Modelling lung tumor motion in Geant4: a Monte Carlo model of the QUASARtm respiratory motion phantom

Full text: Aim Motion of lung tumours due to respiratory motion is a significant problem in radiotherapy. The aim of this work was to develop a Monte Carlo model of a commercially available motion phantom. Method The Geant4 C++ based Monte Carlo package was used to replicate the QUASAR motion phantom from Modus Medical. The physical QUASAR phantom contains moving inserts which represent the target and is capable of numerous dosimetric and imaging quality assurance functions. The Monte Carlo phantom model in this work allows the user to import patient respiratory data recorded with the Varian Real-time Position Management system. The spatial and temporal motion of the virtual phantom is determined by the patient data, therefore, making it ideal for patient specific QA. A user interface was created that allows patient data and scoring options to be assigned as well as media and density selections for all inserts. Results The virtual QUASAR Monte Carlo phantom is able to replicate patient motion and determine the effects of motion on dose distributions. The Monte Carlo model replicates patient superior inferior respiratory motion accurately and creates a platform for patient specific QA and TPS verification. Furthermore, dose calculation within the phantom can be performed with the increased accuracy of Monte Carlo and compared with measurements. Conclusion The added accuracy of dose-calculation afforded by Monte Carlo methods along with the ability to QA motion management protocols makes the virtual QUASAR phantom a useful tool in motion management for radiotherapy.