Purpose: The objective of this study was to evaluate the feasibility of using helical tomotherapy for left‐sided breast cancer patients with involved lymph nodes. Method and Materials: Four left‐sided breast cancer patients treated using conventional multi‐port‐breast technique were retrospectively planned on Tomotherapy planning system. PTVs including chest‐wall/breast, supraclavicular, axillary and internal‐ mammary lymphnodes were contoured. Optimized treatment plans were generated on Tomotherapy TPS using 25mm field‐width with pitch of 0.42. The modulation factors varied from 1.5–2.6. All plans had a prescription of 50.4Gy to 93% and 46.9Gy to 98% of the PTV. Directional blocking was used on the right side to limit the dose to the contra‐lateral‐breast and lung. The optimization goals for planning were to protect the heart and lungs from receiving excessive doses. Resulting plans were compared against a conventional multi‐port breast technique. Lung toxicities using the Lymann‐Kutcher‐Burman model were estimated for tomotherapy plans. The parameters used for these calculations are TD50%=30.8Gy, slope(m)=0.37 and the exponent(a)=1. Results: Tomotherapy increased the minimum dose to the PTV (D99% = 44.6Gy for tomotherapy versus 30.5Gy for 3D) while improving the homogeneity index (HI = 1.16 for tomotherapy and 1.52 for 3D). The mean V20Gy for the left lung decreased from 32.6% (3D) to 16.4% (tomotherapy) while keeping the mean right lung dose well under 4Gy. However, the mean V5Gy volume increased from 26.4% (3D) to 42.6% (tomotherapy). The mean V35Gy for the heart decreased from 6.5%–2.5%, while the mean heart dose increased from 9.5Gy–11.3Gy for conventional and tomotherapy, respectively. The estimated NTCP for lung range from 1.4% to 2.4% for tomotherapy plans. Conclusion: Tomotherapy plans have better conformity and dose homogeneity than the 3D‐ plans. Tomotherapy provided improved sparing for the heart and lungs. Conflict of Interest: This work supported in part by Tomotherapy, Inc.