A system plans a radiation therapy treatment of a target volume based on inputs in the form of: a set of candidate beams (B), where each beam defines an arrangement of a therapeutic beam relative to the target volume； a treatment plan (x) for the radiation therapy treatment that uses a subset of the candidate beams (B)； an objective function (F) describing a quality of the treatment plan (x)； and a feasible region (X) describing requirements on the treatment plan (x) that must be fulfilled. The objective function (F) and/or the feasible region (X) also reflect a first complexity criterion (τ(x)≤；{circumflex over (τ)}) limiting a first complexity measure (τ(x)) to be less than or equal to a maximum first complexity ({circumflex over (τ)}). An optimization step is executed repeatedly； whereby, in each iteration, an updated treatment plan (x′；) is calculated by optimizing the treatment plan (x) with respect to the objective function (F) and the feasible region (X). Here, if a termination criterion is fulfilled, a set of selected beams (B*) is calculated based on the updated treatment plan (x′；). The set of selected beams (B*) is a subset of the set of candidate beams (B). Otherwise, the updated treatment plan (x′；) is set to the treatment plan (x)； an updated first complexity criterion (τ′；(x)≤；{circumflex over (τ)}′；) is calculated； the updated first complexity criterion (τ′；(x)≤；{circumflex over (τ)}′；) is set to the first complexity criterion (τ(x)≤；{circumflex over (τ)}), and another iteration of the optimization step is executed.
