Radiation therapy of a lesion within a patient is guided to take into account movement of the lesion caused by respiration and/or cardiac effects by using MRI or other imaging system suitable for locating the lesion to image the patient while on the treatment support and using a PET detection system responsive to a radiation source preferentially taken up by the lesion and registered with the MRI so as to monitor movement of the lesion in real time and thus guide the beam of the RT.
