A novel approach to contamination suppression in transmission detectors for radiotherapy

The current trend in X-ray radiotherapy is to treat cancers that are in difficult locations in the body using beams with a complex intensity profile. Intensity-modulated radiotherapy (IMRT) is a treatment which improves the dose distribution to the tumor whilst reducing the dose to healthy tissue. Such treatments administer a larger dose per treatment fraction and hence require more complex methods to verify the accuracy of the treatment delivery. Measuring beam intensity fluctuations is difficult as the beam is heavily distorted after leaving the patient and transmission detectors will attenuate the beam and change the energy spectrum of the beam. Monolithic active pixel sensors (MAPSs) are ideal solid-state detectors to measure the 2-D beam profile of a radiotherapy beam upstream of the patient. MAPS sensors can be made very thin ( ${\sim }30~\mu \text{m}$ ) with still very good signal-to-noise performance. This means that the beam would pass through the sensor virtually undisturbed ( $2{~\mu }\text{m}$ to $100~{\mu }\text{m}$ are commercially available. Large area devices ( ${\sim }15 \times 15$ cm2) have been produced. MAPS can be made radiation hard enough to be fully functional after a large number of fractions. All this makes MAPS a very realistic transmission detector candidate for beam monitoring upstream of the patient. 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.