High resolution MLC leaf position measurements with a large area MAPS

Multileaf collimators (MLCs) are an essential component in modern radiotherapy that shape the X-ray treatment beam. Currently, MLC leaf position accuracy is verified to ±1 mm every month. However, leaf position accuracy can drift between verification dates and treatment verification only occurs pretreatment. To prevent serious errors, it would be highly beneficial to use a real-time verification system. We are developing a system based on monolithic active pixel sensors (MAPSs). MAPS are radiation hard under photon and electron irradiation, have high readout rates, low attenuation and are suitable for high-resolution applications making them an ideal upstream radiation detector. Here, we report results using the Lassena MAPS, which measures $12\times14$ cm2 and is three-side buttable, allowing full treatment fields to be monitored. The Lassena detector was placed in the treatment field of an Elekta Synergy LINAC which has an MLC leaf width of 0.5 cm. An MLC leaf was extended to ten different positions within the field. Sobel-based methods were used to reconstruct the leaf edge position. Correspondence between reconstructed and set leaf position was excellent and resolutions ranged between 60.6± 8 and $109\pm 12~\mu \text{m}$ for a central leaf with leaf extensions ranging from 1 to 35 mm using ~0.3 s of treatment beam time while the sensor was placed at an SSD of 85 cm.