Intra-fraction motion monitoring during fast modulated radiotherapy delivery in a closed-bore gantry linac.

Abstract

New closed-bore linacs allow for highly streamlined workflows and fast treatment delivery resulting in brief treatment sessions. Motion management technology has only recently been integrated inside the bore, yet is required in future online adaptive workflows. We measured patient motion during every step of the workflow: image acquisition, evaluation and 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). Motion of the chest was tracked by way of 6 degrees-of-freedom surface tracking. Baseline drift and rate of drift were determined. The influence of fraction number, patient and fraction duration were analyzed with multi-way ANOVA. Median fraction duration was 4min 48 s including the IGRT procedure (kV-CBCT acquisition and evaluation) (N=221). Baseline drift at the end of the fraction was -1.8±1.5mm in the anterior-posterior, -0.0±1.7mm in the cranio-caudal direction and 0.1±1.8mm in the medio-lateral direction of which 75% occurred during the IGRT procedure. The highest rate of baseline drift was observed between 1 and 2min after the end of patient setup (-0.62mm/min). Baseline drift was patient and fraction duration dependent (p<0.001), but fraction number was not significant (p=0.33). Even during short treatment sessions, patient baseline drift is not negligible. Drift is largest during the initial minutes after completion of patient setup, during verification imaging and evaluation. Patients will need to be monitored during extended contouring and re-planning procedures in online adaptive workflows.