A radiation therapy planning device comprising: a computer (20, 40) including a display device; and at least one non-transitory storage medium storing instructions readable and executable by the computer to perform operations including: computing an achievability estimate for an intensity modulated radiation therapy (IMRT) geometry, wherein the achievability estimate provides an estimate of the likelihood that an IMRT plan can be generated for the IMRT geometry, and wherein the IMRT geometry includes a target volume to be irradiated, an organ at risk of damage due to undesired radiation exposure, and at least one radiation beam, and wherein the achievability estimate is computed based on relative spatial arrangement of the target region, the OAR, and the at least one radiation beam; and displaying, on the display device, the achievability estimate or information generated based on the achievability estimate, wherein the achievability estimate is computed based on a geometric complexity metric for the IMRT geometry that compares (i) a number N T of beamlets of the at least one radiation beam available in the IMRT geometry for irradiating the target volume that intersect with the target volume and (ii) a number n of the N T beamlets that intersect the target volume and that also intersect the OAR, wherein n is less than N T, and wherein the comparison of the number N T of beamlets that intersect the target volume and the number n of the N T beamlets that intersect the target volume and that also intersect the OAR comprises a ratio of n to N T, and wherein the likelihood that the IMRT plan can be generated for the IMRT geometry decreases the closer n is to N T. a computer (20, 40) including a display device; and at least one non-transitory storage medium storing instructions readable and executable by the computer to perform operations including: computing an achievability estimate for an intensity modulated radiation therapy (IMRT) geometry, wherein the achievability estimate provides an estimate of the likelihood that an IMRT plan can be generated for the IMRT geometry, and wherein the IMRT geometry includes a target volume to be irradiated, an organ at risk of damage due to undesired radiation exposure, and at least one radiation beam, and wherein the achievability estimate is computed based on relative spatial arrangement of the target region, the OAR, and the at least one radiation beam; and displaying, on the display device, the achievability estimate or information generated based on the achievability estimate, wherein the achievability estimate is computed based on a geometric complexity metric for the IMRT geometry that compares (i) a number N T of beamlets of the at least one radiation beam available in the IMRT geometry for irradiating the target volume that intersect with the target volume and (ii) a number n of the N T beamlets that intersect the target volume and that also intersect the OAR, wherein n is less than N T, and wherein the comparison of the number N T of beamlets that intersect the target volume and the number n of the N T beamlets that intersect the target volume and that also intersect the OAR comprises a ratio of n to N T, and wherein the likelihood that the IMRT plan can be generated for the IMRT geometry decreases the closer n is to N T. computing an achievability estimate for an intensity modulated radiation therapy (IMRT) geometry, wherein the achievability estimate provides an estimate of the likelihood that an IMRT plan can be generated for the IMRT geometry, and wherein the IMRT geometry includes a target volume to be irradiated, an organ at risk of damage due to undesired radiation exposure, and at least one radiation beam, and wherein the achievability estimate is computed based on relative spatial arrangement of the target region, the OAR, and the at least one radiation beam; and displaying, on the display device, the achievability estimate or information generated based on the achievability estimate, wherein the achievability estimate is computed based on a geometric complexity metric for the IMRT geometry that compares (i) a number N T of beamlets of the at least one radiation beam available in the IMRT geometry for irradiating the target volume that intersect with the target volume and (ii) a number n of the N T beamlets that intersect the target volume and that also intersect the OAR, wherein n is less than N T, and wherein the comparison of the number N T of beamlets that intersect the target volume and the number n of the N T beamlets that intersect the target volume and that also intersect the OAR comprises a ratio of n to N T, and wherein the likelihood that the IMRT plan can be generated for the IMRT geometry decreases the closer n is to N T.