Radiotherapy planning involves the calculation of dose deposition throughout the patient, based upon quantitative electron density images from computed tomography (CT) scans taken before treatment. Cone beam CT (CBCT), consisting of a point source and flat panel detector, is often built onto radiotherapy delivery machines and used during a treatment session to ensure alignment of the patient to the plan. If the plan could be recalculated throughout the course of treatment, then margins of uncertainty and toxicity to healthy tissues could be reduced. CBCT reconstructions are normally too poor to be used as the basis of planning however, due to their insufficient sampling, beam hardening and high level of scatter. In this work, we investigate reconstruction techniques to enable dose calculation from CBCT. Firstly, we develop an iterative method for directly inferring electron density from the raw X-ray measurements, which is robust to both low doses and polyenergetic artefacts from hard bone and metallic implants. Secondly, we supplement this with a fast integrated scatter model, also able to take into account the polyenergetic nature of the diagnostic X-ray source. Finally, we demonstrate the ability to provide accurate dose calculation using our methodology from numerical and physical experiments. Not only does this unlock the capability to perform CBCT radiotherapy planning, offering more targeted and less toxic treatment, but the developed techniques are also applicable and beneficial for many other CT applications.