We investigated the interfractional dose variation due to seromas in radiotherapy of breast cancer patients. For 3 patients who received seroma aspiration during the period of radiotherapy, 2 sets of computed tomography (CT) scanning images were obtained before and after seroma aspiration. Three sets of plans employing a conventional technique and an intensity-modulated radiotherapy (IMRT) technique were generated: the first set of plans was the optimal plan for the CT images before seroma aspiration, the second set was the plans that applied the treatment parameters with the first plans but used CT images obtained after seroma treatment, and the third set was the optimal plans for CT images taken after seroma treatment. From the analysis of each set of plans, we found that the patient anatomy change had little effect on the prescription dose (1-2% variation for both techniques) but had significant effects on the dose homogeneity in the treatment volume, which increased the dose inhomogeneity up to 13.9% for conventional treatment and 20.7% for IMRT treatment, respectively.