Background: The aim of this study was to evaluate the reduction in excision cavity volume (ECV) during the whole breast radiotherapy (WBRT). The delineating of reduced cavity volume on secondary CT (computed tomography) might decrease the field size of boost plan. Materials and Methods: Twenty patients were treated having breast conservative surgery. At first, primary CT simulation (CT-1) was done prior to WBRT. Treatment planning was performed by a 3D treatment planning system. A CT-2 was performed after WBRT. Then the excision cavity was contoured on CT-1 and CT-2 for each patient. Boost irradiation was planned on both ECV-1 and ECV-2. Results: In Comparison of CT-1 and CT-2: The contoured volumes for ECV-1 and ECV-2 were on average of 42.9 cm3and 23.4 cm3, respectively (p< 0.002). The ECV-2 was reduced in 85% of the patients. Patients with ECV-1 greater than the mean value of ECV-1 (ECV-1> 35.5 cm3) had more reduction in ECV-2 than patient with ECV-1 smaller than the mean value (p < 0.000). The volume of normal breast tissue on CT-2 was decreased from the volume of CT-1 (p< 0.03). Conclusion: A significant reduction in excision cavity volume was shown during WBRT. This volume reduction made smaller sizes of boost field and remaining breast received lower doses. The reduction was more significant in patient with primary large ECV. The Secondary CT for boost planning is suggested for patients with larger cavities after WBRT.