Disclosed is a computer-implemented medical data processing method for determining an irradiation trajectory for the movement of a treatment device for irradiating an anatomical structure with ionising treatment radiation, the method comprising executing, on at least one processor of at least one computer, steps of: a) acquiring (S1), at the at least one processor, medical image data describing a medical image of the anatomical structure, wherein the anatomical structure comprises both a target region which defines a target of the irradiation and non-target tissue, wherein irradiation of the non-target tissue shall be avoided； b) determining (S2), by the at least one processor and based on the medical image data, image concavity data describing a value of a measure of concavity of at least part of the target region described by the medical image； c) acquiring (S3), at the at least one processor, predetermined concavity data describing a predetermined value of the measure of concavity for the at least part of the target region； d) determining (S4), by the at least one processor and based on the image concavity data and the predetermined concavity data, partition data describing whether the at least part of the target region is to be partitioned into target region partitions； e) if the partitioning data describes that the target region is to be partitioned, determining (S5), by the at least one processor and based on the medical image data, partition boundary data describing the position of a partition boundary between the target region partitions, wherein the position of the partition boundary is defined in a reference system in which positions in the medical image are defined； and f) determining (S6), by the at least one processor and based on the partition boundary data, irradiation trajectory data describing at least one trajectory of the treatment device usable for irradiating a target region partition with the ionising treatment radiation, wherein at least one t
