Phased array applicators for deep microwave hyper-thermia struggle to reach satisfying target-to-remaining power absorption ratios in the brain area. In our opinion, this is partly because their design is meant for an overall body region rather than a particular tumor. A paradigm shift in the antenna arrangement design is thus necessary: from regional applicator arrays to target-specific ones. Exploring all the possible array configurations and simulate them, however, is computationally unfeasible. This conference contribution presents an approximated method for determining the field distribution generated by an antenna at any desired location across the surface of the so-called water bolus. This approximated field can be effectively used within an array optimization algorithm, and initial results suggest that the optimal antenna arrangement for treating a child brain tumor differs substantially from that of a regular array.