Electron contamination suppression in transmission detectors for radiotherapy

Objective.Higher energy and intensity radiotherapy beams are being used, in part, due to the increased spatial accuracy of treatments. However, higher intensity beams can result in a larger total dose error, motivating the increasing need for real-time dose monitoring. We are developing a thin, real-time upstream monolithic active pixel sensor based system for beam monitoring with excellent precision on measuring the beam shape. Here we present a method to additionally provide dosimetry by adding thin conversion material in strips to the surface of the detector, a grating structure.Approach.By modulating the thickness of the conversion material to minimally disturb the contamination electron signal while enhancing the photon signal, the difference in these signals can be used to extract a photon-only signal, and hence dose. The simulation software Gate, based on Geant4, is utilised to study whether well functioning gratings can be better made from aluminium or copper and to optimise the thickness of a copper grating.Main results.It is possible to enhance the photon signal by a factor 6.7 (7.7) compared to the bare sensor for a 5.8 (6.7) MV beam, without modulation of the signal due to beam electrons.Significance.The grating can be used to perform dosimetry in real-time using a thin upstream detector.

Performance of a Full-Scale Upstream MAPS-Based Verification Device for Radiotherapy

Intensity-modulated radiotherapy is a widely used technique for accurately targeting cancerous tumours in difficult locations using dynamically shaped beams. This is ideally accompanied by real-time independent verification. Monolithic active pixel sensors are a viable candidate for providing upstream beam monitoring during treatment. We have already demonstrated that a Monolithic Active Pixel Sensor (MAPS)-based system can fulfill all clinical requirements except for the minimum required size. Here, we report the performance of a large-scale demonstrator system consisting of a matrix of 2 × 2 sensors, which is large enough to cover almost all radiotherapy treatment fields when affixed to the shadow tray of the LINAC head. When building a matrix structure, a small dead area is inevitable. Here, we report that with a newly developed position algorithm, leaf positions can be reconstructed over the entire range with a position resolution of below ∼200 μm in the centre of the sensor, which worsens to just below 300 μm in the middle of the gap between two sensors. A leaf position resolution below 300 μm results in a dose error below 2%, which is good enough for clinical deployment.