Within our activities on accelerator-based boron neutron capture therapy (BNCT) carried out at the Tandar Laboratory (Comisión Nacional de Energía Atómica, Argentina) we present here the study and design of a tomographic imaging system for the measurement of the spatial distribution of the absorbed dose during a BNCT session. The ¹⁰B(n,α)⁷Li boron neutron capture reaction produces a 478 keV gamma ray in 94% of the cases. In BNCT a large fraction of this radiation escapes from the patient body. Its detection is thus very attractive for a noninvasive boron concentration measurement and an online absorbed dose evaluation since the dose absorbed by the tumor and healthy tissue strongly depends on the boron uptake and the neutron flux. For this purpose, a dedicated tomographic imaging system based on SPECT (Single Photon Emission Computed Tomography, a diagnostic medical imaging modality used in nuclear medicine) is proposed since standard SPECT cameras cannot be used due to the photon energy (478 keV) and the particular background gamma field in BNCT. A detection system (collimator and detectors) is proposed. Monte Carlo numerical simulations are used for an implementation of a statistical algorithm used for the tomographic image reconstruction.