The present invention provides a system 10 for irradiating a breast 20 of a patient 22. The system 10 comprises a gantry 12 rotatable about a horizontal axis 14 and comprising a radiation source 16 for generating a radiation beam 18 and a detector 24 spaced from the radiation source 16, and a barrier 26 disposed between the patient 22 and the gantry 12. The barrier 26 is provided with an opening 30 adapted to allow a breast 20 passing therethrough to be exposed to the radiation beam 18. In some embodiments, the barrier 26 is provided with an opening 30 adapted to allow both the breast 20 and the tissue leading from the breast to axilla and the muscle tissue of the adjacent chest wall passing therethrough to be exposed to the radiation beam 18.