**4.4 Optimization objectives and procedures**

The setting of dose objective is a crucial step in inverse planning because it defines the doses to be delivered to various delineated structures. The computer then calculates the intensity modulation of the treatment field based on the definition of dose objectives [62]. While both dose-volume based objectives and biological objectives can be input in the current commercially available system, dose-volume based objectives were more commonly used. This is because it has been demonstrated that the use of generalized equivalent uniform dose (gEUD) objectives would lead to poorer homogeneities [63]. Inverse planning was first proposed in 1982 [64], in which the dose distribution was defined by planners for the calculation of beam intensity to deliver the desired dose. It is an "inverse" process when compared with the conventional "forward" approach, in which the planners define beam parameters for the calculation of dose distribution [62]. There are upper objective, lower objective and mean objective in the definition of dose-volume based objectives for a structure. A priority number is assigned for each objective to indicate their relative importance. Because the objectives to achieve target dose coverage and to avoid dose to OARs sometimes oppose to each other, the setting of priority provides information for the computer system to decide the "trade-off" between conflicting objectives.
