The inter- and intrafraction variability of liver position was assessed in patients
with liver cancer treated with kilovoltage cone-beam computed tomography (CBCT)-guided
stereotactic body radiotherapy.
A total of 314 CBCT scans obtained in the treatment position immediately before and
after each fraction were evaluated from 29 patients undergoing six-fraction, non-breath-hold
stereotactic body radiotherapy for unresectable liver cancer. Off-line, the CBCT scans
were sorted into 10 bins, according to the phase of respiration. The liver position
(relative to the vertebral bodies) was measured using rigid alignment of the exhale
CBCT liver with the exhale planning CT liver, following the alignment of the vertebrae.
The interfraction liver position change was measured by comparing the pretreatment
CBCT scans, and the intrafraction change was measured from the CBCT scans obtained
immediately before and after each fraction.
The mean amplitude of liver motion for all patients was 1.8 mm (range, 0.1-5.7), 8.0
mm (range, 0.1-18.8), and 4.3 mm (range 0.1-12.1) in the medial-lateral (ML), craniocaudal
(CC), and anteroposterior (AP) directions, respectively. The mean absolute ML, CC,
and AP interfraction changes in liver position were 2.0 mm (90th percentile, 4.2),
3.5 mm (90th percentile, 7.3), and 2.3 mm (90th percentile, 4.7). The mean absolute
intrafraction ML, CC, and AP changes were 1.3 mm (90th percentile, 2.9), 1.6 mm (90th
percentile, 3.6), and 1.5 mm (90th percentile, 3.1), respectively. The interfraction
changes were significantly larger than the intrafraction changes, with a CC systematic
error of 2.9 and 1.1 mm, respectively. The intraobserver reproducibility (sigma, n
= 29 fractions) was 1.3 mm in the ML, 1.4 mm in the CC, and 1.6 mm in the AP direction.
Interfraction liver position changes relative to the vertebral bodies are an important
source of geometric uncertainty, providing a rationale for prefraction soft-tissue
image guidance. The intrafraction change in liver position from the beginning to the
end of each fraction was small for most patients.