A sensing device for a radiation therapy apparatus, the apparatus comprising an accelerator and a beam-shaping device the beam shaping device being a multi-leaf collimator (MLC) (2) having a plurality of pairs of leaves, and a rotatable gantry, the sensing device comprising:
• a transmission electronic detector (1) comprising an array of ionization chambers. The ionization chambers are defined by a bias electrode (11a, 11b, 34,42) on the one hand and by a planar array of conductive strips (40) or strip assemblies (30) on the other hand. The strips or strip assemblies are associated to the leaf pairs of the MLC. The strips are the collecting electrodes of the ionization chambers. Each strip assembly or in the case of one particular embodiment, each strip, yields two currents which allow to determine the position of the leaves of a leaf pair associated with the strip or strip assembly,
• a gantry sensor configured to determine at least one position associated with a gantry angle; and
• a processor, adapted to determine the position of at least one pair of leaves of the MLC using the currents i 1 and i 2 obtained from the collecting electrode strips.