A device for verification of radiation beam delivery to a patient with a radiation therapy apparatus, said radiation beam delivery comprising one or more treatment fractions delivered on various treatment days, said treatment fraction comprising one or more radiation beams, said radiation therapy apparatus being configurable for a given radiation beam configuration, said radiation therapy apparatus comprising an imaging system supplying updated patient images obtained prior, during or after each treatment fraction, said device comprising: • means to receive radiation beam configurations for a given treatment fraction; • a transmission electronic 2D detector device capable of measuring 2D responses of said radiation beam in a plane perpendicular to the central axis of said radiation beam; • means to acquire in real time the 2D detector responses caused by a radiation beam delivered by said radiation therapy apparatus being configured with said radiation beam configuration; • means to import said updated patient images from said imaging system, said updated patient images comprising a description or image of the patient's anatomy comprising the 3D shape, density distribution and position of the target volume and/or organs at risk; • a fluence reconstructing engine FRE, capable of computing, for each delivered radiation beam of the treatment fraction, the delivered photon fluence distribution corresponding to the delivered radiation beam based on the measured 2D detector responses, said delivered radiation beam being specified by said radiation beam configuration; • a dose calculation engine DCE capable of computing the delivered 3D dose distribution within the patient, said 3D dose distribution comprising the contributions from each radiation beam of the treatment fraction, said computing the delivered 3D dose distribution being based on said radiation beam configuration, said delivered photon fluence distribution and said imported updated patient images; • display means for visualizing the said delivered 3D dose distribution in the patient's anatomy; • means to store the said delivered 3D dose distribution from a single treatment fraction; • means to accumulate the delivered 3D dose distributions delivered during consecutive treatment fractions; • said display means to visualise the accumulated 3D dose distribution in the patient's anatomy. • means to receive radiation beam configurations for a given treatment fraction; • a transmission electronic 2D detector device capable of measuring 2D responses of said radiation beam in a plane perpendicular to the central axis of said radiation beam; • means to acquire in real time the 2D detector responses caused by a radiation beam delivered by said radiation therapy apparatus being configured with said radiation beam configuration; • means to import said updated patient images from said imaging system, said updated patient images comprising a description or image of the patient's anatomy comprising the 3D shape, density distribution and position of the target volume and/or organs at risk; • a fluence reconstructing engine FRE, capable of computing, for each delivered radiation beam of the treatment fraction, the delivered photon fluence distribution corresponding to the delivered radiation beam based on the measured 2D detector responses, said delivered radiation beam being specified by said radiation beam configuration; • a dose calculation engine DCE capable of computing the delivered 3D dose distribution within the patient, said 3D dose distribution comprising the contributions from each radiation beam of the treatment fraction, said computing the delivered 3D dose distribution being based on said radiation beam configuration, said delivered photon fluence distribution and said imported updated patient images; • display means for visualizing the said delivered 3D dose distribution in the patient's anatomy; • means to store the said delivered 3D dose distribution from a single treatment fraction; • means to accumulate the delivered 3D dose distributions delivered during consecutive treatment fractions; • said display means to visualise the accumulated 3D dose distribution in the patient's anatomy.