Session Item

New closed-bore linacs allow for highly streamlined workflows and fast VMAT or IMRT treatment delivery. We measured patient motion during every step of the workflow: image acquisition and evaluation and during treatment delivery using surface scanning. 

Nineteen patients treated for breast, lung or esophageal cancer were prospectively monitored from the end of setup to the end of treatment delivery in the Halcyon linac (Varian Medical Systems) for a total of 221 fractions. Motion of the chest was tracked by way of 6DoF surface tracking at 4 Hz by an in-house built surface guidance system. Cyclic breathing was removed from the signal using a moving average filter. Baseline drift and rate of drift were determined. The contribution of drift during image acquisition and evaluation (IGRT procedure) relative to the total drift was determined. The influence of fraction number, patient and fraction duration were analyzed with multi-way ANOVA.

Median fraction duration was 4 min 48 sec including the IGRT procedure (kV-CBCT acquisition and evaluation). Overall baseline drift of the surface at the end of the fraction was -1.8 ± 1.5 mm in the anterior-posterior, -0.0 ± 1.7 mm in the cranio-caudal direction  and 0.1 ± 1.8 mm in the medio-lateral direction. In 31% of fractions baseline drift was over 3 mm and in 3.6% over 5 mm. The rate of baseline drift was largest between 1 and 2 minutes after the end of patient setup (during the IGRT procedure) namely -0.62 mm/min as shown in figure 1. In 20% of fractions the patients’ position displayed non-monotic behavior by returning to the baseline position after the IGRT procedure resulting in a smaller drift at the end of the fraction. For all other fractions the drift during the IGRT procedure was on average 75% of the total drift, as shown in figure 2. Baseline drift was patient and fraction duration dependent (p < 0.001), but fraction number was not significant (p = 0.33).

Even during brief treatment sessions with a highly streamlined workflow of less than 5 minutes, patient baseline drift is not negligible. Drift is largest during the initial 2 minutes after completion of patient setup, during which verification imaging and evaluation is performed. Surface scanning can detect patient motion during every phase of the intra-bore workflow and will be invaluable for online adaptive treatments.