Dosimetry and Treatment Planning
The section deals with the technical aspects of external beam radiotherapy. This includes regulation of the machine outputs, planning of treatments, designing customised treatment aids, and consulting on the patient set-up for difficult or unusual set-ups. The nature of the work requires that the treatment planning department is a focus for liaison between all the professional groups involved in providing radiotherapy treatments. There are four physicists and six dosimetrists working in the planning department.
Dosimetry refers to the measurement of machine outputs. Radiotherapy machines deliver monitor units but radiotherapy treatments are measured in Grays. When a doctor prescribes a treatment he will define it in terms of dose in Grays required at a point. It is the responsibility of the physics department to calculate the number of monitor units necessary to give the required dose.
Radiation is measured using ion chambers. These are calibrated with respect to an standard ion chamber kept at the National Physics Laboratory in London. Every ion chamber in the country is referenced to this standard chamber. This ensures that a 60 Gy dose given in Leeds is the same as a 60 Gy dose given in Manchester. National physics laboratories of different countries regularly compare their standards to ensure uniformity throughout the World.
Treatment planning refers to the process through which a patients treatment is designed. It begins with the doctor determing the areas to be treated and avoided, and ends with the doctor prescribing a planned treatment. The complexity ofa plan can vary from a simple single field plan, to a complex plan with nine non-coplanar beams.
The role of the physics department is to turn the doctors instructions into a viable plan. For the more common types of treatment this will consist of applying beam weights and modifiers to a standard beam arrangement, whereas for a more advanced treatment the doctor will specify a target and any areas to be avoided, and the planner will determine all the treatment parameters.
Treatment plans used to be calculated by hand but of course modern centres now all use computers for all but the most simple plans. To cope with the millenium bug and help prepare for the introduction of more sophisticated treatment protocols the Cookridge Cancer Centre Appeal has provided us with two new planning systems. These are being commissioned at the moment and will come online next year as the old planning system is being phased out