Comparison of electron and X-ray beams for tumor bed boost irradiation in breast-conserving treatment

Purpose: This study aimed to compare the dosimetric profiles of electron beams (EB) and X-ray beams (XB) for boosting irradiation in breast cancer patients who underwent breast-conserving surgery and postoperative radiotherapy. Methods: For 131 breast cancer patients who underwent breast-conserving surgery, we compared plans for EB and XB boost irradiation after wholebreast irradiation. The organs at risk (OAR) included the cardiac chambers, coronary arteries, ipsilateral lung, and skin. The conformity index (CI), inhomogeneity index (IHI), and dose-volume parameters for the planning target volume (PTV), and OAR were calculated. Postradiotherapy chest computed tomography scans were performed to detect radiation pneumonitis. Results: XB plans showed a significantly better CI and IHI for the PTVs, compared to the EB plans. Regarding OAR sparing, the XB reduced the high-dose volume at the expense of an increased low-dose volume. In 33 patients whose radiation fields included nipples, IHI was higher in the EB plans, whereas the presence of a nipple in the radiation field did not interfere with the XB. EBtreated patients developed more subclinical radiation pneumonitis. Conclusion: XB plans were superior to EB plans in terms of PTV coverage (homogeneity and conformity) and high-dose volume sparing in OAR when used as boost irradiation after breastconserving surgery. A disadvantage of the XB plan was an increased low-dose volume in the OAR, but this was offset by the increased electron energy. Consequently, tailored plans with either XB or EB are necessary to adapt to patient anatomic variance and tumor bed geometric properties.