Exhale fluctuation in respiratory-gated radiotherapy of the lung: A pitfall of respiratory gating shown in a synchronized internal/external marker recording study

PurposeFor optimal respiratory-gated radiotherapy, exhale fluctuation was assessed by monitoring internal fiducials in a synchronized internal/external marker detection system.MethodsSynchronized internal/external position data were collected during the entire course of treatments for 12 lung patients with 24 fiducials. Baseline was determined in the exhale phase during pre-treatment observation time, and a gating level of external waves was set in each treatment session in a simulation of respiratory-gated radiotherapy. Patients were treated under a real-time tumor-tracking (RTRT) system with an external (abdominal) respiratory motion detector. In the simulation, external gating windows were defined as those below the 30% amplitude level (i.e., imaginary beams would be triggered when part of the respiratory wave falls into this window). Exhale fluctuation (EF) was defined as the phenomenon in which the lowest point of the external wave crossed downward past the pre-determined baseline. Gating efficiency (GE) was defined as the ratio between the amount of gate-ON time and the total treatment time.ResultsEF occurred in 18.4% of total measurements. EF varied depending on the patient, fiducial sites, and treatment session. The mean incidence of EF for each patient varied from 2.9% to 37.5% (18.4 ± 9.9). The EF magnitude was 0.2–12.2 mm in the left–right direction, 0.7–12.7 mm in the cranio–caudal direction, and 0.4–9.7 mm in the anterior–posterior direction. Total fiducial movement was 0.5–28.7 mm. GE was 36.1–69.2% (55.4 ± 11.0). EF magnitude correlated with total fiducial movement.ConclusionThis study showed that EF is not a rare phenomenon and needs to be taken into consideration for individualized precise 4D radiotherapy.