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Purpose.--Three-dimensional (3D) treatment planning has reduced the cardiac dose in postoperative radiotherapy for breast cancer; however, the overall cardiac toxicity is still an issue because of more aggressive adjuvant treatment. Toxicity models have suggested that a reduction of the heart volume treated to high doses might be particularly advantageous. We compared aperture-based multifield intensity-modulated radiotherapy (IMRT) plans to 3D-planned tangent fields using dose-volume histograms, cardiac toxicity risk, and the robustness to positioning errors. Methods and Materials.--For 14 computed tomography data sets of patients with left-sided breast cancer (unfavorable thoracic geometry), a 3D treatment plan and an IMRT plan were created. The dose-volume histograms were evaluated for the target and risk organs. Excess risk of cardiac mortality was calculated for both approaches using a relative seriality model. Positioning errors were simulated by moving the isocenter. Results.--IMRT reduced the maximal dose to the left ventricle by a mean of 30.9% (49.14 vs. 33.97 Gy). The average heart volume exposed to >30 Gy was reduced from 45 cm³ to 5.84 cm³. The mean dose to the left ventricle was reduced by an average of 10.7% (10.86 vs. 9.7 Gy), and the mean heart dose increased by an average of 24% (from 6.85 to 8.52 Gy). The model-based reduction of the probability for excess therapy-associated cardiac death risk was from 6.03% for the 3D plans to 0.25% for the IMRT plans. Conclusion.--Aperture-based IMRT for left-sided breast cancer significantly reduces the maximal dose to the left ventricle, which might translate into reduced cardiac mortality. Biological modeling might aid in deciding to treat with IMRT but has to be validated prospectively. [ABSTRACT FROM AUTHOR]