Experimental characterization of megavoltage beams for orthogonal ray imaging

OrthoCT (orthogonal computed tomography) is a potential new imaging technique that aims to acquire images of the volume to be irradiated immediately before or during a radiotherapy treatment. It potentially provides imaging with very low to eventually null dose, allowing to check if the morphology/anatomy of the patient and tumour are in agreement with the planned one. This technique relies on the detection of photons that are scattered in the patient and are emitted perpendicularly to the incident beam direction. To acquire the OrthoCT morphological images the scanning of the volume to be irradiated is done using pencil-like mega-voltage beams. The corresponding scan profile requires: (1) high homogeneity, so that variations can be associated only to dose/morphological alterations, and (2) high velocity, which favors multi-leaf collimator-based scans in respect to jaw-based ones. To compare the variability of a homogeneous beam with the variability of a scanned profile two scans with a cross-section of 5mm × 5mm (MLC-collimated) and 6mm × 6mm (jaw-collimated) were experimentally evaluated. The transverse profiles obtained with MLC-collimation in this work reveal a homogeneity with an intensity variability inferior to 1%, thus supporting OrthoCT imaging with morphology/anatomy changes superior to that value.