Impact of tumor contrast in orthogonal ray imaging: A prostate irradiation study

A new very-low-dose imaging technology to assist external-beam radiotherapy treatments has been proposed. This technique, called orthogonal ray imaging, does not require X-ray source rotation around the target. It is based on the detection of photons escaping the target at almost right angles with respect to the incoming photon flux. Modern image-guided radiation therapy techniques allow an accurate positioning of the patient, consequently improving the treatment accuracy. However, some of these techniques, such as electronic portal imaging and mega-voltage cone-beam computed tomography provide poor resolution in soft tissues. Other techniques like kilovoltage cone-beam computed tomography, allow a good visualization of these type of tissues but at the cost of an increase of the dose delivered to the patient, consequently increasing the risk of possible side-effects on the surrounding healthy tissues. One way to enhance the contrast of the lesion region is by injecting a contrast agent. The aim of this study was to evaluate through simulation the benefit of iodinated-contrast agent (Ultravist 370) administration in cases of prostate cancer in OrthoCT using the Geant4 simulation toolkit and the anthropomorphic phantom NCAT. We conclude that (1) in all studied scenarios it was possible to obtain OrthoCT images with good visual agreement with the simulated dose as well as the phantom pelvic structures with a maximum dose of the order of 2mGy and (2) the administration of 100mL of Ultravist 370 is not enough to allow the visualization of the tumor or the normal prostate tissue.