A positioning assistance device for assisting in positioning of catheters in a focal radiation therapy with respect to a pre-determined target volume and a given catheter insertion point, the positioning assistance device comprising: a candidate catheter providing unit, which is configured to provide candidate catheter data associated with the given catheter insertion point and defining candidate catheters by a respective candidate inserted-catheter position, respective candidate dwelling positions of radiation sources inside a given candidate catheter, and respective associated candidate dwelling times; a catheter pre-selection unit, which is configured to ascertain and provide a gradient with respect to dwelling time of a composite constraint function for each current candidate catheter, using the current candidate dwelling times at associated current candidate dwelling positions of the candidate catheter, the composite constraint function combining at least two linear constraints representing clinical objectives with respect to radiation dose governing the focal radiation therapy; and to pre-select a predetermined number of one or more candidate catheters from the candidate catheters, which are associated with a steepest descent value of the gradient among the candidate catheters; a dwelling-time pre-selection unit, which is configured to ascertain and pre-select, using the current pre-selected candidate catheters, those current candidate dwelling times for the respective candidate dwelling positions, which achieve a minimum of a scalar composite constraint function of dwelling time combining the at least two linear constraints; a positioning assistance control unit, which is configured to alternatingly drive operation of the catheter pre-selection unit and the dwelling-time pre-selection unit over a plurality of iteration cycles, and to provide, upon detecting that in a current iteration cycle the catheter pre-selection unit has provided a non-negative value of the gradient of the composite constraint function for at least one of the candidate catheters, an output indicative a next target catheter position as that of the at least one current pre-selected candidate catheter. a candidate catheter providing unit, which is configured to provide candidate catheter data associated with the given catheter insertion point and defining candidate catheters by a respective candidate inserted-catheter position, respective candidate dwelling positions of radiation sources inside a given candidate catheter, and respective associated candidate dwelling times; a catheter pre-selection unit, which is configured to ascertain and provide a gradient with respect to dwelling time of a composite constraint function for each current candidate catheter, using the current candidate dwelling times at associated current candidate dwelling positions of the candidate catheter, the composite constraint function combining at least two linear constraints representing clinical objectives with respect to radiation dose governing the focal radiation therapy; and to pre-select a predetermined number of one or more candidate catheters from the candidate catheters, which are associated with a steepest descent value of the gradient among the candidate catheters; ascertain and provide a gradient with respect to dwelling time of a composite constraint function for each current candidate catheter, using the current candidate dwelling times at associated current candidate dwelling positions of the candidate catheter, the composite constraint function combining at least two linear constraints representing clinical objectives with respect to radiation dose governing the focal radiation therapy; and to pre-select a predetermined number of one or more candidate catheters from the candidate catheters, which are associated with a steepest descent value of the gradient among the candidate catheters; a dwelling-time pre-selection unit, which is configured to ascertain and pre-select, using the current pre-selected candidate catheters, those current candidate dwelling times for the respective candidate dwelling positions, which achieve a minimum of a scalar composite constraint function of dwelling time combining the at least two linear constraints; a positioning assistance control unit, which is configured to alternatingly drive operation of the catheter pre-selection unit and the dwelling-time pre-selection unit over a plurality of iteration cycles, and to provide, upon detecting that in a current iteration cycle the catheter pre-selection unit has provided a non-negative value of the gradient of the composite constraint function for at least one of the candidate catheters, an output indicative a next target catheter position as that of the at least one current pre-selected candidate catheter.