An intensity-modulated radiation therapy planning and monitoring system for a subject including a target and/or an organ at risk (32, 34), the system comprising: at least one surrogate (18, 20, 22, 24) and a dosimeter (26, 28, 202, 204, 206, 208, 210, 212), the surrogate (18, 20, 22, 24) being configured to act as a surrogate for the target and/or the OAR (32, 34), and the dosimeter (26, 28, 202, 204, 206, 208, 210, 212) being configured to measure dose delivered to the target and/or the OAR (32, 34); and at least one processor (98, 100) programmed to: receive a planning image of a region of the subject; determine a reference location and shape in the planning image for one or more of the target and/or the OAR (32, 34); determine a reference location and/or shape from the planning image of each of the surrogate (18, 20, 22, 24) and the dosimeter (26, 28, 202, 204, 206, 208, 210, 212); determine deviations between: the reference location and/or shape of the surrogate (18, 20, 22, 24) and a current location and/or shape of the surrogate (18, 20, 22, 24); and, a planned dose distribution and a delivered dose distribution, the delivered dose distribution determined from dosimetric data and the reference location and/or shape of the dosimeter (26, 28, 202, 204, 206, 208, 210, 212); characterised in that the surrogate (18, 20, 22, 24) is an optical fiber (18, 20) and one or more of: the reference location and/or shape of the surrogate (18, 20, 22, 24); and the current location and/or shape of the surrogate (18, 20, 22, 24), are determined using fiber Bragg gratings of different Bragg wavelengths along the length of the optical fiber (18, 20). at least one surrogate (18, 20, 22, 24) and a dosimeter (26, 28, 202, 204, 206, 208, 210, 212), the surrogate (18, 20, 22, 24) being configured to act as a surrogate for the target and/or the OAR (32, 34), and the dosimeter (26, 28, 202, 204, 206, 208, 210, 212) being configured to measure dose delivered to the target and/or the OAR (32, 34); and at least one processor (98, 100) programmed to: receive a planning image of a region of the subject; determine a reference location and shape in the planning image for one or more of the target and/or the OAR (32, 34); determine a reference location and/or shape from the planning image of each of the surrogate (18, 20, 22, 24) and the dosimeter (26, 28, 202, 204, 206, 208, 210, 212); determine deviations between: the reference location and/or shape of the surrogate (18, 20, 22, 24) and a current location and/or shape of the surrogate (18, 20, 22, 24); and, a planned dose distribution and a delivered dose distribution, the delivered dose distribution determined from dosimetric data and the reference location and/or shape of the dosimeter (26, 28, 202, 204, 206, 208, 210, 212); receive a planning image of a region of the subject; determine a reference location and shape in the planning image for one or more of the target and/or the OAR (32, 34); determine a reference location and/or shape from the planning image of each of the surrogate (18, 20, 22, 24) and the dosimeter (26, 28, 202, 204, 206, 208, 210, 212); determine deviations between: the reference location and/or shape of the surrogate (18, 20, 22, 24) and a current location and/or shape of the surrogate (18, 20, 22, 24); and, a planned dose distribution and a delivered dose distribution, the delivered dose distribution determined from dosimetric data and the reference location and/or shape of the dosimeter (26, 28, 202, 204, 206, 208, 210, 212); the reference location and/or shape of the surrogate (18, 20, 22, 24) and a current location and/or shape of the surrogate (18, 20, 22, 24); and, a planned dose distribution and a delivered dose distribution, the delivered dose distribution determined from dosimetric data and the reference location and/or shape of the dosimeter (26, 28, 202, 204, 206, 208, 210, 212);