Rotation-Free Scattered-Radiation Imaging with a Radiotherapy X-Ray Linac

The OrthoCT (acronym for orthogonal computed tomography) concept, based on orthogonal ray imaging, is a new low-dose imaging technique under investigation to potentially assist external-beam radiation therapy treatments. It consists in detecting radiation scattered in the patient and emitted at approximately 90 degrees with respect to the direction of the incoming beam. Such radiation can be collected by a 1D-detector system with a multi-sliced collimator positioned parallel to the incident beam axis. This system can be potentially useful for on-board imaging with the patient positioned and ready for treatment, or for real-time treatment monitoring. In this work, a multi-pixel, small OrthoCT detector prototype was developed and tested experimentally. This system is based on gadolinium orthosilicate crystals coupled to photomultiplier tubes and a collimator made of lead slices. The experimental measurements were performed with a heterogeneous phantom of acrylic with an air cavity inside, using a TrueBeam linac operated at 6 MV in the flattening-filter-free modality. The results allow concluding that this new imaging technique is capable to provide, in 1.3 s, morphological images of the phantom without the need to rotate the X-ray source around the object to be irradiated, showing the feasibility of such system.