A comparison of dosimetric variance for external-beam partial breast irradiation using three-dimensional and four-dimensional computed tomography

Objective: To investigate the potential dosimetric benefits of four-dimensional computed tomography (4DCT) compared to three-dimensional CT (3DCT) in the planning of radiotherapy for external-beam partial breast irradiation (EB-PBI). Methods: Three-DCT and 4DCT scan sets were acquired for 20 patients who underwent EB-PBI. For each patient a conventional 3D conformal plan (3D-CRT) was generated based on end-inhalation phase (EI). The treatment plan based on the 4DCT EI phase images was copied and applied to the end-exhalation phase (EE) and 3DCT images (defined as EB-PBI(EI), EB-PBI(EE), EB-PBI(3D), respectively). Results: The median volumes of the tumour bed based on 3DCT, EI and EE were 20.99 cm(3,) 19.28 cm(3,) and 18.78 cm(3,) respectively. The tumour bed volume based on 3DCT was significantly greater than that of EI and EE volumes (P<0.05). The planning target volumes (PTV) coverage of EB-PBI(3D), EB-PBI(EI) and EB-PBI(EE) were 96.85%, 97.51%, 97.03%, respectively. The planning target volume (PTV) coverage of EB-PBI(3D) was significantly less than that of EB-PBI(EI) and EB-PBI(EE) (P<0.05). The median homogeneity indexs (HI) based on 3DCT, EI and EE were 0.13, 0.13, 0.13, respectively. The median conformal indexs (CI) based on 3DCT, EI and EE were 0.68, 0.69, 0.68, respectively. The median mean doses (D(mean)) based on 3DCT, EI and EE were 36.20 Gy, 36.20 Gy, 36.22 Gy, respectively. However there were no significant differences in the homogeneity index, conformity index and the mean dose of PTV between the three treatment plans (P>0.05). The EB-PBI(3D) plan resulted in the largest organs at risk dose (P<0.05). Conclusion: There was a significant benefit when using 4DCT to plan 3D-CRT for EB-PBI with regard to reduced non-target organ exposure, and might result in poor dose coverage when the PTV is determined using 3DCT.