An efficient direct parameter optimization (DPO) approach is provided, in which a dosimetry gradient-based iterative greedy technique is applied to determine positioning assistance information for catheter insertion in focal radiation therapy, by toggling between optimization of candidate catheter positions and candidate radiation source dwelling times. In the specific case of free hand delivered catheters, the insertion point may be directly selected by the human expert. Using monitoring of the catheter position by additional imaging modalities such as magnetic resonance imaging or ultrasound imaging, the method enables real-time orientation guidance and adaptive dose correction in the course of treatment delivery.