A novel approach to contamination suppression in transmission detectors for radiotherapy

A novel trend in radiotherapy is to treat cancers that are in difficult locations in the body using beams with complex intensity profiles. Intensity Modulated Radiotherapy (IMRT) improves the dose distribution to the tumour whilst reducing the dose to healthy tissue. Such treatments require increasingly complex methods to verify the accuracy and precision of the treatment delivery. Monolithic Active Pixel Sensors (MAPS) are ideal detectors to measure the 2D beam profile of a radiotherapy beam upstream. MAPS sensors can be made very thin (~20 μm) with still very good signal-to-noise performance, hence the beam would pass through the sensor virtually undisturbed (<0.1% attenuation). Pixel pitches between 2x2 μm2 and 100x100 μm2 are commercially available. Large area devices (~15x15 cm2) have been produced. MAPS can be made radiation hard enough to be fully functional after a large number of fractions. A remaining challenge for thin, upstream sensors is that the detectors are sensitive to the signal of both therapeutic photons and electron contamination. Here a method is presented to distinguish between the signal due to electrons and photons and thus provide real-time dosimetric information in very thin sensors that does not require Monte Carlo simulation of each linear accelerator treatment head.