Using tomotherapy to deliver adjuvant radiation therapy for breast cancer treatment requires more precise immobilization than can be achieved using gravity alone. We evaluated the use of a thermoplastic shell to immobilize the patient's torso during breast cancer treatment. To measure intrafraction breathing motion, 4DCT scans were performed for eight post-lumpectomy or post-mastectomy breast cancer patients with the thermoplastic shell in place. The 4DCT scans were then analyzed to determine the magnitude of motion of the breast surface, chest wall, and heart over the breathing cycle. Maximum surface motion was typically less than 2mm, with a maximum of 4mm. Maximum displacement of the chest wall was less than 3mm with a maximum of 5mm in a single patient. Comparison with the setup errors recorded prior to repositioning the patients suggests that, with the thermoplastic shell in place, patient setup error will be a more significant source of uncertainty in patient position than breathing motion.