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

PURPOSE: To investigate the potential dosimetric benefits from four-dimensional computed tomography (4DCT) compared with three-dimensional computed tomography (3DCT) in radiotherapy treatment planning for external-beam partial breast irradiation (EB-PBI). PATIENTS AND METHODS: 3DCT and 4DCT scan sets were acquired for 20 patients who underwent EB-PBI. The volume of the tumor bed (TB) was determined based on seroma or surgical clips on 3DCT images (defined as TB(3D)) and the end inhalation (EI) and end exhalation (EE) phases of 4DCT images (defined as TB(EI) and TB(EE), respectively). The clinical target volume (CTV) consisted of the TB plus a 1.0 cm margin. The planning target volume (PTV) was the CTV plus 0.5 cm (defined as PTV(3D), PTV(EI), and PTV(EE)). For each patient, a conventional 3D conformal plan (3D-CRT) was generated (defined as EB-PBI(3D), EB-PBI(EI), and EB-PBI(EE)). RESULTS: The PTV(3D), PTV(EI), and PTV(EE) were similar (P=0.549), but the PTV coverage of EB-PBI(3D) was significantly less than that of EB-PBI(EI) or EB-PBI(EE) (P=0.001 and P=0.025, respectively). There were no significant differences in the homogeneity or conformity indexes between the three treatment plans (P=0.125 and P=0.536, respectively). The EB-PBI(3D) plan resulted in the largest organs at risk dose. CONCLUSION: There was a significant benefit for patients when using 3D-CRT based on 4DCT for EB-PBI with regard to reducing nontarget organ exposure. Respiratory motion did not affect the dosimetric distribution during free breathing, but might result in poor dose coverage when the PTV is determined using 3DCT.