In this paper we present a report on our custom 3D CT extension capable of producing fully 3D tomographic studies from conventional radiotherapy simulator cone-beam fluoro output. The extension consists of a common PC system on which a proprietary software toolkit provides the appropriate environment for reconstruction and acquisition of cone-beam data provided by commercial simulator fluoro video chains. The extension may compare favorably with CT options offered by simulator manufacturers: in particular, multi-slice single-scan reconstruction seems to be achievable while the current commercial solutions are based on single-slice single-scan. Radiotherapy applications include on-line treatment planning, patient treatment verification and registration.