A conventional radiotherapy simulator is a Kv x-ray machine and detector, that is attached to a machine that emulates the movements of a radiotherapy treatment machine (linear accelerator (linac)). Traditionally, it's role was to plan all types of radiotherapy treatment. It takes 2D x-rays which the planning department can convert into a 3D drawing.
The machines main limitation was its inability to accurately distinguish the different densities of areas,
such as bone and air, making treatment panning more of an art form than a science! Because of this, their role in planning radical radiotherapy treatments, is mainly redundant in many centers across the UK and the sale of such machines is now almost non-existant in the UK and North America. That being said they are useful for checking radiotherapy plans and planning palliative treatments very quickly and efficiently.
CT Simulators took over from the conventional simulators because they have the major advantage of being able to construct a 3D image of the patient. They are able to accurately locate and record the different densities of tissue, organs and bones in the human body meaning these can be accounted for in the radiotherapy planning. These 3D images and detailed information, enable the treatment planners to create plans that are tailored very specifically to each individual patients specific size and shape.