Monitoring Tumor Lung Irradiation With Megavoltage Patient-Scattered Radiation: A Full System Simulation Study

We analyze by simulation a fully-3-D, low-dose imaging system aiming at assisting external-beam radiotherapy, either for on-board patient imaging, or for real-time radiotherapy monitoring. The system consists in detecting megavoltage patient-scattered radiation that is emitted at right angles with respect to the beam axis. Since photon scattering in the patient occurs with higher intensity in tissues of higher density, a multislice photon detection system positioned perpendicularly to the beam axis yields a signal correlated with patient morphology, including the tumor. We thus report on GEANT4 simulations carried out with an anthropomorphic phantom in order to analyze the capability of the system to detect pertinent and clinically relevant scenarios, e.g., lung tumor deviation and tumor regression/progression. The signal distribution obtained with a realistic full system (including the multislice collimator, the scintillator crystals, and the charge electronic readout mode) show a very high visual agreement both with the simulated, prescribed dose, and with the tumor location/size, as well as with the phantom structures. The capability of this system to obtain morphological images without X-ray source rotation potentially allows to highly reduce dose in healthy tissues and organs at risk in respect to other existing image-guided radiation therapy techniques, thus complementing them.