IMRT has greatly enhanced our ability to conform dose to target structures while minimizing dose to normal tissues. This improved conformality mandates that targets be clearly defined and target motion be critically assessed. We set out to analyze breast motion due to respiration using 4D CT scans for women undergoing IMRT breast radiotherapy at Fox Chase Cancer Center (FCCC) and to determine whether our current margins are adequate to cover the entire breast in all phases of the respiratory cycle during normal quiet breathing. 20 women, 10 with left breast cancer and 10 with right breast cancer, who were simulated for breast radiotherapy at FCCC underwent a free breathing 4D CT scan. An average intensity projection scan (ave) was constructed from the 4D dataset. The 4D scan was analyzed in detail in order to identify the two phases of the respiratory cycle which represented the extremes of respiration during normal quiet breathing–maximum inspiration (MI) and maximum expiration (ME). The breast, ipsilateral lung and, for left-sided breast cancers, the heart, were contoured on the average scan as well as on the two phases mentioned above. Breast motion was measured on the 4D scan in three planes: axial, sagittal and coronal. The axial motion was measured at the chest wall, the coronal motion was measured at the inframammary fold and the sagittal motion was measured at the breast surface. These landmarks represented the points of maximum motion observed on the 4D scan in each plane. In order to assess the coverage of the breast and the doses to the normal structures at the extremes of the respiratory cycle, IMRT plans were generated for the ave scan as well as for the MI and ME phases. The IMRT plan generated from the ave scan was used to treat the patient. Each IMRT plan was then analyzed and compared using the parameters which are used to evaluate all breast IMRT plans at FCCC. For the 20 patients analyzed, the mean motion in the axial direction was 1.9 mm (range 0–4 mm), in the coronal direction was 0.7 mm (range 0–2 mm) and in the sagittal direction was 2.0 mm (range 0–4 mm). For 7 patients, the sagittal motion was greatest and for 8 patients, the axial motion was the greatest. For all but 1 patient, the coronal motion was the least. PTVs were defined as breast tissue plus 2 cm superiorly and inferiorly and 7 mm posteriorly. Dose volume parameters were analyzed for the first 10 patients. The median PTV volume for the ave scan was 748 cc, for the MI phase was 733 cc and for the ME phase was 728 cc. The V95 for the PTV for the ave plan 98.4% vs. 96.3% for the MI plan and 96.5% for the ME plan. (The FCCC goal for the V95 is ≥95%). None of the other dose-volume parameters analyzed (V100, V105, D99, D1, mean dose to PTV, min dose to PTV and max dose to PTV) were significantly different between the ave and the MI and ME plans. The V20 for the lung was also analyzed and was not significantly different between the ave, MI and ME plans. 4D CT simulation is unnecessary for breast radiotherapy treatment planning, as the motion of the breast is minimal during normal quiet breathing. The present IMRT methods being utilized at our institution are sufficient and no additional margin to account for respiratory motion is necessary.