Radiotherapy MLC Upstream Leaf Edge Detection Using Lassena Large Area MAPS

Multileaf collimators (MLC) are an integral component in modern radiotherapy as they shape the X-ray treatment beam and therefore need to be closely controlled. Currently, MLC leaves are calibrated to ± 1 mm every month, however leaves can drift beyond this during calibration dates and treatment verification only occurs pre-treatment. MAPS are radiation hard in photon and electron irradiation, have high readout speeds and low attenuation which makes them an ideal upstream radiation detector. Here, we report results using the Lassena MAPS, which is 12×14 cm
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, three side buttable and is capable of clinical deployment. We have previously shown that the Achilles MAPS can reconstruct 1 cm leaf edge positions with a 52 ± 4 μm precision in 0.1 s of treatment beam. The Achilles however, is sized 6×6 cm
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, is not buttable and hence, cannot be used for real time clinical monitoring. Additionally, Achillies has a pixel pitch of 14.5 μm whilst the Lassena has a 50 μm pixel pitch. The Lassena detector was placed in the treatment field of an Elekta Synergy LINAC and an MLC leaf of width 0.5 cm, positioned stepwise. Sobel-based methods were used to calculate leaf edge positions, resolutions and plot as a function of set leaf position. Correspondence between reconstructed and set leaf position was excellent and resolutions ranged between 52.9 ± 8.5 μm and 142 ± 31.6 μm for 0.3 s of treatment beam, which is significantly lower than the ± 0.5 mm uncertainty in LINAC set position.