To evaluate the dosimetric effects of target volume delineation with metal clip and seroma, alone or in combination, on external-beam partial breast irradiation (EB-PBI) based on four-dimensional computed tomography (4DCT).

Twenty female patients undergoing EB-PBI from 2009 to 2013 were enrolled as subjects. The gross tumor volumes (GTV_S), GTV_C, GTV_S, and GTV_{C+ S}, were delineated on 4DCT images at 10 phases using metal clip, seroma, and both of them, respectively. The GTV_S on 4DCT images at 10 phases were fused to generate the internal gross tumor volumes (IGTV_S), IGTV_C, IGTV_S, and IGTV_{C+ S}. The planning target volumes (PTV_S), PTV_C, PTV_S, and PTV_{C+ S}, were obtained via expansion of margin by 15 mm. The three-dimensional conformal radiotherapy plans were made by one physician based on PTV_C, PTV_S, and PTV_{C+ S} on end-inhalation images. The target volume, homogeneity index (HI), conformity index (CI), and doses to organs at risk were compared between the three groups.

The _{C+ S} group had the largest IGTV, PTV, and the ratio of PTV to diseased breast volume, which was followed by the C group and the S group (all P<0.05). The S group had significantly lower doses to the ipsilateral normal breast and lung than the C group and the _{C+ S} group (all P<0.05). There were no significant differences in HI or CI between the three groups (all P>0.05).

The volume variation caused by target volume delineation on 4DCT images based on different references has little impact on dose distribution in target volume. However, it has substantial impact on radiation doses to the ipsilateral normal breast and lung.

External-beam partial breast irradiation; Tmography, X-ray computed, four-dimensional; Surgical clips; Seroma; Target determination