A conical-reflector scintillation device for 3-dimensional measurement of radiotherapy beams: a feasibility study

This study presents a novel optical dosimetry system designed to support three-dimensional characterisation of radiotherapy beams using a single-camera imaging geometry. The system combines a large-volume cylindrical plastic scintillator with a conical reflector, enabling scintillation light emitted during beam irradiation to be captured as both direct emission and secondary reflection arcs using a single camera. These reflected signals encode spatial information about the beam's position, size, and intensity, forming the basis for extracting clinically relevant parameters such as beam geometry and intensity from a single 2D image. Controlled experiments using a 6 MV photon beam investigated the system's response to changes in beam position, lateral width, and penumbra shape. Analysis of reflection arc geometry and intensity demonstrated strong linear correlations between beam position and arc radius over lateral and longitudinal shifts of up to 60 mm and 70 mm respectively, and a clear non-linear relationship between the average arc intensity and lateral beam field widths between 10 and 80 mm. Additionally, penumbra widths derived from reflection arc profiles were linearly proportional to those measured directly in the scintillator, with the arc penumbra approximately [Formula: see text] times wider than the underlying scintillator observed beam penumbra. This proof-of-concept study demonstrated that reflection arc geometry and intensity in a single 2D image provided sufficient information to infer spatial beam position, lateral field width, and penumbra characteristics for static irradiations. The simplicity, symmetry, and static design of the system support future development towards volumetric beam reconstruction and, through sequential image acquisition, potentially time-resolved treatment verification.