Orthogonal ray imaging with megavoltage beams: Simulated results with an anthropomorphic phantom

Orthogonal ray imaging is a new technique under investigation by our groups. It shows good potential for (1) dose verification in external beam radiotherapy and (2) very-low-dose computed tomography (CT-like) imaging. We have simulated with Geant4 the performance of four flat-panel-like perfect detectors for evaluating the capabilities of orthogonal ray imaging and portal imaging for assisting external beam therapy. The four detectors were positioned surrounding the head of the patient, three parallel and one perpendicular to the beam axis. Each detector covers an area of 185 mm × 185 mm and the simulation scores every particle seen by every detector. This allows for a second-stage investigation of optimum acceptance angles and energy thresholds, presented here. For demonstration purposes one small rectangular, sub-therapeutic beam with a front area of 20 mm × 5 mm, maximum entrance dose in the buildup region of 1.3 mGy, and tumor dose of 0.8 mGy, was shot at the region of the pituitary gland of an anthropomorphic phantom. Despite the low dose, visual inspection shows a remarkable agreement both with the predicted dose and with patient bone structures, collected with the orthogonal ray detectors. The portal imaging detector could not provide comparable information based on a single shot. In addition, seven small rectangular beamlets irradiating the region of the pituitary gland of the phantom with 4.6 mGy and simulating an intensity modulated radiation therapy treatment-like scenario were also analyzed, showing equally a visual agreement with the planed dose distribution. We finally evaluate the possibility of using rotation-free, scanned megavoltage orthogonal ray imaging (multi-slice collimation) for patient morphologic imaging. The technique provides images of high visual correlation with phantom morphology, therefore being of potential usefulness e.g. for low-dose, on-board patient imaging prior to a radiotherapy treatment, among other applicati- ns.