Interfraction and Intrafraction Changes in Amplitude of Breathing Motion in Stereotactic Liver Radiotherapy

Purpose Interfraction and intrafraction changes in amplitude of liver motion were assessed in patients with liver cancer treated with kV cone beam computed tomography (CBCT)-guided stereotactic body radiation therapy (SBRT). Methods and Materials A total of 314 CBCTs obtained with the patient in the treatment position immediately before and after each fraction, and 29 planning 4DCTs were evaluated in 29 patients undergoing six-fraction SBRT for unresectable liver cancer, with ( n = 15) and without ( n = 14) abdominal compression. Offline, the CBCTs were sorted into 10 bins, based on phase of respiration. Liver motion amplitude was measured using liver-to-liver alignment from the end-exhale and end-inhale CBCT and four-dimensional CT reconstructions. Inter- and intrafraction amplitude changes were measured from the difference between the pre-SBRT CBCTs relative to the planning four-dimensional CT, and from the pre-SBRT and post-SBRT CBCTs, respectively. Results Mean liver motion amplitude for all patients (range) was 1.8 (0.1–7.0), 8.0 (0.1–18.8), and 4.3 (0.1–12.1) mm in the mediolateral (ML), craniocaudal (CC), and anteroposterior (AP) directions, respectively. Mean absolute inter- and intrafraction liver motion amplitude changes were 1.0 (ML), 1.7 (CC), and 1.6 (AP) mm and 1.3 (ML), 1.6 (CC), and 1.9 (AP) mm, respectively. No significant correlations were found between intrafraction amplitude change and intrafraction time (range, 4:56–25:37min:sec), and between inter- and intrafraction amplitude changes and liver motion amplitude. Intraobserver reproducibility (σ, n = 29 fractions) was 1.3 (ML), 1.4 (CC), and 1.4 (AP) mm. Conclusions For the majority of liver SBRT patients, the change in liver motion amplitude was minimal over the treatment course and showed no apparent relationships with the magnitude of liver motion and intrafraction time.