To evaluate the performance of ten different treatment-planning systems when intensity modulated (IMRT) plans are designed for breast treatments that include the irradiation of the internal mammary chain. A dataset of five patients (CT images and volumes of interest) was distributed to design IMRT plans on the ten systems. To minimise biases, the same geometry and clinical planning aims were imposed on the individual plans. Results were analysed in terms of dose distributions and dose volume histograms. For target coverage, the volume receiving more than 95% of the prescribed dose ranged from 77% (OTP) to 91% (Eclipse and Pinnacle), the volume receiving more than 107% ranged from 3.3% (Hyperion) to 23.2% (OTP). The mean dose to ipsilateral lung ranged from 13Gy (Eclipse) to 18Gy (OTP). The volume of the contralateral breast receiving more than 10Gy ranged from 3% (Pinnacle) to 26% (Precise). The volume of heart receiving more than 20Gy ranged from 7% (Eclipse) to 47% (Precise), the maximum significant dose to heart ranged from ∼27Gy (XiO) to ∼49Gy (Precise). The maximum significant dose to healthy tissue ranged from ∼51Gy (Eclipse) to ∼62Gy (OTP). It was also possible to show that the treatment geometry proposed here enables to minimise contralateral breast irradiation while keeping minimal ipsilateral lung (or heart) involvement and satisfactory target coverage.