Correlation between respiratory-induced target motion and dosimetric variance on EBRT

During radiation therapy, it is crucial to realise adequate dose coverage of the target at depth. However, physicists must overcome many challenges before they can achieve better accuracy. Respiratory-induced organ motion (RIOM) remains a challenge that negatively contributes to external beam radiotherapy (EBRT) accuracy. Although the magnitude of respiratory motion amplitude is a major factor in maximising or minimising the dosimetric effect of respiratory motion, other factors may negatively or positively contribute to this effect. This study investigated the impact of respiratory motion and tumour parameters, amplitude, and volume, on gross tumour volume (GTV) coverage during EBRT. In addition, it analysed the impact of tumour volume on the dosimetric effect of RIOM. A GEANT4 code was used for simulating the in vivo interaction of matter with ionising radiation was used to provide a model of respiratory-induced organ motion and voxel-level dosimetry of a simulated target. Thirty different setups were simulated using the GEANT4 simulation toolkit. Each setup demonstrated how a specific combination of respiratory and tumour parameters, amplitude, and size impact GTV coverage. The results show that the dosimetric effect of respiratory motion depends more on respiratory parameters i.e. on the respiratory motion amplitude. The reduction in GTV coverage for a tumour of 5 mm diameter can vary from approximately 28%-67%. At a certain respiratory amplitude, the tumour size can strongly affect GTV coverage. This effect becomes more dominant when the respiratory amplitude exceeds the tumour diameter. This study shows that any approach to compensate for RIOM in EBRT should consider the role of tumour size.