LYSO scintillators coupled to phototransistors for orthogonal ray imaging: Experimental results at 4 and 6-MV linacs

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. Here, we report on scanned, collimated measurements performed with LYSO crystals coupled to a phototransistor (PT), the latter fully substituting the photomultiplier tube (PMT). The very small dimensions of the PT allowed its insertion inside the collimation hole. Consequently, the well-known, harsh background flux characteristic of linacs used for radiotherapy is strongly reduced. This could allow for eliminating the need for PMT-dedicated shielding during the measurements, which provides a large reduction of complexity to all measurements. For this purpose, an electronics assembly was developed for signal formatting and acquisition. The setup was tested in 4-MV and 6-MV linacs, irradiating a PMMA phantom with a 80 mm × 80 mm squared beam. Despite the correlation obtained between the experimental profiles and the simulated dose, the PT showed insufficient sensitivity to detect an air cavity inside the phantom. Due to this reason we conclude that the PT is not a good alternative to the PMT.