A radiotherapy system is configured to determine in vivo dose deposition of a radiotherapy treatment beam. The system includes the following components. A bi-planar magnetic resonance imaging (MRI) apparatus comprising a pair of spaced apart magnets. One of the magnets includes a hole proximal the centre thereof. A treatment beam source configured to generate a radiotherapy treatment beam. The treatment beam source is positioned to transmit the treatment beam through the hole in the magnet. A patient support configured to position a patient with the system so that a treatment target is proximal the treatment beam. A Positron Emission Tomography (PET) detector configured to obtain PET data of the treatment beam impacting the patient. The PET detector is positioned so that a transverse section of the patient that includes the treatment target lies between opposing portions of the PET detector.