In some embodiments, an image-guided radiotherapy apparatus and method is provided in which a radiotherapy radiation source and a gamma ray photon imaging device are positioned with respect to a patient area so that a patient can be treated by a beam emitted from the radiotherapy apparatus and can have images taken by the gamma ray photon imaging device. Radiotherapy treatment and imaging can be performed substantially simultaneously and/or can be performed without moving the patient in some embodiments. The gamma ray photon imaging device can be coupled and movable with respect to any part of a building structure, can be located on a portable frame movable to and from the radiotherapy radiation source and patient, or can take other forms. In some embodiments, the gamma ray photon imaging device can be used for imaging in connection with other types of medical interventions.