First demonstration of real-time in-situ dosimetry of X-ray microbeams using a large format CMOS sensor.

Microbeam radiotherapy is a novel type of radiotherapy in which narrow beams of radiation (typically less than 500 mu m) are spatially fractionated, delivering a non-uniform distribution to the target tumour volume. Due to the very high dose gradients and very small beams involved, new dosimetric techniques are required for translation into clinical practise. Current real-time beam monitoring is typically performed using 1 dimensional silicon strip detectors or wire chambers, with 2D beam information measured offline using radiochromic film (requiring a minimum of 24 h to self-develop).Using an Xstrahl SARRP X-ray irradiation device with a bespoke microbeam collimator at the Technical University of Munich, Germany, the newly developed vM1212 detector was exposed to a variety of microbeams (220 kV, nominal slit widths 0-100 mu m) for evaluation of in vivo real time verification.The performance of the detector was assessed by changing the collimator slit width (and thus microbeam FWHM) mid-irradiation. Microbeam FWHMs of 130-190 mu m could be measured in this manner in addition to temporally monitoring other basic parameters such as the radiation intensity. More advanced parameters could be calculated as the tungsten slits within the microbeam collimator opened and closed such as the rate of change of FWHM; the peak-valley-dose-ratio (PVDR); and the sub-pixel movement of each microbeam peak.This work demonstrates the potential of radiation hard CMOS sensors in radiotherapy for in vivo real-time monitoring of X-ray microbeams FWHM, intensity and position