Dose to contralateral breast from whole breast irradiation by automated tangential IMRT planning: comparison of flattening-filter and flattening-filter-free modes

BACKGROUND: The most common secondary cancer is contralateral breast (CLB) cancer after whole breast irradiation (WBI). The aim of this study was to quantify the reduction of CLB dose in tangential intensity modulated radiotherapy (t-IMRT) for WBI using flattening-filter-free (FFF) beams. MATERIALS AND METHODS: We generated automated planning of 20 young breast cancer patients with limited user interaction. Dose-volume histograms of the planning target volume (PTV), ipsilateral lung, heart, and CLB were calculated. The dose of PTV, the most medial CLB point, and the CLB point below the nipple was measured using an ionization chamber inserted in a slab phantom. We compared the two t-IMRT plans generated by FFF beams and flattening-filter (FF) beams. RESULTS: All plans were clinically acceptable. There was no difference in the conformal index, the homogeneity for FFF was significantly worse. For the ipsilateral lung, the maximum dose (D(max)) was significantly higher; however, V(20) showed a tendency to be lower in the FFF plan. No differences were found in the D(max) and V(30) to the heart of the left breast cancer. FF planning showed significantly lower D(max) and mean dose to the CLB. In contrast to the calculation results, the measured dose of the most medial CLB point and the CLB point below the nipple were significantly lower in FFF mode than in FF mode, with mean reductions of 21.1% and 20%, respectively. CONCLUSIONS: T-IMRT planning using FFF reduced the measured out-of-field dose of the most medial CLB point and the CLB point below the nipple.