- BACKGROUND AND PURPOSE: To prospectively compare diagnostic ultrasound to the 'gold standard' of surgical clips for localization of the lumpectomy site for electron boost irradiation. PATIENTS AND METHODS: Consecutive breast cancer patients referred following lumpectomy underwent diagnostic ultrasound in radiation treatment position 21-100 days post-surgery. All patients had 3-6 surgical clips defining the excision cavity. The site was marked on the skin and depth was measured. Target depth was the deepest aspect of the cavity plus a 1 cm deep margin. Treatment fields were prescribed with a 2 cm margin on the cavity, and electron energy was chosen to cover the target depth. Surgical clip position was assessed on orthogonal simulator films. RESULTS: Localizations were performed in 54 breasts (52 women). The mean interval post-surgery was 53 (SD 17) days. Overall, 35/54 (65%) of localizations were adequate, 15/54 (28%) were marginal and 4/54 (7%) were inadequate. Regression showed that lower patient weight (r=-0.37, P=0.006) predicted adequacy of localization, with better accuracy in lighter women. CONCLUSIONS: The accuracy rate for ultrasound exceeds the 20-50% reported for clinical localization. Diagnostic ultrasound may be used to improve the accuracy when surgical clips are not present.