OrthoCT for tumor head irradiation: A simulation study

A new imaging technique, called orthogonal ray imaging, has been proposed to assist external-beam radiation therapy treatments. This technique consists in detecting radiation scattered in the patient and emitted perpendicularly to the incident beam axis. Since photon scattering in the patient occurs with higher intensity in tissues of higher density, a detection system (constituted by a multi-slice collimator and a photon detector) positioned perpendicularly to the beam axis yields a signal proportional to the photons that escaped the patient (i.e., a signal correlated with patient morphology). Unlike some of the other imaging techniques applied in treatments based on image-guided radiotherapy such as cone-beam computed tomography, OrthoCT does not require rotational irradiation of the target. This technique provides CT-like images with very-low dose, potentially sparing the healthy tissues around the target to an unnecessary irradiation (since rotation of X-ray source is not required). This system can potentially be useful to (1) on-board imaging, or (2) real-time radiotherapy monitoring. In this work, we report Geant4 simulation results with an anthropomorphic phantom to analyze the capability of OrthoCT in detecting pertinent and clinically relevant morphological variations during head irradiation. The counts distributions obtained show a good visual agreement with the simulated dose and the phantom structures.