Background:  After breast conserving surgery, most recurrences occur around the primary tumor site. This justifies the use of boost radiotherapy to the primary site of tumor. Surgical scar is not always a good surrogate for the location of the lumpectomy cavity. The aim of this study was to evaluate ultrasonic guidance for detection of the lumpectomy cavity after breast conserving surgery for electron beam boost field planning. Materials and Methods: 35 breast cancer patients who treated with whole breast irradiation after breast conserving surgery underwent ultrasonic evaluation for measurement of depth and size of lumpectomy cavity. Results of ultrasonic imaging were used to define electron boost field size and depth of treatment. These results were compared with clinical measurements that made by two expert radiation oncologists. Results: The operative bed was well visualized in all of the patients. In only 21 patients (60%) cavity was located on the scar. Depth determined by ultrasonography was not significantly correlated with depth determined by clinical impression (r=0.304, P<0.01). This means that the results of the two methods are quite different (these results are also true for other variables). In 88% of patients, the depth of treatment and electron beam energy that were selected by clinical measurements were changed. Conclusion: Ultrasound is found useful tool for measurement of depth and size of lumpectomy cavity and could be used for boost planning. It is easily available, non-invasive and inexpensive.