The VANILLA sensor as a beam monitoring device for X-ray radiation therapy

• MAPS sensors are very thin (~20μm) detectors that still have appreciable signal-to-noise ratio for X-ray radiation.
• As the sensors are so thin they can be placed upstream of a patient without disturbing the beam.
• When placing the VANILLA sensor in an X-ray beam, it was shown that the collimated beam position can be reconstructed with approximately 64μm precision.
• The above demonstrates that MAPS are an excellent candidate for beam monitoring in X-ray radiation therapy.

Cancer treatments such as intensity-modulated radiotherapy (IMRT) require increasingly complex methods to verify the accuracy and precision of the treatment delivery. In vivo dosimetry based on measurements made in an electronic portal imaging device (EPID) has been demonstrated. The distorting effect of the patient anatomy on the beam intensity means it is difficult to separate changes in patient anatomy from changes in the beam intensity profile. Alternatively, upstream detectors scatter and attenuate the beam, changing the energy spectrum of the beam, and generate contaminant radiation such as electrons. We used the VANILLA device, a Monolithic Active Pixel Sensor (MAPS), to measure the 2D beam profile of a 6 MV X-ray beam at Bristol Hospital in real-time in an upstream position to the patient without clinically significant disturbance of the beam (0.1% attenuation). MAPSs can be made very thin (~20μm) with still a very good signal-to-noise performance. The VANILLA can reconstruct the collimated beam edge with approximately 64μm precision.