Evaluation of beam-on time and number of breath-holds using a flattening-filter-free beam with the deep inspiration breath-hold method in left-sided breast cancer.

We performed a dosimetric study to evaluate the benefits of using a flattening-filter-free (FFF) beam with the deep inspiration breath-hold (DIBH) method for left-breast cancer. We used data from 30 previous patients with treatment plans that included DIBH for left-breast cancer with a flattened beam. FFF beam plans were calculated from previous treatment plan images and compared to the original plans in terms of monitor units (MU), number of segments, beam-on time, and breath-holds. Beam-on time was calculated by adding the traveling time of 1.5 second between segments to the time calculated from the MU and dose rate. Breath-holds were calculated based on the beam-on time, assuming 15 s per hold. The FFF beam had increased MU in all cases (mean ± SD: flattened beam, 122.4 ± 9.8 MU; FFF beam, 160.2 ± 17.5 MU). Furthermore, the number of segments increased with the FFF beam in all cases (median [range]: flattened beam, 2 [1 to 3]; FFF beam, 5 [3 to 7]). However, in most cases, the beam-on time was reduced using the FFF beam (mean ± SD: flattened beam, 27.8 ± 7.4 seconds; FFF beam, 13.2 ± 1.7 seconds), although when a 6 MV flattened beam was used there was not a large increase. There were fewer breath-holds in most cases with the FFF beam. Cases using a 4 MV flattened beam also had fewer breath-holds; however, the number of breath-holds was consistent or increased in cases that used a 6 MV flattened beam (median [range]: flattened beam, 3 [1 to 3]; FFF beam, 1 [1 to 2]).