Time-resolved plastic scintillator dosimetry in a dynamic thorax phantom

Motion managed and dynamic radiotherapy of lung cancer patients is increasingly complex and subject to challenges related to respiratory motion and heterogeneous tissue densities. This puts high demands on methods for quality assurance and especially time-resolved dose verification of the treatment delivery. The aim of this study was to develop a novel dynamic thorax phantom for time-resolved plastic scintillator dosimetry. The in-house developed phantom has a well-known geometry mimicking a lung cancer patient with a reproducible (within 0.04 mm), respiratory-like motion of a tumor embedded in a lung. The phantom motion was controlled by a script in-house developed using LabVIEW (National Instruments) and synchronized with the in-house developed ME40 scintillator dosimetry system (DTU Nutech). The dose in the center of the tumor was measured, using a BCF-60 plastic scintillator detector (Saint-Gobain Ceramics & Plastics Inc.), during dynamic 6 MV half-arc treatments on a TrueBeam linear accelerator (Varian Medical Systems). Deviations of ∼2% from the corresponding dose calculated by the treatment planning system (TPS) were detected. The results emphasize the shortcomings of commercial TPSs to handle respiratory motion and lack of lateral charged particle equilibrium, motivating quality assurance based on a system like the one presented in this study. It has specifically been demonstrated that reliable time-resolved scintillator dosimetry in a dynamic thorax phantom can play an essential role in dose verification of lung cancer radiotherapy.