Displacement of the lumpectomy cavity defined by surgical clips and seroma based on 4D-CT scan for external-beam partial breast irradiation after breast-conserving surgery: a comparative study

Objective: To compare the displacements of the lumpectomy cavity delineated by the surgical clips and the seroma based on four-dimensional CT (4D-CT) for external-beam partial breast irradiation (EB-PBI) after breast-conserving surgery (BCS). Methods: 14 breast cancer patients after BCS were recruited for EB-PBI and undertook 4D-CT simulation. On the 10 sets of the 4D-CT images, all the surgical clips in the cavity were delineated. The gross tumour volume (GTV) formed by the clips, the seroma, and both the clips and the seroma were defined as GTV(c), GTV(s) and GTV(c+s), respectively. The displacements of the centre of mass (COM) of the clips, GTV(c), GTV(s), GTV(c+s) and the selected clips in the three-dimensional (3D) directions were recorded and compared. Results: In the left-right, anterior-posterior and superior-inferior directions, the displacements were 2.20, 1.80 and 2.70 mm for the clip COM; 0.90, 1.05 and 1.20 mm for GTV(c); 0.80, 1.05 and 0.80 mm for GTV(s); and 0.90, 1.20 and 1.40 mm for GTV(c+s), respectively. In the 3D directions, the displacements of the clip COM were greater than the GTV(c), GTV(s), GTV(c+s), and the displacements of the clip COM, GTV(c+s), GTV(c) and GTV(s) were significantly greater than the displacements of the selected clips (p<0.05). Conclusion: The displacements of the clip COM were greater than that of the GTV(c), GTV(s), GTV(c+s) and the four selected clips. The optimal internal target volume should be defined based on the boundary displacements. Advances in knowledge: When the GTV was delineated using the clips and/or the seroma, there was displacement difference between the lumpectomy cavity centre and the boundary for the EB-PBI. The optimal internal target volume should be defined based on the boundary displacements of the lumpectomy cavity.