), and RL (0.048 cm vs. 0.033 cm, p = 0.88) directions (Table 2).
Height and weight correlated with mean BHL (r = 0.605 and 0.502, p = 0.001 and 0.007, respectively). The Pearson correlation coefficient between mean BHL and positional error in the CC direction was − 0.346 (p = 0.071). BHL was not correlated with positional errors in the AP (r = 0.270, p = 0.165) or RL (r = 0.244, p = 0.211) direction. There was no correlation between ΔBHL and positional errors in the AP (r=-0.147, p = 0.456), CC (r = 0.031, p = 0.874), or RL (r = 0.024, p = 0.902) directions.