This disclosure is related to an apparatus and method for commissioning or performing a quality assurance (QA) verification of a radiation therapy (RT) device. The device may comprise: (a) a motorised water phantom; (b) a RT controller configured for obtaining operation parameters of fields, and causing the RT device to emit a beam according to said operations parameters; (c) a QA controller having a memory for storing a measurements plan, the measurements plan including data defining a sequence of fields, and d) a reference radiation detector adapted and positioned for intercepting said radiation beam and for measuring the dose rate of said radiation beam. The reference radiation detector may be substantially transparent to the radiation beam. The QA controller may include an acquisition interface for acquiring and storing the dose rate from the reference radiation detector. The device may also include a processor configured to check the synchronism between the dose rate from the field radiation detector and the dose rate from the reference radiation detector.