An achievability estimate is computed for an intensity modulated radiation therapy (IMRT) geometry (32) including a target volume, an organ at risk (OAR), and at least one radiation beam. Namely, a geometric complexity (GC) metric is computed for the IMRT geometry that compares a number NT of beamlets of the at least one radiation beam available in the IMRT geometry for irradiating the target volume and a number n of these beamlets that also pass through the OAR. A GC metric ratio is computed of the GC metric for the IMRT geometry and the GC metric for a reference IMRT geometry for which an IMRT plan is achievable. If the clinician is satisfied with this estimate then optimization (38) of an IMRT plan for the IMRT geometry (32) is performed. Alternatively, a reference IMRT geometry is selected by comparing the GC metric with GC metrics of past IMRT plans.