1. A dosimeter for use on a radiotherapy machine having a radiation source, the dosimeter comprising: a scintillation assembly configured to generate a light output in response to incident radiation distribution thereon from the radiation source; and at least one photo-detector operable to convert optical energy transmitted by the scintillation assembly to electrical signals for determining at least one of actual two-dimensional (2D) or actual three-dimensional (3D) dose distribution using tomographic reconstruction algorithms; wherein the scintillation assembly is composed of a plurality of scintillating optical fibers that are embedded in and span lengthwise across a phantom medium; wherein the scintillating optical fibers are made of a material that has been conditioned using UV light in order to increase their optical attenuation.