To assess the benefit of beam fringe (50%-90% dose level) sharpening for lung tumors, we performed a numerical simulation in which all geometrical errors (breathing motion, random and systematic errors) are included. A 50 mm diameter lung tumor, located centrally in a lung-equivalent phantom was modeled. Treatment plans were designed with varying number and direction of beams, both with and without the use of intensity modulation to sharpen the beam fringe. Field size and prescribed dose were varied under the constraint of a constant mean lung dose of 20 Gy, which yields a predicted complication probability of about 10%. After numerical simulation of the effect of setup errors and breathing, the resulting dose distribution was evaluated using the minimum dose and the equivalent uniform dose (EUD) in the moving clinical target volume (CTV). When the dose in the CTV was constrained between 95% and 107% of the prescribed dose, the maximum attainable EUD was 71 Gy for a four-field noncoplanar technique with simple conformal beams. When penumbra sharpening was applied using a single beam segment at the edge of the open field, this EUD could be raised to 87 Gy. For a hypothetical infinitely steep penumbra, further escalation to an EUD of 104 Gy was possible. When the dose in the CTV was not constrained, a large escalation of the EUD was possible compared to the constrained case. In this case, the maximum attainable EUD for open fields was 115 Gy, using the four-field noncoplanar technique. The benefit of penumbra sharpening was only modest, with no increase of the EUD for the single-segment technique and a small increase to 125 Gy for the infinitely steep penumbra. From these results we conclude that beam fringe sharpening in combination with field-size reduction leads to a large increase in EUD when a homogeneous target dose is pursued. Further escalation of the EUD is possible when the homogeneity constrained is relaxed, but the relative benefit of beam-fringe sharpening then decreases.