The present invention provides a radiation detector (30, 100) which comprises a tube (102) and detector elements (106) distributed over the tube. Such an arrangement can be used to provide information on a radiation beam prior to interaction with a patient and after interaction with the patient. The detector is particularly compact and therefore suited to use with apparatus where space is restricted. By a particular arrangement of detectors and optical fibres (104, 108), the detector can provide data at all angles of rotation.
