planning system that can deliver sufficient radiation to a tumour while both sparing critical organs and minimising the dose to healthy tissue. The required decisions include how many radiation beams to use, which angles each will be delivered from, whether attenuating wedges be used, and which MLC configuration will be used to shape the radiation from each beam. Once the treatment planner has made an initial plan, the treatment planning system calculates the required monitor units to deliver a prescribed dose to a specific area, and the distribution of dose in the body this will create. The dose distribution in the patient is dependent on the anatomy and beam modifiers such as wedges, specialized collimation, field sizes, tumor depth, etc. The information from a prior CT scan of the patient allows more accurate modelling of the behaviour of the radiation as it travels through the patient's tissues. Different dose calculation models are available, including pencil beam, convolution-superposition and monte carlo simulation, with precision versus computation time being the relevant trade-off. This type of planning is only sufficiently adept to handle relatively simple cases in which the tumour has a simple shape and is not near any critical organs. Inverse planning In inverse planning a radiation oncologist defines a patient's critical organs and tumour, after which a planner gives target doses and importance factors for each. Then, an optimisation program is run to find the treatment plan which best matches all the input criteria. In contrast to the manual trial-and-error process of forward planning, inverse planning uses the optimiser to solve the Inverse Problem as set up by the planner. See also Brachytherapy planning Image-guided radiation therapy References Radiation therapy Medical physics