OrthoCT for tumor lung irradiation: A simulation study

The OrthoCT (standing for orthogonal computed tomography) imaging technique consists in detecting radiation scattered in the patient and emitted at right angles with respect 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). This low-dose morphological imaging can potentially be useful to (1) on-board imaging for assisting radiotherapy, or (2) real-time radiotherapy monitoring. In this work we report Geant4 simulation results with an anthropomorphic phantom to analyze the capability of OrthoCT to detect pertinent and clinically-relevant lung tumor deviation and lung tumor regression/progression. Tumor deviation or progression potentially leads to a tumor under-dosage. On the other hand, the detection of tumor shrinkage in lung irradiation may allow for a reduction of the dose field, which represents an important relief of the dose delivered to the surrounding healthy lung. The counts distributions obtained with this technique show a good visual agreement both with the simulated dose, the tumor location/size, as well as the phantom structures. The capability of OrthoCT to obtain morphological images with no X-ray source rotation potentially allows minimizing dose to healthy tissues and organs at risk. Therefore, this technique is likely to represent a high potential asset for onboard imaging (i.e. imaging after patient positioning, just before the treatment), or eventually, during the treatment.