A system configured to be used with a radiotherapy apparatus comprising a source of therapeutic radiation adapted to deliver a beam of therapeutic radiation into an imaging volume containing at least one object, the system comprising: a scanner for producing images of the at least one object within the imaging volume; a Treatment Planning System for controlling the source so as to deliver a beam of therapeutic radiation in accordance with a predetermined plan for treating a patient and with images of the or each object from the scanner of the imaging volume, and one or more devices within and/or adjacent the imaging volume, wherein the or each device comprises at least one marker which is/are adapted to be visible to the scanner and at least one MR marker which is/are adapted to be visible to an MR scanner, and wherein the system further comprises a database containing data including the markers associated with a device and at least the geometric and density characteristics of the device; wherein the Treatment Planning System is adapted, from the at least one MR marker and/or the disposition/orientation of the at least one MR marker, to identify the device from among the range of different devices in the database and determine the position and orientation of the device relative to the imaging volume within the scanner where radiotherapy is to be applied; the Treatment Planning System is adapted then to calculate, using the position/orientation of the device and the geometric and density characteristics of the device in the database, the effect on the therapeutic radiation beam which would be caused by the presence of the device, and to adjust the predetermined treatment plan so as to ensure that the therapeutic radiation may be delivered to the patient in accordance with the predetermined treatment plan. a scanner for producing images of the at least one object within the imaging volume; a Treatment Planning System for controlling the source so as to deliver a beam of therapeutic radiation in accordance with a predetermined plan for treating a patient and with images of the or each object from the scanner of the imaging volume, and one or more devices within and/or adjacent the imaging volume, wherein the or each device comprises at least one marker which is/are adapted to be visible to the scanner and at least one MR marker which is/are adapted to be visible to an MR scanner, and wherein the system further comprises a database containing data including the markers associated with a device and at least the geometric and density characteristics of the device; wherein the Treatment Planning System is adapted, from the at least one MR marker and/or the disposition/orientation of the at least one MR marker, to identify the device from among the range of different devices in the database and determine the position and orientation of the device relative to the imaging volume within the scanner where radiotherapy is to be applied; the Treatment Planning System is adapted then to calculate, using the position/orientation of the device and the geometric and density characteristics of the device in the database, the effect on the therapeutic radiation beam which would be caused by the presence of the device, and to adjust the predetermined treatment plan so as to ensure that the therapeutic radiation may be delivered to the patient in accordance with the predetermined treatment plan.